by Theodore G. Schurr
Recent genetic studies are painting surprising, if tentative, new pictures of the peopling of the Americas — complex images with multiple waves of colonists from a number of homelands, the first of them arriving in the New World much earlier than generally believed.
Genes are housed within a cell's nucleus, but mitochondria (bottom left), which carry mitochondrial DNA, are outside the nucleus. The mtDNA is inherited only from the mother (ring on top left).
Molecular genetic research in the past five years suggests the first Americans arrived well before the Clovis culture, the traditional benchmark for human occupation of the New World at about 11,500 radiocarbon years ago (13,350 cal BP). And the data imply that several geographic expansions of ancient peoples contributed to the genetic diversity now seen in aboriginal Siberian and Native American populations.
Several genetic studies have suggested that a single migration from southeastern Siberia and Mongolia populated the New World. However, research by my colleagues and I suggest multiple migrations to the Americas, as well as additional and/or alternative source areas for ancestral Paleoindians, including the lower Amur River between Russia and China.
But whether a single colonizing event or just the first of several population expansions, the initial migration into the New World seems to have occurred at least 20,000 years ago, and perhaps as long ago as 40,000 years. Later, well after the last glacial maximum at about 18,000 years ago, other groups moved into northern North America. These people probably had been inhabiting the Bering land bridge, the then-dry region between Alaska and Siberia that is called Beringia.
Furthermore, while most of the genetic lineages present in modern Native Americans have their antecedents in East Asia and Siberia, recent work has shown that one lineage appears to have its roots in Eurasia, leading to speculation about how it was brought to the Americas.
The two primary genetic systems that have been used in these groundbreaking studies are the mitochondrial DNA (mtDNA), short pieces of genetic material found outside a cell’s nucleus (where the genetic chromosomes are located), and the Y chromosome, which is present only in males.
These genetic systems have a number of properties that make them invaluable tools for molecular-anthropological studies. First, each is inherited only from one parent: The mtDNA is passed from mother to daughter, and the Y chromosome from father to son. These inheritance patterns mean their analyses are essentially the study of female and male genetic histories within human populations.
Second, these genetic systems generally accumulate mutations in a more or less linear fashion through time, which means permanent and detectable genetic changes occur at a roughly predictable rate. This allows researchers to trace the evolution and divergence of female and male genetic lineages in human groups.
Third, many of the mutations detected in mtDNAs and Y chromosomes correlate with the geographic region in which they first occurred. This makes it possible to reconstruct ancient migration patterns based on the distribution of these mutations, or genetic markers, in different populations.
Finally, these genetic systems are very sensitive to such random processes as genetic drift — the changes in gene frequencies that occur in a population over generations. These changes can result from geographic isolation, migration, or population splits, in which some of the people leave to found a new group. Hints of such past events are often recorded in a population’s mtDNA and Y chromosome sequences.
The mtDNAs of Native American populations separate primarily into four haplogroups, or lineages, designated A, B, C, and D. Each is defined by a specific set of genetic markers, so their identification is relatively straightforward. Statistical analyses indicate that haplogroups A, C, and D originated about 35,000 to 25,000 years ago in both Siberia and America. By one measure, haplogroup B appeared to be much younger in America (about 15,000 years ago), but other work suggests that it was present in East Asia by at least 30,000 to 24,000 years ago and may have entered the New World during this period. Thus, the molecular data suggest that the four primary haplogroups in Native Americans were brought to the New World before the last glacial maximum.
Most recently, a fifth founding haplogroup, genetically linked to the rare European haplogroup X, has been identified among Native Americans. While present in Native Americans, haplogroup X has not been detected in any East Asian or Siberian population — the putative source populations for ancestral Native Americans — in whom haplogroups A, C and D commonly occur. It has, however, been observed at low frequencies in a number of European, Middle Eastern, and West Asian groups, suggesting it arose somewhere in this general region.
In addition, haplogroup X is found primarily in North America, whereas the other four haplogroups are found throughout the Americas, albeit at various frequencies. Furthermore, haplogroup X appears to be a relatively ancient arrival, at 30,000 to 15,000 years ago, in the New World.
Collectively, the evidence suggests that haplogroup X originated in a region outside of eastern Siberia and was somehow brought to the Americas, where it became part of the genetic makeup of Paleoindian populations. And it argues against a single migration from southeastern Siberia bearing these mtDNA lineages to the New World; the data suggest instead that the haplogroups present in today’s Native Americans arrived with several human migrations that originated in different areas.
