Dance Of Human Evolution Was Herky-Jerky, Fossils Suggest

toggle caption Sylvain Entressangle/Science Source

A trio of anthropologists has decided it's time to rewrite the story of human evolution.

That narrative has always been a work in progress, because almost every time scientists dig up a new fossil bone or a stone tool, it adds a new twist to the story. Discoveries lead to new arguments over the details of how we became who we are.

But anthropologists generally agree on this much: A little more than 2 million years ago in Africa, the human lineage emerged. Smithsonian 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 or die, leave the forest and embrace the savanna — and in doing so, they evolved into something more like us.

"The traditional package of traits," Potts explains, "including elongated legs, large brain, culture, a whole variety of traits, were thought to have come together with the origin of the genus Homo. We're saying no, that's not the case."

What's different is that 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.

Potts is curator of human origins at the Smithsonian's National Museum of Natural History. He and his collaborators, Susan Anton of New York University and Leslie Aiello of the Wenner-Gren Foundation, have analyzed fossils discovered over the last few decades. They say the human animal didn't come together quite as quickly and neatly as commonly thought.

"What's different," Aiello says of this new narrative, "is that 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."

Instead, these scientists say, traits that make 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 seems to have had different features that ended up in more modern humans. And, in fact, one particularly "human" feature — longer legs — actually seems to have first emerged in our ape-like ancestor Australopithecus (known by most people via the famous fossil Lucy), well before humans arrived on the scene.

In a way, human evolution was like the development of the modern automobile: You can trace its numerous and various advancements back to their origins in the Model T frame, diesel engines, 1950s luxury designs, or compact hatchbacks.

So what drove this piecemeal construction of humans? Potts says it wasn't adapting to the drying out of Africa. In fact it wasn't a single event at all. Instead, what forced our evolution was frequent climate change.

"The kind of unpredictability and uncertainty in the environment — that was the evolutionary driver for the origin of larger body size, and large brains, the use of tools," Potts says.

As a result, he says, what made us uniquely human was the ability to adapt quickly to any new environment — wet to dry, cold to hot, whatever came down the pike. "I would say that the origin of our human lineage in fluctuating conditions led us to be versatile — we're 'versatilists,' if you will."

Potts and his colleagues lay all this out in this week's issue of the journal Science, where other anthropologists will likely line up to pick out flaws. Adrienne Zihlman at the University of California at Santa Cruz says more data might make the group's conclusions less "speculative."

William Jungers, an anatomist and anthropologist at the State University of New York at Stony Brook, says the group may be reaching by attributing so many traits to different human species with so little physical evidence. He also wonders why the evolution of this "super-adaptability" of humans hasn't shown up in other mammals, which faced the same climate fluctuations.

That said, Jungers adds, "I think this is an enormous undertaking to try to synthesize all this information, much of it in the last decade. And, you know, every new fossil you find sort of requires getting back to the drawing board."

And in the case of human evolution, the drawing on that board keeps getting more complex.