19September 2018 natural history

The Origin of UsArchaeological and fossil finds shed light on the emergence and spread of modern humans.

By Natalie O’Shea and Eric Delson

In 1868, the first well-preserved fossils of our own species, Homo sapiens, were found in Les Eyzies, southwestern France. At first they were termed “Cro-Magnon Man” and thought to be a distinct variety or even species, but in fact they were just early Europeans [see photograph above

right]. Dating from about 28,000 years ago, those fossils are some 12,000 years younger than the oldest H. sapiens fossils now known from western Europe. In the middle of the twentieth century, researchers argued over whether these modern humans had a long independent history in the region or if they evolved from early representatives of Nean-derthals, an archaic species that had been recognized since 1856. Either way, the focus remained on Europe as the sup-posed birthplace of modern humans. Only in the late 1970s did accurate dating of early H. sapiens indicate an origin in Africa, followed by dispersal to Eurasia and beyond.

In the 1980s and 1990s, paleoanthropology became dominated by a debate between two evolutionary models: African replacement (or “Out of Africa”) and multiregion-alism. The first suggested that our species originated in Africa and then spread across the globe, replacing more archaic Homo species with little to no interbreeding be-tween moderns and archaics. On the other end of the spectrum, the multiregionalists proposed that H. sapiens evolved more or less independently in several regions of the world from more archaic hominin ancestors (a hom-inin is a member of the evolutionary lineage including modern humans since our split from chimpanzees). This model emphasized the importance of continuous genetic and cultural exchange among populations around the world since the initial dispersal of hominins out of Africa nearly 2 million years ago.

As often happens in paleoanthropology, as more evi-dence accrued it became clear that the story was more complicated than either original model could support. A range of more nuanced intermediate models that had pre-viously been proposed became more tenable. Some aligned with the African replacement hypothesis in emphasizing the largely African nature of modern human origins (but, for example, allowing significant hybridization with the older regional forms). Others sustained the multiregional focus, proposing a degree of interchange among popula-

tions but emphasizing adaptation by immigrants to re-gional environments.

Beginning in 2010, geneticists have shown that out-side Africa, all H. sapiens now alive derive 1 to 4 percent of their genome from Neanderthals. This confirmed that modern and archaic hominins did interbreed, which had been a matter of debate. And a hominin species discovered only recently, the Denisovans (Asian relatives of Nean-derthals), apparently exchanged genetic information with the ancestors of living Melanesian populations in the last 100,000 years.

As a consequence of these genetic insights, the focus of research has shifted from determining whether modern hu-mans evolved “from” regional archaic forms—or did not—to the timing and mode of the emergence and dispersal of our species and its interaction with other hominins. In this en-deavor, the traditional search for and dating of fossils and artifacts goes hand in hand with the latest in genetic analysis.

Many paleoanthropologists long claimed that the origin of our species could be traced back 200,000 years. That is the estimate of when an ancestral version of mitochondrial DNA (mtDNA) existed,

back to which all living humans can trace their own vari-ants of mtDNA. A component of human cells, mtDNA is passed down through maternal lines and accumulates variations at a fairly predictable rate, thus allowing scien-tists to estimate when that ancestral version was present. The ballpark figure of 200,000 years ago was seemingly supported in the fossil record by the appearance of some of the earliest anatomically modern humans between 195,000 and 160,000 years ago, at the Ethiopian sites of Herto and Omo Kibish [see top left photograph on next page]. However, the mtDNA estimate only gives us a mini-mum time of divergence. Our species likely arose earlier and possessed many mitochondrial lineages that simply did not leave descendants. Therefore, the fossil evidence of our species’ origin is likely to prove more conclusive.

As recently as the summer of 2017, research on both new

Above: A 28,000-year-old cranium discovered in southwestern France, once thought to belong to a distinct human variety or species called Cro-Magnon, is actually the same as that of a modern European.

