How far back along the evolutionary tree do we have to go to find divergence to a completely different species to Hom. sapiens? I don't count Neanderthals because it seems that part of our genome contains fragments of Neanderthal DNA, and therefore, according to the species definition, Humans and Neanderthals could not have been an entirely different species. What about Denisovans? Is enough known about their DNA to consider them a completely different species? In fact, is it even possible to distinguish 'a species' from the DNA alone?
The erectus line that gives rise to antecessor is your best bet, at about 1.2 MYA. http://www.nature.com/nature/journal/v485/n7396/full/485033a.html part of the problem is you are hitting the limit of where the term species is useful, once you get really close to the speciation event the term breaks down becasue everything is a ring species once time is included. Erectus includes both our ancestors and populations wholly isolated from us. Its like trying to draw a line between yellow and orange on a spectrum.
Do you also consider tiger and lions as different species? (Limited) Gene flow between the two groups has occurred. (Due to female hybrids being fertile.)
There is evidence that gene flow between humans and neanderthals was also restricted. Neanderthal genes regulating fertility, speech, neural activity are absent in human (No Neanderthal Y chromosome has ever been found in human population either), although neanderthal genes for immunity, hair and skin and coagulation have been retained. This selective-ness is pretty much a sign that hybrid fertility was reduced and that there was selection pressure to remove Neanderthal genes that govern fertility from the human population. And this is the hall mark of different species.
If you still discount neanderthal as a separate species due to limited gene flow between the two groups. Then Denisovans are also not another species because there has been limited gene flow from Denisovans into humans (Melanesians and Aboriginal Australians)
In fact, is it even possible to distinguish 'a species' from the DNA alone?
It is possible if you have a pure nonhybrid sample of all three species. Which was obtained from human (africans do not have neanderthal DNA), neanderthal (from various samples), Denisovans (from a single finger bone of a female from the Denisova Cave in the Altai Mountains in Siberia)
I guess the next homo species we are aware of and have no evidence of gene flow is homo erectus (mainly because we don't have DNA from Homo Erectus)... however there is some indication that Denisovans did interbreed with an older and still unknown population of homo...
And if true you start getting into a ring species problem.... both ends of the ring are infertile and by definition are different species... yet if all member populations of the ring are still alive... gene flow between the two species is possible. So are they different species or are they not.
That's something that's measured by generations.
If the neanderthals bred with humans after divergence times in a range of 631–789 KY, that's roughly 30,000 generations distance, at which stage their genetics converged again.
The notion of a species is so flexible, that Neanderthals and Sapiens are considered different species, and also Sapiens is a hybrid of both.
Horses and zebras can still nearly interbreed after 900,000 generations divergeance and 4.5 million years.
What counts is the number of chromosomes. If they have the same number of chromosomes, the species can mix genetically. If they have odd numbers, the species wouldn't have a chance to converge again.
Mammals mutate more diversely than other tetrapods and birds, i.e. bats and whales are more different than any two species of bird. Mammals have less protein diversity and gene diversity than species that can hybridize after longer speciations(some birds can be viable after 20mn years), and they have more gene control variations which regulate complex metabolism traits which have to be compatible in both parents. https://en.wikipedia.org/w/index.php?title=Hybrid_inviability&action=edit§ion=1