Since the birth of archaeology, one of its central preoccupations has been the search for evidence of the first humans in the Americas.  We are unified in our wonder at the questions of where and when the great human migration began, and where and when it ended.  The discovery of Lucy in Africa's Great Rift Valley gave humanity of a sense of common provenance.  Now, the mystery of Brazil's New World "Luzia" offers a sense of the great prehistoric movement to encircle the planet reached an end.

The idea that humans first entered the western hemisphere along a land route in the high northern latitudes was first suggested by a Spanish priest four centuries ago.  In 1933, with the discovery of 11,500-year old spear points embedded in the remains of a woolly mammoth near the New Mexico town of Clovis, the first detailed picture of America's first peoples took shape.  The Clovis culture appeared to be a formidable and highly specialized population of big-game hunters.  The estimated appearance of Clovis in Arizona and the Great Plains 11,500 years ago also appeared to neatly coincide with the retreat of the vast continental glaciers at the end of the last Ice Age, when a land route linking Beringia and ice-free North America finally opened.

For most of the 20th century, the "Clovis-first" theory of the peopling of the Americas dominated North American archaeology, led by an American archaeological establishment relatively impervious to criticism from without its walls.  Certainly the idea of mammoth hunters racing down from Beringia between the retreating glaciers to populate North America was a compelling vision of the ancestral First Nations.  Blame for the sudden disappearance of Pleistocene mammals, from mastodons and woolly mammoths to giant ground sloths and sabre-toothed tigers, was hung on the Clovis – the "overkill thesis" of Pleistocene extinctions.  The Clovis-first theory was an American-centric vision of New World, a proposal that all later cultures in the western hemisphere traced their roots to an aggressive hunting people from the American Southwest.

The archaeological establishment was, in retrospect, blind to the accumulating evidence, some coming from other scientific disciplines, casting doubt on the plausibility of the Clovis-first theory.  Decades ago, Canadian geologists established that no ice-free continental route could have existed until a millennium after the purported appearance of the Clovis farther south.  Linguists observed that the diversity of modern aboriginal languages in the Americas could not result from a single migration from Siberia less than 12,000 years ago.  Most compelling of all was a growing number of 'pre-Clovis" archaeological sites throughout North and South America, which many of the mandarins of American archaeology fought to discredit with great determination. 

In fact, so entrenched is the orthodoxy of "Clovis-first" that it remains fixed in the public imagination to this day.  In 2003, the BBC's Natural History Unit and Discovery USA even produced a high-budget international docudrama portraying the Clovis as first peoples called Monsters We Met.

Although largely ignored by American archaeology, South American archaeologists began discovering credible pre-Clovis sites of human occupation in the 1960s.  It eventually took an American archaeologist named Tom Dillehay working on a site in southern Chile to catch the attention of the scientific establishment.  Discovered in 1977, Monte Verde "proved out" as a creek-side encampment inhabited 14,500 years ago, 2000 years earlier than the arrival of Clovis in the Americas.  Dillehay led 10 years of meticulous excavation with contributions from 40 specialists from an array of disciplines, published a 1,400-page report on the site, and in 1997 hosted a delegation of archaeologists on a site visit to examine the evidence first-hand.  Overawed by the evidence, his American colleagues agreed with Dillehay's dates – after seven decades, the first serious cracks appeared in the Clovis-first theory.

Monte Verde cast a generation of archaeological consensus into serious confusion, a condition that persists to the present.  The problem is, although Monte Verde is clearly a pre-Clovis site, archaeologists are still looking for the "pathway" linking distant southern Chile to the Bering Strait, the entry route of early migrations.  Now, with the Clovis-first orthodoxy crumbling, we are witnessing a foment of competing new ideas and tantalizing new evidence.

The documentary Code Breakers takes up the story just as a series of discoveries promises to clarify this confused picture and rewrite our understanding of when and how the first migrants reached the Americas.  Techniques for precisely dating human artefacts and human remains are rapidly improving.  At the same time, with breakthroughs in the study of ancient DNA, we are entering a Golden Age for the study of prehistoric human migrations.  The documentary crew had unique access to two research projects using the power of ancient DNA analysis and carbon dating to challenge the Clovis-first theory and thereby introduce a much different set of ideas about the first humans in the New World.

The earliest evidence of human presence anywhere close to the Bering Strait (Beringia) is found in modern Mongolia and south-central Siberia.  Lake Baikal has been an oasis for human cultures going back at least 30,000 years, and genetic evidence from modern and ancient human samples now suggests the Baikal region was a "starting block" for the peopling of the New World.  On its shorelines, archaeologists have uncovered the world's greatest concentration of prehistoric hunter-gatherer gravesites, many yielding intact human skeletons bearing DNA.  The documentary focuses on the work of Professor Andrzej Weber, who leads the decade-old Baikal Archaeology Project.  During a summer-2009 shoot, Weber unearthed a 5000-6000 year old human skeleton from deep within a karst cave above the estuary of a river on the western shore of Lake Baikal.  Weber has retrieved DNA from 68 different skeletons, which amounts to the oldest and largest body of human genetic material ever found in Siberia.  He is now bringing these bones back to his labs at the University of Alberta, where he is work a team of geneticists to extract DNA, possibly establishing connections with ancient human remains in North America. 