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and previously known fossils from Jebel Irhoud, Morocco, by Jean-Jacques Hublin of the Max Planck Institute for Evo-lutionary Anthropology in Leipzig and colleagues, revealed that they display a number of anatomically modern mor-phological features [see photograph below]. These fossils date from approximately 315,000 years ago, predating the previously suggested “origin” of modern humans by more than 100,000 years. Furthermore, recent discoveries from the Olorgesailie Basin in southern Kenya have also reset the beginnings of the Middle Stone Age, the archaeological “industry” of tool types most closely associated with early African H. sapiens. Credit for this work goes to Alison S. Brooks of George Washington University, Richard Potts of the Smithsonian National Museum of Natural His-tory, both in Washington, D.C., and Alan L. Deino of the Berkeley Geochronology Center in Califor-nia, and their colleagues. The new estimate, more than 300,000 years ago, is strikingly compara-ble to that suggested by the Jebel Irhoud fossils.

In combination with other new analyses, the revised date for early modern human fossils and artifacts highlights the “mosiac” nature of our origin. That is, dif-ferent parts of our anatomy and our toolkits evolved at different rates, rather than appearing all at once as a complete package.

These findings have altered the

traditional origins narrative, providing additional support for placing certain controversial specimens within our spe-cies, such as the 260,000-year-old Florisbad cranium from South Africa [see top photograph on opposite page]. In addition, the evidence that H. naledi fossils from South Af-rica date from 335,000 to 236,000 years ago demonstrates that a very primitive hominin survived in Africa around the time of modern human origins. Adding to the complexity, archaeological and fossil evidence from West Africa, for instance from the 10,000-year-old site of Iwo Eleru, point to the persistence in parts of Africa both of archaic tech-

nologies (Middle Stone Age tools, which elsewhere disappeared 50,000 to 30,000 years ago) and of anatomical features (such as low foreheads and elongate brain-cases reminiscent of those in fos-sils more than 300,000 years old) [see photograph above right]. While much work remains to bet-ter understand population dy-namics within Africa during the last few hundred thousand years, exciting new finds and interpreta-tions have clarified the movement of early modern humans out of Africa.

Current views of early H. sapiens dispersal from Af-rica can be divided into two broad categories: those

which propose a single successful dispersal event and those which

A partial fossil cranium, above left, from the Ethiopian site of Omo Kibish, dates from 195,000 years ago and is considered one of the earliest specimens of Homo sapiens or a late member of a precursor variety. At right is a cranium from the site of Eleru in West Africa that is only 10,000 years old, yet has a similar—long and low—archaic appearance.

A Homo sapiens cranium from Jebel Irhoud, Morocco, dating from about 315,000 years ago, is the earliest known fossil of our species.

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21September 2018 natural history

support multiple successful dispersals out of Africa. One of the key points of contention among these models is the ancestry of certain hypothetical “relict” populations that may retain shared ancestral traits from an early dispers-ing population. These putative relict populations include Australians, Melanesians, Papuans, Dravidian speakers of South Asia, and the short-statured “Negrito” peoples of is-land Southeast Asia.

The original multiple-dispersal model suggested that there were two major waves of dispersal out of Africa, the first leading to the establishment of these relict popula-tions, and the second accounting for the ancestry of all other living humans. While most single-dispersal models do not support any special ancestry for hypothetical rel-ict groups, some allow that these populations may have diverged from other Eurasians not in Africa, but some-where in southwest Asia, with populations ancestral to Australo-Melanesians following a southern route along the Indian Ocean rim.

The clearest evidence of an early-dispersing population of early modern humans comes from Israel. Several rela-tively intact specimens were recovered from the Skhul and Qafzeh caves beginning in the early 1930s and later found to date from between 120,000 and 90,000 years ago [see photographs below]. Those specimens were long believed to be the oldest evidence of our species outside the African continent. However, newly excavated material from the nearby Misliya Cave, reported by Is-rael Hershkovitz of Tel Aviv University in Israel and col-leagues, dates from around 180,000 years ago, pushing back the first H. sapiens occupation of the region by more than 50,000 years. While paleoanthropologists agree that these specimens can be attributed to our species, it has been unclear whether the populations of which they

were a part survived and contributed to the genetic ances-try of any living humans.

The same team discovered a 55,000-year-old partial skull from Manot Cave, Israel, that shows a resemblance to some of the earliest European populations, but not to the earlier fossils from Israel at Skhul and Qafzeh. That suggests that the earlier population may have gone extinct, and the region was later recolonized. Meanwhile, the re-

cent discovery of a finger bone and stone artifacts from the site of Al Wusta, Saudi Arabia, dating from 85,000 years ago, pres-ents a new piece of the puzzle. It could support either continuous occupa-tion of southwest Asia or potentially another, sepa-rate, early dispersal event.

