The similarity between different vertebrate embryos is indeed remarkable. Since the early 1800s, embryologists have been struck by the parallel between early development in various animal species and their evolutionary relationship, a resemblance conveniently abbreviated by the saying \"ontogeny recapitulates phylogeny.\" Of course, each of us does not really pass through a \"lizard\" stage on our way to a fully developed human form. But it is true that animals who are more closely related in terms of evolution will resemble each other for a longer period of embryonic development. At four weeks, a human embryo is barely distinguishable from any other vertebrate embryo—bird, reptile, or mammal—but by six weeks it resembles only other mammalian embryos, and by seven weeks, only certain primate embryos. such as monkeys.

The similarity between ontogeny and phylogeny shows that the strategy of early development has been highly conserved in evolution. This makes sense. if you think about the precise timing and series of events necessary to turn a single fertilized egg into many different complex organ systems; it\'s simply much easier to add changes at the end of a common developmental sequence than to alter things from the outset. A slight change early in neurulation, for example, could invalidate all kinds of later, subtly timed cues, throwing off the whole process of brain formation. (Just such a problem occurs in spina bifida, a relatively frequent condition in which part of the spinal cord is not fully enclosed because of a defect in the early neural tube.) It has been much easier for evolution to take an existing structure, like a forelimb, and turn it into a wing, or a primate cerebral cortex, and enlarge it into the human cortex, than to start with a whole new game plan for each species. Evolution proceeds through the selection of random mutations, and the later in development such a change occurs, the likelier it is to produce a viable offspring than a horrible mistake. Indeed, this is why miscarriages are more common in early pregnancy.

Living things go through the forms of their ancestors, not specifically but generally, because it is easier for evolution to add a mutation to the end of a complex sequence of developments than to re-engineer earlier in the process.

What a piece of work is the mammalian skeleton. I don't mean it is beautiful in itself, although I think it is. I mean the fact that we can talk about 'the' mammalian skeleton at all: the fact that such a complicatedly interlocking thing is so gloriously different across the mammals, in all its parts, while simultaneously being so obviously the same thing throughout the mammals. Our own skeleton is familiar enough to need no picture, but look at this skeleton of a bat. Isn't it fascinating how every bone has its own identifiable counterpart in the human skeleton? Identifiable, because of the order in which they join up to each other. Only the proportions are different. The bat's hands are hugely enlarged (relative to its total size, of course) but nobody could possibly miss the correspondence between our fingers and those long bones in the wings. The human hand and the bat hand are obviously - no sane person could deny it - two versions of the same thing. The technical term for this kind of sameness is 'homology'. The bat's flying wing and our grasping hand are 'homologous'. The hands of the shared ancestor - and the rest of the skeleton - were taken and pulled, or compressed, part by part, in different directions and by different amounts, along different descendant lineages.

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The skeletons of all mammals are identical, but their individual bones are different. The resolution of the paradox lies in my calculated usage of 'skeleton' for the assemblage of bones, in ordered attachment one to the other. The shapes of individual bones are not, on this view, properties of the 'skeleton' at all. 'Skeleton', in this special sense, ignores the shapes of individual bones, and is concerned only with the order in which they join up: 'bone to his bone' in the words of Ezekiel, and, more vividly, in the song that is based upon the passage:

Your toe bone connected to your foot bone,
Your foot bone connected to your ankle bone,
Your ankle bone connected to your leg bone,
Your leg bone connected to your knee bone,
Your knee bone connected to your thigh bone,
Your thigh bone connected to your hip bone,
Your hip bone connected to your back bone,
Your back bone connected to your shoulder bone,
Your shoulder bone connected to your neck bone,
Your neck bone connected to your head bone,
I hear the word of the Lord!

The point is that this song could apply to literally any mammal, indeed any land vertebrate, and in far more detail than these words suggest. For example your 'head bone', or skull, contains twenty-eight bones, mostly joined together in rigid 'sutures', but with one major moving bone (the lower jaw*). And the wonderful thing is that, give or take the odd bone here and there, the same set of twenty-eight bones, which can clearly be labelled with the same names, is found across all the mammals.

There are corresponding bones across species, evolved into other functions.