Our prehistoric ancestors journeyed out of Africa on contested roads
It was the most momentous immigration ever, a population realignment that marked a startling departure for our species, Homo sapiens. After emerging in eastern Africa close to 200,000 years ago, anatomically modern people stayed on one continent for roughly 140,000 years before spreading out in force around the world. Then, from 40,000 to 35,000 years ago, our forerunners advanced into areas stretching from what is now France to southeastern Asia and Australia.
For the past century, scientists have tried to retrace the intercontinental paths followed by ancient human pioneers and to reconstruct what happened when these intrepid travelers encountered other humanlike species, such as Neandertals, or Homo neanderthalensis. Neandertals originated in Europe around 130,000 years ago, having evolved from ancestors who had already lived there for hundreds of thousands of years. Researchers fiercely debate how H. sapiens came to dominate. Some propose that modern people drove related species to extinction and replaced them, thanks to mental and social advantages. Others say that the African emigrants interbred with their northern kin as intellectual equals and assimilated into their cultures before genetically overpowering them with larger populations.
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A third idea, known as multiregional evolution, posits that modern humans evolved simultaneously in Africa, Asia, and Europe through periodic population movements and interbreeding. From this minority perspective, modern people originated as early as 2 million years ago, not 200,000 years ago.
Two new studies, both published in the Jan. 12 Science, inject new elements into the replacement-versus-interbreeding dispute. One investigation focuses on the South African “Hofmeyr” skull, recently dated to 36,000 years ago. The investigators conclude that modern humans emigrated from sub-Saharan Africa in the late Stone Age and produced European and western Asian populations that made rapid cultural strides. A second paper describes evidence for the presence of modern H. sapiens in western Russia more than 40,000 years ago. That would have been several thousand years before their spread across the rest of Europe.
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The authors of both studies suspect that modern humans advanced speedily out of Africa in various directions, out competing and hastening the demise of Neandertals and others. “The new discoveries … reinforce the argument that the break between the Neandertals and modern humans is a sharp one,” says archaeologist John F. Hoffecker of the University of Colorado at Boulder.
Archaeologist João Zilhão of the University of Bristol in England contests that conclusion. He says that Hoffecker and other scientists have twisted the new findings to accommodate their preconceived notion that modern humans swarmed out of Africa, wiped out and replaced the Neandertals more than 30,000 years ago, and spread a uniquely advanced culture. More likely, in Zilhão’s view, these species interbred, and their two complex cultures intermingled. “The [replacement] model has become an obstacle to modern-human–origins research,” he says.
In 1952, archaeologists working in a dry river channel near the town of Hofmeyr, South Africa, unearthed a partial H. sapiens skull, filled with hardened sediment. They tried to obtain a radiocarbon age for a piece of the fossil, but it was unsuitable for such measurement.
A team led by anthropologist Frederick E. Grine of Stony Brook (N.Y.) University has now taken a different approach to gauge the Hofmeyr skull’s age. After measuring the rate of decay of radioactive material in the sediment packed into the skull, the researchers concluded that the specimen dates to about 36,000 years ago, a time when modern humans were leaving Africa in droves.
In a series of anatomical comparisons, Grine and his coworkers have also found that the Hofmeyr skull looks more like European H. sapiens skulls from 30,000 to 20,000 years ago than like the skulls of Africans, Asians, or Europeans from the past few centuries.
Moreover, the Hofmeyr skull’s relatively long head and flat face contrasts with Neandertals’ shorter heads and sloping faces, the researchers say.
Grine’s data fit a scenario in which “36,000 years ago, modern [human] populations of sub-Saharan Africa and Europe shared a very recent common ancestor, one that likely expanded from eastern Africa 60,000 years ago,” remarks anthropologist Ted Goebel of Texas A&M University in College Station. Migrating groups of modern humans spread out in separate directions, Goebel asserts. Some headed south into what’s now South Africa; others forged east into Asia, reaching Australia by around 50,000 years ago.
