And that shift coincides with the seeming disappearance of the Neanderthals. The bulky-browed primates, the scientists speculate, were unable to adapt their hunting skills to small game. And that was not a small thing, because big game are just that: big. Hunting larger animals -- chasing them, felling them, hauling them home -- expends considerable resources of time and energy. Small game, on the other hand, is less demanding of hunters. It might take more cunning to catch a rabbit, but it generally takes less physical energy. And this discrepancy might have made an important evolutionary difference, the thinking goes, particularly as large animals reduced in numbers. "We suggest," the authors write, "that hunters that could shift focus to rabbits and other smaller residual fauna, once larger-bodied species decreased in numbers, would have been able to persist." Neanderthals, on the other hand, "may have been less capable of prey-shifting."

The evolutionary status of Neanderthals, it's worth noting, remains a subject of debate among paleontologists: Were they a distinct species, or simply a less-evolved version of humans? Should we call them, properly, Homo neanderthalensis, or is it more accurate to classify them as Homo sapiens neanderthalensis, a subspecies of Homo sapiens? (Fa and his colleagues break down the distinction as "Neanderthal" versus "Anatomically Modern Human," or AMH.) These are taxonomic niceties, though. What's clear is that humanity, Darwinistically speaking, won. We became what we are, and Neanderthals stayed stuck in their evolutionary moment.

And though we don't know why, exactly, that was the case, we know that it came down to our ancestors' ability to relate -- violently or cooperatively -- to their fellow species. The bunny-based explanation of human hegemony is only the latest in a long line of proposals adding context to something we already know: that we humans owe our current human-ness to our fellow creatures. Evolution, like so much else, is relative. And many of the most interesting hypotheses in that respect, particularly from a technological perspective, have come down to animals -- to early humans' ability to use animals, essentially, as cooperators (less romantically: as tools) in survival. Whether as partners or as prey -- whether the creatures in question were big dogs or tiny bunnies -- animals helped us to become what we are.

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