[ELI5] Why do mammal embryos look the same when they are developing inside their mothers?
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Embryos growing are kind of like looking back at past common ancestors in evolutionary history. At a point humans even have gill slits as embryos, and you’d be hard pressed to tell the difference between a human/elephant/pig embryo at early enough stages. Using that example gill slits are seen in fish, reptile, bird and mammalian embryos. Comparing embryos shows a lot of evolutionary developmental programming.
Evolution just builds on top of the last thing that worked, and there are a ton of things that were built on to get to where we are now. While we don’t develop gills, we share that dna with fish, and it develops early on in our development and it then reprogrammed to turn into things that become the inner ear, thyroid/cricoid cartilage and various other things in the face/neck.
This is known as the recapitulation theory by Ernst Haekel i believe, according to which, all animals metaphorically go through thousands of years of evolution in their few months of gestation
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
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.
It’s not just mammals, the early stages of most vertebrates are very similar. The closer in relation you get to humans, the longer the embryos will look similar to human embryos. This is due to the fact that all life that exists today evolved from the same life form billions of years ago. So the genetic instructions used to create new species will be similar, but the further you’re separated from other life forms on the tree of life, the less similar you’ll be. For example, plants obviously don’t develop like humans. That said, vertebrates are relatively close to each other on the tree of life, and mammals are all very close to each other.
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