Tool Module: The Connection between Ontogeny
The evolution of the human brain over millions of years and its development over the course of one lifetime are inextricably linked. In fact, the best way to get an overview of the stages through which our brain passed in the course of evolution is to look at those through which it passes as an individual develops.
To understand the link between phylogeny and ontogeny (in other words, between the evolution of a species and the development of an individual), one must understand that a species can evolve from a series of small mutations in the development program encoded in its individuals’ genes. The earlier that these mutations occur in an embryo’s development, the more likely they are to be lethal, because of the fundamental changes that they will involve. That is why we tend to see more mutations in the later stages of development, and why various species show similarities in their early embryonic stages. But sometimes a mutation in the program at an early stage of development will still leave the embryo viable, resulting in a differentiation of these early stages that erases any strict correspondence with the phylogeny of this species.That is why a strict interpretation of Haeckel’s law of recapitulation does not withstand close empirical scrutiny.
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