system
Given the following post, respond as if you're commenting on Reddit:
human
If Humans and Neanderthals are separate species how was there interbreeding?

The way my college bio professor explained it is that crossbreeding between species creates infertile offspring if at all. I have heard that scientists believe some of our DNA can be traced back to crossbreeding with them. I asked my professor but he didn't seem to have and answer. He just said sometime there are exceptions.
gpt
Species aren't always as cut and dried as that. Some interspecies hybrids are fertile. Homo Sapiens and Homo Neandertalensis both have 23 pairs of chromosomes, and there's a commonality of about 99.7% between the two genomes. The concept of species is a human invention, has nebulous boundaries, and is especially difficult to accurately apply to extinct specimens.