Eccentricity, Obliquity, Precession, and Glaciation

The glacial cycles of the last few million years were highly determined by earth's orbital mechanics.

Naturalism as a philosophy came into its own when Newton explained the heavens as a machine, not a pantheon. It was stunning to realize that age-old mysteries were thoroughly explicable and that, if we kept at it with a bit of diligence and intellectual openness, we could attain ever-widening vistas of understanding, which now reach to the farthest reaches of the universe. 

In our current day, the mechanics of Earth's climate have become another example of this expansion of understanding, and, sadly, another example of resistance to naturalism, to scientific understanding, and ultimately to the stewardship of our environment. It has dawned on the scientific community (and anyone else willing to look) over the last few decades that our industrial production of CO2 is heating the climate, and that it needs to stop if the biosphere is to be saved from an ever-more degrading crisis. But countervailing excuses and interests abound, and we are now ruled by an adminstration in the US whose values run toward lies and greed, and which naturally can not abide moral responsibility.

The Cenozoic, our present age after the demise of the dinosaurs, has been characterized by falling levels of CO2 in the atmosphere. This has led to a progression from very warm climates 50 mya (million years ago) to ice ages beginning roughly 3 mya. The reasons for this are not completely clear. There has been a marked lack of vocanism, which is one of main ways CO2 gets back into the atmosphere. This contrasts strongly with ages of extreme volcanism like the Permian-Triassic boundary and extinction events, about 250 mya. It makes one think that the earth may be storing up a mega-volcanic event for the future. Yeet plate tectonics has kept plugging along, and has sent continents to the poles, where they previously hung out in more equatorial locations. That makes ice ages possible, giving glaciers something to glaciate, rather than letting ocean circulation keep the poles temperate. Additionally, the uplift of the Himalayas has dramatically increased rock exposure and weathering, which is the main driver of CO2 burial, by carbonate formation. And on top of all that has been the continued evolution of plant life, particularly the grasses, which have extra mechanisms to extract CO2 out of the atmosphere.

CO2 in the atmosphere has been falling through most of the Cenozoic.

All this has led to the very low levels of CO2 in the atmosphere, which have been stable at about 300 ppm over the last million years, very gradually declining prior to that time. Now we are pushing 420 ppm and beyond, which the biosphere has not seen for ten million years or more, and doing so at speeds that no amount of evolution can accommodate. The problem is clear enough, once the facts are laid out.

But what about those glaciations, which have been such a dramatic and influential feature of Earth's climate over the last few million years? They have followed a curious periodicity, advancing and retreating repeatedly over this time. Does that have anything to do with CO2? It turns out that it does not, and we have to turn our eyes to the heavens again for an explanation. It was Milankovitch, a century ago, who first solidified the theory that the changing orbital parameters of Earth, and particularly the intensity of the sun in the Northern hemisphere, where most of the land surface of Earth lies, that causes this repetitive climatic behavior.  

Cycles of orbital parameters and glaciation, over a million years.

It was in 1976 that a more refined analysis put a mathematical model and better data behind the Milankovitch cycles, showing that one major element of our orbit around the sun- the variation of eccentricity- had the greatest overall effect on the 100,000 year periodicity of recent glacial cycles. Eccentricity is how skewed our orbit is from round-ness, which varies slightly over time, due to interactions with other planets. Secondly, the position of the Earth's tilt at various points of this eliptical orbit, whether closer to the sun in northern summer, or father away, has critical effects on net solar input and on glaciation. The combined measure is called the precessional index, expressing the earth-sun distance in June. The eccentricity itself has a period of about 93,000 years, and the precessional index has a periodicity of 21,000 years. As glacial cycles over the last 800,000 years have had a strong 100,000 year periodicity, it is clearly the eccentricity alone that has the strongest single effect.

Lastly, there is also the tilt of the Earth, called obliquity, which varies slightly with a 40,000 year cycle. A recent paper made a major claim that they had finally solved the whole glaciation cycle in more detail than previously, by integrating all these cycles into a master algorithm for when glaciations start/end. They were curious about exactly what drives the deglaciation phase, within the large eccentricity-driven energetic cycle. The rule they came up with, again using better data and more complicated algorithms, is that it reaches its maximum rate when, after a minimum of eccentricity, the precession parameter (the purple line, below) has reached a peak, and the obliquity parameter (the green line, below) is rising. That is, when the Earth's degree of tilt and closeness to the sun in Norther summer are mutually reinforcing. There are also lags built into this, since it takes one or two thousand years for these orbital effects to build heat up in the climate system, a bit like spring happening annually well after the equinox.

"We find that the set of precession peaks (minima) responsible for terminations since 0.9 million years ago is a subset of those peaks that begin (i.e., the precession parameter starts decreasing) while obliquity is increasing. Specifically, termination occurs with the first of these candidate peaks to occur after each eccentricity minimum."

 

 

Summary diagram from Barker, et al. At the very top is a synopsis of the orbital variables. At bottom are the glacial cycles, marked with yellow dots (maximum slope of deglaciation), red dots (maximum extent of deglaciation) and blue dots (maximum slope of reglaciation, also called inception). Above this graph is an analysis of the time spans between the yellow and red dots, showing the strength of each deglaciation (gray double arrows). They claim that this strength is proportion to an orbita parameter illustrated above with the T-designation of each glacial cycle. This parameter is precession lagged by obliquity. Finally in the upper graph, the orbital cycles are shown directly, especially including eccentricity in gray, and the time points of the yellow nodes are matched here with purple nodes, lagged with the preceeding (by ~2,000 years) rising obliquity as an orange node. The green verticle bars were applied by me to ease the clear correlation of eccentricity maxima vs deglaciation maxima.

I have to say that the communication of this paper is not crystal clear, and the data a bit iffy. The T5 deglaciation, for instance, which is relatively huge, comes after a tiny minimum of eccentricity and at a tiny peak of precession, making the scale of the effect hard to understand from the scale of the inputs. T3 shows the opposite, with large inputs yielding a modest, if extended, deglacial cycle. And the obliquity values that are supposed to drive the deglaciation events are quite scattered over their respective cycle. But I take their point that ultimately, it is slight variations in the solar inputs that drive these cycles, and we just need to tease out / model the details to figure out how it works.

There is another question in the field, which is that, prior to 800,000 years ago, glacial cycles were much less dramatic, and had a faster cadence of about 40,000 years. This is clearly more lined up with the obliquity parameter as a driver. So while obliquity is part of the equation in the recent period, involved in triggering deglaciation, it was the primary driver a million years ago, when CO2 levels were perhaps slightly higher and the system didn't need the extra push from eccentricity to cycle milder glaciations. Lastly, why are the recent glacial cycles so pronounced, when the orbital forcing effects are so small and take thousands of years to build up? Glaciation is self-reinforcing, in that higher reflectivity from snow / ice drives down warming. Conversely, retreat of glaciers can release large amounts of built-up methane and other forms of carbon from permafrost, continental shelves, the deep ocean, etc. So there may be some additional cycle, such as a smaller CO2 or methane cycle, that halts glaciation at its farthest extent- that aspect remains a bit unclear.

