Out of Africa (or not) The incredibly complex and mostly true story of the origin and dispersal of Homo sapiens throughout the World.

There are currently two competing theories of the origin of anatomically modern Homo sapiens

The Replacement model, which states that there was a single origin for Homo sapiens (in Africa), and these anatomically modern humans subsequently radiated out from Africa and replaced other species of Homo as they came in contact with them throughout Europe and Asia.

The Regional Continuity model, which states that modern Homo sapiens developed from regional populations of archaic Homo sapiens populations that in turn evolved from regional populations of Homo erectus.

The replacement model of Christopher Stringer and Peter Andrews proposes that modern humans evolved from archaic Homo sapiens 200, ,000 years ago in Africa and then some of them migrated into the rest of the Old World replacing all of the Neanderthals and other late archaic Homo sapiens. If this interpretation of the fossil record is correct, all modern people share relatively modern African ancestry. All other lines of humans that had descended from Homo erectus presumably became extinct. From this view, the regional anatomical differences that we see among humans today are recent developments--evolving only in the last 50,000-40,000 years. This hypothesis is also referred to as the out of Africa and the Noah's ark model.

The regional continuity (or multiregional) model of Milford Wolpoff at the University of Michigan proposes that modern humans evolved more or less simultaneously in all major regions of the Old World from local archaic Homo sapiens populations. For example, modern Chinese are seen as having evolved from Chinese archaic Homo sapiens and ultimately from Chinese Homo erectus. This would mean that the Chinese and some other peoples in the Old World have great antiquity in place. Advocates of this model believe that the ultimate common ancestor of all humans was Homo erectus in Africa more than a million years ago. Since then, however, it is proposed that there was sufficient gene flow between Europe, Africa, and Asia to prevent reproductive isolation and the subsequent evolution of distinct regional species.

Regional Continuity Model Replacement Model

Obviously, whether you believe in one theory or the other has a great impact on the question of early dispersal and migration of Homo sapiens. The replacement model necessitates long distance dispersal and migration and adaptation to new environments, and the multi-regional model suggests in situ development and adaptation over a longer period of time.

The following figure is a family tree for the phylogenetic Class Primates. Although somewhat out of date, it does give an idea of the closeness of genetic relationships among the different families and the approximate timing of their divergence from the trunk of the primate family tree. Although there is at least one big problem with this particular diagram that might cause a skeptic to question the veracity of the whole thing. See if you can spot the problem…

(Hominidea) (Hominoidea)

Yikes! Showing Modern humans
as different branches is not accurate--we’re all the same species Yikes!

Dispersal corridors opened out of Africa and across the Middle East into South and East Asia during the late Pliocene. Corridors formed primarily along coastal land masses, the product of expanding polar ice caps and resultant drop in sea-level. Incipient development of the Red Sea rift also established departure routes through the Middle East. The drop in sea level in island Southeast Asia would have connected Sumatra, Java and Borneo with the mainland. Evolutionary analysis of fossil species indicates that large mammals dispersed along these routes sporadically during the late Pliocene and early Pleistocene. New fossil finds suggest that early Homo arrived in Asia some 1.8 to 1.9 million years ago, after departing Africa at least 100,000 years earlier.

Hominids now known as Homo erectus were found on Java, Indonesia, in 1891, and at Zhoukoudian, near Beijing, in the 1930s. As Homo erectus was clearly more primitive than hominid fossils known in Europe, human beings were initially thought to have emerged in East Asia and dispersed westward. Since the early 1960s, numerous fossils from African localities in the eastern Rift Valley, Lake Malawi and South Africa have demonstrated an African emergence for Homo . In the 1990s, advances in dating methods and new finds at Dmanisi (Georgia), Riwat and Pabbi Hills (Pakistan), Sangrian and Mojokerto (Java) and Longgupo (China) show that early Homo had arrived in East Asia by just after 2 million years ago. The following map shows dispersal corridors that would have been available due to lowered sea level in the at the Pliocene-Pleistocene boundary.

