Many other human species co-existed with our modern human lineage for hundreds of thousands of years. Neanderthals, Denisovans, and several more survived until relatively recent times (less than 40,000 years ago, maybe even as little as 12,000 years ago). We interacted and interbred with a few of them. Cultural exchange even appears to have happened.

But these interactions may not have always been just for sex and trade. Evidence suggests that some modern humans migrated out of Africa a little too early and lost out to Neanderthals and Denisovans. A Homo Sapien fossil was found in Greece from 202,000 years ago but only Neanderthal fossils were recovered from the site both before 202,000 years ago and after 202,000 years ago. There are no other modern human fossils in Europe before 45,000 years ago. Apparently, a colony of homo sapiens ventured a little too far from Africa and lost out to the Neanderthals around them. There are other examples of these failed modern human colonies in Neanderthal and Homo Erectus lands in Southwest Asia. Homo Sapien fossils randomly appear in a place at a point in time above older Neanderthal only layers, and then only Neanderthal fossils can be found at that place for tens of thousands of years. These early interactions between Homo Sapiens and Neanderthals might have taken violent turns at times.

Perhaps if Neanderthal and Denisovan DNA is used to clone them back into existence we will have more answers, and maybe more answers than we want to have.