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The story of how humans evolved has always been, well, a work in progress because when scientists dig up a new fossil bone or stone tool, it often adds a new twist to the story. Well, today a trio of anthropologists has decided it is time to rewrite that narrative in a big way. As NPR's Christopher Joyce reports, they're bringing us a new take on how we became the creatures we are and what drove that process.

CHRISTOPHER JOYCE, BYLINE: Anthropologists argue about the details of human evolution. But they generally agree that about 2 million years ago in Africa, the human lineage emerged. Anthropologist Rick Potts says the conventional wisdom is that much of Africa changed about then, from forest to dry savanna. Our ape-like ancestors had to adapt, so they evolved into something more like us.

RICK POTTS: The traditional package of traits, includes elongated legs, large brain, culture - a whole variety of traits all were thought to have come together with the origin of the genus homo. We're saying, no, that's not the case.

JOYCE: Potts is curator of human origins at the Smithsonian's National Museum of Natural History. He and his collaborators have analyzed fossils discovered over the past few decades, and they say the human animal did not come together in one fell swoop. Anthropologist Leslie Aiello runs the Wenner-Gren foundation in New York.

LESLIE AIELLO: What's different about it is the whole package that makes us human - long, linear bodies, very large body size, delayed growth and development for the kids - didn't evolve at the same time.

JOYCE: Instead, these scientists say traits that made us human arose separately in a herky-jerky fashion. There were at least three different types of early humans running around in Africa then - each had different features. In a way, human evolution was like the development of the modern automobile. You can trace its various advancements back to their origins in the Model T, 1950s luxury cars, station wagons or hatchbacks. Potts argues also that what drove this piecemeal construction of humans wasn't a single event like the drying out of Africa. Instead, it was frequent climate change

POTTS: The kind of unpredictability - uncertainty in the environment - that was the evolutionary driver for the origin of larger body size and large brains, the use of tools.

JOYCE: Potts says we did not evolve to adapt to any single change. Instead, we evolved the ability to adapt quickly to any new environment - wet to dry, cold to hot, whatever came down the pike.

POTTS: I would say that the origin of our human lineage in fluctuating conditions led us to become versatile; we're versatilists, if you will.

JOYCE: And our versatility is what makes us special. Potts and his colleagues lay all this out in the journal Science, where other anthropologists will likely line up to pick out the flaws. William Jungers at the State University of New York at Stony Brook says the group makes assumptions based on pretty fragmentary evidence, and he wonders why the super-adaptability of humans doesn't show up in other mammals which face the same climate fluctuations. That said, however...

WILLIAM JUNGERS: I think this is an enormous undertaking to try to synthesize all this information, much of it in the last decade to be sure. And, you know, every new fossil you find sort of requires getting back to the drawing board.

JOYCE: And in the case of modern evolution, the drawing on that board keeps getting more complex. Christopher Joyce, NPR News.

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