Archaeological and fossil finds from caves in the Levant, particularly the Skhul and Qafzeh caves in northern Israel, have often been cited as evidence to support the popular theory that early modern humans left Africa in a single wave perhaps 100,000+ years ago and occupied locations in the Levant, only to be snuffed out due to environmental change and the lack of more advanced stone tool and weapon technology before their descendants could colonize further eastward into greater Southwest Asia and the Far East. It has been portrayed as a short-lived movement, with a more successful dispersal out of Africa occurring later, around 60 - 70,000 years ago, based on the interpretation of other finds and genetic evidence. Recent findings in other parts of the world, including China and other regions, however, have turned up evidence for early modern human occupation that challenge the dating of the latter dispersal model by thousands of years.

Enter here a recent study of a collection of lithic artifacts from Skhul Cave, a total of 270 samples, currently housed at the Pitt River Museum (PRM), University of Oxford. In that study, Huw S. Groucutt of the University of Oxford and colleagues analyzed 56 stone cores, 85 Levallois flakes, 47 non-Levallois flakes, 81 retouched stone tools and 1 hammerstone, providing the first in-depth, complete examination and description of the assemblage using modern techniques of analysis. In the process, they also compared them to lithics excavated at other paleolithic sites and integrated their findings with the paleontological and chronometric (dating) data from previous research of the Skhul and Qafzeh caves. The latest chronological data from Skhul and Qafzeh suggests the artifacts were made and used about 130 - 120,000 and 95,000 years ago, respectively. This places the artifacts, and the early modern human fossils excavated in the caves, among the first early modern humans outside of Africa known to date. 

By analyzing the artifacts within the context of finds from other sites, the stratigraphic context of the cave, and paleo-climate data, the authors hypothesize that the dispersal of and occupation by early modern humans in the Levant may have been longer and more complex than previously modeled. Write Groucutt and colleagues, “accumulating the multidisciplinary data, including the analysis of the PRM Skhul lithic assemblage we have presented, increasingly suggest that there were multiple dispersals of Homo sapiens out of Africa in MIS 5 [between 130,000 and 80,000 years ago], corresponding with humid phases, and alternating with phases of aridity.”* With this conclusion, the authors also recognize that other hypotheses could explain their results, such as the evolution of the Skhul and Qafzeh hominids from an indigenous Middle Pleistocene hominid population in southwest Asia.

Skhul Cave was initially excavated by Theodore McCown in 1931 and 1932. Excavations eventually yielded lithic tools and other finds, including human fossil remains representing seven adults and three children, some of whom were suggested by the excavators as having been intentionally buried. In addition to the lithic tools, perforated Nassarius shells thought to have been imported to the caves from a different location were also discovered. The discoverers have suggested that these people may have collected and used them as decorative beads, a behavior distinctly more ‘advanced’ than what is attributed to much earlier hominid species. The morphological features of the fossils show characteristics identified with both neanderthals and modern humans, and though they were initially classified as neanderthals, today the generally accepted classification is that of archaic (early) modern human. The specimens are thought to represent some of the earliest modern humans to occupy regions outside of Africa. The finds in both the Skull and Qafzeh caves provide evidence to support the theory that early modern humans and neanderthals lived concurrently in Eurasia for a time, and may even have interbred in the Levant. 

Although the initial investigations at Skhul in the 1930’s employed techniques and methodology that are considered inadequate by modern standards, and the collection analyzed does not represent the complete assemblage of all lithics recovered from the site, Groucutt and colleagues determined nevertheless that the artifact sampling met the requirements for making some valid observations and drawing conclusions for a hypothesis worth testing at other digs and through future studies. “We have described the Skhul lithic assemblage acknowledging its limitations in terms of excavation methods and so on, and formulated hypotheses that can be tested by multidisciplinary analyses of new sites,” the study authors concluded.*   

Macaw skeletons from three prehistoric pueblos in New Mexico bear signs of feather harvesting, according to analysis presented on 31 March at a meeting of the Society for American Archaeology in Vancouver, Canada. But the skeletons also hint that the macaws’ handlers went to great lengths to care for their demanding charges. “People were doing their utmost to keep them alive,” says Randee Fladeboe, an archaeologist at the University of Florida in Gainesville who analysed the macaw bones.

