DNA USA

by Bryan Sykes

  It was at this point, with the animals uncertain what to do, that the older men leaped down into their midst and thrust their spears into the chests of the panic-stricken beasts: To the low grunts of confusion were added the high-pitched bellows of pain. Two of the speared bison fell where they stood, the flints having found their target of the heart. Others appeared unaffected by the missiles, except for the stabs of pain that panicked them into trying to push past the bodies of their fallen companions. The men had returned, each armed with a fresh spear, and continued with a second round of slaughter, this time concentrating on the young animals whose bewilderment was all too plain to see from their rolling eyes. Three of them fell instantly, the other two reversing out of the canyon with the spears snapping off as they broke against its hard walls. The young men, armed with spears, followed them back onto the flat grassland and ran after them, knowing that eventually they would be brought down by loss of blood.

  That was enough. The spearmen climbed silently back up onto the canyon walls. The panic subsided, and the twenty or so animals left alive managed to get past the bodies of the fallen and out of the canyon. It was never the band’s intention to kill the entire herd. They knew very well that they needed to leave a majority to replenish the herd for the future. Another of the young men followed a wounded female as she stumbled down the canyon. It would not be long before she, too, collapsed. All told the band had killed eight bison: Two adult males were killed instantly; of the five young, three were killed in the ambush and two others mortally wounded and followed to their deaths a few miles distant; and the wounded female who fell to her knees soon after reaching the canyon’s mouth. As the sun went down behind the mountains in the west, the campfires were burning and the air was filled with the mouthwatering smell of roasting meat. Their faces lit by the flickering light of the fire, the hunters bade farewell to their quarry as their spirits floated upward into the sky.

  The bones of the slaughtered bison lay where they were. The hunters moved on down to the river valley for the winter. A sudden rainstorm collapsed the canyon wall onto the bones and there they stayed, undisturbed for millennia, until another flood washed them out of their resting place. Though the details of the ambush thousands of years ago are imagined, there is no invention in recounting what happened next. Sifting through the bones, Jesse Figgins brushed the congealed yellow dust from a dark shape, and within five minutes held in his hand the same flint spear point that had been thrust into the rib cage of the living, snorting beast so many years before.

  The effect of finding the Folsom spear point in Wild Horse Arroyo in the summer of 1926 was explosive, but the fuse was slow to burn. At first the find was dismissed by the experts at the Smithsonian Institution in Washington because the excavation had not been performed by an experienced archaeologist. In their opinion this meant that it was not certain that the bison bones and the spear point were in exactly the same layer, and therefore absolutely contemporaneous. To his credit Figgins, who acknowledged his lack of proper training, took notice of the objections and returned to the arroyo. The following year he found what he was looking for, another point embedded in the rib cage of an extinct bison. This time he left it where it was and sent a telegram to the Smithsonian and other leading museums. The Smithsonian immediately dispatched its own archaeologists, who soon arrived to see the evidence for themselves. Naturally eager to discover their own artifacts, within a few weeks they had unearthed an additional seventeen spear points alongside the bones of a total of twenty-three bison in and around the canyon. There was no longer any room for doubt. The first Americans had been there a very long time indeed and were not, as the weight of scientific opinion believed at the time, far more recent arrivals.

  Seven years later the time line had been pushed farther back when the excavations at Blackwater Draw, still in New Mexico but almost two hundred miles south of Wild Horse Arroyo, got under way in 1933. The scorching winds that today scour the area had uncovered the gravel bed of an ancient river, long since gone, and among the bison bones the archaeologists discovered an array of Folsom-style spear points. However, beneath the Folsom layer there was another, containing the bones not just of bison but of mammoths too. Here, lying next to the ribs of one of these huge creatures, was another spear point, larger and heavier than its Folsom successor, but equally finely manufactured. This was the Clovis point, just like the one I held in my hand. And since mammoths had disappeared from America by the time the Folsom points were being made, the Clovis point must be from an even earlier time.

  In the succeeding decades Clovis points have been found all over North America. There are several examples in the collection that belonged to that inveterate antiquarian Thomas Jefferson, though he had no idea how very old they really were. The widespread distribution of Clovis-style points shows that the hunters, or at the very least their technology, had spread rapidly across the whole continent, and into South America as well. Effective spear points can only be made from stone with the right properties of fracture and hardness, usually flint or chert. Clovis points are often found hundreds of miles from the closest sources of tool stone, and in some parts of the eastern United States more than a thousand miles separate archaeological sites where spear points have been found from the nearest location of the stone from which they were made1. All this goes to show that, as in Stone Age Europe, long journeys, maybe along established trade networks, were also commonplace in early America. If this comes as a surprise, it is only because we constantly underestimate the capabilities of our ancestors.

  Accurate dating of the bones in which the Clovis points were embedded to about 11,500 years BP (Before Present) unsurprisingly raised the question of whether this date marked the time when the first Americans arrived. Equally unsurprising was the animosity of the exchanges between those archaeologists who believed in “Clovis First” and their opposite numbers, who considered the manufacturers of these elegant weapons to be latecomers, and that America was settled much earlier. Two archeological sites have come to epitomize the evidence in favor of the latter school of thought, the Meadowcroft Rockshelter in Washington County, Pennsylvania, and Monte Verde in Chile. There are several other sites for which claims of pre-Clovis occupation have been made, but it is strongest in these two.