Overall, the earlier paper of Hays et al. found that summer insolation varies by at most 10% over Earth's various orbital cycles. That is not much, yet it drives glaciation of ice sheets thousands of feet thick, and reversals back to deglaciation that uncovers bare rock all over the far north. It shows that Earth's climate is extremely sensitive to small effects. The last time CO2 was as high as it is now, (~16 mya), Greenland was free of ice. We are heading in that direction very rapidly now, in geological terms. Earth has experienced plenty of catastrophes in the past, even some caused biologically, such as the oxygenation of the atmosphere. But this, what we are doing to the biosphere now, is something quite new.

• That new world order we were working on...
• Degradation and corruption at FAA.. what could go wrong?
• Better air.
• Congress has the power, should it choose to use it.
• Ongoing destruction, degradation.
• Oh, Canada!

Proving Evolution the Hard Way

Using genomes and codon ratios to estimate selective pressures was so easy... why is it not working?

The fruits of evolution surround us with abundance, from the tallest tree to the tiniest bacterium, and the viruses of that bacterium. But the process behind it is not immediately evident. It was relatively late in the enlightenment before Darwin came up with the stroke of insight that explained it all. Yet that mechanism of natural selection remains an abstract concept requiring an analytical mind and due respect for very inhuman scales of the time and space in play. Many people remain dumbfounded, and in denial, while evolutionary biology has forged ahead, powered by new discoveries in geology and molecular biology.

A recent paper (with review) offered a fascinating perspective, both critical and productive, on the study of evolutionary biology. It deals with the opsin protein that hosts the visual pigment 11-cis-retinal, by which we see. The retinal molecule is the same across all opsins, but different opsin proteins can "tune" the light wavelength of greatest sensitivity, creating the various retinal-opsin combinations for all visual needs, across the cone cells and rod cells. This paper considered the rhodopsin version of opsin, which we use in rod cells to perceive dim light. They observed that in fish species, the sensitivity of rhodopsin has been repeatedly adjusted to accommodate light at different depths of the water column. At shallow levels, sunlight is similar to what we see, and rhodopsin is tuned to about 500 nm, while deeper down, when the light is more blue-ish, rhodopsin is tuned towards about 480 nm maximum sensitivity. There are also special super-deep fish who see by their own red-tinged bioluminescence, and their rhodopsins are tuned to 526 nm. 

This "spectrum" of sensitivities of rhodopsin has a variety of useful scientific properties. First, the evolutionary logic is clear enough, matching the fish's vision to its environment. Second, the molecular structure of these opsins is well-understood, the genes are sequenced, and the history can be reconstructed. Third, the opsin properties can be objectively measured, unlike many sequence variations which affect more qualitative, difficult-to-observe, or impossible-to-observe biological properties. The authors used all this to carefully reconstruct exactly which amino acids in these rhodopsins were the important ones that changed between major fish lineages, going back about 500 million years.

The authors' phylogenetic tree of fish and other species they analyzed rhodopsin molecules from. Note how mammals occupy the bottom small branch, indicating how deeply the rest of the tree reaches. The numbers in the nodes indicate the wavelength sensitivity of each (current or imputed) rhodopsin. Many branches carry the author's inference, from a reconstructed and measured protein molecule, of what precise changes happened, via positive selection, to get that lineage.

An alternative approach to evolutionary inference is a second target of these authors. That is a codon-based method, that evaluates the rate of change of DNA sites under selection versus sites not under selection. In protein coding genes (such as rhodopsin), every amino acid is encoded by a triplet of DNA nucleotides, per the genetic code. With 64 codons for ~20 amino acids, it is a redundant code where many DNA changes do not change the protein sequence. These changes are called "synonymous". If one studies the rate of change of synonymous sites in the DNA, (which form sort of a control in the experiment), compared with the rate of change of non-synonymous sites, one can get a sense of evolution at work. Changing the protein sequence is something that is "seen" by natural selection, and especially at important positions in the protein, some of which are "conserved" over billions of years. Such sites are subject to "negative" selection, which to say rapid elimination due to the deleterious effect of that DNA and protein change.

Mutations in protein coding sequence can be synonymous, (bottom), with no effect, or non-synonymous (middle two cases), changing the resulting protein sequence and having some effect that may be biologically significant, thus visible to natural selection.

This analysis has been developed into a high art, also being harnessed to reveal "positive" selection. In this scenario, if the rate of change of the non-synonymous DNA sites is higher than that of the synonymous sites, or even just higher than one would expect by random chance, one can conclude that these non-synonymous sites were not just not being selected against, but were being selected for, an instance of evolution establishing change for the sake of improvement, instead of avoiding change, as usual.

Now back to the rhodopsin study. These authors found that a very small number of amino acids in this protein, only 15, were the ones that influenced changes to the spectral sensitivity of these protein complexes over evolutionary time. Typically only two or three changes occurred over a shift in sensitivity in a particular lineage, and would have been the ones subject to natural selection, with all the other changes seen in the sequence being unrelated, either neutral or selected for other purposes. It is a tour de force of structural analysis, biochemical measurement, and historical reconstruction to come up with this fully explanatory model of the history of piscene rhodopsins. 

But then they went on to compare what they found with what the codon-based methods had said about the matter. And they found that there was no overlap whatsover. The amino acids identified by the "positive selection" codon based methods were completely different than the ones they had found by spectral analysis and phylogenetic reconstruction over the history of fish rhodopsins. The accompanying review is particularly harsh about the pseudoscientific nature of this codon analysis, rubbishing the entire field. There have been other, less drastic, critiques as well.

But there is method to all this madness. The codon based methods were originally conceived in the analysis of closely related lineages. Specifically, various Drosophia (fly) species that might have diverged over a few million years. On this time scale, positive selection has two effects. One is that a desirable amino acid (or other) variation is selected for, and thus swept to fixation in the population. The other, and corresponding effect, is that all the other variations surrounding this desirable variation (that is, which are nearby on the same chromosome) are likewise swept to fixation (as part of what is called a haplotype). That dramatically reduces the neutral variation in this region of the genome. Indeed, the effect on neutral alleles (over millions of nearby base pairs) is going to vastly overwhelm the effect from the newly established single variant that was the object of positive selection, and this imbalance will be stronger the stronger the positive selection. In the limit case, the entire genomes of those without the new positive trait/allele will be eliminated, leaving no variation at all.

Yet, on the longer time scale, over hundreds of millions of years, as was the scope of visual variation in fish, all these effects on the neutral variation level wash out, as mutation and variation processes resume, after the positively selected allele is fixed in the population. So my view of this tempest in an evolutionary teapot is that these recent authors (and whatever other authors were deploying codon analysis against this rhodopsin problem) are barking up the wrong tree, mistaking the proper scope of these analyses. Which, after all, focus on the ratio between synonymous and non-synonymous change in the genome, and thus intrinsically on recent change, not deep change in genomes.

• That all-American mix of religion, grift, and greed.
• Christians are now in charge.
• Mechanisms of control by the IMF and the old economic order.
• A new pain med, thanks to people who know what they are doing.

The Climate is Changing

Fires in LA, and a puff of smoke in DC.

An ill wind has blown into Washington, a government of whim and spite, eager to send out the winged monkeys to spread fear and kidnap the unfortunate. The order of the day is anything that dismays the little people. The wicked witch will probably have melted away by the time his most grievous actions come to their inevitable fruition, of besmirching and belittling our country, and impoverishing the world. Much may pass without too much harm, but the climate catastrophe is already here, burning many out of their homes, as though they were made of straw. Immoral and spiteful contrariness on this front will reap the judgement and hatred of future generations.