Archaeologists studying the ancient Native Americans called the Puebloans and nearby groups have found macaw bones and feathers dating from ad 300 to ad 1450 at sites ranging from Utah in the American Southwest to Chihuahua in Mexico. It is likely that many of these birds were imported; there is scanty evidence of macaw breeding, except at one Mexican site, and many macaws are tropical. The highly prized scarlet macaw (Ara macao), for example, lives at least 500 kilometres to the southeast.

Fladeboe examined the wing bones of 17 scarlet and military macaws (Ara militaris) from three pueblos. Fifteen of the birds had small bumps marring the upper surfaces of their wing bones.

A macaw’s flight feathers are rooted in the bone, so pulling them out can cause bleeding and infection, Fladeboe says. Multiple infections, or a combination of infection and malnutrition, lead to bumps like those on the skeletons. Macaws do sometimes yank out their own feathers, but the ancient bones show traces of multiple feather loss along their entire lengths and on both right and left wings. To Fladeboe, it seems unlikely that 15 of the 17 macaws she studied would strip themselves so methodically.

Tough old birds

One macaw had suffered two broken wings, and its beak bore signs of attacks from other macaws. Its bones also show irregularities from either malnutrition or illness. This macaw probably would not have survived without hand-feeding and protection, Fladeboe says.

Fladeboe makes a “good preliminary case” that the birds were plucked by humans, says zooarchaeologist Erin Keenan Early at Texas State University in San Marcos, who thinks the evidence that the birds were well cared for is also credible. Fladeboe plans to do computerized tomography scanning of the bones to confirm her early results.

Macaws that were stressed by captivity and feather removal may have engaged in “self-destructive and otherwise aggressive behavior, making them quite difficult to care for”, says zooarchaeologist Meredith Wismer of the University of Iowa in Iowa City. Fladeboe thinks that the birds’ caretakers probably learned to soothe the macaws; happy macaws have more attractive feathers. 


Few sites had more macaws than Chaco Canyon, a cluster of settlements in New Mexico. Chaco’s Pueblo Bonito even had what seems to be an aviary, complete with a layer of guano 25 centimetres thick. A radiocarbon analysis demonstrated that macaws were living there in the late 1000s and early 1100s, even as the pueblo was heading towards collapse.

Earlier research had shown that birds were imported to the pueblo when it was flourishing¹. But seemingly the pueblo’s access to macaws “continued throughout the rise and decline of Chaco”, illustrating the birds’ importance, says archaeologist Adam Watson of the American Museum of Natural History in New York City, who presented the new dates at the meeting on 30 March.

Macaw keepers at places such as Chaco Canyon clearly had to go to great lengths to procure the water and food the birds needed, Fladeboe says. To ancient Southwestern peoples, the macaws served a role befitting their powers as ambassadors to the underworld and bringers of rain. The macaws’ caretakers “did obviously care for the birds”, she says. “To say that they only cared about them [for] their feather output would be to do them a disservice.”

When exactly Population y arrived in the Americans remains unclear—before, after or simultaneously with the first wave of Eurasians are all possibilities. Reich and his colleagues suspect the line is fairly old, and at some point along the way, Population y probably mixed with the lineage of Eurasian settlers. Amazonian tribes remain isolated from many other South American groups, so that’s probably why the signal remains strong in their DNA.  

The results line up with studies of ancient skulls unearthed in Brazil and Colombia that bear stronger resemblance to those of Australasians than the skulls of other Native Americans. Based on the skeletal remains, some anthropologists had previously pointed to more than one founding group, but others had brushed off the similarities as a byproduct of these groups living and working in similar environments. Bones can only be measured and interpreted so many ways, while genes usually make a more concrete case.

