DNA USA

by Bryan Sykes

  Although the magical Polynesian “motif” is not found in America, the ancestral sequences certainly are; 189 217 and even 189 217 261 are right there at the center of the cluster B sequences of Native Americans. But how did these sequences arrive in America? Unlike A, C, and D which are widespread among today’s Siberians, cluster B is virtually unknown there. That itself is curious, if the only way of getting to America was across Beringia and the land bridge from Siberia to Alaska. Neither is cluster B found more than very sporadically among modern Alaskans and is only encountered in numbers from Vancouver Island south. However, in Central and South America it is the predominant cluster where, if anything, the cluster’s genetic dates are slightly earlier than they are in the North. To me and to other scholars, notably another Polynesian enthusiast, Rebecca Cann from the University of Hawaii, this geographical distribution smacks of a distinctly nautical arrival and suggests that cluster B arrived in America not by land but by sea.6 The calibrated genetic date for cluster B in America is 18,700 years, on a par with the three other Asian clusters, which almost certainly crossed from Siberia. But what were conditions were like at the time?

  America is separated from the rest of the world by two vast oceans, the Pacific and the Atlantic, with only fingers of land in Siberia and Alaska reaching out toward each other, like God and Adam in Michelangelo’s famous mural on the ceiling of the Sistine Chapel in the Vatican. From time to time over the last quarter of a million years these fingers have touched and let go again as the sea around them fell and rose with the cycle of the ice ages. If humans needed dry land to reach America, then they would have to wait for the ice caps of the Arctic to suck enough water from the ocean to expose the causeway.

  According to climate records stored in bubbles of air trapped in ice deep below the surface, the temperature was only low enough for long enough to expose the land for short intervals during the last two hundred thousand years. One was during the last Great Ice Age, which reached its peak about eighteen thousand years ago. This enduring spell of freezing weather lasted for nearly ten thousand years. Glaciers grew from the mountain ranges of the North, connecting to become vast ice domes covering much of North America and Europe. The ancestors of modern Europeans were forced southward to Spain and Mediterranean France, before the gradual thaw drew the herds of game on which they depended north again. Earth’s orbit around the sun wobbled again about thirteen thousand years ago, and the conditions of the Ice Age returned, once more allowing Siberia and Alaska to join hands. But this was a comparatively brief spell and was followed by a rapid warming that announced the stable conditions we have known for the last ten thousand years. If the ancestors of the first Americans arrived only by land then, according to this research, there were just two opportunities to do so, between 23,000 and 17,000 years ago during the Great Ice Age, and later during the mini-ice age, called the Younger Dryas, around thirteen thousand years ago.

  During these cold phases the sea was three hundred feet or more below its present level. The tips of Siberia and Alaska were connected by a broad expanse of land up to a thousand miles across, and most people who have pondered the origin of the First Americans have tied their arrival to one or another of these periods when the land connection between Asia and America was intact and open for traffic. However, I do not see any convincing reason for this restriction. Even today, when the causeway is flooded by the sea, the crossing from one side of the Bering Strait to the other is only just over fifty miles. Not only that, the gap between Siberia and Alaska is not all water. The Diomede Islands, one belonging to Russia, the other to the United States, are conveniently situated around the halfway mark. Nowadays caribou and arctic fox routinely swim between the mainland and the islands, and it would not have been dangerous for humans to set out for the Diomedes from Siberia by boat. Once there, Alaska would have been plainly visible.

  Our ancestors certainly knew about boats by the time the first humans reached the Arctic shores of Siberia. They had reached Australia across the open sea at least fifty thousand years ago. We do not know for sure whether Siberians had boats or not, and there is scant hope of ever finding any direct evidence: Boats were constructed from biodegradable materials like bone, skin, and wood that have long since turned to dust. It is conceivable that a frozen boat, deliberately buried, might one day be discovered, dating to a time when we know that the Bering Strait was open water, but it isn’t likely. Likewise we have no direct proof that the first Australians had boats, only the powerful circumstantial logic that, even when sea levels were at their lowest, any route from Asia to Australia involved a sea crossing of at least thirty miles. And, of course, in the Arctic the sea can freeze over, creating a temporarily solid connection. Even now, in the severest winters, sea ice forms right across the strait and is thick enough to support the weight of a human and a dogsled. Whether on a moonlit ice crossing under the flickering phosphorescence of the northern lights, or by boat in the endless sun of the Arctic summer, the Bering Strait can never have been a serious obstacle to human movement. I see no need at all to restrict the arrival of the first Americans to those periods of time when we know the connection to Siberia was dry land. Our ancestors were far too adventurous to let that stop them.

