Ontogeny Recapitulates Phylogeny

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.