https://www.youtube.com/watch?v=3_VybJT_LFk&t=902s
There is reportedly some debate about whether Neanderthals were a separate species. Google search results say that they were. However, Neanderthals were close enough to us that I like the idea that they were a sub-species.
It is common for people of European ancestry to have a little bit of Neanderthal DNA. Someone I know did a genetic test that showed a higher-than-average percentage of Neanderthal DNA.
Genetic diversity is a good thing regardless of the species. It can help a group survive environmental changes. Under harsh conditions, some individuals die off and some survive. (Mother Nature is a bitch. 🙂)