  The curious geographical distribution of cluster B, absent from higher latitudes but abundant in Taiwan to the west and in Central and South America to the east, does raise the possibility that the ancestors of these Native Americans arrived by boat. The east–west flow of wind and currents across the Pacific makes it very difficult to navigate directly from Asia to America across the open sea. Even the oceangoing galleons of Spanish navigators during the fifteenth and sixteenth centuries were forced to travel along the great circle route to return from Indonesia and the Philippines to their bases in Central America. That meant following the Pacific coast north to China, Japan, and Kamchatka, and then crossing the Bering Strait before turning south and skirting Alaska, Canada, and California before reaching their home ports in Mexico. I find it very reasonable that the same route might have been taken by the ancestors of Native Americans in cluster B who set out from the Pacific coast of South China and Taiwan. Even though the waters were cold, fish and sea mammals were abundant just as they are now. Perhaps the cluster is absent from Alaska and the icebound northern Pacific coasts simply because they did not stop, preferring to follow the sun south toward the more inviting climates of Central and South America. This preference might explain the astonishingly rapid movement of the first Americans from north to south, judging by the early archaeological dates from coastal sites in Chile.

  Even though, just as in Australia, we have no preserved remains of boats, there is hard evidence of marine travel. At Arlington Springs, on Santa Rosa Island off the California coast near Santa Barbara, human remains carbon dated to 11,000 years BP, but with no distinct stone tools, could only have arrived by sea.7 Even at Monte Verde, in southern Chile, microscopic examination of the soil around the hearths, dated to 14,600 years, found tiny fragments of edible and medicinal seaweed, showing that the inhabitants were very familiar with the abundance of coastal resources.8 The same was true for our European ancestors living at the same time and a little later, who preferred the coasts to the interior.

  My own summary, based on an amalgamation of the archaeological and genetic results, is that the first Americans arrived by both land and by sea. The mammoth hunters from Siberia, mainly from mitochondrial clusters A, C, and D, arrived in Alaska either across the causeway of dry land when the sea levels dropped, or covered the short distance by boat or across the ice at other times, probably using the Diomede Islands as a convenient stepping stone. Another group, from the descendants of Ina in mitochondrial cluster B, traveled by boat from much farther south. They did not stop to settle in Siberia or Alaska but carried on to the more moderate climate of Central and South America. There is no need, in this reconstruction, to confine the arrivals to different “waves” of settlement; and, as elsewhere in the world, there w
ould have been movement back and forth. Nonetheless, the genetic echoes of the epic Arctic journeys of their ancestors are still there in the cells of today’s Native Americans.

  Coastal travel down the Pacific coast of America was comparatively easy for the descendants of Ina, but for the Siberians going south by land, the way was blocked by two vast ice sheets. As the orbit and inclination of the earth around the sun shifted slightly around twenty-five thousand years ago, the cycle of ice ages that have been a feature of the earth’s history for the last two million years once more turned the thermostat down. At first, snow that used to melt every summer began to resist the sun and lay all year round. As the temperature continued to drop, these patches of permanent snow grew into small glaciers that in turn coalesced into ice fields. Slowly these ice fields grew in thickness and extent until, by twenty thousand years ago, an enormous dome of ice, the Laurentide, covered most of Canada and the United States north of a line joining New Jersey, Cincinnati, and St. Louis, Missouri. Farther to the west, the mountain glaciers of the Rockies and the Cascades grew and fused into a second ice cap, the Cordilleran, that stretched as far south as Seattle. Only the strip of land between this and the larger Laurentide sheet covering the Midwest was free from permanent ice cover, at least some of the time. Ironically, Alaska, outside the McKinley range, was spared this crushing load as it was just too dry for the snow and ice to build up. Overland access to the interior would have been possible through the ice-free corridor, though exactly when and for how long is still controversial. The other way south was along the coast, where a thin sliver of land was kept free of ice by the warming effect of the oceans. Even where glaciers blocked the coastal route, a short diversion out to sea would have circumvented the obstacle.

