On the superiority of endoskeletons
There are two great instances in the history of life of skeletons appearing on the inside versus the outside of organisms- the vertebrate skeleton, compared with precursors like insects with their exoskeletons, and the eukaryotic cytoskeleton, compared with the cell wall of typical bacteria. Both instances paved the way for great innovations of size and complexity.
It has often puzzled me that squishy naked mammals should have risen to the top of the food chain, rather than the more abundant insects or other, better protected, better architected, and handsome-er species. We seem so vulnerable, with pathetic teeth, muscles, and skin, barely-existent hair, let alone armor. Of course, we've got brains, and that's pretty much it.
At the beginning of life, creatures didn't need protection. Proto-cells had enough to do to survive- no predators existed. And before the pre-Cambrian explosion, the first known animals were flaccid pillows of cytoplasm. But once predators came on the scene, body plans quickly adapted, becoming enclosed in bacterial cell walls in the first instance, and in shells, chitin, and the like among the animals, leading to the long reign of Trilobites and similar creatures.
However the first response to a crisis is not always the best one, and many organisms kept their heads down and did not adopt external protection, making do with other strategies, like burrowing, distastefulness, speed, etc. Often these were predators themselves, like amoebae and jellyfish. External protections can also be discarded in evolutionary time, if they are not too deeply embedded in the body plan, as happened with aplacophora and nudibranches, both subclasses of mollusks.
On the micro scale, bacteria had little in the way of internal structure for billions of years, other than a few optimizations in locating their DNA near their membranes, which helped coordinate cell division with division of the DNA, the membrane, and the cell wall.
But then came along eukaryotes, formed from the fusion of two vastly different types of bacteria- by one theory, for metabolic cooperation, or by another, through simple phagocytosis. At any rate, this fusion created a revolutionary new type of cell that dispensed with the exterior wall (later re-made in fungi and plants from different materials) and developed instead an interior, or cyto-skeleton.
Mitosis, where the cytoskeleton (green microtubules) temporarily reforms to manage division of DNA (blue).
The cytoskeleton makes two basic contributions over the preceding bacterial cell wall- one, allowing cells to become protean- flexible and active protoplasmic blobs that can seek prey, as do many of our immune system cells. And second, helping manage the increased internal complexity of eukaryotes, facilitating their great size with membranous organelles, multiple chromosomes ... bits and pieces that need to be located consistently, moved occasionally, and inherited reliably. Our cells are not just bags of protoplasm, but ecosystems of molecules and larger structures, in which the cytoskeleton plays an organizational role.
The cytoskeleton also provided a cabling system to allow different cells to interact, paving the way for multicellularity. Eukaryotic cells touch at cytoskeletal attachment points, which are not just passive anchors, but also active participants in migration and signaling, helping cells touch and talk with their neighbors.
Later on in the Cambrian, once oxygen levels rose to respectable levels, allowing larger life forms, and Earth's climate got past a devastating series of ice ages, a new kind of skeleton emerged... the organismal endoskeleton. Compared to the armor of insects, skeletons are far more efficient, exchanging a surface-proportional structure for a linear structure with consequent reductions in weight. This allowed animals with skeletons to become far larger (think elephants, dinosaurs, and whales).
But what about the vulnerability of an unarmored exterior? There is where the story gets interesting, since being naked gives priority to reducing harm through smarts or size rather than through armor. Unarmored microbes are the predators and behemoths of their world. Think amoeba and paramecium. On the macro scale, the implications of endoskeletons are similar, with large size becoming its own form of defense. But more importantly, if the exterior skin is turned into a sensitive sensory system, with sensing hairs, whiskers, various forms of touch, all abetted with remote sensors like vision, hearing, and smell, plus a big brain, then the weakness of surface vulnerability can be turned into a strength.
The endoskeleton allowed for a greater range of evolvability, including encouraging organisms to turn themselves into multi-sensory platforms, much as our modern navy has become more effective by developing over-the-horizon sensing and attack rather than turning ships into super-armored tankers. Humans are, of course the ultimate expression of this evolutionary trend, not just mammals with endoskeletons, but nakedly hairless besides, vulnerable to every slight of the elements and enemies. Yet they have succeeded by replacing dumb armor with a combination of efficient and sensitive physiology and formidable intelligence.
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