Embryos don't replay their ancestors as adults
- The idea
- Predict
- See it
- Why
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The idea
“A human embryo passes through the adult forms of its ancestors, first a fish, then a reptile.”
People still say a human embryo grows gills. It grows neck pouches like a fish embryo's, but in us they never become gills.
The short version
The mistake: A human embryo passes through the adult forms of its ancestors, first a fish, then a reptile.
What's actually true: Related species share early embryo stages from a common ancestor. No embryo passes through an adult ancestor.
- Development is inherited like any other trait. Related species share early steps because they got the same developmental genes from a common ancestor.
- Those early steps tend to change little, because so much later growth depends on them. Later on, each lineage reshapes the shared parts.
- The old idea that embryos replay adult ancestors is called recapitulation. It was dropped long ago.
On the exam: Embryos resemble their relatives' embryos, never adult ancestors. Shared early stages mean common ancestry.