But hasn't the biosphere and the climate always been in flux? Such is the awful refrain from the right, in a heartless conservatism that parrots greedy, mindless propaganda. In truth, Earth has been blessed with slowness. The tectonic plates make glaciers look like race cars, and the slow dance of Earth's geology has ruled the evolution of life over the eons, allowing precious time for incredible biological diversification that covers the globe with its lush results.

A stretch of relatively unbroken rain forest, in the Amazon.

Past crises on earth have been instructive. Two of the worst were the end-Permian extinction event, about 252 million years ago (mya), and the end-Cretaceous extinction event, about 66 mya. The latter was caused by a meteor, so was a very sudden event- a shock to the whole biosphere. Following the initial impact and global fire, it is thought to have raised sun-shielding dust and sulfur, with possible acidification, lasting for years. However, it did not have very large effects on CO2, the main climate-influencing gas.

On the other hand, the end-Permian extinction event, which was significantly more severe than the end-Cretaceous event, was a more gradual affair, caused by intense volcanic eruptions in what is now Siberia. Recent findings show that this was a huge CO2 event, turning the climate of Earth upside down. CO2 went from about 400 ppm, roughly what we are at currently, to 2500 ppm. The only habitable regions were the poles, while the tropics were all desert. But the kicker is that this happened over the surprisingly short (geologically speaking) time of about 80,000 years. CO2 then stayed high for the next roughly 400,00 years, before returning slowly to its former equilibrium. This rate of rise was roughly 2.7 ppm per 100 years, yet that change killed off 90% of all life on Earth. 

The momentous analysis of the end-Permian extinction event, in terms of CO2, species, and other geological markers, including sea surface temperature (SST). This paper was when the geological brevity of the event was first revealed.

Compare this to our current trajectory, where atmospheric CO2 has risen from about 280 ppm at the dawn of the industrial age to 420 ppm now. That is rate of maybe 100 ppm per 100 years, and rising steeply. It is a rate far too high for many species, and certainly the process of evolution itself, to keep up with, tuned as it is to geologic time. As yet, this Anthropocene extinction event is not quite at the level of either the end-Permian or end-Cretaceous events. But we are getting there, going way faster than the former, and creating a more CO2-based long-term climate mess than the latter. While we may hope to forestall nuclear war and thus a closer approximation to the end-Cretaceous event, it is not looking good for the biosphere, purely from a CO2 and warming perspective, putting aside the many other plagues we have unleashed including invasive species, pervasive pollution by fertilizers, plastics and other forever chemicals, and the commandeering of all the best land for farming, urbanization, and other unnatural uses. 

CO2 concentrations, along with emissions, over recent time.

We are truly out of Eden now, and the only question is whether we have the social, spiritual, and political capacity to face up to it. For the moment, obviously not. Something disturbed about our media landscape, and perhaps our culture generally, has sent us for succor, not to the Wizard who makes things better, but to the Wicked Witch of the East, who delights in lies, cruelty and destruction.

• The oligarchy is upon us. Oh, and they will help the working class!
• The attention economy.
• Which distorts our public understandings.
• Good coverage will never win in the attention economy.
• Keep Greenland white!

Eeking Out a Living on Ammonia

Some archaeal microorganisms have developed sophisticated nano-structures to capture their food: ammonia.

The earth's nitrogen cycle is a bit unheralded, but critical to life nonetheless. Gaseous nitrogen (N2) is all around us, but inert, given its extraordinary chemical stability. It can be broken down by lightning, but little else. It must have been very early in the history of life that the nascent chemical-biological life forms tapped out the geologically available forms of nitrogen, despite being dependent on nitrogen for countless critical aspects of organic chemistry, particularly of nucleic acids, proteins, and nucleotide cofactors. The race was then on to establish a way to capture it from the abundant, if tenaciously bound, dinitrogen of the air. It was thus very early bacteria that developed a way (heavily dependent, unsurprisingly, on catalytic metals like molybdenum and iron) to fix nitrogen, meaning breaking up the triple N≡N bond, and making ammonia, NH3 (or ammonium, NH4+). From there, the geochemical cycle of nitrogen is all down-hill, with organic nitrogen being oxidized to nitric oxide (NO), nitrite (NO2-), nitrate (NO3), and finally denitrification back to N2. Microorganisms obtain energy from all of these steps, some living exclusively on either nitrite or nitrate, oxidizing them as we oxidize carbon with oxygen to make CO2. 

Nitrosopumilus, as imaged by the authors, showing its corrugated exterior, a layer entirely composed of ammonia collecting elements (can be hexameric or pentameric). Insets show an individual hexagonal complex, in face-on and transverse views. Note also the amazing resolution of other molecules, such as the ribosomes floating about.

A recent paper looked at one of these denizens beneath our feet, an archaeal species that lives on ammonia, converting it to nitrite, NO2. It is a dominant microbe in its field, in the oceans, in soils, and in sewage treatment plants. The irony is that after we spend prodigious amounts of fossil fuels fixing huge amounts of nitrogen for fertilizer, most of which is wasted, and which today exceeds the entire global budget of naturally fixed nitrogen, we are faced with excess and damaging amounts of nitrogen in our effluent, which is then processed in complex treatment plants by our friends the microbes down the chain of oxidized states, back to gaseous N2.

Calculated structure of the ammonia-attracting pore. At right are various close-up views including the negatively charged amino acids (D, E) concentrated at the grooves of the structure, and the pores where ammonium can transit to the cell surface. 

The Nitrosopumilus genus is so successful because it has a remarkable way to capture ammonia from the environment, a way that is roughly two hundred times more efficient than that of its bacterial competitors. Its surface is covered by a curious array of hexagons, which turn out to be ammonia capture sites. In effect, its skin is an (relatively) enormous chemical antenna for ammonia, which is naturally at low concentration in sea water. These authors do a structural study, using the new methods of particle electron microscopy, to show that these hexagons have intensely negatively charged grooves and pores, to which positively charged ammonium ions are attracted. Within this outer shell, but still outside the cell membrane, enzymes at the cell surface transform the captured ammonium to other species such as hydroxylamine, which enforces the ammonium concentration gradient towards the cell surface, and which are then pumped inside.

Cartoon model of the ammonium attraction and transit mechanisms of this cell wall. 

It is a clever nano-material and micro-energetic system for concentrating a specific chemical- a method that might inspire human applications for other chemicals that we might need- chemicals whose isolation demands excessive energy, or whose geologic abundance may not last forever.

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• Insurrection or not insurrection?
• The fluoride is OK.
• Cigarettes and ... wow!
• Bad judgement, ill discipline, and low morals. How did we get here?

Inside the Process of Speciation

Adaptive radiations are messy, so no wonder we have a hard time reconstructing them.

Darwin drew a legendary diagram in his great book, of lineage trees tracing speciation from ancestors to descendants. It was just a sketch, and naturally had clear fork points where one species turns into two. But in real life, speciation is messier, with range overlaps, inter-breeding, and difficulties telling species apart. Ornithologists are still lumping and splitting species to this day, as more data come in about ranges, genetics, sub-populations, breeding behavior, etc. And if defining existing species is difficult, defining exactly where they split in the distant past is even harder.

Darwin's notebook sketch of speciation, from ancestors ... to descendants.