According to the Replacement Model, there is little question that Homo sapiens emerged in Africa, although the date of emergence, the technological associations and the dates for its Eurasian dispersals are debatable. Homo sapiens originated between 200,000 and 100,000 years ago, somewhere in sub-Saharan Africa. Some recent discoveries in Zaire of ancient and finely crafted tool types (such as barbed-bone harpoons) indicate that the technology associated with this emergence may have been very advanced indeed, resembling the much later Upper Paleolithic of Europe. In the Levant, where Homo sapiens is evident about 90,000 years ago, a more archaic Middle Paleolithic technology still held sway. Consequently, we may not yet say whether the European dispersal of Homo sapiens was associated with either its emergence or a new technology.

Genetic Evidence for the Replacement Model
Mitochondrial DNA Y chromosomal DNA

Mitochondrial DNA Mitochondrial DNA offers a quick-ticking molecular clock and by comparing the number of mutations that have collected in separate populations, geneticists can infer when the populations split from each other. When selected sequences of mtDNA from a group of people representing African, Asian, Australian, Caucasian and New Guinean ethnic groups were compared, 133 variants of mtDNA were found. When these different mitochondrial types were arranged into an evolutionary tree, that tree showed a trunk splitting into two major branches.

Mitochondrial DNA One branch consisted only of Africans, the other included some modern Africans and some people from everywhere else. The first branch represents the first modern humans and forms the trunk and longest branch of the tree. The second branch represents a subgroup of modern humans that left Africa and later spread out to the rest of the world.

Mitochondrial DNA It was also found that all of the mtDNA (even from far regions of the world) was similar. This suggested that the molecular clock has not been ticking long enough to accumulate appreciable differences in our DNA. In other words, our species is young. But the African samples had the most mutations. This too implied that the African lineage is the oldest, that all modern humans trace their roots back to Africa.

Map illustrating the timing of the migration of anatomically modern Homo sapiens out of Africa, based on mtDNA evidence.

Y chromosomal DNA An international study of Y chromosomal DNA shows that East Asian populations migrated out of Africa and suggests that little or no interbreeding of Homo erectus and Homo sapiens occurred after the migration. The goal of the study was to test the hypothesis that the common origin of human populations is in Africa, and also to see if there was evidence of archaic admixture of Homo erectus and Homo sapiens.

Y chromosomal DNA The researchers tested 12,127 male individuals from 163 East Asian populations. The Y chromosome was used because it remains the same when passed from father to son. The Y chromosome is was examined because it does not recombine, and so a lot more evolutionary information is available than is found in mitochondrial DNA. Researchers from China, Indonesia, England and the U.S. collected samples, genotyped the Y chromosomes and analyzed the results. They looked for specific mutations at three locations on the Y chromosome and found that every one of the 12,127 samples typed, carried one of these three polymorphisms.

Y chromosomal DNA These three markers can be used to test the completeness of the replacement of modern humans of African origin in East Asia, because finding a male not carrying one of the three polymorphisms would be indicative of a potential ancient origin and possibly leading to the rejection of complete replacement. This result indicates that modern humans of African origin completely replaced earlier populations in East Africa.

In linking the early dispersal of early Homo with its emergence, we are describing a hominid very different from the australopithecines, whose bipedal but still ape-like anatomy must have limited them to wooded locales. Thus the significance of an early dispersal to Asia is manifold. First, the climatic conditions of cool aridity that played a great role in the emergence of Homo itself also drew hominid populations out of Africa and into Asia. Emergence and dispersal are, to a great extent, a product of environmental change.

Nevertheless, early Homo emerged with a radical, yet still generalized, set of characteristics that granted it ecological hegemony across the subtropical Old World. An early intercontinental distribution signifies a hominid not adapted to specific territorial conditions, but adapted to manage many local conditions through physical presence, technology and flexible social organization. Ironically, as the first species to use technology, early Homo colonized much of the subtropical Old World without the benefit of language, symbolic culture or individual consciousness as we know it.