At the Paisley Caves in Oregon's Great Basin desert, archaeologist Dennis Jenkins has found preserved human faeces alongside a wealth of stone and bone tools in layers of cave floor he identifies as 14,500 years old – exactly the age of the mysterious Monte Verde materials in Chile.  If verified, this would be the first bona fide pre-Clovis site in North America.  Its position midway between Beringia and coastal Chile would also plug the gap in a purported New World migration from the Bering Strait to the Monte Verde site.  When Jenkins discovered the preserved human faeces, or coprolites, he approached Eske Willerslev, a young Danish geneticist studying at Oxford University with a growing reputation for retrieving DNA from super-ancient organic materials.  Willerslev was able to directly date the human coprolites to 14,500 years, and to extract a limited amount of mitochondrial DNA.

Paisley Caves, with its claim to pre-Clovis status, has aroused enormous scientific interest – and much scepticism.  Like in the case of Monte Verde, a site visit of eminent experts and Clovis-first supporters was planned in late September 2009 to place the evidence under critical examination.  The documentary crew secured exclusive access to this gathering of scientists, an event that the roughly 30 assembled experts later acknowledged was a pivotal moment in modern archaeology.  Despite the initial opposition of some scientists at the site visit to the camera's presence, the project negotiated access to film the defence of the evidence by Jenkins, Willerslev and a team of other geologist, biochemists and archaeologists.  The principal criticisms of their claims centred on contamination of the coprolites by modern DNA and the true age of the layers in which the human artefacts and coprolites were discovered.  After two days of debate, just like in Monte Verde a decade earlier, a consensus emerged that Paisley Caves was indeed the breakthrough North American archaeology has been waiting for – final, direct evidence of humans before Clovis.

As the Paisley Caves site visit wrapped up, filming concentrated on Willerslev, truly a phenomenon of nature – the youngest Professor appointed to the University of Copenhagen, a rough-hewn tornado of scientific curiosity and an unmatched master at finding and extracting DNA from the unlikeliest sources.  Working alongside him was Tom Stafford, a generation older than Willerslev and a global authority in carbon dating techniques.

Stafford and Willerslev, confident their work cements the pre-Clovis claims of Paisley Caves, fixed their sight on an even greater prize – the Lucy of the New World, whose skeleton has long been suspected, but never proven, to be the oldest ever found in the Americas.  While archaeological remains from human occupation sites like Monte Verde are compelling evidence, Stafford and Willerslev know the gold standard is the direct dating of actual human bones.  Of even greater interest is the promise of intact ancient DNA.  If the two scientists could prove this Brazilian skeleton is the same age as the Paisley Cave coprolites, they would be able to present the first direct evidence of pre-Clovis humans in South and North America simultaneously.  The "pathway" from Beringia to Monte Verde would be complete.

But "Luzia", as she is known in Brazil, has long confounded attempts to reliably establish her age and genetic origins.  Discovered in a rock shelter in the Lagoa Santa karst complex in 1975, her bones were so thoroughly mineralized that scientists believed no organic collagen remained to offer carbon dates and potentially workable DNA.  Nevertheless, the Brazilian archaeologist Walter Neves, based at the University of Sao Paulo, has never abandoned his belief that Luzia is older than 12,000 years old, and therefore almost certainly a member of a pre-Clovis population in the Americas.  Our documentary crew travelled to Brazil to work with Neves, and Willerslev and Stafford enthusiastically agreed to tackle the case of Luzia.  With Neves help, they set out to extract collagen trapped in the enamel layers of Luzia's teeth, and to finally obtain direct proof of her age.

Working with these "code breaker" scientists and their field sites on Lake Baikal, in the Oregon desert, and in Brazil's tropical Lagoa Santa complex, the documentary crew was able to film groundbreaking evidence of early New World humans as it actually emerged for the first time.  Willerslev, Stafford, Neves and Weber are all working toward a common goal: to rewrite the map of human migration into the New World, using dating and DNA analysis techniques to place these movements in time and to establish the genetic origins of First Nations peoples. 

But if the Clovis-first model is now failing comprehensively, what theory of migration is left to take its place?

In the documentary's final scenes, we explore the emergence of a fascinating new theory about the very earliest movements into the Americas.  If there were humans in the New World before Clovis, and possibly as early as 15,000 years ago, they would have faced an unbroken wall of ice in the eastern reaches of modern Alaska – the continental ice sheet created by the last Ice Age.  Now, attention is shifting to the possibility of a coastal migration by a founding population skilled in hunting sea mammals and the manufacture of seafaring boats from their skins.  In the frigid late Pleistocene, these migrants could have skirted the southern coasts of Beringia and moved down the ice-free coastlines of modern Alaska and British Columbia.  The Paisley Caves complex is located above a dry lake bed, but in the Pleistocene era, the camp was on the fringe of a pluvial lake fed by the waters of melting glaciers farther north and connected to the nearby coast by a network of rivers and wetlands.  Monte Verde, in modern Chile, would likewise have been within reach of a culture skilled in sea navigation.  And the modern precedent of the Thule Inuit migration (800 years ago) proves the possibility of long-distance sea migrations over very short spans of time.

Yet the final evidence of a coastal migration remains circumstantial.  The shorelines exposed at the end of the last Ice Age 15,000 years ago have been drowned beneath rising oceans, thus submerging potential archaeological evidence.  Because prehistoric boats were fashioned with perishable materials, the oldest remains ever found are only 8000 years old.  For now, we must rely on circumstantial evidence that the first peoples in the Americas arrived by sea – knowing they arrived when our continent was covered in ice, we understand they could have come by no other route.

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