The advent of molecular data from genomes gave a tremendous boost to the amount of information on which to base phylogenetic inferences. It gave us a whole new domain of life, for one thing. And it has helped sharpen countless phylogenies that not been fully specified by fossil and morphological data. But still, difficulties remain. The deepest and most momentous divergences, like the origin of life itself, and the origin of eukaryotes, remain shrouded in hazy and inconclusive trees, as do many other lineages, such as the origin of birds. It seems to be a rule that when a group of organisms undergoes rapid evolution / speciation, the tree they are on (as reconstructed by us from contemporary data) becomes correspondingly unclear and unresolved, difficult to trace through that tumultuous time. In part this is simply a matter of timing. If dramatic events happened within a few million years a billion years ago, our ability to resolve the sequence of those events is going to be weak in any case, compared to the same events spread out over a hundred million years.

A recent paper documented some of this about phylogeny in general, by correlating times of morphological change with times of phylogenetic haziness, which they term "gene-tree conflict". That is to say, if one samples genes across genomes to draw phylogenetic trees, different genes will give different trees. And this phenomenon increases right when there are other signs of rapid evolutionary change, i.e. changing morphology.

"One insight gleaned from phylogenomics is that gene-tree conflict, frequently caused by population-level processes, is often rampant during the origin of major lineages."

They identify three mechanisms behind this observation: incomplete lineage sorting (ILS), hybridization, and rapid evolution. Obviously, these need to be unpacked a bit. ILS is a natural consequence of the fact that species arise not from single organisms, but from populations. Gene mutations that differentiate the originating and future species happen all over the respective genomes, and enter the future lineage at different times. Some may happen well after the putative speciation event, and become fixed (that is, prevalent) later in that species. Others may have happened well before the speciation event, and die off in most of the descending lineages. The fact is that not every gene is going to march in lock step with the speciation event, in terms of its variants. So phylogenetic inference is best done using lots of genes plus statistical methods to arrive at the most likely explanation of the diverse individual gene trees.

Graphs drawn from different sources relating gene conflicts in lineage estimation, (top), versus rate of morphological change from the fossil record, (bottom), in birds, and over time on the X axis. There are dramatic upticks in all metrics going back towards the end-Cretaceous extinction event.

Similarly, hybridization means that proto-species are still occasionally interbreeding with their ancestors or other relatives, (think of Neanderthals), thereby mixing up the gene trees relative to the overall speciation tree. This can even happen by gene transfer mediated by viruses. "Rapid evolution" is not defined by these authors, and comes dangerously close to using the conclusion (of high morphological change during periods of "gene-tree conflict") to describe their premise. But generally, this would mean that some genes are evolving rapidly, due to novel selective pressures, thus deviating from the general march of neutral evolution that affects most loci more evenly. This rate change can mess up phylogenetic inferences, lengthening some (gene) tree branches versus others, and making a unitary tree (that is, for the species or lineage as a whole) hard to draw.

But these are all rather abstract ideas. How does this process look on the ground? A wonderful paper on the tomato gives us some insight. This group traced the evolutionary history of a genus of tomato (Solanum sect. Lycopersicon) in the South American Andes (plus Galapagos islands just off-shore, interestingly enough). These form a tight group of about thirteen species that evolved from a single ancestor over the last two million years, before jumping onto our lunch plates via intensive breeding by native South Americans. This has been a rapid process of evolution, and phylogenies have been difficult to draw, for all the reasons given above. The tomatoes are mostly reproductively isolated, but not fully, and have various specializations for their microhabitats. So are they real species? And how can they evolve and specialize if they do not fully isolate from each other?

Gene-based phylogenetic tree of Andean tomato species. The consensus tree is in black at the right, while alternate trees (cloud) are drawn from 2,745 windows of 100 kb across the tomato genomes, clearly giving diverse views of the lineage tree. Lycopersicon are the species under study, while Lycopericoides is an "outgroup" genus used as a control / comparison. 

In the graph above, there is, as they say, rampant discord among genomic segments, versus the overall consensus tree that they arrived at:

"However, these summary support measures conceal rampant phylogenetic complexity that is evident when examining the evolutionary history of more defined genomic partitions."

For one thing, much of the sequence diversity in the ancestor survives in the descendent lineages. The founders were not single plants, by any means. Second, there has been a lot of "introgression", which is to say, breeding / hybridization between lineages after their putative separation. 

Lastly, they find a high rate of novel mutations, often subject to clear positive selection. Ten enyzmes in the carotenoid biosynthesis pathway, which affects fruit color in a group that has evolved red fruits, carry novel mutations. A UV light damage repair gene shows strong signs of positive selection, in high-altitude species. Others show novel mutations in a temperature stress response gene, and selection on genes defending plants against heavy metals in the soil. 

Their conclusion (as that of the previous paper) is that adaptive radiations are characterized by several components that scramble normal phylogenetic analysis, including variably preserved diversity from the originating species, post-divergence gene flow (i.e. mating), and rapid adaptation to new conditions along with strong environmental selection over the pre-existing diversity. All of these mechanisms are happening at the same time, and each position in the genome is being affected at the same time, so this is a massively parallel process that, while slow in human time, can be very rapid in geologic time. They note how tomato speciation compares with some other well-known cases:

"Nonetheless, based on our crude estimates within each analysis, we infer that relatively small yet substantial fractions of the euchromatic genome are implicated in each source of genetic variation. We find little evidence that one of these processes predominates in its contribution, although our estimates suggest that de novo mutation might be relatively more influential and cross-species introgression relatively less so. This latter observation is in interesting contrast with several recent studies of animal adaptive radiations, including in Darwin’s Finches [18], Equids [14], and fish [13], where evidence suggests that hybridization and introgression might be much more pervasive and influential than previously suspected, and more abundant than we detect in Solanum."

Naturally, neither of these studies go back in time to nail down exactly what happened during these evolutionary radiations, nor what caused them. They only give hints about causation. Why the stasis of some species, and the rapid niche-finding and filling by others? Was the motive force natural selection, or god? The latter paper gives some clear hints about possible selective pressures and rationales that were at work in the Andes and Galapagos on the genus of Solanum. But it is always frustratingly a matter of abstract reasoning, in the manner of Darwin, that paints the forces at work, however detailed the genetic and biogeographic analyses and however convincing the analogous laboratory experiments on model, usually microbial, organisms. We have to think carefully, and within the discipline of known forces and mechanisms, to arrive at intellectually honest answers to these questions, insofar as they can be answered at all.

• Military procurement and production are hopelessly antiquated, and too slow for an actual crisis.
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• Do wingnuts love the lockdown of educational institutions in Hungary because of the increased quality of research?

Aging and Death

Our fate was sealed a very long time ago.

Why do we die? It seems like a cruel and wasteful way to run a biosphere, not to mention a human life. After we have accumulated a lifetime of experience and knowledge, we age, decline, and sign off, whether to go to our just reward, or into oblivion. What is the biological rationale and defense for all this, which the biblical writers assigned to the fairy tale of the snake and the apple?

A recent paper ("A unified framework for evolutionary genetic and physiological theories of aging") discusses evolutionary theories of aging, but in typical French fashion, is both turgid and uninteresting. Aging is widely recognized as the consequence of natural selection, or more precisely, the lack thereof after organisms have finished reproducing. Thus we are at our prime in early adulthood, when we seek mates and raise young. Evolutionarily, it is all downhill from there. In professional sports, athletes are generally over the hill at 30, retiring around 35. Natural selection is increasingly irrelevant after we have done the essential tasks of life- surviving to mate and reproduce. We may participate in our communities, and do useful things, but from an evolutionary perspective, genetic problems at this phase of life have much less impact on reproductive success than those that hit earlier. 

