I will have a go at giving you a mental picture of what was up with these Denisovans of East Asia, with whom Anatomically Modern Humans mated at multiple points in our history, from 15 - 50,000 years ago. If you have been following the story over the last decades of human genetic descent you likely have the "Out of Africa" model in your head. This is the idea that a small number of one group of East Africans among a variety of near-related hominids migrated from what is now Ethiopia, Eritrea, or Djibouti and gradually settled the whole world beginning about 50,000 years ago.
Don't throw this away. This is still 95% of the story. Anthropologists and geneticists are getting so excited about the 1-4% Neandertal in Eurasian and 3-5% Denisovan in East Asian DNA that they recently uncovered that sometimes leave the main story behind. I like Razib Khan very much, but every few episodes he has to ask a guest "Doesn't this mean we will have to change our definition about what it means to be human? Doesn't this undermine the idea that we are special in some way?" He clearly has some philosophical, cultural, or even political idea attached to this debunking, though I can't tell what it is.
Well, we did settle the entire world and drive out all the other hominid species. That seems pretty special.
For now, put aside that 95% Out of Africa picture, in order to see the remaining bits more clearly.
It is now clear that other hominids left Africa before this, sometimes long before this. We knew about the Neandertals from all our European digging, but gradually, others have emerged. There were hominids discovered in the Levant that go back well before 50,000 years ago, and even possible homo sapiens at other Mediterranean sites from more than 100,000 years ago. The Discovery in Spain of a serious tool-making homonim, looking more Denisovan than Neandertal, and maybe even more like us, is 430,000 years old. That and the discovery ten years ago on the Kazakh border of Russia, in the Denisova Cave of a bone that is clearly homonin, and perhaps even sapiens (categorisation is being argued) has sucked up all the oxygen in the room since. It clearly split from us more than 200,000 years ago. Homo floresiensis, the "hobbit" people of Flores island survived until about 50,000 years ago. There is even more Denisovan ancestry in New Guinea and the Philippines, and now a mandible has been found on the Tibetan Plateau in China. Pull out a globe. These places aren't near each other. And neither are close to Spain and Morocco, where the newer finds are.
In the same cave, there has now been found a specimen that is half Neandertal, half Denisovan, which was deeply unexpected. Spencer Wells wonders humorously whether this cave was some kind of Mos Eisley Cantina where all homonin species in Asia eventually visited. A few genetic researchers had known that something weird was up, especially in Melanesia, as there was genetic material that didn't appear related to any other groups of humans. We now think there were Denisovans all over East Asia, varying widely and occupying different habitats. If we can see how much variation you can get in modern humans in 50,000 years of Inuit, Turks, Maori, and Yamamomo, it's not hard to imagine even greater variation in 200,000 years of this group that left Africa even earlier. Neandertals may only be a newer variant of that larger group. They had different adaptations, different mutations, and likely ran into each other, sometimes mating, sometimes one outcompeting the other for a niche, most likely a combination of both.
Back in Africa there were several varieties of human as well. Most are gone, but some left genetic evidence of themselves in the tribes that outcompeted them. Genes protective against disease are always useful. The Khoi-San people are the best known ot these. They are Homo sapiens, but they have some genetic material that is different from anything anyone else has. Lots of species closely related to us, seemingly everywhere around the world, mostly getting more distinct but sometimes mixing with each other. They had some great adaptations. The Tibetan Denisovans, for example, had a bunch of alleles that helped them adapt to altitude. There are five mutations close together on one gene that are strongly associated with survival at altitude. Five random mutations scattered about the genome could gradually come in at different rates over time, each with a slight advantage. Five that close means they've been together a long time, and work together very well. Hardly anyone has got them.
Let's bring that Homo sapiens (sapiens) tribe back in. They have arisen somewhere in East Africa and probably outcompeted neighbors, moving around becoming a big deal, and some of them start either crossing the Red Sea or moving up the side of it. Not everyone. Most stayed behind. But those who started moving on seemed to be able to adapt to just about any environment. If one group of cousins came and kicked them out they moved on and survived anyway. Whenever the neighborhood got too crowded or resources too scarce, enough of them figured out an alternative strategy.
Along the way they encountered these other humans. Whether they mostly fought or mostly just moved away from each other is unknown. They had enough contact to have interbred, and whatever good genes each had was shared with the other. As the later group eventually eliminated the others they must have had some powerful advantages. These may have even translated into improved survival for all those Denisovan and Neandertal variants - for a while. The genetic advantages they traded back, like surviving on a frigid plateau at 3000 meters, were likely specific to the environmental niche.
The first nomination for the advantage the new group had was improved language skills. The previous groups likely had facial expression, sounds or noises made with mouths or with objects, and very solid communication skills. Yet maybe not quite enough. In the land of the blind, the one-eyed man is king and all that. Anyway, they're gone and we're here, but we interbred with them often enough to get some prizes. There was a second introjection of Denisovans in New Guinea very recently - maybe as little as 12,000 years ago, according to the geneticists attempting to put things together in clear packages, and third or fourth episodes scattered over time and place are considered very possible.
Back in Africa, the group that gave rise to ours did the same, gradually establishing dominance but picking up cool genetic stuff related to specific environments whenever they moved in.
Out Of Africa is still the story of our ancestors. But it has recently gotten more complicated.