Dennis H. O'Rourke

The Late Pleistocene Dispersal of Modern Humans in the Americas

When did humans colonize the Americas? From where did they come and what routes did they take? These questions have gripped scientists for decades, but until recently answers have proven difficult to find. Current genetic evidence implies dispersal from a single Siberian population toward the Bering Land Bridge no earlier than about 30,000 years ago (and possibly after 22,000 years ago), then migration from Beringia to the Americas sometime after 16,500 years ago. The archaeological records of Siberia and Beringia generally support these findings, as do archaeological sites in North and South America dating to as early as 15,000 years ago. If this is the time of colonization, geological data from western Canada suggest that humans dispersed along the recently deglaciated Pacific coastline.

The Human Genetic History of the Americas: The Final Frontier

The Americas, the last continents to be entered by modern humans, were colonized during the late Pleistocene via a land bridge across what is now the Bering strait. However, the timing and nature of the initial colonization events remain contentious. The Asian origin of the earliest Americans has been amply established by numerous classical marker studies of the mid-twentieth century. More recently, mtDNA sequences, Y-chromosome and autosomal marker studies have provided a higher level of resolution in confirming the Asian origin of indigenous Americans and provided more precise time estimates for the emergence of Native Americans. But these data raise many additional questions regarding source populations, number and size of colonizing groups and the points of entry to the Americas. Rapidly accumulating molecular data from populations throughout the Americas, increased use of demographic models to test alternative colonization scenarios, and evaluation of the concordance of archaeological, paleoenvironmental and genetic data provide optimism for a fuller understanding of the initial colonization of the Americas.

Context of maternal lineages in the greater Southwest

We present mitochondrial haplogroup characterizations of the prehistoric Anasazi of the United States (US) Southwest. These data are part of a long-term project to characterize ancient Great Basin and US Southwest samples for mitochondrial DNA (mtDNA) diversity. Three restriction site polymorphisms (RSPs) and one length polymorphism identify four common Native American matrilines (A, B, C, and D). The Anasazi (n = 27) are shown to have a moderate frequency of haplogroup A (22%), a high frequency of haplogroup B (56%), and a low frequency of C (15%). Haplogroup D has not yet been detected among the Anasazi. In comparison to modern Native American groups from the US Southwest, the Anasazi are shown to have a distribution of haplogroups similar to the frequency pattern exhibited by modern Pueblo groups. A principal component analysis also clusters the Anasazi with some modern (Pueblo) Southwestern populations, and away from other modern (Athapaskan speaking) Southwestern populations. The Anasazi are also shown to have a significantly different distribution of the four haplogroups as compared to the eastern Great Basin Great Salt Lake Fremont (n = 32), although both groups cluster together in a principal component analysis. The context of our data suggests substantial stability within the US Southwest, even in the face of the serious cultural and biological disruption caused by colonization of the region by European settlers. We conclude that although sample numbers are fairly low, ancient DNA (aDNA) data are useful for assessing long-term populational affinities and for discerning regional population structure.

Ancient DNA perspectives on American colonization and population history.

Ancient DNA (aDNA) analyses have proven to be important tools in understanding human population dispersals, settlement patterns, interactions between prehistoric populations, and the development of regional population histories. Here, we review the published results of sixty-three human populations from throughout the Americas and compare the levels of diversity and geographic patterns of variation in the ancient samples with contemporary genetic variation in the Americas in order to investigate the evolution of the Native American gene pool over time. Our analysis of mitochondrial haplogroup frequencies and prehistoric population genetic diversity presents a complex evolutionary picture. Although the broad genetic structure of American prehistoric populations appears to have been established relatively early, we nevertheless identify examples of genetic discontinuity over time in select regions. We discuss the implications this finding may have for our interpretation of the genetic evidence for the initial colonization of the Americas and its subsequent population history.

Geographic distribution of consanguinity in Europe

Average consanguinity (alpha) of 20 large regional or national samples derived from Roman Catholic Dispensations or state archives shows a strong negative relationship with latitude (r = -0.738; P less than 0.001) which seems best explained by cultural-historical factors. Isolates show no such pattern and are more variable in their inbreeding levels. Two-way analysis of variance shows the sample division by population size to be significant, the division by geography to be not significant, with no significant interaction between the two factors. In a one-way analysis of variance only the mainstream samples showed significant geographic differences (P less than 0.001). There is no such geographic association in Japan which has greater cultural and historical homogeneity. The clinical patterning in average consanguinity may bias frequency estimates of rare alleles, including genetic disorders.