Populations represented by the Hofmeyr skull then waited awhile before heading north. They needed to become capable of surviving Europe’s cold, dry climate and dealing with Neandertals, in Goebel’s view. DNA studies suggest that sets of genes carried by the first modern humans spread into Europe and western Asia by 45,000 years ago. After “a short period of interaction,” modern humans’ northern thrust led to the Neandertals’ demise, in Goebel’s view.
Recent advances in radiocarbon dating show that the overlap of modern humans with Neandertals in Europe lasted only 1,000 to 2,000 years, according to archaeologist Paul Mellars of the University of Cambridge in England. Incoming H. sapiens populations were better equipped and better organized to deal with severe cold than resident Neandertals were, Mellars argues in the Feb. 23, 2006 Nature.
“This could have delivered the coup de grâce to the Neandertals in many parts of western and central Europe,” Mellars says.
When Zilhão peruses the Hofmeyr skull, he envisions a far different evolutionary scenario than Grine’s team does.
For now, Zilhão says, 36,000 years represents a minimum estimate of how long ago sediment clogged the South African fossil. The skull’s large proportions and thick bones suggest that that it could be up to 100,000 years old, he notes. Only measurements of the decay of radioactive uranium in the skull itself—which have yet to be conducted—can narrow down its age, in his opinion.
If the age estimate made by Grine and his colleagues holds up, then fossil evidence not mentioned in their report challenges their notion of southern Africans moving into Europe and setting off cultural fireworks, Zilhão says. Those fossil finds suggest that the Hofmeyr individual belonged to a group that arrived in southern Africa long after other modern humans had settled there. The group then stayed put rather than relocating to Europe, Zilhão speculates.
For instance, the new data reveal an anatomical gulf between the Hofmeyr individual and present-day hunter-gatherers in South Africa. In the same vein, other researchers have reported that modern hunter-gatherers display skeletal links to human fossils at Border Cave, a 70,000-year-old South African site.
The Hofmeyr skull also differs in many respects from the skulls of H. sapiens previously discovered at two 100,000-year-old sites in Israel, further cementing its status as an African latecomer, Zilhão says. Most researchers regard the Israeli fossils as evidence of an early human migration from eastern Africa that never approached the scale of later population thrusts.
“These facts should lead to the conclusion that the Hofmeyr fossil documents a migration of early modern Europeans into sub-Saharan Africa between 40,000 and 30,000 years ago,” Zilhão concludes. He proposes that large numbers of modern humans, nurtured in the bounty of Africa’s tropical savannas, entered Europe around 40,000 years ago and encountered relatively small numbers of Neandertals. Europe’s harsh glacial conditions slowed Neandertal population growth, but Neandertals’ mental and cultural capabilities matched those of H. sapiens, in Zilhão’s view.
The two species then interbred, producing offspring with an unusual mosaic of skeletal traits, Zilhão says. Within a few thousand years, the genetic contributions of modern humans overwhelmed those of Neandertals, erasing all traces of the smaller population’s DNA.
Anthropologist Erik Trinkaus of Washington University in St. Louis concurs. Fossil evidence indicates that, after leaving eastern Africa, the earliest modern humans interbred with other Homo species that they encountered, Trinkaus holds. This phenomenon would have occurred most often in geographically dead-end regions, including southern Africa and western Europe, where various immigrant populations accumulated.
This would explain why the Hofmeyr skull exhibits not only modern-human traits but also so-called primitive characteristics, such as a brow ridge and large cheek teeth, Trinkaus says.
Similarly, a 40,000-year-old H. sapiens skull found in Romania 2 years ago contains a mix of modern-human and Neandertal features. Trinkaus and his colleagues describe the skull in the Jan. 23 Proceedings of the National Academy of Sciences. He mentions that other finds, such as a 25,000-year-old skeleton of a youth found in Portugal, also include a montage of H. sapiens and Neandertal traits suggestive of interbreeding.
“Far more-complex population interactions occurred as modern humans dispersed into southern Africa and Europe than many researchers have recognized,” Trinkaus says.
Researchers have long focused on western Europe as a destination for modern humans leaving Africa. But Colorado’s Hoffecker says that new evidence puts H. sapiens in what’s now southern Russia between 45,000 and 42,000 years ago, a few millennia prior to any incursions into the rest of Europe. In his view, the Russian discoveries underscore unique cultural advances achieved by modern humans but not by Neandertals. This idea serves as a linchpin of the replacement model of human evolution.