Critiques of the Replacement Model
Critics of this genetic argument say that the rate of mutation is not necessarily constant and that there were flaws in the computer program that was used to construct the human family trees; for instance, the results of the study varied with the order in which the data were entered. Further genetic studies carried out since the mid 1990's have both supported and undermined an African origin for modern humans. John Relethford, of the State University of New York College at Oneonta, has pointed out that Africa could have had the greatest diversity in mtDNA simply because there were more people living there during the last several hundred thousand years.

Critiques of the Replacement Model
Researchers from the University of Chicago and Yale University have discovered that variations in the DNA of the Y chromosome and chromosome 12 have the greatest diversity among Africans. This is consistent with the replacement model. However, geneticists from Oxford University have found that the human betaglobin gene is widely distributed in Asia but not in Africa. Since this gene is thought to have originated more than 200,000 years ago, it undercuts the claim that an African population of Homo sapiens sapiens replaced East Asian archaic Homo sapiens.

The Regional Continuity Model
Fossil evidence is used to support the regional continuity model. Its advocates claim that there has been a continuity of some anatomical traits from archaic Homo sapiens to modern humans in Europe and Asia. In other words, the Asian and European physical characteristics have antiquity in these regions going back over 100,000 years. They point to the fact that many Europeans have relatively heavy brow ridges reminiscent of Neandertals. Similarly, it is claimed that Chinese facial characteristics can be seen in Asian archaic Homo sapiens dating to 200,000 years ago. Like Homo erectus, East Asians today commonly have shovel-shaped incisors while Africans and Europeans rarely do. This supports the contention of direct genetic links between Asian Homo erectus and modern Asians.

Alan Thorne of the Australian National University believes that Australian aborigines share key skeletal and dental traits with people who inhabited Indonesia at least 100,000 years ago. The implication is that there was no replacement by modern humans from Africa 60,000-50,000 years ago. However, the evidence does not rule out gene flow from African populations to Europe and Asia at that time and before. David Frayer of the University of Kansas believes that a number of European fossils from the last 50,000 years have characteristics that are the result of archaic and modern Homo sapiens interbreeding.

Part of the mitochondrial DNA was extracted recently from the bones of a 60,000 year old modern Homo sapiens skeleton found in 1974 on the shores of Lake Mungo in Southeastern Australia. This is the oldest DNA that has been extracted from a human so far. Comparison of this DNA with that of nine other ancient Australian skeletons, 2 Neanderthals, and 3,453 contemporary people from around the world indicates that "Mungo Man" had a unique genetic marker. This indicates that a now lost genetic line of modern Homo sapiens existed in Australia prior to the arrival of later Australian Aborigines. This evidence provides significant support for rejecting the "out of Africa" complete replacement model of modern Homo sapiens evolution.

Alan Templeton, a geneticist at Washington University, has reported that a new computer based analysis of 10 different human DNA sequences indicate that there has been interbreeding between people living in Asia, Europe, and Africa for at least 600,000 years. These data suggest that the complete replacement model of Homo sapiens origin is incorrect. According to Templeton, "humans expanded again and again out of Africa, but these expansions resulted in interbreeding, not replacement, and thereby strengthened the genetic ties between human populations throughout the world." This view is gaining support among paleoanthropologists, but critics say that Templeton's sample is still too small to be conclusive.

Expansion Out of the Old World
Dude is carrying a spear, and his old lady is hauling a VW bug in a backpack!

The world population of modern Homo sapiens began to grow rapidly after 50,000-40,000 years ago. It was around this time they expanded their territory by migrating into new regions. Their movement into northern areas coincided with the end of a long cold period that had begun about 75,000 years ago. By 60,000 years ago, modern humans apparently moved into Australia for the first time. Around 35,000-30,000 years ago, they moved into Northeastern Siberia.

