New Species?

The recent discovery of hominin (human-like) bones in a South African cave has sparked a lot of excitement. The skeletal remains are virtually intact and reveal a people who have an interesting blend of characteristics - curved fingers which suit them for arboreal life and modern feet which indicates that most of their time was spent walking upright. An article in The Guardian elaborates on the find. Here's an excerpt:

Another team led by William Harcourt-Smith at the City University of New York analysed 107 pieces of Homo naledi foot bone. Writing in the journal, they describe how the foot is similar to those of Neanderthals and modern humans, but with a number of subtle differences. The toe bones are slightly curved, which may have helped Homo naledi a little when it took to the trees. The arch of the foot is low, or absent entirely, making Homo naledi flat-footed.

“It was unequivocally spending more time walking upright than not,” said Harcourt-Smith. “But you can imagine it spending time in the trees to gather fruit, or perhaps nesting in trees, or going there when there are predators around.” The curved toe bones are thought to be skeletal adaptations that Homo naledi inherited from its more arboreal ancestors and had not lost.

Until the bones can be dated, one of the major questions surrounding Homo naledi will remain: did the species emerge millions of years ago and live in successful isolation, perhaps even overlapping with modern humans? That is one possibility. Another is that Homo naledi is an evolutionary side-branch, a sister species of a known human ancestor, such as Homo erectus.

“You can imagine this lineage emerging early on, close to the origins of the Homo genus, and hanging on for a long period of time,” said Harcourt-Smith. “But that’s speculation. Evolution is messy. There is lots of experimentation going on, and lots of dead ends.”

However long ago H. naledi lived a question still remains that I've not seen anyone answer. Why is it assumed that these are the bones of an organism that is an entirely different species from H. sapiens? For that matter why is it assumed that H. erectus and H. sapiens are different species? The definition of a species is (or was) a reproductively isolated population of organisms. In other words, if two organisms can copulate and produce fertile offspring they're considered to be members of the same species. If they can't produce fertile offspring then they're assigned to different species.

So the question is how do we know that H. naledi, H. erectus, and H. sapiens could not interbreed? Even if they were separated in time that doesn't mean that they were a different species any more than a H. sapiens today is a different species than a H. sapiens which lived 50,000 years ago. It's true that all of the hominins are anatomically different but why would that make them different species? After all, every breed of dog, as different as they all are, are all the same species.

I'm eager to be instructed on this point, but until I am it seems to me that calling any of these hominins anything other than morphological variations of the same species is simply unwarranted on the basis of the evidence we have. What reason do we have to think that just because these different populations of hominins were separated by time or morphology that they would've been incapable of producing fertile offspring?