[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.
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