Possibly as early as 30,000 years ago and certainly by 11,500 years ago, they migrated into North America via the Bering Land Bridge (or Beringia ). That intercontinental land connection appeared between Siberia and Alaska as a result of sea levels dropping more than 300 feet during the last ice age. Until that time, all human evolution had occurred in the Old World. The rate of human population growth has continued to accelerate until now. The current world population is over six billion and intercontinental migration and gene flow are at higher levels than ever before.

A tragic consequence of human migrations into new regions of the world has been the extinction of many animal species indigenous to those areas. By 11,000 years ago, human hunters in the New World apparently had wiped out 135 species of mammals, including 3/4 of the larger ones. Most of these extinctions apparently occurred within a few hundred years. It is likely that the changing climate at the end of the last ice age was also a contributing factor.

However, the same cannot be said for the animal extinctions that occurred following the arrival of aboriginal people in Australia and Polynesians in New Zealand. In both cases, humans were instrumental in wiping out easily hunted species. Vulnerable marsupials were the main victims in Australia. In New Zealand, it was mostly large flightless birds that were driven to extinction by hunters. The Moa, a very large and apparently quite tasty flightless bird. It became extinct within centuries of the Polynesian arrival in New Zealand

Of course, the ultimate destruction of all giant flightless birds in New Zealand made this island nation safe for naked bungee jumping.

The Peopling of the New World
Early theories regarding the peopling of the New World were based on the concept of “Clovis First,” which stated that the first culture to enter the New World were big game hunters of the Clovis tradition, which first appears in the archaeological record in North America in the last part of the Pleistocene at around 11,600 years ago. Clovis hunters were thought to have crossed into North America from northeast Asia across a land bridge, which was a broad area of land up to a thousand miles wide that connected Siberia and Alaska. This land bridge, called Beringia, was exposed when sea levels were lower during the last glacial period of the Pleistocene and the shallow floor of the Bering sea was exposed.

According to the this theory, groups of people that were culturally adaptated to the cold environments of northeast Asia would have followed the herds of the large mammals (like mammoth) that they relied on for the majority of their subsistence. As these herds moved across Beringia, groups of hunters and their families would have followed them into the New World. This was a slow process that could have taken generations to accomplish. The migration to the New World was not a conscious decision, but a natural consequence of subsistence.

Studies of pollen, fossil insects and peat from cores taken from the floor of the Bering Sea indicate that Beringia was covered with tundra similar to that found in the arctic areas of modern Alaska, and dates from peat indicate that at least parts of the bridge were above water as recently as 11,000 years ago.

Archaeologists excavating the remains of a mammoth bone hut in the Ukraine. Huts of this sort have been found in eastern Europe and Asia and predate the Clovis culture, but they are a good example of the sorts of cold environment adaptations that Clovis ancestors would have possessed.

The superstructure of bones would have been covered
with mammoth hide.

Once across Beringia, the ancestors of the Clovis hunters could occupy an ice-free area in what is now Alaska. How did these people get down into the lower part of North America? Geological evidence suggests that as the glaciers began to recede at the end of the Pleistocene, an ice-free corridor opened up between the Laurentide ice sheet on the east and the Cordilleran ice sheet on the west. This corridor is supposedly how the Clovis people migrated south.

Map of Beringia showing the ice free area of Alaska and the migration route (in red) through the ice-free corridor between the Laurentide and Cordilleran ice sheets in North America. The exposed continental shelf is in light green.

The journey through the ice-free corridor was possible, though, only before 21,000 years ago and after 12,000 years ago. At about 21,000 to 19,000 years ago the Cordilleran Ice Sheet and Laurentide Ice Sheet coalesced thereby blocking the corridor and preventing passage. Although glacial ice retreated and the corridor re-opened about 18,000 years ago, the landscape was forbidding, a cold, semi-arid steppe with scant precipitation and only percent of the land bearing sparse grass and sagebrush. Until about 12,000 years ago, such harsh climate, sparse vegetation, and minimal fauna probably would not have sustained human population.