All this is embodied in the "disposable soma" theory of aging, which is that our germ cells are the protected jewels of reproduction, while the rest of our bodies are, well, disposable, and thus experience all the indignities of age once their job of passing on the germ cells is done. The current authors try to push another "developmental" theory of aging, which posits that the tradeoffs between youth and age are not so much the resources or selective constraints focused on germ cell propagation vs the soma, but that developmental pathways are, by selection, optimized for the reproductive phase of life, and thus may be out of tune for later phases. Some pathways are over-functional, some under-functional for the aged body, and that imbalance is sadly uncorrected by evolution. Maybe I am not doing justice to these ideas, which maybe feed into therapeutic options against aging, but I find this distinction uncompelling, and won't discuss it further.

A series of unimpressive distinctions in the academic field studying aging from an evolutionary perspective.

Where did the soma arise? Single cell organisms are naturally unitary- the same cell that survives also mates and is the germ cell for the next generation. There are signs of aging in single cell organisms as well, however. In yeast, "mother" cells have a limited lifespan and ability to put out daughter buds. Even bacteria have "new" and "old" poles, the latter of which accumulate inclusion bodies of proteinaceous junk, which apparently doom the older cell to senescence and death. So all cells are faced with processes that fail over time, and the only sure bet is to start as a "fresh" cell, in some sense. Plants have taken a distinct path from animals, by having bodies and death, yes, but being able to generate germ cells from mature tissues instead of segregating them very early in development into stable and distinct gonads.

Multicellularity began innocently enough. Take slime molds, for example. They live as independent amoebae most of the time, but come together to put out spores, when they have used up the local food. They form a small slug-like body, which then grows a spore-bearing head. Some cells form the spores and get to reproduce, but most don't, being part of the body. The same thing happens with mushrooms, which leave a decaying mushroom body behind after releasing their spores. 

We don't shed alot of tears for the mushrooms of the world, which represent the death-throes of their once-youthful mycelia. But that was the pattern set at the beginning- that bodies are cells differentiated from the germ cells, that provide some useful, competitive function, at the cost of being terminal, and not reproducing. Bodies are forms of both lost energy and material, and lost reproductive potential from all those extra cells. Who could have imagined that they would become so ornate as to totally overwhelm, in mass and complexity, the germ cells that are the point of the whole exercise? Who could have imagined that they would gain feelings, purposes, and memories, and rage against the fate that evolution had in store for them?

On a more mechanistic level, aging appears to arise from many defects. One is the accumulation of mutations, which in soma cells lead to defective proteins being made and defective regulation of cell processes. An extreme form is cancer, as is progeria. Bad proteins and other junk like odd chemicals and chemically modified cell components can accumulate, which is another cause of aging. Cataracts are one example, where the proteins in our lenses wear out from UV exposure. We have quite intricate trash disposal processes, but they can't keep with everything, as we have learned from the advent of modern chemistry and its many toxins. Another cause is more programmatic: senescent cells, which are aged-out and have the virtue that they are blocked from dividing, but have the defect that they put out harmful signals to the immune system that promote inflammation, another general cause of aging.

Aging research has not found a single magic bullet, which makes sense from the evolutionary theory behind it. A few things may be fixable, but mostly the breakdowns were never meant to be remedied or fixed, nor can they be. In fact, our germ cells are not completely immune from aging either, as we learn from older fathers whose children have higher rates of autism. We as somatic bodies are as disposable as any form of packaging, getting those germ cells through a complicated, competitive world, and on to their destination.

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Putting Body Parts in Their Places

How HOX genes run development, on butterfly wings.

I have written about the HOX complex of genes several times, because they constitute a grail of developmental genetics- genes that specify the identity of body parts. They occupy the middle of a body plan cascade of gene regulation, downstream from broader specifiers for anterior/posterior orientation, regional and segment specification, and in turn upstream of many more genes that specify the details of organ and tissue construction. Each of the HOX genes encodes a transcriptional regulator, and the name of one says it all- antennapedia. In fruit flies, where all this was first discovered, loss of antennapedia converts some legs into antennae, and extra expression of antennapedia converts antennae on the head into legs.

The HOX complex (named for the homeobox DNA binding motif of the proteins they encode) is linear, arranged from head-affecting genes (labial, proboscipedia) to abdomen-affecting genes (abdominal A, abdominal B; evidently the geneticist's flair for naming ran out by this point). This arrangement is almost universally conserved, and turns out to reflect molecular mechanisms operating on the complex. That is, it "opens" in a progressive manner during development, on the chromosome. Repression of chromatin is a very common and sturdy way to turn genes off, and tends to affect nearby genes, in a spreading effect. So it turns out to be easy, in some sense, to set up the HOX complex to have this chromatin repression lifted in a segmental fashion, by upstream regulators, whereby only the head sections are allowed to be expressed in head tissues, but all the genes are allowed to be expressed in the final abdominal segment. That is why the unexpected expression of antennapedia, which is the fifth of eight HOX genes, in the head, leads to a thoracic tissue (legs) forming on the head.

A recent paper delved a little more deeply into this story, using butterflies, which have a normal linearly conserved HOX cluster and are easy to diagnose for certain body part transformations (called homeotic) on their beautiful wings. The main thing these researchers were interested in is the genetic elements that separate one part of the HOX cluster from other parts. These are boundary or "insulator" elements that separate topologically associated domains (called TADs). Each HOX gene is surrounded by various regulatory enhancer and inhibitor sites in the DNA that are bound by regulatory proteins. And it is imperative that these sites be directed only to the intended gene, not neighboring genes. That is why such TADs exist, to isolate the regulation of genes from others nearby. There are now a variety of methods to map such TADs, by looking where chromatin (histones) are open or closed, or where DNA can be cut by enzymes in the native chromatin, or where crosslinks can be formed between DNA molecules, and others.

The question posed here was whether a boundary element, if deleted, would cause a homeotic transformation in the butterflies they were studying. They found, unfortunately, that it was impossible to generate whole animals with the deletions and other mutations they were engineering, so they settled for injecting the CRISPER mutational molecules into larval tissues and watching how they affected the adults in mosaic form, with some mutant tissues, some wild-type. The boundary they focused on was between antennapedia (Antp) and ultrabithorax (Ubx), and the tissues the forewings, where Ubx is normally off, and hindwings, where Ubx is normally on. Using methods to look at the open state of chromatin, they found that the Ubx gene is dramatically opened in hindwings, relative to forewings. Nevertheless, the boundary remains in place throughout, showing that there is a pretty strong isolation from Antp to Ubx, though they are next door and a couple hundred thousand basepairs apart. Which in genomic terms is not terribly far, while it leaves plenty of space for enhancers, promotes, introns, boundary elements, and other regulatory paraphernalia.

