American Scientist

Contrary to the way they are depicted in popular culture, archaeologists don’t always place the highest value on locating an ancient site before any of their colleagues. Often they make major discoveries while reexploring sites that have already been found, by bringing new techniques and new tools to the excavation. A current case in point is Bacho Kiro Cave, which has yielded fossils of modern Homo sapiens dating from at least 46,000 years ago—the earliest physical evidence of our species that has yet been found in the mid-latitudes of Europe. Jean-Jacques Hublin, director at the Max Planck Institute for Evolutionary Anthropology (MPI-EVA), in Leipzig, Germany, is the lead author of a May 21 article in Nature that gives an account of the work.

The age of the Bacho Kiro fossils corresponds to a particularly intriguing time in European prehistory, when Neanderthal populations were on the wane after inhabiting the continent for almost 400,000 years. Researchers have long debated the circumstances that led to the Neanderthals’ extinction: The hypotheses include conquest or slaughter at the hands of newly arriving modern humans, competition with the newcomers for scant resources, and reproductive bottlenecks that arose from Neanderthals living in small, widely scattered groups; most likely, more than one factor was involved. Even so, with a long-established population dying off while a new population begins moving into the area, it’s reasonable to want to look very carefully for any clues as to how those two groups dealt with each other. Bacho Kiro is well situated, both geographically and chronologically, to offer such clues, although their interpretation is still a matter of debate.

Radiocarbon dates can now be narrowed down to within 300 years.

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Located in central Bulgaria about 70 kilometers south of the Danube River, Bacho Kiro Cave was partially excavated in the 1930s and again in the 1970s. Unfortunately, any fossils recovered from these earlier digs were subsequently lost, and some apparent disturbance of the stratigraphic layers left the dating of the site unresolved. In 2015 an international team based at the MPI-EVA set out to clarify the chronology of Bacho Kiro by cutting two new trenches adjacent to the earlier excavation area. This time, the researchers found not only fossils but also numerous tools and other objects made of stone and animal bone, as well as animal bones bearing the cut-marks characteristic of butchering.

The new Bacho Kiro study is notable for the solidity of its radiocarbon dating. The Leipzig team obtained no fewer than 100 dates, constituting “one of the largest carbon-14 datasets from a single Paleolithic site produced by one team,” according to a companion paper in the June 4 issue of Nature Ecology & Evolution.

The utility of radiocarbon dating is limited by the age of the material being analyzed—in this case, the collagen contained in human and animal bone. Collagen tends to break down gradually over time, so samples as old as 40,000 years often retain too little collagen to analyze. For the human bones at Bacho Kiro, however, an innovative approach developed by postdoctoral researcher Helen Fewlass and her colleagues made it possible to obtain accurate radiocarbon readings from much smaller samples of collagen than are needed for the conventional technique, making the new method much less destructive (an important consideration when sampling priceless ​fossils).

“The core question is, When did Homo sapiens arrive in Europe?”

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The technique Fewlass and her collaborators introduced also improved precision. Sahra Talamo, professor of chemistry at the University of Bologna and principal investigator of the radiocarbon part of the study, explains, “Conventional carbon-14 analysis of samples older than about 40,000 years usually gives dates with a plus-or-minus margin of 1,000 or even 1,700 years. That would be like a mathematician saying that Archimedes was around at the same time as Einstein! The new technique uses a much slower process with the latest equipment, but it produces dates that are much more precise: We now have a margin of error of only 300 years.”

To achieve accurate radiocarbon dates first requires demineralizing a sample of bone in weakly acidic water. The Leipzig researchers do this slowly, often over several weeks, but in the end it allows for the extraction of a larger-than-usual portion of collagen as a single, gelatinous piece. The gelatinized collagen undergoes ultrafiltration to minimize the risk of contaminating the sample (a crucial step, because any contamination introduces the risk that the age of the sample may be underestimated by thousands of years) and is then analyzed by accelerator mass spectrometry.