Hoffecker and an international team of coworkers base this conclusion on work at ancient sites clustered near the town of Kostenki, about 250 miles south of Moscow. Russian investigators first discovered Stone Age material at Kostenki in 1872. Twenty-one prehistoric locations have now been identified there.
Hoffecker’s group dated the oldest Kostenki occupations by radiocarbon measurements of burned wood and assessments of the decay of radioactive substances in the soil. Also, artifacts in these deposits lie just beneath ash that was created in a massive volcanic eruption dated elsewhere in Europe to 40,000 years ago.
In the oldest parts of Kostenki, the researchers uncovered tools and other objects that they consider remnants of a unique late Stone Age culture. Modern humans probably fashioned all the artifacts discovered so far, Hoffecker says.
Remains include bone points and narrow, sharpened stone implements, which might have been used to carve bone. The team also found a piece of carved ivory that might represent a person’s head, and shells with holes punched in them. Much of the stone used for tools came from areas about 100 miles from Kostenki, the scientists hold. They say that shells at Kostenki were probably imported from the Black Sea, more than 300 miles away.
Some artifacts found below the ancient ash consist of relatively simple stone tools typical of those used by H. sapiens and Neandertals more than 50,000 years ago. Such artifacts, along with the bones of reindeer and other large animals, also turn up in Kostenki sediment from about 30,000 years ago.
Hoffecker regards these finds as modern-human tools for butchering game after hunting expeditions. Similar prehistoric implements have been unearthed inside the Arctic Circle in northern Russia, where only H. sapiens could have survived the brutal cold, he contends.
Moreover, two teeth uncovered among Kostenki’s oldest artifacts appear to have come from modern humans, Hoffecker adds.
Hoffecker suspects that modern humans who arrived at Kostenki more than 40,000 years ago came from further east, perhaps central Asia. H. sapiens emigrating to southern Russia at that time probably avoided any extensive dealings with Neandertals that already inhabited the only direct route north from the Near East.
Wherever Kostenki’s ancient residents came from, Goebel agrees that they were “a pioneering group of modern humans.”
Efforts to portray Stone Age Kostenki as the exclusive preserve of modern humans—apparently because they wielded cultural advances that allowed them to adapt to a harsh locale avoided by Neandertals—rankle other researchers.
“No one has a realistic idea of which biological group, Homo sapiens or Neandertals, was responsible for the Kostenki material,” Trinkaus says. Since modern humans and Neandertals made comparable tools beginning around 40,000 years ago, only the discovery of substantial skeletal remains at Kostenki will reveal who first settled the region, he asserts. The two teeth found at Kostenki could have come from either species, in his opinion.
Anthropologist John Hawks of the University of Wisconsin–Madison suggests on his blog (http://www.johnhawks.net/weblog) that people and Neandertals simultaneously inhabited Kostenki more than 40,000 years ago. Basic stone tools found at the Russian sites resemble those already attributed to central European Neandertals, whereas other ancient-Kostenki finds have more in common with European H. sapiens material, he says.
In contrast, Zilhão suspects that Neandertals monopolized Kostenki at first. The oldest remains uncovered by Hoffecker’s group “represent a Russian example of the northward extension of the Neandertal range,” he says.
Neandertal sites in England, France, and Poland contain artifacts similar to those that Hoffecker attributes to modern humans at Kostenki, Zilhão contends. These Neandertal finds include small, sharpened stone blades, decorated bone tools, and bone ornaments that apparently held symbolic significance for their makers.
After hosting Neandertals for a few thousand years, Kostenki shows no sign of occupation between 36,000 and 32,000 years ago, Zilhão notes. After that, stone and bone implements like those made by European modern humans began to appear. This pattern fits with his view that Neandertals and modern humans developed comparably complex cultures, with Neandertals eventually getting absorbed by larger H. sapiens populations.
To make a long, ancient story short, the Hofmeyr skull and the Kostenki remains won’t resolve quarrels over how modern humans spread from Africa to all corners of the world. But they may encourage scientific clashes along some intriguing new routes.