Analysis of the site-to-site chromosomal closeness and accessibility across the HOX locus of the butterfly Junonia coenia. The genetic loci are noted at the bottom, and the site-to-site hit rates are noted in the top panels, with blue for low rates of contact, and orange/red for high rates of contact. At top is the forewing, and at bottom is the hindwing, where Ubx is expressed, thus the high open-ness and intra-site contact within its topological domain (TAD). Yet the boundary between Ubx and Anp to its left (dotted lines at bottom) remains very strong in both tissues. In green is a measure of transcription from this DNA, in differential terms hindwing minus forewing, showing the strong repression of Ubx in the forewing, top panel.

The researchers naturally wanted to mutate the boundary element, (Antp-Ubx_BE), which they deduced lay at a set of binding sites (featuring CCCTC) for the protein CTCF, a well-known insulating boundary regulator. Note, interestingly, that in the image above, the last exon (blue) of Ubx (transcription goes right to left) lies across the boundary element, and in the topological domain of the Antp gene. This means that while all the regulatory apparatus of Ubx is located in its own domain, on the right side, it is OK for transcription to leak across- that has no regulatory implications. 

Effects of removing the boundary element between Ubx and Antp. Detailed description is in the text below. 

Removal of this boundary element, using CRISPER technology in portions of the larval tissues, had the expected partial effects on the larval, and later adult, wings of this butterfly. First, note that in panel D insets, the wild type larval forewing shows no expression of Ubx, (green), while the wild type hind wing shows wide-spread expression. This is the core role of the HOX locus and the Ubx gene- locate its expression in the correct body parts to then induce the correct tissues to develop. The larval wing tissue of the mosaic mutant, also in D, shows, in the forewing, extensive patchy expression of Ubx. This is then reflected in the adult (different animals) in the upper panels, in the mangled eyespot of the fully formed wing (center panel, compared to wild-type forewing and hindwing to each side). It is a small effect, but then these are small mutations, done in only a fraction of the larval cells, as well.

So here we are, getting into the nuts and bolts of how body parts are positioned and encoded. There are large regions around these genes devoted to regulatory affairs, including the management of chromatin repression, the insulation of one region from another, the enhancer and repressor sites that integrate myriad upstream signals (i.e. other DNA binding proteins) to come up with the detailed pattern of expression of these HOX genes. Which in turn control hundreds of other genes to execute the genetic program. This program can hardly be thought of as a blueprint, nor a "design" in anyone's eye, divine or otherwise. It resembles much more a vast pile of computer code that has accreted over time with occasional additions of subroutines, hacks, duplicated bits, and accidental losses, adding up to a method for making a body that is robust in some respects to the slings and arrows of fortune, but naturally not to mutations in its own code.

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The Long Tail of Genome Duplication

A new genomic sequence of hagfish tells us a little about our origins.

Hagfish- not a fish, and not very pretty, but it occupies a special place in evolution, as a vertebrate that diverged very early (along with lampreys, forming the cyclostome branch) from the rest of the jawed vertebrates (the gnathostome branch). The lamprey has been central to studies of the blood clotting system, which is a classic story of gradual elaboration over time, with more steps added to the cascade, enabling faster clotting and finer regulation.

A highly schematic portrayal (not to scale!) of the evolutionary history of animal life on earth.

A recent paper reported a full genome sequence of hagfish, and came up with some interesting observations about the history of vertebrate genomes. At about three billion nucleotides, this genome is about as large as ours. (Yet again, size doesn't see, to matter much, when it comes to genomes.) They confirm that lampreys and hagfish make up a single lineage, separate from all other animals and especially from the jawed vertebrates. For example, though lampreys have 84 chromosomes to the hagfish's 17, this resulted from repeated splitting of chromosomes, and each lamprey chromosome can be mostly mapped to one hagfish chromosome, accepting that a lot of other gene movement and change has taken place in the roughly 460 million years since these lineages diverged. 

Hagfish (bottom) and lamprey (top) chromosomes pretty much line up, indicating that despite the splitting of the lamprey genome, there hasn't been a great deal of shuffling over the intervening 460 million years.

The most important parts of this paper are on the history of genome duplications that happened during this early phase of vertebrate evolution. Whole genome duplications are an extremely powerful engine of change, supplying the organism with huge amounts of new genetic material. Over time, most of the duplicated genes are discarded again (in a process they call re-diploidization). But many are not, if they have gained some foothold in providing more of an important product, or differentiated themselves from each other in some other way. Our genomes are full of families, some extremely large, of related genes that have finely differentiated functions. Many of these copies originated in long-ago genome duplications, while others originated in smaller duplication accidents. It is startling to hear from self-labeled scientists in the so-called intelligent design movement that there is some rule or law against such copying of information, by their ridiculous theories of specified information. Hagfish certainly never heard of such a thing.

At any rate, these researchers confirm that the earliest vertebrate lineage, around 530 million years ago, experienced two genome duplications which led to a large increment of new genes and evolutionary innovation. What they find now is that the cyclostome lineage experienced another genome three-fold duplication (near its origin, about 460 million years ago, leading to another round of copies and innovation. And lastly, the gnathostome lineage separately experienced its own genome four-fold duplication around the same time, after it had diverged from the cyclostome lineage. One might say that the gnathostomes made better use of their genomic manna, generating jaws, teeth, ears, thymus, better immune systems, and the other features that led them to win the race of the animal kingdom. But hagfish are still around, showing that primitive forms can find a place in the scheme of things, as the biosphere gets larger and more diverse over time.

A classic example of gene replication is the Hox cluster, which are a set of genes that have the power of dictating what body part occurs where. They are gene regulators that function in the middle of the developmental sequence, after determination of the overall body axis and segmentation, and themselves regulating downstream genes governing features as they occur in different segments, such as limbs, parts of the head, fingers, etc. Flies have one Hox cluster, split into two parts. The extremely primitive chordate amphioxus, which far predates the cyclostomes, also has one complete Hox cluster, as diagrammed below. Most other vertebrates, including us, have four Hox clusters, amounting to over thirty of these transcription regulators. These four clusters arose from the inferred genome duplications very early in the vertebrate lineage, prior to the advent of the cyclostomes. 

Hox clusters and their origins, as inferred by the current authors. The red/blue points at the left mark whole genome duplications (or more) that have been inferred by these or other authors. More description is in the main text below.

The inferred genome duplications during early chordate evolution, noted on the far left of the diagram above, led to duplicated clusters of Hox genes. Amphioxus (top) is the earliest branching chordate, and has only one full Hox cluster of transcription regulators, which, in general terms, control, during development, the expression of body parts along the body axis, with the order of genes in the cluster paralleling expression and action along the body axis. Chicken as a gnathostome has four copies of the cluster, with a few of the component genes lost over time. Hagfish have six copies of this Hox cluster, some rather skeletal, stemming from its genome duplication events. Clearly several whole clusters have also been lost, as in some cases the genome duplications experienced by the cyclostomes resolved back to diploidy without leaving an extra copy of this cluster. The net effect is to allow all these organisms greater options for controlling the identity and form of different parts of the body, particularly, in the case of gnathostomes, the head.

Genome duplications are one of those fast events in evolution that are highly influential, unlike the usual slow and steady selection and optimization that is the rule in the Darwinian theory. Unlike mass extinction, another kind of fast event in evolution, genome duplications are highly constructive, providing fodder on a mass (if microscopic) scale for new functions and specializations that help account for some of the more rapid events in the history of life, such as the rise of chordates and then vertebrates in the wake of the Cambrian explosion.

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Where Did Flowers Come From?

Are angiosperms 135 million years old, or 275 million years old?

