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E. James Dixon
University of Colorado
Geography during the last Ice Age limited possible migration routes available to the first humans to colonize the Americas. The preponderance of linguistic and biological evidence indicates that Native Americans most likely originated somewhere in northeastern Asia. Two possible routes have been identified for the first humans to enter the Americas from Northeast Asia: by watercraft along the Northwest Coast, or by a pedestrian terrestrial route across the Bering Land Bridge and then south through central-western Canada. There is a growing body of evidence suggesting that the mostplausible route for the initial colonization of the Americas may have been along the Northwest Coast, beginning possibly as early as 14,000 radiocarbon years ago (16,300 calendar years BP).
Evaluating the evidence for the first human colonization of the Americas rests on dating geological events, human remains, artifacts, and other archaeological evidence. In some publications radiocarbon dates are used, and in other articles dates are recalibrated to indicate calendar years before the present (cal BP). When radiocarbon dates are calibrated to calendar years it is apparent that there is more time for human colonization and settlement than is suggested by using only the radiocarbon time scale. For consistency both dates are presented in this article. Because it is difficult to calibrate radiocarbon determinations older than about 15,000 years, these dates are reported only as radiocarbon dates.
The Continental Route: Traditionally scientists have believed that the first humans to colonize the Americas arrived during the last Ice Age, or Pleistocene, via the Bering Land Bridge. The climate was much colder then, and massive glaciers in eastern and western Canada formed a huge ice sheet covering most of the country. The ice blocked access between what is today Alaska and the contiguous United States. Polar sea ice extended south into the Atlantic, covering Greenland, Iceland, and all but the southernmost areas of Ireland and England (fig.1). Because much of the earths water was trapped in glacial ice, sea level was lower. The continental shelves and the floor of the Bering and Chukchi Seas were exposed, creating the Bering Land Bridge, which, along with adjacent regions of Siberia and North America, formed a geographic area known as Beringia.
The idea that humans may have first entered the Americas via a land route in the high northern latitudes was first suggested by a Spanish priest more than 400 years ago and over time this concept has become deeply rooted in scientific thought. It is believed that these early colonists were adapted to the interior environments of Asia and hunted large Ice Age animals, including the extinct woolly mammoth, as well as other species that did not become extinct such as bison. According to this theory, after these early people had crossed the Land Bridge into Alaska they then moved south into central-western Canada and from there to more southern areas of North America and eventually into South America.
During the height of the last Ice Age, massive glaciers covered much of northern North America. The first humans to enter the region would have been confronted by a seemingly endless icescape reaching from Canadas Yukon Territory in the north to the Great Lakes in the south. The continental glaciers would have blocked human migration southward until the ice melted sufficiently to enable plants and animals to colonize the deglaciated land.
Several researchers have hypothesized that an ice-free corridor may have existed in the region of central-western Canada. They further theorized that this hypothetical corridor could have provided an avenue through which humans may have passed from Beringia to the more southern regions of the continent prior to the end of the last Ice Age. This ice-free corridor concept was used to explain the discovery of archaeological evidence south of the continental glaciers believed to be older than 11,500 BP (13,050 cal BP) and predating the melting of the glaciers.
[Fig.1a,b: Maps of the Western Hemisphere, illustrating the hypothetical coastal migration route that may have been open to human colonists using watercraft possibly as early as14,000 radiocarbon years ago (16,250 cal BP).]
Exciting new discoveries are causing some researchers to question many of these traditional concepts. For example, geologists working in Canada have recently demonstrated that the ice-free corridor did not exist, and that connections between eastern Beringia and areas south of the continental glaciers probably were not established until about 11,000 BP (12,550 cal BP). Radiocarbon dates previously used to support the existence of an ice-free corridor or that suggest early deglaciation of central Canada have proven inaccurate (MacDonald 1987). The distribution of quartzite erratics (large rocks transported by glaciers) derived near the headwaters of Albertas Athabasca River (Jackson et al. 1997) demonstrates the coalescence of the Cordilleran and Laurentide ice over an extensive area in southern Alberta (Jackson et al. 1996) and that the coalesced continental glaciers did not melt in southern Alberta until sometime about 12,000-11,000 BP (13,600-12,550 cal BP) (Jackson et al. 1996:223).