“The problem so far was that there has never been strong genetic evidence to support this notion,” says Mark Hubbe, an anthropologist at Ohio State University who was not affiliated with the latest study.

But even genetic evidence is subject to skepticism and scrutiny. Cecil Lewis Jr., an anthropological geneticist at the University of Oklahoma, cautions that Amazonian groups are low on genetic diversity and are more susceptible to genetic drift. “This raises very serious questions about the role of chance … in creating this Australasian affinity,” he says.

Another group led by Eske Willerslev and Maanasa Raghavan at the University if Copenhagen reports in Science today that Native Americans descend from just one line that crossed the land bridge no earlier than 23,000 years ago. While they didn’t look at Amazonian groups in-depth, the team did find a weak link between Australasians and some South American populations, which they chalk up to gene flow from Eskimos. 

There’s just one problem: Evidence of Population y doesn’t persist in modern Eurasian groups, nor does it seem to show up in other Native Americans. If Aleutian Islanders or their ancestors had somehow mixed with an Australasian group up north or made their way south to the Amazon, they'd leave genetic clues along the way. “It’s not a clear alternative,” argues Reich. 

Both studies therefore suggest that the ancestry of the first Americans is a lot more complicated than scientists had envisioned. “There is a greater diversity of Native American founding populations than previously thought,” says Skoglund. “And these founding populations connect indigenous groups in far apart places of the world.”

But given the relative scarcity of early Homo fossil remains, comparatively less is known about the earliest Homo postcranial morphologies. A recent study by an international team ofHomo fossil remains uncovered in Kenya, however, has provided a few more clues reflecting on the diversity and complexity of early Homo differentiation during the earliest periods of the emergence of humans from the still obscure primordial mix of a time when some species of Australopithecus are thought to have coexisted with their 'more advanced' Homo counterparts. The team examined a partial ilium (the uppermost and largest bone of thepelvis), and a femur (thigh bone) found at the famous hominin fossil site of Koobi Fora, Kenya, dating to 1.9 Ma (millions of years ago). They found that the specimens featured attributes commonly associated with the genus Homo. But they also found morphological characteristics not typically seen in eastern African early Homo erectus fossils:  "The geometry of the femoral midshaft and contour of the pelvic inlet do not resemble that of any specimens attributed to H. erectus from eastern Africa," summarized the study authors in their report, which will soon be published in the Journal of Human Evolution. "This new fossil confirms the presence of at least two postcranial morphotypes within early Homo, and documents diversity in postcranial morphology among early Homo species that may reflect underlying body form and/or adaptive differences."*

Koobi Fora has long been known as a key region containing hominin fossils that have shed light on human evolution over the last 4.2 million years. It is described as a ridge or outcrop of Pliocene/Pleistocene sediments that preserve a prolific record of mammal fossils, including early hominin species. The ridge is being eroded into a badlands terrain by rivers draining into modern Lake Turkana. Anciently, Lake Turkana provided a good habitable lake environment for a variety of mammals, including early humans. In 1968, Richard Leakey, the son of famous paleoanthropologist Louis Leakey and Mary Leakey, established the Koobi Fora Base Camp, a field center for hominin studies, at Lake Turkana.

Bone mass was around 20% higher in the foragers -- the equivalent to what an average person would lose after three months of weightlessness in space.

After ruling out diet differences and changes in body size as possible causes, researchers have concluded that reductions in physical activity are the root cause of degradation in human bone strength across millennia. It is a trend that is reaching dangerous levels, they say, as people do less with their bodies today than ever before.

Researchers believe the findings support the idea that exercise rather than diet is the key to preventing heightened fracture risk and conditions such as osteoporosis in later life: more exercise in early life results in a higher peak of bone strength around the age of 30, meaning the inevitable weakening of bones with age is less detrimental.