The mtDNA data also revealed strong evidence for a post-Ice Age re-expansion of ancient Beringian populations into northern North America. One mutation within haplogroup A (16111T) marks the emergence of these ancestral Beringian populations that gave rise to Native American groups. After the initial occupation of the New World, however, these populations in Beringia became isolated from the Paleoindian groups in the south, perhaps by expanding glaciers. This isolation lasted long enough for the Beringians to diverge genetically, including the appearance of a distinct haplogroup A mutation (16192T).
This genetic marker is found only among the Koryaks, Chukchi, and Eskimos in Siberia and the northwestern tip of North America, and the Na-Dene Indians, including the Navajo in the American Southwest.
Research into both mtDNA and Y chromosome markers is greatly enriching the story of the first Americans. Additional work on these and other genetic systems will likely further refine the story, perhaps revealing new and exciting details about the peopling of the New World.
THEODORE G. SCHURR is at the Department of Genetics at the Southwest Foundation for Biomedical Research in San Antonio, Texas.
by Dennis Stanford and Bruce Bradley
For half a century, archaeologists have assumed that ancestors of the Clovis people — long considered the first Americans — crossed the Bering Land Bridge from northeast Asia some 12,000 radiocarbon years ago (14,000 calendar years BP), then spread southward across the continent.
Solutrean seafarers may have followed the edge of an icesheet that covered the North Atlantic during the last Ice Age to reach the coast of North America. Years of research in eastern Asia and Alaska have produced little evidence of any historical or technological connection between the Asian Paleolithic (Stone Age) and Clovis peoples. Also, the southeastern United States has produced more Clovis sites than the West, and a few radiocarbon dates suggest some of them may predate those in the western states. If correct, that hardly fits the notion that Clovis technology originated in northeast Asia or Alaska.
Over the years, various scholars have noted similarities between Clovis projectile points and “Solutrean” points, the product of a Paleolithic culture on the north coast of Spain between 22,000 and 16,500 years ago. Little credence has been given to suggestions of a direct connection between these technologies because of the 4,500-year time gap between the last of Solutrean and the first of Clovis, and because of doubts that people of the Upper Paleolithic could navigate the Atlantic Ocean.
But indirect evidence for Paleolithic ocean travel has been mounting. Although no boats have been found, we now know that by at least 40,000 years ago, watercraft carried a founding population to Australia. By 28,000 years ago, flintknappers were collecting raw materials from islands far off the Japanese coast. And closer to Spain, Paleolithic peoples inhabited some of the Mediterranean islands at least 14,000 years ago.
Solutrean peoples could have used this knowledge of watercraft to travel and exploit marine resources, which would have been especially important during the last glacial maximum, about 18,000 years ago, when most of Europe was covered with ice and competition for diminishing land resources must have been intense. Given these facts, we believe the hypothesis of a western Old World ancestry for Clovis should be reconsidered.
To determine whether the idea was worth additional study, we examined archaeological collections in Spain, France, and Portugal, looking for technological affinities between the European Upper Paleolithic and Clovis. Our cursory examination revealed an amazing correspondence between Solutrean and Clovis; in fact, Solutrean has more in common with Clovis than with Paleolithic technologies that followed it in Europe.
Solutrean and Clovis flintknappers used nearly identical stoneworking technologies. We observed a high degree of correspondence between stone and bone tools, as well as engraved limestone tablets, and caching of extra large bifaces and other tool stock. The Solutrean toolkit is, with a few exceptions, nearly identical to that of Clovis. Although some of the Solutrean concave-base projectile points are heavily thinned, none that we saw exhibited a well-developed Clovis-style flute. Clovis assemblages lack shouldered points and the Solutrean laurel-leaf knife.
A Solutrean origin for the Clovis culture seems a more parsimonious explanation of the evidence than an Asian ancestry. Certainly, if Solutrean industries were found in Siberia, no one would question their historical relationship with Clovis.
The ultimate test of this hypothesis may be found in genetic research on ancient human remains. Michael Brown and colleagues reported in 1998 that mitochrondrial-DNA haplogroup X (a genetic marker of population groups) is found in low frequencies in both European and Native American populations, but not among Asians. This indicated to them that some of the American founders may have come from Europe between 36,000 and 12,000 years ago.
Regardless of whether a Solutrean-Clovis link is eventually proven, exploring this hypothesis should increase our understanding of the development of technological innovations and broaden our knowledge of early peoples of the New World.
DENNIS STANFORD is Chairman of the Anthropology Department at the National Museum of Natural History, Smithsonian Institution.
BRUCE BRADLEY is President of Primitive Tech Enterprises, Inc., in Cortez, Colorado, and Adjunct Professor at Augustana College in Sioux Falls, South Dakota.