Around 50,000 years ago, modern humans suddenly started leaving Africa. Over the course of the next 20,000 years, Homo sapiens pushed other hominid species to extinction and established populations in Australia, Asia, and Europe.
What spurred Homo sapiens to leave Africa after living there for at least 70,000 years? What made modern humans successful?
Archaeologists and anthropologists specializing in human origins tackled those questions this past weekend at the American Association for the Advancement of Science annual meeting held in Denver, Colorado. In a symposium titled Revolution and Evolution in Modern Human Origins: When, Where, and Why?, several differing perspectives were presented.
There is general agreement that Homo erectus, the precursor to modern humans, evolved in Africa and gradually expanded to Eurasia beginning about 1.7 million years ago. Anatomically modern humans-humans that more or less looked like us-arose in Africa at least 130,000 years ago.
By 100,000 years ago, several species of hominids populated the Earth. H. sapiens-us-lived in Africa, H. erectus in Southeast Asia and China, and Neandertals in Europe. Around 50,000 years ago, there was a sudden explosion of human migration out of Africa, and by about 30,000 years ago, we were the last ones standing.
What propelled modern humans out of Africa, and what gave us the edge over other species?
The debate is complicated by differing definitions of what constitutes "modern" behavior, and differing interpretations of the archaeological record.
Common elements used to define modern behavior include the ability to plan ahead; technological innovation, establishing social and trade networks; adapting to changing conditions and environments; and exhibiting symbolic behavior like cave painting, bead making (used to show status or group identity), or burying the dead.
The crux of the argument comes down to whether these abilities resulted from a sudden biological and genetic revolution, or a more gradual evolution of abilities that culminated around 50,000 years ago and propelled the exodus from Africa.
Evolution versus Revolution
One school of thought argues that Homo sapiens emerged with the abilities needed to be modern, and it simply took 70,000 years to hone the technological and social skills needed before they could successfully venture out and populate the rest of the world.
"Humans have had the capacity for space travel, or designing computers, for several thousand years-nothing about us has really changed," said Richard Potts, director of the human origins program at the Smithsonian Institution in Washington, D.C., and a speaker at the conference. "The problem of discovering when modern human behavior arose is that significant innovations are manifested after the cognitive and social capacities to innovate have already evolved."
Potts considers the ability to adapt to change, particularly to environmental conditions, as a key factor in modern human evolution. Adaptability and innovation in response to climatic change can be seen occurring as far back as 400,000 years ago, he says.
A less widely accepted theory holds that humans underwent a dramatic genetic change in brain function-perhaps increased memory or improved language skills-around 50,000 years ago which gave them an enhanced ability to innovate and enabled the exodus from Africa.
"The alternate argument [gradual evolution of skills] is based on the assumption that the historic present is key to the past, that because we've never seen a modern change in cognition it didn't occur in the past. I don't see any reason to make that assumption," said Richard Klein, a paleoanthropologist at Stanford University.
Revolutionary changes in behavior also occurred roughly 2.5 million years ago, and again at 1.8 million years ago, he says.
"There were events that occurred back then that were certainly unique," he said. "Four, five, six million years ago, hominid brains were about half the size of ours [scaling for size]. Increased brain size was not a gradual process. Greater intelligence was naturally selected for and occurred quickly. It was not a continuous process."
Klein believes that genetic evidence might ultimately provide answers. Researchers have recently isolated a gene called foxp2 that is associated with human speech and language.
Pity the Poor Neandertals
No one, it seems, thinks the Neandertals, our closest relatives, were modern. Neandertals evolved in Europe, emerging in full-blown form by 130,000 years ago. By 80,000 years ago populations stretched from Spain to western Asia. However, "fossils show that the African contemporaries of the Neandertals between 130,000 and 50,000 years ago were more modern in anatomy," said Klein.
Physically, Neandertals had shorter limbs, a more massive trunk, and heads that were larger and had several distinctive features that differentiate them from modern humans.
However, from an archaeological viewpoint-stones, not bones-the skills and behaviors of Neandertals and early modern humans in Africa weren't that different until the sudden change around 50,000 years ago, says Klein.
The two species shared some behavioral traits, including some tool making skills, sporadic burial of the dead, control over fire, and heavy dependence on meat acquired by hunting. There is also some evidence that both groups took care of their old and sick.
But the gap between the two cultures widened rapidly during the roughly 20,000 years that Homo sapiens and Neandertals coexisted in Europe, says Klein. In the end the Neandertals died out because they lacked the ability to either compete with or even imitate modern human behaviors.
"The Neandertals don't show fundamentally different adaptations across Eurasia. They don't respond to changes, and couldn't cope [with] them," said Steven Kuhn, an anthropologist at the University of Arizona in Tucson.
An alternate view is the possibility that "modernity" in cognition evolved more than once, says Francesco D'Errico, a paleontologist at the Institut du Quaternaire in France. He suggests that Neandertals were capable of many modern behaviors, but that modern humans were able to grow their population faster and overwhelmed the Neandertals technologically.
Winning the Arms Race
Alison Brooks, a paleoanthropologist at George Washington University in Washington, D.C., disagrees with Klein's interpretation of the African archaeological record. A proponent of the gradual evolution of skills school of thought, she argues that there is a well documented history of continuous innovation.
There is ample evidence in Africa from 120,000 years to 50,000 years ago of extensive trade networking, improved technology, and advanced social strategies, says Brooks.
"Survivorship and population expansion ultimately depend on a reliable food supply, which in turn depends on economic as well as social organization," Brooks wrote in an e-mail communication.
Archaeological evidence suggests that modern humans in Africa were transporting raw materials distances up to 300 kilometers (186 miles). This is more indicative of trade than seasonal migratory patterns, she says. And the ability to trade goods is a hedge against hard times.
Also, she says, the tool kit in Africa included smaller, lighter weapons that could be propelled. While the Neandertals risked major bodily harm going nose to nose with a giant wooly mammoth, for instance, modern humans had figured out a way to hunt them from a more comfortable distance, considerably improving their odds for getting back home for dinner.
To Brooks and colleague Sally McBrearty, evidence in Africa of fishing with bone points, advanced weaponry, and the ability to continue to innovate indicates that modern cognitive abilities developed early.
And the Answer Is:
Scientists say an exciting aspect about their research into the origins of our species is that new finds and new technologies will only enhance what we know today.
Just like with early modern humans, improved technology and lots of trial and error might eventually yield answers to questions posed by paleontologists today.