An alternative to the ice-free corridor was first suggested by archaeologist Knut Fladmark. Fladmark hypothesized that travel would have been much more rapid by boat along the coast of the exposed continental shelf. Confirmation of this hypothesis is difficult, because the archaeological sites that would document the journey are now under 80 meters or more of water. However, there are other pieces of evidence that may support this model. For instance, the distribution of native American languages had their greatest clustering and distinctiveness along the West Coast, suggesting a longer period of time to diversify, compared to languages in the interior.

The coastal resources of northeastern Asia are very similar to those of northwest North America, and once people had adapted to hunting and gathering these resources, moving along that coastline would have been easy. It would not require much invention of new technologies or adaptation to drastically different climates, even in the course of a migration of thousands of kilometers. The possibility that the colonizers used boats during their entry into the New World has also been proposed.

It would have been much easier and safer for sizable groups of migrating people, including children, pregnant women and the elderly, to move along the coast by boat. Also, the environment along the coast would have provided many more resources (i.e.. shellfish) that could have been gathered by all members of the group. By contrast, inland groups slogging across the tundra through the ice-free corridor would be dependent on the few adult, male big-game hunters in their band. During the best of times resources other than large game would not have been abundant.

Using boats would have aided greatly in the speed and safety of the coastal journey. Early use of boats has been demonstrated--boats would have been necessary for the colonization of Australia,which occurred 60,000 years ago, and there is also evidence of possible Homo erectus use of some sort of water craft to colonize islands that would not have been otherwise accessible. So it isn’t a stretch to suppose that the technology was available to the colonizers of the New World. Colonization by boat has another advantage as a mode of colonization. It does not require an ice-free corridor or a completely ice free continental shelf, and so could have occurred any time over the last 60,000 years.

Strong yet flexible skin boats that could carry more than 40 people is a technology found among the ancient, northern sea peoples of Europe, Asia and North America. Eskimo skin boats

It has also been proposed, based on technological similarities in lithic technology that the origin of the Clovis culture is due to settlement in North America by people of the Upper Paleolithic Solutrean culture (21,000-16,500 B.P.) of southern France and the Iberian Peninsula. These first settlers were participating in a hypothesized “Paleo-Arctic Maritime Tradition,” whose economy was based on the hunting of large marine mammals and fishing from hide covered boats along the glacial ice margin in the north Atlantic.

A comparison of Solutrean and Clovis flaking techniques-
The outre passe technique is when the flake scar travels the width of the biface to the opposite edge. If not executed precisely, it can destroy the tool being flaked. A comparison of Solutrean and Clovis flaking techniques- the outre passé flake. .

Some of these sailors eventually worked their way over to the New World and settled in what is now the southeastern U.S., where the concentration of Clovis sites is greatest and where the oldest Clovis sites are located. Needless to say, there is contentious debate associated with this theory.

Other Evidence for the Colonization of the New World
Dental Morphology Linguistics Mitochondrial DNA Other Genetic Data

Other Evidence Synthesis of linguistic, dental and genetic data suggest three migrations from Asia to America, with each wave leading to a separate linguistic group. Dental variation is greater in the north, and that there are three Native American dental (and parallel linguistic) clusters, Na-Dene, Aleut-Eskimo, and Amerind (Paleoamericans). The Aleut-Eskimo is the most recent, the Na-Dene (Athabaskan) the next oldest, and the Amerind the oldest.

Other Evidence There are limitations to mtDNA studies, such as the fact that molecular divergence can precede population divergence. When molecules in the mtDNA chain diverge, it only reflects when a population’s genetic composition diverges, but does not necessarily coincide with when a population became genetically isolated. Mutations evidenced today may predate divergence, and statistical change may be due more to population dynamics than temporal depth.

