so that the number of separate hominid species found so far is still disputed, as is the line that led to us.
Why do you think I need to read more Dawkins?
If you want an example of how the "when fertile offspring can no longer be produced" definition isn't completely clear, try this:
Were Neanderthals a different species?
Hybrid males descended from both branches tend to be infertile, like mules. That’s because males have only one X chromosome, and if it happens to be one that impairs their fertility, then they may not reproduce. Females have two X chromosomes, so even if one is impaired, if the other one is normal, it can rescue her ability to bear young.
“So this suggests that the male hybrids might not have been fertile, whereas the females might have been fully fertile,” Svante Pääbo told Richard Harris of National Public Radio. Pääbo, the grand old man of ancient DNA based at the Max Planck Institute for Evolutionary Anthropology in Leipzig, was an author of the other paper, which appeared in Nature. We might have inherited most of our Neanderthal genes through hybrid females, he said.
...
“This underlines that modern humans and Neanderthals are indeed different species,” Fred Spoor told New Scientist. Spoor is also at the Leipzig Max Planck but was not a part of the Neanderthal research. Other scientists are more cautious about making so firm a declaration, but it’s clear that many lean toward that same conclusion, that Neanderthals were not Homo sapiens neanderthalensis but, rather, Homo neanderthalensis.
Darren Curnoe, a human evolutionary biologist at the University of New South Wales, blogged, “The latest findings from genome comparisons reinforce the status of Neanderthals and modern humans as distinct species. Those anthropologists who continue to regard Neanderthals as members of Homo sapiens now face a stronger challenge than ever reconciling their position with the DNA.”
http://www.geneticliteracyproject.org/2014/02/04/were-neanderthals-a-different-species/So, although non-Africans (and, thanks to very recent mixing, many Africans as well) have some Neanderthal DNA, many scientists regard them as a separate species from Homo sapiens, despite the offspring from the 2 groups being sufficiently fertile to be some of the ancestors of most of humanity.
The classification of older hominids - like Ardipithecus, Sahelanthropus, Paranthropus and Australopithecus - is even more up for discussion, because we can't use DNA in the same way for them.
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