[ELI5] Why do mammal embryos look the same when they are developing inside their mothers?

Question

[ELI5] Why do mammal embryos look the same when they are developing inside their mothers?

In: 27

Answer 1

Embryos of different species don’t look *exactly* the same, but it’s true that they’re close.

The basic logic here is that evolution works by combining a bunch of *small* mutations, each of them individually beneficial. But a mutation that affects an embryo early in gestation tends to have *big* effects rather than small ones. So the mutations that survive are *usually* ones that don’t affect the embryo until late in gestation.

For example, suppose you’ve got a simple cake recipe that looks like the following:

1. Put the dry ingredients in a bowl.
2. Stir.
3. Add the wet ingredients.
4. Stir again.
5. Bake.

Now, what if you want to modify this to make it a *birthday* cake, by adding a “Put candles on top of it” step? You’ll find that if you insert this extra step too early in the list, the cake won’t turn out well–it may taste waxy, and/or fail to rise properly–but if you insert it at the end of the list, it’ll be fine.