system
Given the following post, respond as if you're commenting on Reddit:
human
Why were offspring of cross-bred hominids (e.g., Homo sapiens and homo neanderthalensis) viable to breed, but other cross-bred species like mules or ligers are infertile?
gpt
Answer:  i don't know the current terminology, but the last common ancestor of horses and donkeys lived 4 to 5 million years ago, so they have evolved to be farther apart than homo sapiens and homo neanderthalis, which had a much more recent common ancestor.
 As a result, horses and donkeys do not yield fertile offspring, because the parents are too far apart genetically. But many hominids could interbreed.
I consider mules and hinnies to be a real time 'experiment' proving evolution.