Other Evidence That said, most of the statistical measures based on models for mtDNA mutation rate assumptions suggest that the Amerindian colonization of the New World occurred between nineteen and seventy-eight thousand years ago. This by itself should suggest that the Clovis First theory for the peopling of the New World doesn’t hold water. But when the growing body of archaeological evidence indicating greater time depth of occupation is added to the mix, then debate of whether the New World was occupied during pre-Clovis times becomes moot.

Antiquity of New World Archaeological Sites
Pedra Furada, Brazil – 32,000 B.P. Meadowcroft Rockshelter, Pennsylvania – 16,200 B.P. Topper, South Carolina 16,000+ B.P. Cactus Hill, Virginia – 15,070 B.P. Monte Verde, Chile 14,500 B.P.

Monte Verde projectile points and a wood foreshaft

Human footprint from the 14,500 B.P. level at the Monte Verde site

Until 1997 no site was widely accepted as pre-dating the Clovis culture (11,000 to 11,500 radiocarbon years before present). That year, a blue-ribbon commission of Paleoindian specialists visited Monte Verde, a site in Chile with dates averaging 12,500, and declared it to be valid. Other possible pre-Clovis sites include Hebior and Schaefer, Cactus Hill, and Topper. Meadowcroft and Pedra Furada have also been proposed as pre-Clovis. Additional early sites include Taima-Taima, Pedra Pintada, Santa Barbara in the Channel Islands, Quebrada Tacahuay, and Quebrada Jaguay. (Map by Joe LeMonnier).

Other Evidence By various measurements of genetic distance, New World populations have more similarities to east Asian populations that they do to other populations around the world (no real surprise). However, there are some intriguing differences between the populations. Native Siberians lack one peculiar mtDNA mutation that appeared in the Amerinds 6,000 to 10,000 years ago. This particular mutation pattern is also found in aboriginal populations in Southeast Asia and in the islands of Melanesia and Polynesia. This suggests contact between these populations, but how? The route by which this gene found its way into the population is unknown. It either came across the Pacific to Central and South America or up the east coast of Asia and across the northern Pacific to Alaska and Canada.

Other Evidence There are also genetic similarities between New World populations and the indigenous Ainu of Japan, which exhibit more genetic similarities to European populations than to other Japanese or mainland Asian populations (shades of Kenniwick Man). These similarities have been interpreted variously as either representing a common origin for these two populations (more likely) or a Jomon fishing boat that was blown off course (not so much). There are even some investigators that have suggested contact with Africa, based on the cranial morphology of a skeletal population from an archaeological site in Brazil (!).

Other Evidence The one thing that should become obvious from the above discussion, is that deducing the timing and nature of migration and diffusion is never a clean cut process. The theories about the origin of Homo sapiens and the subsequent radiation of the species is still a topic that is debated, and as the above evidence demonstrates, it is unlikely that any long-term migration is the product of a unilinear process. Several different groups have colonized the The New World over time and contributed their genes to the populations that lived here, and it is just as likely that many other groups arrived over time and perished without contributing to the genetic makeup of the population.

Hypothesized prehistoric migration routes into the New World, including the ice-free corridor, the coastal route, the Solutrean entry, and the Polynesian and Australian mariners.

We’ve looked at human migration at a global scale, let’s examine a case of human migration on a smaller scale.

So, What Happened Once People Arrived in the New World?
Well, they didn’t just spread out, settle down and stay put for the next 15,000 years, THAT’S for sure.

A Late Prehistoric Example of Migration in North America: The Numic Expansion
Which offers problems no less complex or less hotly debated than those cited for the migration scenarios discussed above.

The Numic Expansion The Numa are several groups of people that speak related languages of the Uto-Aztecan Family, which arrived in the southern Sierra Nevada-Mohave Desert area from northern Mexico approximately 5000 years ago. This area (southwestern or central Great Basin) is considered to be the ancestral Numic homeland. It is from here that the Numic groups expanded north and east into the Rocky Mountains, Basin and Range and Northern Plains regions. The Great Basin

The Numic Expansion The Numa can be divided into three groups based on language: the western group consisting of the Mono and Northern Paiute, the Central group that contains the Panamint, Western, Northern and Eastern Shoshone, and the Southern group, which consists of the Kawaiisu, Chemehuevi, Southern Paiute and Ute. The environment of the Numa homeland is particularly arid and harsh, and traditional economies have always been based on highly mobile hunting and gathering.