Recent studies demonstrate that Beringia and the unglaciated areas of North America remained separated by the continental glacier until about 11,000 BP (12,550 cal BP) when the glaciers had melted enough to enable people to move from eastern Beringia southward into the lower latitudes of North America (Rutter 1984, Clague et al. 1989, Jackson and Duk-Rodkin 1996, Jackson et al. 1997). Because the ice sheets blocked this southern route, the North American continent south of the glaciers could not have been colonized by humans on foot until sometime about 11,000 BP (12,550 cal BP). Support for this conclusion has been presented by paleontologists working in central Canada. They have found no animal bones dating between about 21,000 to 11,500 BP (until 13,050 cal BP) in the region formerly believed to be the ice-free corridor (Burns 1996). This demonstrates fairly conclusively that the ice-free corridor did not exist and that the glaciers prevented the area from being inhabited by animals during the last Ice Age. This evidence precludes a mid-continental route for human entry before about 11,500 to 11,000 BP (13,050-12,550 cal BP).
While this geological research has been taking place in Canada, important archaeological evidence has been accumulating at sites across North and South America. There are numerous sites throughout the Americas believed by some researchers to be older than 11,500 BP (fig.1). While the age and cultural origin of these sites are considered controversial by some researchers, this growing body of data strongly supports the concept that humans were widespread throughout the Americas prior to the melting of the continental glaciers approximately 11,500-11,000 BP (13,100-12,550 cal BP).
Human Remains: One way to evaluate the timing of the first human colonization of the Americas is to analyze the ages of the oldest reliably dated human skeletal remains from North America. The three oldest sets of well dated human remains from North America are from Fishbone Cave in western Nevada; Arlington Springs on Santa Rosa Island off the coast of California; and the Anzick site in Montana. Radiocarbon dates of 11,555 +/- 500 BP (13,540 cal BP) and 10,900 +/- 300 BP (12,900 cal BP) were reported from a stratigraphic level at Fishbone Cave containing the burned bones of a human foot, clavicle (collar bone), and a fibula (lower leg bone). On Santa Rosa Island human bones have recently been redated by Johnson and Stafford to 10,970 +/- 80 BP (12,980 cal BP) (Johnson et al. 2000). Bone sampled from a human cranium stained with red ochre found at the Anzick rock shelter in Montana has two somewhat conflicting radiocarbon dates, one of 10,680 +/- 50 BP (12,840 cal BP) and another of 11,550 +/- 60 BP (13,470 cal BP).
The early radiocarbon dates from all three individuals demonstrate that humans had arrived in North America by about 11,500-11,000 BP (13,050-12,550 cal BP). By this time human population density in North America had achieved a level sufficient to assure the survival and discovery of fossil remains over broad geographic areas. It is unlikely that these examples represent the remains of North Americas earliest inhabitants, because the initial human population was probably very small. As the number of individuals increased over time, the chances of discovering their remains increase. It is extremely unlikely that the remains of the very first inhabitants of North America will ever be found. Radiocarbon dated human skeletal materials provide limiting dates for the arrival of humans in the Americas sometime prior to circa 11,500 BP (13,050 cal BP), with no reliably dated human remains from the Americas before then. This and other evidence suggests humans may have first arrived in the Americas no more than approximately 14,000 BP (16,250 cal BP) or 15,000 BP (16,950 cal BP), near the end of the last Ice Age.
Coastal Migration Hypothesis: If the first Americans could not walk across the continental glaciers south into the New World, how could they have gotten there prior to the melting of the ice sheets? A few researchers (Fladmark 1979, 1983, Dixon 1993, Gruhn 1994) suggest that the earliest human migration to North America may have occurred with the use of watercraft along the southern margin of Beringia and then southward along the northwest coast of North America (fig.1.) This would have enabled humans to enter southern areas of the Americas prior to the melting of the mid-continental glaciers. Some scientists believe migration from Beringia into the Americas may have begun as early as 50,000 BP, or possibly earlier, while others suggest it may have been as late as 12,000-11,000 BP (13,550-12,550 cal BP).
The coastal migration hypothesis has gained increased support in recent years, because the remains of black and brown bears and other terrestrial animals have been found in Southeast Alaska dating between 12,500 and 10,000 BP (14,150-10,900 cal BP) (Heaton 1995, 1996, Heaton and Grady 1993, Heaton et al. 1996). Because the diets of the large omnivores are very similar to the diets of humans, these discoveries demonstrate that sufficient subsistence resources were available to support people. The remains of land and sea mammals including caribou, seals, birds, and fish dating to this time have also been discovered along the Northwest Coast. These finds provide additional evidence that there were ample resources for people to survive.
Prior to the early 1970s it had been assumed that the Cordilleran ice extended westward to the margins of the continental shelf (Coulter et al. 1965, Nasmith 1970, Prest 1969) thus creating a barrier to human migration. More recently, geologists working in Southeast Alaska and British Columbia have discovered that deglaciation along the Northwest Coast of North America had begun by about 14,000 BP (16,800 cal BP) and was sufficiently advanced to enable humans using watercraft to colonize coastal areas by 13,000 BP (15,350 cal BP).
