Well, fire up your time machine and set it for a few hundred thousand years in the past and you might find out.
Most of us know about our Neanderthal brothers. They lived between about 30,000 and 130,000 years ago, and were probably just as adept at tool-making as modern humans.
Where we differed the most was in the social aspects of life.
Denisovans are a lot less known, and scientists don't even know that much about them.
They likely split from Neanderthals around 300,000 years ago. Little is known about how Denisovans lived and what they looked like.
Until recently, those are the only groups that we knew about. Now, scientists think these is a fourth group that lived among us, Neanderthals and Denisovans.
It's been known that modern humans interbred with Neanderthals and Denisovans. Scientists were trying to create a much more detailed sequence of Neanderthal and Denisovan genomes when they came across genetic traces of a yet-unknown population of human ancestors.
It seems as though all four groups lived during the same time and would all interbreed.
It's still unknown why modern humans were the only group to survive to the present.
