RENEE MONTAGNE, host:
Something big happened to our species during the middle of the Stone Age. We got smarter. At least that's what scientists deduced from artifacts they've dug up from that period. And many of those scientists believe that some change in our culture caused this flowering of intelligence. NPR's Christopher Joyce reports.
CHRISTOPHER JOYCE: Remember that moment in the movie "2001: Space Odyssey" when the apes saw the big black monolith from outer space, and suddenly they picked up some bones and voila?
(Soundbite of music, "Also Sprach Zarathustra")
JOYCE: They had tools, and human intelligence took off. Well, no. Here's another explanation. Let's say a guy in a cave, we'll call him Org, invents a cool new spear point with barbs on it so it'll stick better. If he lives in a small group, there's a good chance his pals just won't get it. Org's invention dies when he gets eaten by a cave bear.
But if he's in a big community, chances are good that there's someone else who'll say, hey, Org. I think I can do that, too. That's what Mark Thomas, an evolutionary geneticist at University College London, thinks.
Dr. MARK THOMAS (Evolutionary Geneticist, University College London): So if there are more people in the population, then more complex skills can be maintained in the population without that decay.
JOYCE: Because there are more teachers?
Dr. THOMAS: No, because there are more good teachers.
JOYCE: Writing in the journal Science, Thomas and his colleagues say the leap in human intelligence in the middle Stone Age came about because people started living in bigger communities. Thomas outlines a mathematical model that explains how this works. He says you'd need at least 200 or so people in an area 35 square miles to set the stage for the creative explosion. But he says you don't need the math to understand the basic idea.
Dr. THOMAS: If you take random people from around the world, 20 people, and ask somebody to play the guitar, there might be somebody there who has a little strum. If you take 100,000 people and look for the person who plays the guitar the best, on average they're going to be considerably better than somebody from a room of 20 people, right?
JOYCE: But Thomas says invention apparently had two mothers: population density and lots of separate communities. Rick Potts agrees with that. Potts heads the Human Origins program at the National Museum of Natural History in Washington, D.C. He says history shows that when different groups come in contact, they tend to trade ideas.
Dr. RICK POTTS (Human Origins program, National Museum of Natural History): One of the neat things about this study is that it takes into account not only how many teachers there are, how many learners there are, but also the interconnectivity between a group that may have an innovator to other groups that may have receptive learners.
JOYCE: Thomas's idea helps explain something that has long puzzled anthropologists. Modern humans came on the scene about 180,000 years ago, but for a long, long time, says Thomas, we pretty much sat on our hands.
Dr. THOMAS: Even though we appear to be what we are today, you know, anatomically modern humans with our nice, great, big fat brains, we just didn't seem to do anything interesting for a long time.
JOYCE: Not until 90,000 years ago in Africa, and then again about 45,000 years ago in Europe. That's when humans started painting on cave walls, fashioning better weapons like slings and boomerangs and making bone tools. Some scientists still believe that the human brain suddenly did change at that point. But Thomas and colleagues say their theory better explains why intelligence seems to have taken off in so many places at once.
And the idea doesn't rule out the possibility that something did change in our brains, but Thomas says his hypothesis suggests that it wasn't necessarily some magic spark that made us smart.
Christopher Joyce, NPR News.