We live among a hodgepodge of plants from different evolutionary epochs, with flowering plants being the most recent, (including the even more recently evolved grasses), alongside the more ancient conifers, cycads, ferns, mosses, and lichens. All have a place in diverse ecosystems, but what is their true history? This has been difficult to establish in more than broad outlines, due, as usual, to the patchy nature of the fossil record, and the difficulties of aligning it with what we now have as the molecular record. Angiosperms have been a particular sore spot, ever since Darwin, who recognized that the sudden appearance and radiation of flowering plants, roughly 130 million years ago, was a problem for evolutionary theory.

A paper from a few years back offered a carefully aligned molecular and fossil analysis of angiosperms, coming to the conclusion that they actually originated ~275 million years ago (MYA), and must have persisted in some cryptic fashion through the ensuing 150 MY before making a splash in the fossil record. How is this kind of analysis done? First, the best early fossils are tabulated, with secure dating and clear characteristics that include them among angiosperms. The most ancient example is a sample of pollen, from roughly 125 million years, which looks strongly like it came from angiosperms. These fossils are also assigned to plant lineages, so that their appearance can inform the branching points of the phylogenetic diagram, whether that diagram is based purely on these fossils and their morphology, or based on molecular data.

Then a set of gene sequences is collected, which are conserved between all the surveyed species, and aligned so that their changes can be fed into a program that counts all the differences. It was clear through this work that some lineages changed faster than other ones (the faster ones are marked with blue flares towards the right. Since the sampled species are all ones that exist now (time 0), being able to provide DNA, and since the branch points are in any case shared between the lineages that descend from them(at their origination points), faster change / evolution in one lineage vs another will be readily apparent, and the researchers just have to make up some rules to judge where to come down in time assignments when faced with such discrepancies. The more serious problem is that such different speeds can totally derange this kind of analysis, making a faster-changing lineage seem much older than it is. So pinpointing the branch points between lineages is extremely important to pin down such hard-to gauge branch lengths. 

Biologically, it is now well known that lineages vary substantially (up to ten fold, less so in longer lineages and time spans) in their speed of molecular change.. molecular evolution is not a clock. Faster change tends to happen when populations are small, and when big evolutionary transitions have happened. For example, plants, and specifically angiosperms, have gone through whole-genome duplications that represent major evolutionary watersheds. These duplications supplied raw material for countless diversifications and specializations of genes, with especially rapid change in molecular sequences either released from previous selective constraints, or subject to new ones via new roles.

Integrated phylogenetic diagram of the evolution of angiosperms, marking key fossils that inform branch point timing (lettered blue circles), and ranges of possible branch points derived from the molecular alignments (red circles). At bottom is time, in millions of years before present.

What can explain the big gap in estimated angiosperm origins? There are three basic hypotheses. First is that the molecular data is correct, which implies that there is an extremely long (150 million years) history of cryptic angiosperms that have not (yet) been detected in the fossil record. There are smatterings of findings in the literature that suggest that such fossils may be (or may have been) found, but I don't think these have been widely accepted yet (especially when they come from highly questionable sources).

The second hypothesis is that something about the very early evolution of angiosperms (like the very early evolution of eukaryotes, and the very early diversification of macroscopic animals) was accelerated in molecular terms, (as discussed above in terms of differing rates between lineages), rendering the apparent molecular phylogeny much longer than the real one. That includes the prodigious radiation of the many lineages in the diagram above, all before the first fossil is found.

The third hypothesis, much beloved of creationists, is that god did it. A mystery like this is ripe for invoking the solution to all mysteries, which is it does not need to be explained in the normal mechanistic terms of the natural world, but rather can be chalked up to the author of all things, god. While this hypothesis, at least for believers, solves this one nagging mystery, it brings on a few others. Why does the rest of biology through this vast lineage still follow the plodding path of gradual (if uneven) development? Why jump in to create this mystery when so many other lineages in the fossil record do not present similar mysteries? Why did god insist upon, (presumably for the ultimate appearance of us as humans), the whole four billion year process of life's plodding development, when the whole thing could have been authored at once and at the start? What amazing societies we could have developed with a four billion year head start!

It is clear, therefore, that some hypotheses create more problems than they solve. Inviting scientists to consider and comment on harebrained hypotheses is not going to end well. The solution to this problem is going to be some combination of the first two hypotheses, of rapid molecular evolution at the start of a major radiation, and some as-yet missing material in the fossil record. Innovative organisms are very likely to be rare, though whole cryptic lineages surviving for many tens of millions of years is hard to posit without more evidence. Yet it is also a given that fossils will necessarily appear after the actual events of lineage branching, thus will always post-date the calculated molecular branching point.

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Ruffling the Feathers of Dinosaurs

The origin of birds remains uncertain, as does the status of feathers on dinosaurs. Review of "Riddle of the Feathered Dragons", by Alan Feduccia. 

As regular readers can surmise, I was raised (scientifically) in an empirical, experimental tradition- that of molecular biology. In that field there is little drama, since any dispute can be taken back to the lab for adjudication. No titanic battles of conflicting interpretations happen, and extremely high standards pervade the field, since any lapse is easily discovered and replicated. Despite the dominant position of molecular biology in the major journals, due to its high productivity, it is thus rarely in the public spotlight. It has been a bit of a culture shock to realize that other areas of science have significantly different standards and epistemology. Many fields (such as astronomy or paleontology) are at heart observational rather than experimental, or have other restrictions or conflicts (medical science, nutrition studies) that impair their ability to find the truth, leading to a great deal more interpretation, drama, and sometimes, rampant speculation.

Paleontology, and the study of the past in general, has an intrinsic lack of data. If the fossils are missing, what can one do but to wonder and speculate what could have happened during that gap? And when fossils do turn up, they still lack alot of information about their unfortunate contributor- they are only the bare bones, after all. They may be in bad condition and particularly hard to interpret. Whole genera or above may be represented by a tooth or single bone. Millions of years may go by with nothing to show for it. No wonder speculation fills the gap- but is that science? Incidentally, I have to thank the Discovery Insitute, with its keen nose for scientific controversy, for pointing me to today's author, who disputes the now-conventional view that birds arose from dinosaurs. While Alan Feduccia has nothing to do with Creationism and its offshoots, and is a perfectly respectable paleontologist, he is, in the course of at least four books on the subject, (of which this is the third), clearly frustrated with the reigning interpretations of his field, which has jumped to what he regards as unwarranted conclusions that have led to a flurry of portrayals of feathered dinosaurs.

The Archaeopteryx fossil, Berlin specimen, which dates to roughly 150 million years ago.

The first bird, more or less, was Archaeopteryx. Its Berlin fossil, found about 1875, and dating to roughly 150 million years ago (mya) is perhaps the most beautiful, and informative, fossil ever found- a complete bird, with fully spread flight feathers on its arms and legs, a long tail, claws on its hands,and teeth in its mouth. It had the precisely the in-between characteristics of both reptiles and birds that gave immediate validation to Charles Darwin's theory of evolution by gradual change and natural selection. But where did it come from? That is the big question. While a great many other fossil birds have been found, none substantially predate Archaeopteryx, (other than perhaps Anchiornis, very similar to Archaeopterix, and dating to roughly 160 mya), and thus we really do not know (as yet) from fossils how birds originated. 

