Photograph by Brent Stirton, Getty

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A skull of Australopithecus sediba, one of Africa's prehuman species, rests on a rock outcrop.

Photograph by Brent Stirton, Getty

Climate Swings Drove Patchwork-Like Human Evolution, Study Says

Unique human traits evolved over time among humanity's myriad predecessors.

Stone tools, big brains, and long legs made for walking—the hallmarks of humanity—arose at different times and in patchwork fashion in Africa millions of years ago, not in one big jump as previously thought, evolution experts suggested on Thursday.

Scientists have long thought that as forests retreated and savannas expanded in East Africa more than two million years ago, our apelike ancestors adapted to a  more terrestrial lifestyle, including a dedicated bipedal gait that freed up the hands for toolmaking.

But the report in the journal Science suggests that rapid fluctuations between wet and arid conditions, rather than a steady progression from wet to dry, may have set the stage for the emergence of the genus Homo. (Related: "Oldest Skeleton of Human Ancestor Found.")

New fossil findings and the revelations about these 20,000-year swings in moisture in Africa more than two million years ago have scrambled the picture of what led to Homo erectus.

What preceded Homo erectus—the early human species marked by big brains, modern body proportions, and use of stone tools—wasn't a series of steadily improving prehuman lineages, the review suggests. Instead, a mosaic of at least three early human species lived amid frequently changing patterns of monsoons and savanna expansion in Africa from 2.5 million to 1.5 million years ago.

"We really were extremely lucky to have made it," says paleoanthropologist Leslie Aiello of the Wenner-Gren Foundation in New York. "We evolved to be the best at adapting to changing conditions."

Better Lucky Than Good

The authors of the new study argue that the prehuman Homo rudolfensis (named after a lake in Kenya) and Homo habilis ("handy man") species of the era overlap in skull, teeth, and jaw shapes with Homo erectus. That points to traits that look uniquely "human" popping up in disparate creatures at that time.

They also note the discovery in South Africa (by National Geographic Society explorer Lee Berger) of Australopithecus sediba, a bipedal species from about 1.98 million years ago possessed of apelike arms and a small brain, but with small teeth and other more humanlike traits.

The team suggests such finds point to evolution of human features such as bigger brains, smaller teeth, and full bipedalism ebbing and flowing from 2.5 million to 1.5 million years ago among many lineages of the earliest humans.

Out of Africa

Elsewhere, recent Homo erectus finds at the site of Dmanisi in the Republic of Georgia add to the argument, Aiello says. Wide ranges in body size, skull shape, and brain size existed among the early humans who lived there about 1.8 million years ago, a jumble of features seen in older and more recent early human species. (See "Beautiful Skull Spurs Debate on Human History.")

"What we consider as 'modern' traits seem to have been assembled piecemeal in Africa over a long period of time," says paleoanthropologist Chris Stringer of London's Museum of Natural History, who was not on the review team.

"There were a lot of ways we could have become human," Aiello says. "Look at all the varieties of monkeys in South America. You might have seen something similar in prehumans."

Our ancestors survived a gauntlet of changing environmental conditions, the authors contend, because they didn't specialize, but instead used their evolving larger brains and tool-using hands to become supreme generalists in their diet. They ate millet and related grasses, for example, cereals absent from the diet of their predecessors.

"If you are a specialist and your food goes away, you die," Aiello says. "If you can change what you eat, you can muck through."

Climate Conundrum

Although the pattern of rapidly changing rainfall and woodlands rising and falling may have played a role in shaping human adaptations two million years ago, Stringer adds that "we should be cautious in just considering climate as the driver here."

Stone tools and a fondness for meat were innovations that could have readily spread across all sorts of prehuman lineages regardless of the tempo of climate change, he suggests. That would have catalyzed evolutionary changes, "including increasing brain size and complexity, greater terrestrial mobility, and a reduction in the size of the jaws and teeth," Stringer says.

If the review's picture of patchwork prehuman evolution is correct, discovering the evolutionary path that led to our early ancestors will be much harder, he adds, "but certainly not impossible."

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