  We may never know. The sea now covers the evidence for coastal migrations, just as it does in so many different parts of the world where rising water levels have swamped and obliterated all traces of human habitation. An encampment with the discarded bones of deep-sea fish would prove an aquatic capability, even if the boats needed to catch them have long since perished. My admiration for the resourcefulness and resilience of our ancestors has grown to the extent that I find it hard to believe that their eventual discovery of the rich pastures of the United States south of the ice domes would have been thwarted by any of the temporary inconveniences that the ice ages might have put in their way. Where there is food, they will come.

  Imagine the southerly progress of the first Americans, by land or by sea. Long before the rising temperature, the first thing they would have noticed was the light. The sun no longer sank to the horizon in the winter months, and it climbed higher into the sky in the middle of the day. Eventually they would have reached the southern limit of the ice and seen, stretching out before them, a land of wide tundra and rolling hills. Rivers swelled by ice melt gushed from under the glacial fringe, carrying frothing water gray with the suspended particles of tortured rock ground down to dust. Away to the south the rivers carried the pulverized rocks of Canada, until they settled out onto the flatlands of the Midwest, the waters flowing slowly now as the young Mississippi spewed out, exhausted, into the Gulf of Mexico.

  The land was empty of humans but not of life. The animals were big, much bigger than they are today. The wild menagerie that greeted the pioneers who first encountered the rolling country beyond the ice contained a fair share of travelers who had, like themselves, arrived from distant lands. Three types of ancient elephant, all now extinct, had begun their own journey from Africa to the New World at least four million years before, following the same route as the humans who trekked through Central Asia and across to Alaska during one of the periodic openings of the causeway. The solitary forest-dwelling mastodon, the more sociable open-country mammoth, and its heavily insulated cousin, the woolly mammoth, were even larger than today’s African elephant, and all of them reached America. Also much larger than its modern equivalent, the Ice Age Bison antiquus roamed over the entire continent. Musk oxen were the cattle of the frozen tundra, much as they are today, and like the bison, they had traveled across from Asia.

  Some species that, you might have thought, would also have benefited from intercontinental travel stayed firmly put on the Asia side of Beringia. Both the yak and the woolly rhinoceros, though plentiful in Asia and certainly well adapted to the extreme cold, never made it to America. From what I have written so far you would be forgiven for thinking that the Siberian-Alaskan highway only took one-way traffic, but there were many species that originated in America and then emigrated to Asia and beyond. Both horses and camels first evolved in America from where they crossed Beringia eventually to become the two-humped Bactrian camels of Mongolia’s Gobi Desert, and the one-humped dromedaries of the Arabian Desert and the Sahara. While camels survive in South America as llamas, alpacas, and the vicuñas of the Andes, wild horses became extinct in America and only re-appeared with the Spanish conquistadores in the sixteenth century.

  Travel between North and South America only became possible between three and four million years ago, when the Isthmus of Panama rose above the waves thanks to the collision of two tectonic plates that thrust the seabed that connected an island chain upward and out into the open air. Until the two continents were linked, the evolution of South American mammals had taken an entirely different course from that of their cousins in most of the rest of the world. These were the marsupials, whose tiny embryos are forced early from the womb and suckled in a warm pouch. This is the fundamental difference between marsupial and placental mammals like ourselves, who emerge from the womb much more fully formed. Both independent evolutionary pathways produced very much the same type of mammal—herbivores, predatory carnivores, and so on—in either marsupial or placental form, but when the two were able to advance into the other’s territory it was the placental versions that usually triumphed. The cat-like marsupial predators of South America were replaced by the placental ancestors of the jaguar. In Australasia, the only other place where isolation allowed marsupials to flourish, placental dingos replaced marsupial wolves (the last specimen nicknamed “Benjamin,” passed into history in Hobart Zoo, in Tasmania, on September 7, 1936). Even so, some marsupials had crossed into North America and survived to greet the new human arrivals. None was more impressive than the giant ground sloth, which reached upward of eighteen feet at the shoulder.