Traditional Paiute and Shoshone lifeways in the Great Basin.

The modern distribution of Numic-speaking peoples in western North America

There has been considerable debate in the archaeological community concerning the place of origin and timing of the migration of the Numa throughout the West. Most archaeologists (but not all) agree that the likely Numa homeland was in either the southwestern or central Great Basin. There are also several schools of thought regarding the timing of the expansion out of this homeland. These can be roughly divided into the early and late schools. There is less agreement on why the expansion occurred and who the people were that the Numa replaced as they expanded.

Some archaeologists believe that the expansion out of the Numic homeland occurred soon after the arrival of the Uto-Aztecans from Mexico. This would have occurred just after a mid-Holocene climatic arid episode formerly known as the Altithermal.

Numic speakers would have come into contact with other hunter-gatherer groups, and it is unknown if they replaced these peoples or intermixed with them. Regardless, in this scenario the Middle and Late Archaic populations throughout the Intermountain West would have been Numic in makeup.

Other archaeologists believe that the expansion occurred much later, at around 1000 B.P. This would have been well into the Late Prehistoric period, and it would have brought the Numa into contact with sedentary Horticultural cultures.

This later expansion has some support from archaeological data, especially in the documentation of the entry of the Ute and southern Paiute in the southern area. This later expansion makes sense for several reasons.

The big question is, how did the Numa expand relatively rapidly into an area that was already inhabited? There are theories that invoke an adaptation that was more efficient than that possessed by the native populations that were replaced, but they do not address the problem of differences in population density. As was pointed out above, the Mohave Desert and the Great Basin as a whole are harsh environments, and historically supported native population densities lower than anywhere in North America other than the Arctic. How could a relatively small population with low density push into areas with larger, denser populations?

Data suggest that there were significant and precipitous decreases in population throughout western North America starting approximately A.D. 850 and continuing through the Anasazi abandonment of the Four Corners region in the late 13th century. This depopulation may have been associated with climatic deterioration and increased frequency of drought prior to A.D. 1200, and elsewhere has been correlated with the Medieval Climatic Anomaly. Depopulation would have set the scene for a rapid radiation of a relatively small group of people that were pre-adapted to harsh environments. Now that we have examined the nature of population dynamics on very large and moderately large scales, let’s examine the nature of population dynamics on a smaller regional scale.

Population Dynamics of Prehistoric Hunter–Gatherer Groups in Eastern Colorado

Recognition of a Regional Pattern: Peaks in Late Prehistoric Radiocarbon Age Frequency
Platte River Basin ( BP). Arkansas River Basin ( BP). Northern Colorado River Basin ( BP). Wyoming ( BP).

From Gilmore et. al. (1999)

Platte Basin Radiocarbon Ages
Adapted from Gilmore et. al. (1999)

Arkansas Basin Radiocarbon Ages
Adapted from Zier and Kalasz 1999

Northern Colorado River Basin
From Reed and Metcalf (1999)

Wyoming From Frison (1993)

Platte Basin Curve Peaks B.P. Arkansas Basin Curve Peaks B.P.

The Data Set

Distribution of archaeological sites and isolated finds in the Platte and Arkansas River Basins (N=17,812)

Distribution of sites with radiocarbon ages between 3000 and 100 BP
In general the distribution of radiocarbon ages mirrors the distribution of all sites in space, and the numbers of radiocarbon dated sites per cultural period is roughly proportional to the number of site with relative dates (artifactual) that fall into the cultural periods.

Distribution of archaeological sites with associated radiocarbon ages
BP C-14 ages n=621 Components n=534 So, lets use the distribution of radiocarbon dated sites represent the distribution of all sites.

