Are vertebrates likely to be rare in the universe? On Earth there are only two types of animals with internal, articulated skeletons, the vertebrates and the brittle stars (Ophiuroidea, Echinodermata), and the brittle star’s endoskeleton is simple compared to that of the vertebrates. Most other animal phyla are limited in size, and brain size, because they lack an internal skeleton for support. The vertebrates, with their strong skeletons, have produced some sizeable animals like the blue whale and man. Man’s big, internal skeleton helps to support his large brain.

 

If vertebrates are scarce in the cosmos, although life of other sorts may be common, it may explain why no aliens have come knocking. They’re too small to support large brains and raise a civilization.

 

Kevin Fitzgerald

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Yeah, and a lot of that may have do with the availability of oxygen on an alien planet. Only the metabolic energy released by burning oxygen (i.e. eating carbon and breathing oxygen) is capable of "large" animals (by large I mean visible to naked eye). So if there is life on another planet, and there is oxygen for several billion years, and the environment is not too chaotic, then maybe that life could evolve to support a skeleton and a brain? Maybe. Maybe not. Recent research shows that this relationship is not straightforward. See attached for further reading!

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What do you mean by "this relationship is not straightforward" ?

That the presence of oxygen was the direct cause for animals. The article I attached in my previous message has the details. Reading the abstract and the conclusion will give you the gist :) There are other factors that likely contributed (temperature, selective pressures driving evolution, etc..) 

Life becoming larger in response to higher levels of oxygen and life becoming larger by evolving vertebrate skeletons are two separate issues

I'm not sure they are entirely separate: where did the biochemical energy come from to create the larger structure if not by burning oxygen? I'm sure it's more complicated than this, but oxygen surely had some sort of driving role?

The internal brain of the external vertebrate skull seems comparable to an invertebrate structure.  Would it be possible for invertebrates to evolve a vertebrate type skeleton after evolving an encased brain first?  According to Neil Shubin in his book Your Inner Fish, there is evidence that skulls evolved from and after teeth. 

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