Geologic and paleoecological studies document deglaciation and the existence of ice-free areas throughout major coastal regions of British Columbia by about 13,000 BP (14,850 cal BP) (Blaise et al. 1990, Bobrowsky et al. 1990). It is now clear that areas of continental shelf and offshore islands were not covered by ice during and toward the end of the last glacial period. Vast coastal stretches may have been deglaciated beginning about 16,000 BP. Except for a 400 km coastal area between southwest British Columbia and Washington State, the Northwest Coast of North America was largely free of ice by about 16,000 BP (Mann and Peteet 1994). The exposed continental shelf and offshore islands were available as a migration route between 13,500 and 9,500 BP (14,850-10,150 cal BP) (Josenhans et al. 1997), possibly enabling people to colonize ice-free regions along the continental shelf exposed by lower sea level.
[Fig.2: Microblades characteristic of the Northwest Coast Microblade tradition found at 49-PET-408, Prince of Wales Island, Southeast Alaska (photo: E. James Dixon).]
The earliest archaeological evidence is represented by a group of artifacts, called the Northwest Coast Microblade tradition, found at a number of archaeological sites along the Northwest Coast (fig.2). The Northwest Coast Microblade tradition is documented as early as circa 9,500 BP (10,150 cal BP) in British Columbia and Southeast Alaska. It has not been determined, however, when this tradition first appeared along the Northwest Coast. Rising sea level inundated most coastal sites older than ca. 9,500 BP (10,150 cal BP).
Archaeological sites ascribed to this tradition share the use of microblades, small parallel-sided stone artifacts used for a variety of specialized purposes (fig.4). These sites exhibit a marine economy documented by limited terrestrial faunal remains and ecological settings on ancient beaches, and other geographic features oriented to exploiting marine resources. Northwest Coast Microblade tradition subsistence practices were adapted to an environment characterized by year round open water; rugged coast with fjords, islands, and rocky headlands; calving glaciers; major salmon runs; and intertidal shellfish. These complex traits were probably preceded by a long developmental process, suggesting that regional adaptation began much earlier.
Because previous geologic interpretations indicated that the region had been entirely glaciated until about 10,000 BP (10,900 cal BP), very little archaeological work has been undertaken to explore this region as a possible migration route. archaeological evidence necessary to evaluate the coastal migration hypothesis is difficult to detect because rising sea level at the close of the Pleistocene inundated much of the continental shelf. Some pioneering efforts are under way, however, including archaeological and paleontological excavations at an important cave site known as 49-PET-408, on Prince of Wales Island in Southeast Alaska (figs.5-9).
Site 49-PET-408: Excavations at 49-PET-408 (On Your Knees Cave) provide important new insights into the character and timing of the climatic transition at the end of the Pleistocene, as well as the human response to this climate change. The objectives of the interdisciplinary research program developed from cooperation between Native people, scientists, and federal resource managers. This synergy has led to the discovery of new evidence important to evaluating the Northwest Coast migration hypothesis and supports the concept that the first human colonization of the Americas at the end of the last Ice Age may have occurred with the use of watercraft along the north Pacific Rim.
The most extensive use of the cave occurred between approximately 9,500 and 8,500 BP (10,150-9,050 cal BP). This stratigraphic level contains microblades made of obsidian and other exotic types of stone, bifaces (fig.7), flakes, cores, and other tools.
The oldest human skeletal remains known from Alaska or Canada are also associated with this level. The partial remains of an adult male have been found scattered throughout the cave. The pelvis, or hip, has been dated to 9,880 +/- 50 BP (11,230 cal BP) and the mandible, or jaw, to 9,730 +/- 60 BP (11,150 cal BP) (Dixon et al. 1997, Dixon 1999). Isotopic analysis demonstrates that the individual was raised on a diet of marine foods and that the radiocarbon age should be adjusted to about 9,200 radiocarbon years based on the regional marine carbon reservoir (Josenhans et al. 1997). Radiocarbon dates suggest that the human remains are contemporaneous with the oldest microblades, bifaces, and other stone tools recovered from the cave.
Trace element analysis of some of the artifacts made from volcanic glass, or obsidian, demonstrates that the obsidian came from at least two different sources, Mount Edziza on the British Columbia mainland and Sumez Island in Southeast Alaska (Lee 2001) (fig.3). Because Prince of Wales Island was not connected to the mainland at the time the cave was occupied, it could only have reached the site by being transported by humans using watercraft.
