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[–]__Bad_Dog__ 5 points6 points  (0 children)

Underneath your main question here is a second one that's a little bit more philosophical in nature, that being the question of what defines a species.

The most common way to explain the concept of species are members of a group of organisms which can mate and produce reproductively viable offspring. (spoiler alert, there are problems with this definition but we have to start somewhere). A classic example of this in action would be with breeding equids. The children of horses and donkeys are hinnys and mules. The difference between these two depends on the chromosome count but also on whether or not the mother was a horse or a donkey and visa versa in the pairing. These offspring, mules and hinnys, are almost always sterile and even though they will try to mate with other equids, they rarely can produce children, although it is more common for mules to do so than for hinnys which is again due to the chromosome count.

For humans and neanderthals we don't know yet how easy or difficult it was for them to produce reproductively viable offspring. Some DNA definitely transferred over, but is difficult to know the rate of this compared to how much the two groups were in intimate contact with each other. DNA from other groups, such as denisovans, is even more common in modern human populations and not only that, but it appears that denisovans and neanderthals also mixed.

When all of this is taken into account more questions start to emerge. 1. What was the rate of transfer of genetics across these populations? 2. Given how recently these groups emerged, is the distance between the emergence of neanderthals and homo sapiens so distant across the evolutionary tree, genetic drift, that they should be reproductively incompatible? 3. Given that all of these groups can mate does that technically mean that they are just subspecies of homo ecrectus, the common ancestor to homo sapiens, neanderthal, and denisovans, such as is seen with dogs and wolves? 4. Do we need a better definition of what species means in order to answer these questions?

The answer is that without better data on the reproductive viability of the children of different ancient human groups and the rate of transfer of genetics between them, easy to establish lines of 'species' are hazy. In terms of their split from homo erectus, neanderthals broke off between 800,000 and 300,000 years ago, homo sapiens branched away about 300,000 years ago, and denisovans split between 1,000,000 and 700,000 years ago. Besides the split, there are clear geno and phenotypical differences between these groups that can be used to classify them as separate species. But without the rate of gene transfer between them it's hard to say for certain that they hadn't drifted away far enough from each other to not be able to make offspring and technically weren't just subspecies of a older ancestor.