[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

Because we share relatively close common ancestors with mammals, we use a lot of the same building blocks.

We all have the same amount of vertebrae, we all have leg bones and tail bones.
In very early stages of development, it doesn’t matter if your legs need to become a dog’s, a whale’s or a human’s, what matters is that they become legs at all. And then our legs have at least the same bone structure and the same muscles, even the rudimentary legs on a whale.

First the basic outline is built, then the details are added