Evidence of early modern human oc-cupation of regions beyond western Asia

is somewhat sparser. Re-cently described modern human teeth from Fuyan Cave (Daoxian county)

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The 260,000-year-old specimen from the site of Florisbad, South Africa,was thought to be too old, as well as too different, to be a modern hu-man cranium. With recognition that modern humans have been around atleast that long, its high forehead is getting some new respect.

Crania from the Israeli sites of Skhul, left, and Qafzeh, right, date from between 120,000 and 90,000 years ago.Discovered in the 1930s, until recently they were thought to be, with their small faces and rounded braincases, the earliest evidence of Homo sapiens outside the African continent.

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in southern China are dated between 120,000 and 80,000 years old, based on the associated geological deposits. But some of the teeth have cavities, a feature that is notably rare in the fossil record prior to the development of ag-riculture, leading to concern that these teeth may have in fact originated from more recent deposits than the ones in which they were found. Another early candidate is a partial skeleton from Liujiang, also in southern China. It has been variously dated to be between 140,000 and 40,000 years old, but its exact geological origin is unclear and it appears quite modern, suggesting a younger age.

The next oldest modern human re-mains are documented at Lida Ajer, Sumatra, and date from 70,000 years ago. And there are artifacts and other archaeological evidence from the site of Madjedbebe that humans had ar-rived in Australia not long after, by 65,000 years ago. That, incidently, sets a minimum age for the dispersal of modern humans out of Africa: there is no fossil evidence that any other hominin ever reached Australia, and so only modern humans could have manu-factured the artifacts that were found. On the other hand, the archaic H. floresiensis (the “Hobbits”) inhabited near-by Flores Island, Indonesia, between 100,000 and 50,000 years ago. They remain a puzzle, as certain details of their anatomy connect them to much older hominins.

In light of these recent discoveries there is growing agree-ment that there were earlier dispersals of modern humans out of Africa, but it remains unclear whether these popu-lations made any significant genetic contribution to living people. Combined genetic and morphological research on living humans (including several “relict” populations) sup-ports the view that Australo-Melanesian peoples retain the genetic signal of an early-dispersing population, but that the genomes and morphology of other groups have been “swamped” by later dispersals. Geneticists have yet to re-solve this issue. One recent study concluded that 2 percent of the Papuan genome was derived from an early-dispersing population, while another found no evidence for special an-cestry of Australians, Papuans, and Andamanese.

But none of these details answers the big question: why were modern humans so successful? Many paleoanthro-pologists believe our propensity for symbolic culture may have been the key development that allowed us to out-compete other hominins by enabling us to maintain larger social networks. Genetic evidence does indeed support

the existence of larger social networks in modern humans compared with Neanderthals, and high levels of morpho-logical variability in the earliest modern humans suggests that this complex population structure may have been present at the origin of our species.

Upcoming technological advances can be expected to yield new strategies. One of these is paleoproteomics (the study of ancient proteins, which survive much longer than DNA), which will allow investigators to determine the species identity of even the smallest bone fragment. Com-bined with a wealth of new fossil discoveries, such tools set the stage for what may be the most exciting time yet in modern human origins research.

Modern humans likely dispersed from Africa by following a northern route (red), a southern route (blue), or some combination of the two. According to one model, an earlier dispersal took the southern route, and some so-called relict populations now living along that route may still bear genetic traces of that history. But such traces have been genetically confirmed only in Australia and Melanesia (blue dots).

Natalie O’Shea is a PhD candidate in anthropology at the Graduate Center of the City University of New York (CUNY). Her dissertation research seeks to model early modern human demography and biogeography in Africa by exploring past population dynamics in savanna-dwell-ing monkeys. Eric Delson is a professor of anthropology at the Graduate Center and at CUNY’s Lehman College. He is also a research associate in the Division of Paleontology at the American Museum of Natural History. Delson ed-its the Springer book series Vertebrate Paleobiology and Paleoanthropology, for which he is currently co-editing a volume about field research at the 2-million-year-old fossil mammal site of Senèze, central France. His research has focused on fossil monkeys of Africa and Eurasia, as well as hominin and ape evolution.

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