It’s common to hear that we share 98% of our DNA with chimps. It is also common to hear that early humans interbred with Neanderthals or Denisovans because we share 1-2% of our DNA with them. Presumably these percentages are being calculated differently, unless we share more DNA with a mouse than another hominid. How are these percentages arrived at? Also, how do we differentiate shared DNA with a Denisovan or Neanderthal from the DNA we share widely across the animal kingdom. In essence, how do we know THAT DNA came from interbreeding rather than just being part of our common heritage that we share with much of the animal kingdom?
In: Biology
The chimp percentage is [kind of a hand-wave](https://youtu.be/IbY122CSC5w), and it’s based on how much of our total DNA appears to be the same sequences plus or minus identifiable mutations. Pretty much every living thing on Earth needs to have a fair amount of DNA in common to code for basic functions like digesting sugars and copying DNA.
The inter-hominid percentage is based on how much of our DNA it looks like we got specifically from interbreeding, and is presumably based on analysis of human populations in different regions where interbreeding wasn’t possible or didn’t happen. You can track some DNA through different species by following mutations. For example, if we all have some chimp DNA with mutation A, and Neanderthals all had the same chimp gene with mutation B but *not* mutation A, then as soon as humans start having a chromosome with mutation B instead of mutation A you have a pretty good idea of where it came from. Especially if it looks like it happened around the time when we had another hominid species nearby. It’s all ultimately from our common ancestor, this is more about which path the gene took to get to us.
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