[Fig.3: Map depicting the location of 49-PET-408 on Prince of Wales Island and obsidian sources on Sumez Island and Mt. Edziza (Eric Parrish).]
There is additional tantalizing information from the cave. An undated stratigraphic unit underlying the level containing the microblades and other artifacts contains a few stone flakes. This could be possible evidence of an earlier human occupation. A tool made from the bone of a terrestrial mammal found in an interior chamber of the cave has been radiocarbon dated by the AMS method (accelerator mass spectrometry, providing accurate dates using smaller samples) to 10,300 +/- 50 BP (12,070 cal BP) and is the oldest reliably dated artifact from the Northwest Coast of North America (fig.4).
The research at 49-PET-408 suggests that humans were adapted to the maritime environments of the Northwest Coast by ca. 9,500 BP (10,150 cal BP), and probably much earlier. Collectively, the data from this important site indicate that humans along the Northwest Coast were experienced coastal navigators with an economy based on maritime subsistence and established trade networks for obsidian by 10,000-9,000 BP (10,900-9,600 cal BP). This further implies even earlier human occupation in order to establish this broad regional adaptation by circa 10,000 BP (10,900 cal BP).
Other Discoveries: Additional evidence of early maritime adaptation and the use of watercraft is beginning to accumulate from a variety of sites along the west coast of the Americas. A basalt blade-like artifact was recovered from an underwater site on the continental shelf of British Columbia. Based on the local rise in sea level at the end of the last Ice Age, this locale would probably have been covered by the rising ocean about or shortly before 10,000 BP (10,900 cal BP), thus suggesting the site may have been occupied prior to that time (Fedje, this issue, p.28).
Further south along the coast of California, Arlington Man, found on Santa Rosa Island, provides the earliest evidence for the use of watercraft in North America, at approximately 11,000 BP (12,550 cal BP). As Santa Rosa Island was not connected to mainland North America during the last Ice Age, the only plausible way this individual could have reached it was with the use of watercraft, or to have been the descendent of individuals who arrived by watercraft (Erlandson 1994:183).
[Fig.4: Bone tool found in a chamber of cave site 49-PET-408 dated by accelerator mass spectrometry (AMS) to 10,300 radiocarbon years before present, which documents late Pleistocene use of the cave (photo: E. James Dixon).]
The coastal migration hypothesis is also supported by evidence from several archaeological sites discovered along the west coast of South America (fig.1b). At the Las Vegas site in southwestern Ecuador, marine fish and shellfish have been recovered from a habitation site dating between 10,840 and 10,100 BP (circa 11,750 cal BP) (Stothert 1998, Wisner 1999). Further south at Quebrada Tachahuay, Peru, located less than 0.5 km (0.3 mi) from the Pacific Ocean, a human occupation dating between 11,000 BP (12,550 cal BP) and 10,500 BP (11,750 cal BP) contains the remains of fish, marine birds, and stone tools (Keefer et al. 1998). At another Peruvian site, Quebrada Jaguay, there are remains of marine fish, shells, cordage (possibly fishnet), stone flakes, and tools (Sandweiss et al. 1998). At the time the site was occupied, it would have been located approximately 7 or 8 km (4 or 5 mi) from the ocean. Collectively these sites are beginning to reveal a glimpse of very early coastal adaptations along the Pacific coast of the Americas at the end of the last Ice Age.
Summary: There is a growing body of data that suggests the first human colonization of the Americas could have occurred along the west coast of the Americas with the use of watercraft, possibly skin boats, near the end of the last Ice Age, perhaps as early as 14,000 BP (16,250 cal BP). This hypothesis is supported by recent archaeological discoveries along the west coast of the Americas that suggest a widespread pattern of maritime adaptation and coastal navigation as early as 11,000 to 10,000 BP (12,550-10,900 cal BP). If the theory is correct, the oldest archaeological sites in the Americas should be located along the western coast.
The interior ice-free corridor model for human colonization requires an economy based on hunting terrestrial mammals, fresh water fishing, and pedestrian travel. By contrast, the coastal hypothesis suggests an economy based on marine mammal hunting, salt water fishing/shellfish gathering, and the use of watercraft. It is important to address these hypotheses because each requires different types of adaptations by the New World founding population. Understanding the timing and processes of colonization of the Americas is important because the cultural adaptations of the New Worlds first populations established the foundation for all subsequent cultural development and for the rich and diverse cultures that followed. If the coastal colonization hypothesis is to be fully evaluated, the late Pleistocene coastal archaeology of western North America will require additional and sustained research efforts equivalent to those that have traditionally focused on mid-continental North America.
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