  All these animals were hunted by the first Americans. But there were very substantial dangers awaiting the new human arrivals: animals that would have come to know the taste of human flesh. Lions, larger and stockier than today’s African and Asian survivors, were to be found all over America, both South and North, along with their fearsome cousin the Smilodon, known more descriptively, though inaccurately, as the saber-toothed tiger. But even these dangerous predators posed less of a threat to the human newcomers than did the quintessential distillation of fear and danger, Arctos simus, better known as the short-faced bear. These were not the ambling ground-hugging grizzlies, but long-legged bears built for speed. They were as tall as a moose and towered to fifteen feet when rearing on two legs. They could move as fast as the horses and bison that they hunted, and though a grizzly can put on a good spurt in pursuit of a wounded elk, the short-faced bear was a Bentley Continental to the grizzly’s Nissan Micra.

  How large a part the first Americans played in the extinction of the indigenous megafauna is open to question. Certainly, from the evidence of the fossil record, many species died out soon after they arrived, although a direct causal link is impossible to prove. What we can be sure of is that whether they came by land or sea, the ancestors of today’s Native Americans overcame enormous challenges to settle the new continent. It is too easy to forget how resilient and courageous all our ancestors, wherever they lived, must have been to survive and to pass on their DNA to us, their descendants.

  4

  The Mystery of Cluster X

  Grand Canyon, Arizona.

  While the genetic evidence that we have covered s
o far does not agree with every aspect of traditional academic thinking about the origin of the first Americans, it does at least confirm that they arrived from Asia. Of the four predominant mitochondrial clusters found among Native Americans, all have their closest matches in Asia, even if three (A, C, and D) originated in Siberia and one (B) farther south in Taiwan and China. However, several years after the discovery of the four main clusters, signs of a fifth were found among some American Indian tribes. In all genetic surveys that I have been involved with, there are always one or two individuals who have mDNA sequences that appear to be completely out of place for their surroundings. A Polynesian sequence in Edinburgh or a Korean mDNA in Iceland are examples that I remember finding during the course of my own research. I call these “accidentals,” borrowing a phrase from the world of ornithology, where the term describes the freak appearance of a bird in unfamiliar surroundings—an American robin, T. migratorius, blown off course to the west coast of Ireland, or a Wandering Albatross joining a colony of gannets on the Shetland Islands off the north coast of Scotland—that sort of thing. However exotic the explanation for the human “accidentals”—the elopement of a Tahitian princess or a pirate raid in the South China Sea—they are incidental to the overall genetic pattern of a region.

  It was only in 1997, six years after the Nuu-Chah-Nulth work defined the mDNA sequences of the four main clusters, that a number of rare and apparently sporadic sequences that had been found in Native Americans were recognized as belonging to a separate cluster in their own right. By then the alphabetical notation of clusters had moved on a long way from A, B, C, and D, with global coverage having almost reached the end of the alphabet. With a fitting air of mystery, the new cluster was called X. The catalyst was a study of the Ojibwa.1 The Ojibwa are the third-largest Indian nation in America, surpassed in numbers only by the Navajo and the Cherokee. They live on the borders of the United States and Canada in the general area of the Great Lakes. The mDNA of a quarter of Ojibwa volunteers belonged to the newly recognized cluster, far too high a proportion to be explained away as “accidental.” Cluster X reaches its highest frequency among the Ojibwa, but elsewhere members of the cluster are found among the Sioux of the Great Plains, the Yakima of Washington State, and the Navajo of the Southwest, where the frequency of cluster X reaches 4 percent. The genetic connection between Ojibwa and Navajo also mirrors their related languages and may be an echo of a southward migration of the Dine (the name the Navajo give to themselves) mentioned in their mythology. Cluster X is found only in North America and did not find its way south of the Mexican border. But where had cluster X come from originally?