The article is very interesting and informative, and comes from a good source. But I have to quibble about describing Denisovan and Neandertal hominins who were ancestors of current human beings as coming from a "different species," because by one of the main concepts of speciation, if two organisms can mate and have viable offspring, and then the offspring can further reproduce, you say the two organisms are part of the SAME species. There are, of course, full-length books about the details of defining species among the common descendants of the earliest living things, and about the mechanisms that bring about speciation among the descendants of a common ancestor species,[1] but right now we don't always know for sure when we dig up old hominin bones which belong to relatives of direct ancestors of living human beings and which do not--that is part of what the studies of ancient DNA are intended to find out.
To make the point I am bringing up here, I would rewrite the second sentence of the helpful article kindly submitted here to read, "As the individuals who provided the main genetic contribution to modern humans began to spread out of Africa roughly 50,000 years ago, they encountered other hominin clades that looked remarkably like them — the Neanderthals and Denisovans, two groups of archaic humans that shared an ancestor with us roughly 600,000 years earlier."
Lions and tigers can mate and have fertile offspring yet no one would consider them the same species [1]. Actually there is evidence that the offspring of ancient African and Neanderthal matings [2] followed Haldan's rule [3].
If this discussion was about anything other than humans there would be no question that Ancient Africans, Neanderthals and Denosovians were anything other than different species.
That conception of what a species is has been my understanding as well for a long time. It's a very simple and clear concept but apparently biologists have given up on it for various reasons. There's actually a whole wikipedia article about it:
That's because the ability to mate and have viable offspring is not anything like a simple and clear concept in some cases. For example: https://en.m.wikipedia.org/wiki/Ring_species
"If two organisms can mate and have viable offspring, and then the offspring can further reproduce, you say the two organisms are part of the SAME species."
Firstly, that's not true.
Even if that were true, there is no evidence of reproductive success between human males and Neanderthal females.
To make the point I am bringing up here, I would rewrite the second sentence of the helpful article kindly submitted here to read, "As the individuals who provided the main genetic contribution to modern humans began to spread out of Africa roughly 50,000 years ago, they encountered other hominin clades that looked remarkably like them — the Neanderthals and Denisovans, two groups of archaic humans that shared an ancestor with us roughly 600,000 years earlier."
[1] http://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3D...