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A photo of the skull of the young male Australopithecus sediba.

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

Photograph by Brent Stirton, Getty

Dan Vergano

National Geographic

Published July 3, 2014

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."

Follow Dan Vergano on Twitter.

13 comments
Galen Horton
Galen Horton

"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 new tools & change in diet wrought evolutionary changes, what evolutionary changes can we expect from the tools & diet we have come to know in the past 100 years that is substantially different from all previous generations before?

Larry S.
Larry S.

Reading the political comments brought to mind the opening water hole sequence from 2001: A Space Odyssey.So, I guess those comments were unintentionally on topic at least for me..

D C
D C

Global warming!  We're all going to die!

Eric Sorensen
Eric Sorensen

Our tool-making ability has evolved to the point that we have the instruments to destroy the world, either instantly or over the next hundred or so years. Perhaps we could learn more about how we evolved our sense of joy, our appreciation of beauty, and our desire to help those in need. 

Philip Olson
Philip Olson

The social interactions of early primates varied as well as the physical traits.

Modern Chimps and Bonobos are more alike than Erectus and Habilis but their cultural attitudes to others in their species has few similarities.  In fact the bonobos are closer to Sapiens in social structure.

Born with intrinsic preferences the variability the article speaks of includes the ability for small group's behavior to be tailored to specific survival conditions.

Peace is great in times of plenty but sharing when not enough food exists for all to eat means merciless killing of others is necessary for survival.

The odd men out were first to go when times were tough.  The short span political differences arose much later when agriculture allowed large groups to develop the means to let this make a difference.

Nomadic hunter gatherers did not have the numbers or resources to allow the existence of our modern divisions of religion and politics.

Agriculture's discovery 10,000 years ago changed the equation so profoundly we still do not have a grasp on the transition.  But one thing is certain, without modern culture none of the modern divides could exist.

John Eller
John Eller

which lineage led to republicans? how could it possibly have survived changes - were there areas with unchanging climate and "culture"? how did the narrow skills of just throwing rocks and being able to believe in anything at all (no matter how far separated from reality) enable long-term survival?

Tom Carberry
Tom Carberry

@Galen Horton Geneticists know that humans have gone through more genetic changes in the last 10,000 years or so since the agricultural evolution, than at any time in human history.  For some reason, many 


Most for the worse.  Humans have shrunk enormously from their pre-agricultural ancestors, including brains that average more than  25% smaller than our ancestors.  


Why?  Basically the stresses of adapting to malnutrition from eating foods humans never ate before, particularly grains such as wheat, rice, and corn.  As scientists have started to learn recently, grains destroy the human brain and lead to dementia.  


During the 19th and 20th centuries, humans in certain parts of the world received much better nutrition, particularly in the US, where land speculators would advertise in European papers for people to come and eat meat every day.


But post-WWII industrial food has reversed the gains in nutrition, as anyone can see looking at our current population.


The agricultural revolution came after the end of the last ice age.  No one knows for sure why it ended.  And the age of agriculture brought us human myths from all over the world of great catastrophes, of the gods warring among themselves, of people dying in huge numbers and only handfuls surviving in pockets around the world.


Something bad happened to the planet, but we don't know exactly what.

Philip Olson
Philip Olson

@John Eller

This comment as well as the two previous replies to the comment cause me to wonder why the three of you even bothered to read the article.

Do any of you have any concept at all of what was being discussed?

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