Complete Columbian mammoth mitogenome suggests interbreeding with woolly mammoths

BackgroundLate Pleistocene North America hosted at least two divergent and ecologically distinct species of mammoth: the periglacial woolly mammoth (Mammuthus primigenius) and the subglacial Columbian mammoth (Mammuthus columbi). To date, mammoth genetic research has been entirely restricted to woolly mammoths, rendering their genetic evolution difficult to contextualize within broader Pleistocene paleoecology and biogeography. Here, we take an interspecific approach to clarifying mammoth phylogeny by targeting Columbian mammoth remains for mitogenomic sequencing.ResultsWe sequenced the first complete mitochondrial genome of a classic Columbian mammoth, as well as the first complete mitochondrial genome of a North American woolly mammoth. Somewhat contrary to conventional paleontological models, which posit that the two species were highly divergent, the M. columbi mitogenome we obtained falls securely within a subclade of endemic North American M. primigenius.ConclusionsThough limited, our data suggest that the two species interbred at some point in their evolutionary histories. One potential explanation is that woolly mammoth haplotypes entered Columbian mammoth populations via introgression at subglacial ecotones, a scenario with compelling parallels in extant elephants and consistent with certain regional paleontological observations. This highlights the need for multi-genomic data to sufficiently characterize mammoth evolutionary history. Our results demonstrate that the use of next-generation sequencing technologies holds promise in obtaining such data, even from non-cave, non-permafrost Pleistocene depositional contexts.

Ancient DNA: Methods, progress, and perspectives

The advent of the polymerase chain reaction as a standard molecular genetic technique and the demonstration that nucleic acids are routinely preserved in prehistoric material have led to a dramatic increase in molecular approaches to archaeological problems. These genetic approaches to long‐standing problems in prehistory hold considerable promise to clarify issues of population origins, migrations, and settlement patterns, as well as ancestor/descendant relationships. The evolving methods for manipulating and analyzing ancient DNA (aDNA) are reviewed here, as are more recent applications of these methods to anthropologically relevant samples. In addition, new preliminary material is presented on mtDNA variation in Anasazi samples from the U.S. Southwest. The initial samples analyzed indicate similarity to contemporary populations of the Greater Southwest, as evidenced by the modest frequency of a 9bp deletion in Region V of the mtDNA molecule, and the possible absence of haplogroup D.

Patterns and correlates of genetic variation in South Amerindians

Gene frequency data from six polymorphic blood group systems in 70 South American Indian populations are used to derive synthetic gene frequency maps that document the geographical pattern of genetic variation. Additional analyses are directed toward the elucidation of mechanisms that give rise to or maintain the observed distributions. Variables of local ecology do not appear to explain gene frequency distributions in South America. Instead, local isolation and the action of stochastic forces appears to be the most parsimonious explanation of the observed geographical patterns. This is distinctly different from the geographical patterns of genetic variation seen in other continents.

Out of Beringia

A shrub tundra refugium on the Bering land bridge may have played a pivotal role in the peopling of the Americas. Based on the distribution of tundra plants around the Bering Strait region, Eric Hultén proposed in the 1930s that the now-submerged plain between Chukotka and Alaska—the Bering land bridge—became a refugium for shrub tundra vegetation during cold periods (1), which include the last glacial maximum (LGM) between ∼28,000 and 18,000 cal BP (calibrated radiocarbon years before the present). Adjoining areas to the west and east supported drier plant communities with a higher percentage of grasses during glacial periods. According to Hultén, when warmer and wetter conditions returned to these areas, the land bridge, which he named Beringia, became a center of dispersal for tundra plants. Now it appears that it also may have been a glacial refugium and postglacial center of dispersal for the people who first settled the Americas.

Beringia and the global dispersal of modern humans

Until recently, the settlement of the Americas seemed largely divorced from the out‐of‐Africa dispersal of anatomically modern humans, which began at least 50,000 years ago. Native Americans were thought to represent a small subset of the Eurasian population that migrated to the Western Hemisphere less than 15,000 years ago. Archeological discoveries since 2000 reveal, however, that Homo sapiens occupied the high‐latitude region between Northeast Asia and northwest North America (that is, Beringia) before 30,000 years ago and the Last Glacial Maximum (LGM). The settlement of Beringia now appears to have been part of modern human dispersal in northern Eurasia. A 2007 model, the Beringian Standstill Hypothesis, which is based on analysis of mitochondrial DNA (mtDNA) in living people, derives Native Americans from a population that occupied Beringia during the LGM. The model suggests a parallel between ancestral Native Americans and modern human populations that retreated to refugia in other parts of the world during the arid LGM. It is supported by evidence of comparatively mild climates and rich biota in south‐central Beringia at this time (30,000‐15,000 years ago). These and other developments suggest that the settlement of the Americas may be integrated with the global dispersal of modern humans.