Feathers are one diagnostic feature in this lineage. Archaeopterix and many later birds found in China and Mongolia from the Cretaceous have feathers, clearly marking them as birds and as lineally related. But other allied fossils have been found, nominally described as dinosaurs, which are described to have feathers as well. One is shown below. 

Fossil of Sinosauropteryx, which dates roughly to 124 million years ago.

Closeup of the hair-like impressions on the tail of Sinosauropteryx.

Whether these structures are feathers, or related to them, is quite debatable. They look more like hairs, and Feduccia claims that they do not even occur outside the body wall. Some experimentation by others has shown that sub-dermal collagen can form this kind of hair-like fuzz during some forms of decay and fossilization, given proper squashing. Current conventional wisdom, however, describes them as filament-like feathers used for insulation or display. My take, looking closely at these pictures, is that they are not feathers, but are outside the body wall, which, on the tail certainly, would have been very close to the bone. Additionally, as these specimens are all rather late, they could easily be descended from birds, while being large and flightless. Feduccia points out that while land-based animals have never gained/regained flight, flightlessness has evolved many times through the bird lineages. Similarly, extensive lineages of secondarily flightless birds may have developed in the Mesozoic, that conventional paleontologists call dinosaurs, (often with feathers), and posit as evidence that the reverse happened- that birds evolved from dinosaurs. For example, the conventional view of dinosaurs and birds draws on many later fossils from the Cretaceous (such as Deinonychus), which had both bird-like and dinosaur-like features: the "raptors". 

Another character at issue is flight itself. If birds are basal- that is, they arose prior to or separately from the other dinosaurs- then they could easily have developed from small arboreal lizards that learned to glide from place to place. On the other hand, dinosaurs are all relatively large and bipedal. So conventional paleontologists have labored to come up with ways that flight could have developed "from the ground up". Such theories as insect trapping by nascent small wings, or occasional tree climbing with tiny wings, to escape predators, have been invoked as rationales for feathers and wings to develop in terrestraial bipedal dinosaurs. Feduccia counters that in the whole history of flight, all animals (birds, bats, squirrels, others) have developed flight from gliding, not from the ground up. Indeed, there are countless flightless birds, and none of them have resumed flight, despite presumably having much of the genetic wherewithal to do so.

Given patchy data, the leading method to make sense of it and organize organisms from the fossil record into a phylogenetic story is the cladistic method. Practitioners choose a wide range of "characters", (such as the lengths, angles, holes, and other morphologies in the available bones) and tabluate their values from all the proposed species. Then they can mathematically just total up who is more distant from whom. Feducci emphasizes that this is an excellent method for ordering closely related genera and species. But over the long run, evolution repeats itself alot, making numerous flightless birds, for example, or similarly shaped swimming animals, not all of which are as closely related as they might look morphologically. Cladistics is a classic case of garbage-in-garbage-out analysis, and has routinely been overturned by molecular evidence when, among extant species, genomic data is available. Sadly, genomic data is not available for the fossils from the Mesozoic (the age of the dinosaurs, which encompasses, in order, the Triassic (245 mya to 208 mya), the Jurassic (208 to 144 mya) and the Cretaceous (144 to 65 mya) periods), nor from any living descendants of the dinosaurs... other than their putative decendants, birds.

As an aside, molecular phylogenies are also at heart cladistic in their theory and method. They just have a lot more "characters"- i.e. the letters of the DNA sequences in homologous / aligned sequences. But even more importantly, since a large proportion of these characters are neutral, (to natural selection), and thus vary (in sort-of clock-like fashion) no matter what convergent evolution might happen morphologically, molecular phylogenies can easily resolve difficult questions of phylogeny on the short to medium geologic terms. When it comes to the deepest phylogenies, however, going over a billion years, neutral characters become wholly useless due to homogenization by the vast times that have passed, so for such time periods these methods become less incisive.

Crude cladogram illustrating the alternative hypotheses- that birds are descended from theropod dinosaurs, or that birds arise from a basal lineage of their own, directly from the common stem of archosaurs. In the latter hypothesis, numerous bird-like lineages currently construed as dinosaurs might be secondarily flightless birds.

It is cladistics (along with other evidence) that has enshrined birds within the dinosaur lineage, finding that theropods came first, and the avians came later on. (With a contrasting view, and a critique of the contrasting view.) Theropods and birds are certainly similar, compared to their crocodilian / archosaur antecedents. They are bipedal, with similar hip structures, neck structures, and hands/feet reduced from five to three toes. But if much of what we take to be the dinosaurs (those with feathers and the whole so-called "raptor" class), are actually secondarily flightless birds, then one can make a lot of sense of some of these similarities, while casting the origin of birds quite a bit father back in time, more or less co-incident with the origin of true dinosaurs. Such as in the diagram above.

The problem with all this is again time. The early Jurassic and Triassic, amounting to almost one hundred million years before Archaeopterix, provide a lot of evidence for dinosaurs. They first appear roughly 240 mya, and flourish after the major exinction event that ended the Triassic, at 201 mya. The stark lack of evidence for birds, and widespread evidence for dinosaurs, including the lineage (theropods) that are most related to birds, suggests strongly that birds did not originate back in the Triassic, in parallel with the core dinosaur lineages. It suggests, rather, that among the many theropod dinosaurs during the ten or twenty million years before Archaeopterix were some small enough to take to the trees, grow longer arms, and be in position for flight. There were doubtless plenty of insects up there, at least until just about this time of the late-Jurassic, when birds started to eat them! Fossil record gaps are treacherous things, but this one indicates strongly that birds evolved in the middle Jurassic, along with (and within) the wider adaptive radiation of dinosaurs.

"Yet Archaeopteryx is still the classic urvogel- the oldest well-studied bird yet discovered, perhaps some 25 or more million years older than most of the Early Cretaceous Chinese fossils. As we saw in chapter 3, the Solnhofen urvogel is a mosaic of reptilian and avian features, a true bird, and the more it is studied, the more and more birdlike it is revealed to be. Ignoring the element of geologic time, however, many paleontologists have proposed that the Liaoning fossils provide evidence for all the stages of the evolution not only of birds and bird flight but also of feathers, from fiberlike protofeathers to pennaceous, asymmetrical flight remiges. Such a claim is remarkable and would be astounding in any fauna, but is especially so for a fauna so temporally removed from the time of avian origins, presumably before the Middle Jurassic and perhaps well back into the Triassic. 

University of Pennsylvania paleontologist Peter Dodson, remarking on the inadequacies of cladistic methodology, tells us: 'To maintain that the problem of the origin of birds has been solved when the fossil record of the Middle or Late Jurassic bird ancestors is nearly a complete blank is completely absurd. The contemporary obsession with readily available computer-assisted algorithms that yield seemingly precise results that obviate the need for clear-headed analysis diverts attention away fron the effort that is needed to discover the very fossils that may be the true ancestors of birds. When such fossils are found, will cladistics be able to recognize them? Probably not.'"

Feducci makes a lot of insightful points and hits some sensitive marks, in addition to all the trash-talk. Cladistics has problems, hairs are not feathers, and Cretaceous birds don't tell us much about the evolution of bird flight, which doubtless began as gliding between trees tens of millions of years earlier. And he is right that the hunt for clear antecedents of Archaeopterix, whether far in the past or near, should be the focus of this field. But overall, it is hard to fully credit the "birds early" story. 

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