A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why did only one intelligent species evolve on earth?

October 25, 2013

Dear Cecil:

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? Is there something inherently unlikely with the alien-planet scenario?

Cecil replies:

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 — surely one of the more poignant conflicts in human history. One imagines a Cro-Magnon watching the Neanderthals flee after another doomed, pointless 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 while our forebears camped out mainly 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 all 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. (Then again, the same might be said of Goliath.) 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. (Or so it was long thought; some question has since been raised about this.) 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 just couldn’t cope with an evolving world.
  • We outcompeted them. Here we get to the heart of your question. 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.

Granted the evidence is largely circumstantial, but come on. Neanderthals had survived for hundreds of thousands of years. Then 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 from us. Unlike the Neanderthals, they 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.

Related Posts with Thumbnails


Bjorklund, David F. "Mother knows best: Epigenetic inheritance, maternal effects, and the evolution of human intelligence." Developmental Review 26.2 (2006): 213-242.

Connor, Richard C. "Dolphin social intelligence: complex alliance relationships in bottlenose dolphins and a consideration of selective environments for extreme brain size evolution in mammals." Philosophical Transactions of the Royal Society B: Biological Sciences 362.1480 (2007): 587-602.

Dunbar, Robin IM. "The social brain hypothesis." Brain 9 (1998): 10.

Flinn, Mark V., David C. Geary, and Carol V. Ward. "Ecological dominance, social competition, and coalitionary arms races: Why humans evolved extraordinary intelligence." Evolution and Human Behavior 26.1 (2005): 10-46.

Liu, Yang, et al. "Cetaceans on a molecular fast track to ultrasonic hearing." Current Biology 20.20 (2010): 1834-1839.

Marino, Lori, et al. "Cetaceans have complex brains for complex cognition." PLoS Biology 5.5 (2007): e139.

Miller, Geoffrey F., and Lars Penke. "The evolution of human intelligence and the coefficient of additive genetic variance in human brain size." Intelligence 35.2 (2007): 97-114.

Miller, Greg. “Is a Dolphin a Person?” Science 21 February, 2010. Online edition, accessed 6 October, 2013. http://news.sciencemag.org/brain-behavior/2010/02/dolphin-person

Roth, Gerhard, and Ursula Dicke. "Evolution of the brain and intelligence." Trends in Cognitive Sciences 9.5 (2005): 250-257.

Sagan, Carl. Dragons of Eden: Speculations on the evolution of human intelligence. Random House Digital, Inc., 2012.

Viegas, Jennifer “Dolphin Intelligence Explained” Discovery News 22 January, 2010. Online edition, accessed 6 October, 2013. http://news.discovery.com/animals/dolphin-intelligence-explained.htm

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