Most of the artifacts found at Bacho Kiro clearly were made for everyday use—flakes, blades, animal bones sharpened to a point—but the site has also yielded some unusual objects that appear to be the world’s first example of European jewelry: a single bead of ivory along with the teeth of cave bears and ungulates, perforated to be worn as pendants. “These are the oldest body ornaments of this kind that we have found in Europe,” says Hublin, who puts their age at possibly 47,000 years.

At many Eurasian sites, stone artifacts of such an age have been found. Although the makers remain unknown, these artifacts have generally been attributed to late Neanderthals. Bacho Kiro breaks with this pattern, however: Here artifacts are found together with teeth and bone fragments belonging to H. sapiens, as determined by protein mass spectrometry and ancient DNA. Given this association, the authors of the Nature paper infer that the artifacts were made by early members of our own species.

Yet the cultural context of these objects is far from clear. The animal-bone tools and body ornaments that most closely resemble those of Bacho Kiro come from a site in central France called Grotte du Renne, some 1,700 kilometers to the west. A recent proteomic analysis of bone fragments from Grotte du Renne has identified the inhabitants of the site as Neanderthals. The age of this site is significant, however: Radiocarbon dating puts it at 2,000 to 3,000 years more recent than Bacho Kiro. Is it possible that the Neanderthals of Grotte du Renne not only survived contact with modern humans but even interacted, and perhaps interbred, with them? Could the Neanderthals have borrowed from transitory groups of H. sapiens the techniques for sharpening animal bones to make awls and perforating animal teeth to wear as pendants? Hublin and his colleagues put forward this explanation, pointing out that artifacts of this type tend to turn up only at Neanderthal sites dating from about 45,000 years ago or less—never at older Neanderthal sites.

“The core question is, When did Homo sapiens arrive in Europe?” says archaeologist William Banks, a Bordeaux-based research director with the French National Center for Scientific Research. Banks, who was not involved in the Bacho Kiro excavation but wrote a commentary accompanying the Nature paper, explains that determining exactly when modern humans first appeared on the continent, and therefore how long their presence in Europe overlapped with that of Neanderthals, is key to assessing how much potential there may have been for cultural interaction between the two peoples.

If some dwindling number of Neanderthals inhabited various sites in Europe 40,000 years ago (as is generally agreed), and if at least one group of modern humans was also present in Europe 46,000 years ago (as the fossils and artifacts from Bacho Kiro seem to attest), then the two populations may have shared portions of the continent for 6,000 years or more. According to Hublin, such a lengthy period of overlap “means that the replacement of Neanderthals by Homo sapiens was a long and complex process,” probably with initial waves of H. sapiens advancing onto European terrain in small groups, some of them dying out and others spreading farther or mixing with Neanderthals.

Then again, the relationship between H. sapiens and Neanderthals may not have been a matter of replacement at all. Research professor João Zilhão, of the Catalan Institution for Research and Advanced Studies (ICREA) at the University of Barcelona, proposes instead that Neanderthals, Homo sapiens, and other early humans may all have been variants of a single human species. “There is the definition of ‘species,’ and then there is the reality, which is usually more complicated,” Zilhão says. He points out as one example the recent consensus that among most inhabitants of Europe today, 2 to 4 percent of the genome is of Neanderthal origin. “You cannot say that Neanderthals and modern humans interbred and then also say that Neanderthals and modern humans belong to different species,” he adds.

For Zilhão, the material recovered from the new Bacho Kiro excavation is emblematic of a site located at a long-ago crossroads, with “genetic data suggesting a connection to Africa and cultural evidence suggesting a connection to Western Europe.” At the same time, as Hublin says, his findings demonstrate the great potential of reexploring an old excavation with new technology: “We are facing a molecular revolution in terms of the amount of information we can extract and analyze from ancient organic material. Bacho Kiro is complicated not because the site itself is complicated but because we are now using so many different kinds of information.” On that last point, at least, everyone can agree.