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?