There is, in fact, no anatomical reason why a person born today could not achieve the bone strength of an orangutan or early human forager, say researchers; but even the most physically active people alive are unlikely to be loading bones with enough frequent and intense stress to allow for the increased bone strength seen in the 'peak point' of traditional hunter-gatherers and non-human primate bones.

"Contemporary humans live in a cultural and technological milieu incompatible with our evolutionary adaptations. There's seven million years of hominid evolution geared towards action and physical activity for survival, but it's only in the last say 50 to 100 years that we've been so sedentary -- dangerously so," said co-author Dr Colin Shaw from the University of Cambridge's Phenotypic Adaptability, Variation and Evolution (PAVE) Research Group.

"Sitting in a car or in front of a desk is not what we have evolved to do."

The researchers x-rayed samples of human femur bones from the archaeological record, along with femora from other primate species, focusing on the inside of the femoral head: the ball at the top of the femur which fits into the pelvis to form the hip joint, one of the most load-bearing bone connections in the body.

Two types of tissue form bone: the cortical or 'hard' bone shell coating the outside, and the trabecular or 'spongy' bone: the honeycomb-like mesh encased within cortical shell that allows flexibility but is also vulnerable to fracture.

The researchers analysed the trabecular bone from the femoral head of four distinct archaeological human populations representing mobile hunter-gatherers and sedentary agriculturalists, all found in the same area of the US state of Illinois (and likely to be genetically similar as a consequence).

The trabecular structure is very similar in all populations, with one notable exception: within the mesh, hunter-gatherers have a much higher amount of actual bone relative to air.

"Trabecular bone has much greater plasticity than other bone, changing shape and direction depending on the loads imposed on it; it can change structure from being pin or rod-like to much thicker, almost plate-like. In the hunter-gatherer bones, everything was thickened," said Shaw.

This thickening is the result of constant loading on the bone from physical activity as hunter-gatherers roamed the landscape seeking sustenance. This fierce exertion would result in minor damage that caused the bone mesh to grow back ever stronger and thicker throughout life -- building to a 'peak point' of bone strength which counter-balanced the deterioration of bones with age.

Shaw believes there are valuable lessons to be learnt from the skeletons of our prehistoric predecessors. "You can absolutely morph even your bones so that they deal with stress and strain more effectively. Hip fractures, for example, don't have to happen simply because you get older if you build your bone strength up earlier in life, so that as you age it never drops below that level where fractures can easily occur."

Other theories for humans evolving a lighter, more fragile skeleton include changes in diet or selection for a more efficient, lighter skeleton, which was never reversed.

While the initial switch to farming did cause a dip in human health due to monoculture diets that lacked variety, the populations tested were unaffected by this window in history. "Of course we need a level of calcium to maintain bone heath, but beyond that level excess calcium isn't necessary," said Shaw.

The research also counters the theory that, at some point in human evolution, our bones just became lighter -- perhaps because there wasn't enough food to support a denser skeleton. "If that was true, human skeletons would be entirely distinct from other living primates. We've shown that hunter-gatherers fall right in line with primates of a similar body size. Modern human skeletons are not systemically fragile; we are not constrained by our anatomy."

"The fact is, we're human, we can be as strong as an orangutan -- we're just not, because we are not challenging our bones with enough loading, predisposing us to have weaker bones so that, as we age, situations arise where bones are breaking when, previously, they would not have" Shaw said.

While the 7,000-year-old foragers had vastly stronger bones than the 700-year-old farmers, Shaw says that neither competes with even earlier hominids from around 150,000 years ago. "Something is going on in the distant past to create bone strength that outguns anything in the last 10,000 years."

The next step for Shaw's research team will be to look at how different types of loading and mobility shape bodies and bones by cross-referencing archaeological records with testing on modern ultra-marathon runners, who cover punishing distances over a range of terrains -- from the Himalayas to the Namibian desert. He hopes this future work will provide insight into the kind of mobility that gave our ancient ancestors such powerful physical strength.