  The sandstone shelter at Meadowcroft is located forty feet up on a south-facing slope of the densely wooded valley of Cross Creek, a tributary of the Ohio River. It is forty-five feet wide and equally high, but only twenty feet deep, thus meriting its description as a rock shelter rather than a cave. It was when, as a young archaeology lecturer at the University of Pittsburgh, John Adovasio was looking for a largely untouched site to demonstrate excavation techniques to his classes that he heard about the rock shelter on the nearby Meadowcroft property. Checking on its suitability for his purpose, and before he began any excavations, he found the floor littered with distinctly modern artifacts: beer cans, syringes, and a hash pipe. Clearly Meadowcroft had lost none of its appeal as a temporary shelter or hideout of a sort.

  That was in 1973. Adovasio and his colleagues spent the next six seasons working on Meadowcroft, logging an impressive 417 excavation days of twelve to fourteen hours each. Ever since, he has been defending himself and his findings against the school of “Clovis First.” Excavating the successive layers of the rock shelter floor, he and his students came across an array of familiar and unsurprising objects down to a level of about ten thousand years, at which point the floor was littered with rocks from a major roof collapse. The feature that made Meadowcroft special was the collection of artifacts that Adovasio found beneath the layer of fallen rocks. The first object he and his students came across under the rocks was a spear point. It was about three inches long, and its cutting edges had been repeatedly sharpened by retouching, the process of knocking off small chips of stone with a bone or wood hammer to reveal a new sharp edge. However, it was not of the familiar Clovis design. For one thing, it lacked the fluting that served to anchor the Clov
is point so firmly to the split wooden spear shaft. Adovasio named this new spear point the Miller point, after the owner of the Meadowcroft property, Albert Miller, who had encouraged the excavation of the rock shelter. Continuing the excavation into deeper levels, Adovasio found other tools, of presumably older date, made of both bone and stone. The deepest layer contained a two-inch-long stone knife, far more crudely made than the Miller point, but an effective cutting tool nonetheless. This he named the Mungai knife, after a farm a few miles to the east.

  Although Adovasio believed from the style of the tools he had found that he was excavating much older layers than the eleven-thousand-year-old Clovis points, he needed an independent method of dating the finds. Fortunately he also came across the remains of campfires that, unlike the stones themselves, could be carbon dated. Briefly, carbon exists in nature in two forms, a stable form with atomic weight 12 (or C12), and a mildly radioactive form with atomic weight of 14 (or C14). All living things, be they bacteria, plants, or humans, contain a lot of carbon. While alive, we and all the rest are literally radioactive, but as soon as we die the unstoppable process of radioactive decay slowly reduces the amount of C14 in our remains while the amount of C12 stays the same. By comparing the level of these two forms of carbon in the remains, archaeologists can estimate how long ago any organism died. At Meadowcroft as elsewhere, the campfires contain charcoal, the carbonized remains of burned wood cut down at, or shortly before, the time the fire was lit. The oldest carbon date, at the level where Adovasio found the Mungai knife, was sixteen thousand years BP.

  No sooner had the Meadowcroft findings been published, in the prestigious British scientific journal Nature, than the attacks began.2 Briefly, there were three separate charges: First, the carbon dates could have been severe overestimates because, so critics argued, flecks of coal might have contaminated the charcoal. As coal, a fossil fuel, is at least 100 million years old and thus completely devoid of radioactive C14 which has long since decayed, even a small amount of contamination would have pushed back the carbon dates for the charcoal. Second, and this is a familiar criticism of any excavation, the layers might have been disturbed by burrowing animals or plant roots, thereby burying the stone points at a deeper “older” layer. Third was the climatic argument that sixteen thousand years ago Meadowcroft was only a few miles south of the Laurentide ice sheet that covered most of North America, and thus the area was too cold for human survival. Adovasio answered his critics on all these points. Experts on the analysis of sediments found no trace of coal, the layer of rock from the roof collapse effectively sealed off the lower layers and would have prevented any vertical movement, and, lastly, Adovasio argued that living near the ice sheet was perfectly possible, just as many people live close to glaciers today.

  The other archaeological site that seriously challenges the “Clovis First” school lies some five thousand miles distant from Meadowcroft, on the banks of a small creek in Chile. While John Adovasio was excavating in Ohio, another American archaeologist, Tom Dillehay, was teaching in the Universidad Austral de Chile in Valdivia, about five hundred miles south of the capital, Santiago. One day a student brought him the tooth of a mastodon, an extinct smaller relative of the mammoth. Along with the tooth he also brought in a collection of other bones on which Dillehay noticed some cut marks that were consistent with deliberate butchering. Dillehay traveled to the site at Monte Verde, which lies on the banks of the Chinchihuapi Creek about ten miles from the coast of a marine inlet near the town of Maulin. Resting in the sandy terraces on the banks of the creek, he found more animal bones with apparent cut marks as well as stone tools and the always helpful remains of campfires. Imagining these to be no more than ten thousand years old, he was very surprised when the carbon dates from the charcoal and the bones came back at around 13,800 years. Older than Clovis certainly, but still in the same ballpark.

