Credit: Slug Signorino

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In sci-fi stories, alien planets often have multiple species of indigenous intelligent life forms, whereas Earth has only one species that is much more advanced than others. Why didn’t multiple species evolve comparable upper-echelon intelligence at the same time? —Ken in Sherborn, MA

What makes you think having multiple intelligent species around at the same time is science fiction? On the contrary, some researchers believe, two intelligent species once competed to dominate the Earth. Much as today we have normal people duking it out with House Republicans, Homo sapiens not too long ago may have engaged in a long twilight struggle with Homo neanderthalensis. One imagines a Cro-Magnon watching the Neanderthals flee after another doomed battle and thinking: Won’t those dumb bastards ever give up?

The thing is, the Neanderthals may not have been all that dumb. Although the name has become a synonym for mouth-breathing dimwit, archaeological research suggests that, at least in terms of brain size, Neanderthals were comparable to us. In other respects, however, they were ill-adapted to the modern age.

To be sure, any discussion of the hominid family tree involves about three parts speculation to one part fact. Here’s what we know.

Neanderthals and modern humans diverged from a common ancestor perhaps 400,000 years ago, with Neanderthals living primarily in Europe and our forebears camping out in Africa. Homo sapiens began spreading out of Africa around 60,000 years ago and reached Europe maybe 45,000 years ago. A relatively short time after that, archeologically speaking, the Neanderthals were gone. Just how short is a matter of debate—some researchers think it may have been as little as 5,000 years.

What happened? Some theories:

We killed them. Author Jared Diamond among others suggests we may have wiped out the Neanderthals just as Europeans did with indigenous peoples, via war and disease. One never knows, but Neanderthals whatever their other deficiencies were stocky and muscular and would have been formidable foes in close combat. As for disease, European pathogens depopulated the New World catastrophically fast—the Taino culture encountered by Columbus in the 1490s was virtually extinct just six decades later. The fact that the Neanderthals hung on for 5,000 years suggests that, whatever the differences in mortality, this wasn’t a case where we annihilated the natives primarily with our germs.

We assimilated them. Also not likely. Genome studies suggest some interbreeding occurred, most likely between male Neanderthals and female humans, but probably not a lot. The amount of Neanderthal DNA in the genomes of most present-day Europeans and Asians is no more than around 4 percent, and it’s even lower for Africans, whose ancestors stayed home and thus had less Neanderthal contact.

They couldn’t adapt. The trendy version of this line of thinking is that Neanderthals couldn’t adapt to the changing climate, although climatic conditions at the time they disappeared from the fossil record were seemingly favorable. The issue of timing aside, many have argued that Neanderthals lacked sophisticated social organization and hunting skills (they apparently never domesticated dogs, for example), were awkward and slow, and generally couldn’t cope with an evolving world.

We outcompeted them. The maladaptation theory suggests Neanderthals would have gone extinct whether we’d been on the scene or not. The competition theory, in contrast, says that, even if we didn’t necessarily destroy them in open warfare, by outgunning them in the battle for scarce resources we pushed them over the brink.

The evidence is largely circumstantial, but come on. Neanderthals had survived for hundreds of thousands of years. We show up, and 5,000 years later they’re gone. Some cite this as an example of the competitive exclusion principle: Two species can’t occupy the same ecological niche; one will eventually drive out the other.

That’s not to say you can only have one intelligent species at a time. Consider what some claim is the second-most intelligent animal on our planet: the dolphin. Dolphins have the second-largest brain-to-body-weight ratio of any terrestrial creature. They form large social groups, communicate, use tools, and exhibit altruistic behavior. Some researchers say they have so much on the ball they should be considered non-human persons. The difference is that dolphins occupy a separate ecosystem and don’t compete with us for the same resources.

OK, we’re dealing with an extremely small data set—until such time as the search for extraterrestrial intelligence pays off, this hypothesis is untestable. Still, it’s tempting to conjecture that a planet has room for one intelligent apex predator, and we’re Earth’s. —Cecil Adams