  Working their way through earlier levels, Dillehay and his colleagues discovered more stone tools. Most of them were not reworked and looked as though they might have been found and selected as usefully sharp objects. But a few did show clear signs of improvement, in particular a piece of fine-grained basalt about four inches by two and with unmistakable signs of deliberate flaking. This was far less sophisticated than a Clovis point, being worked only on one side and lacking its beautiful symmetry and fluting. Of course, the key question was, as usual, how old was it? Once again it was the ashes from two hearths lying close to where the stones were found that gave the answer, and it was completely stunning. Flecks of charcoal from one hearth came back from the radiocarbon lab at 33,730 years old, and a small pieced of burned wood from the other gave a carbon date of between 33,020 and 40,000 years. In other words, three times as ancient as the Clovis points.

  The effect was dramatic, and, just as at Meadowcroft, the critics were ready with a barrage of alternative explanations that undermined what appeared to me to be the careful and understated conclusions of Dillehay’s original Nature article.3 As I write, more than twenty years after the paper was first published, the 13,800 years BP dates for Monte Verde have been supported by additional finds, most recently in 2008, and are now widely acknowledged as being correct.4 However, the significance of the older lower levels at Monte Verde and at Meadowcroft is still extremely controversial. This is despite years of active and often pugnacious defense of their positions by both John Adovasio and Tom Dillehay that has clearly exhausted them both. At various times the debate crossed the boundary from appropriate academic intensity to outright viciousness, with accusations ranging from professional incompetence to downright forgery. Dillehay even went so far as to say that if he had to do it all again, he would not. “It hasn’t been worth the agony,” he is quoted as saying.5

  And there, more or less, the matter rests. From time to time other fragments of evidence emerge that suggest that there were people in North America before Clovis. Other archaeological sites, but always lacking actual human remains, surface from time to time. Most recently, in 2008, a site at Paisley Caves in south-central Oregon yielded a few stone tools, butchered animal bones, and human coprolites, the euphemism for fossilized feces.6 The organic remains, including the coprolites, gave carbon dates of up to 12,300 years BP, slightly older than Clovis but not by much. In another study published in Science in 2007, careful redating of material from Clovis and other sites showed that unambiguously identified Clovis artifacts were confined to a very narrow range of only two hundred years or so.7 The authors were persuaded that this was far too short a time for people to have colonized the vast areas between the scattered sites across America where Clovis points have been found and that therefore America must already have been inhabited and it was the technology that spread. But even that conclusion was fiercely challenged in a strongly worded response to Science a few months later, which argued that the new data were completely irrelevant.8

  The whole debate about the timing and origin of the first Americans has the familiar feel of a stagnant intellectual circus, still balanced between entrenched academic foes who will never agree. This, I have realized over the years, is the natural equilibrium that sets in when a field has reached an impasse and where the rigid stance of personalities and their fiefdoms, rather than evidence, has become the deciding factor in an argument. Although this state of affairs is the antipathy of science as a branch of philosophy, where evidence alone is king, it is surprisingly widespread. When a field stagnates like this, the cycle can be broken only by a completely independent kind of evidence. Which is where genetics comes in.

  2

  The Nature of the Evidence

  A mitochondrion. The powerhouse of the cell.

  Although scientists have been using genetics to interpret the past for almost a hundred years, it is only in the last two decades that DNA itself has been recruited to the quest. Before that, anthropologists had studied how blood groups and other genetically inherited features varied between people from different parts of the world, and had drawn conclusions
about their origins. This was a start, but the tools were very crude and often led to explanations that we now know to be completely mistaken. It is only thanks to the really remarkable technical progress over the past twenty years that scientists have been able to read and interpret the record of the past written in DNA, the language of the genes. In this chapter I will explain how I and other scientists go about doing that. Too many books have been written on DNA for it to warrant a full exposition here, but I do want to spend a little time exploring what makes it such a superb witness of past events—events like the arrival of the first Americans, for example.

  In the briefest of terms, DNA is the stuff we get from our parents that instructs our cells to build and run our bodies. It comes, therefore, from an earlier generation. And since our parents received their DNA in just the same way, from their parents, who got it from theirs, it comes down to us from our ancestors, from the past. It is for this very simple reason that the DNA in all of us—the people alive today—can be a witness to events in human history that occurred hundreds, even thousands of years before we were born. How DNA is used and interpreted I shall explain in greater detail during the course of this book. The other remarkable property of DNA is that, under the right circumstances, it is extremely durable and survives for thousands of years in well-preserved fossils. As a witness to past events we have the DNA of living people, which is technically straightforward to analyze but can be hard to interpret, and the DNA of the dead, which is very difficult to work with but considerably easier to interpret.