If changes in the number of radiocarbon dates are accepted as representing changes in relative population, then it is also reasonable to assume that changes in the spatial distribution of these dated occupations represents changes in the spatial distribution of population.

Geographic Mean Centers for Archaeological Sites by Cultural Period
This shows the progress south and east of the geographic mean centers of sites assigned to cultural periods by temporally diagnostic artifacts. Middle Archaic ( BP) Early Ceramic ( BP) Middle Ceramic ( BP) Late Archaic ( BP) All Middle Ceramic Cultures Apishapa Phase ( BP)

Geographic Mean Centers for Archaeological Sites by Century, 2500–100 BP

The back and forth movement of the red centroids suggests that the movement is random and not culturally meaningful.

Period of Relative Stability

The movement of the orange centroids more or less consistently south for the following period suggests a period of actual movement over the time represented.

Population shifts south

The northward trend of the yellow centroids during the next period is also probably meaningful, but the samples are pretty small for some of sets represented by the points, so I wouldn’t bet the farm on the meaning!

Significance? Difference of means tests comparing the locations (UTM northings and eastings) of the set of all stable sites ( BP) to the locations of the set of all sites falling within the hypothesized period of population movement south ( BP) are significant at p=.05

Significance? Not only is the movement south significant, but the movement from west to east is also significant. What does this mean?

Are we looking at Population Movement, or Differential Rates of Natural Increase?

Probably Some of Both

As the climate gradually became warmer and wetter between A. D
As the climate gradually became warmer and wetter between A.D. 1 and 900, two things apparently happened. The Plains became more productive; plants were more abundant and more productive, and so animals were also more abundant. People spent more of their time at lower elevations, because the resources that the mountains could provide were not as critical as they had been. And human population increased.

As you can see from the following radiocarbon frequency curves, population apparently began to decrease first in the mountains between about 1600 to 1300 BP (top), followed by the Foothills at 1300 (middle), and then finally on the Plains at 850 BP. Based on the slope of the respective curves, the drop in population was more gradual in the mountains (900 years), more rapid in the Foothills (500 years), and precipitous on the Plains (300 years). This may indicate a relatively rapid out migration from the Plains, where the more gradual drops in the mountains and Foothills suggests a decrease in the frequency and duration of visits.

After about A.D. 900, population begins to drop precipitously, perhaps in response to a return to more arid conditions. Instead of returning to a pattern where the resources in the mountains became more important relative to decreasing resources on the Plains, population also decreases in the mountains. This is a pattern that is apparently reflected across much of the west.

One possible explanation is that this dry period was not hot and dry, but cold and dry. This might explain why both the plains and the mountains were apparently abandoned at the same time. It was too dry on the Plains, which which resulted in decreased resource availability, and in the mountains it was too cold, which also resulted in decreased resources. There wasn’t anywhere to go that was productive enough to support the population that had increased in size and density over the past 900 years.

A measure of the relative mobility of prehistoric people called the the Component Complexity Index (CCI) can help to determine if there was movement from the Platte basin to the Arkansas basin as a response to climate change. A higher aggregate CCI for all the sites in a region during a particular century suggests a combination of longer occupations, and/or increased frequency of visits and/or more people occupying sites during occupations dated to that century.

In contrast, a lower aggregate Component Complexity Index for a region during a particular time is indicative of greater residential mobility and possibly smaller populations and/or lower population densities.

Component Complexity Index

Although the peaks in the component curves are offset by 200 years between the Platte and Arkansas basins, the Component Complexity Index curves are concordant, suggesting that even though population was fluctuating, the relative mobility of the populations in both places was following the same trends at the same time.

There is a trough in both CCI curves between 1250 and 950 BP, which corresponds to the period between the peak in radiocarbon frequency in the Platte basin at BP and the peak in the Arkansas at BP. This indication of greater mobility in both basins during a period of decreasing population in the Platte and increasing population in the Arkansas could represent migration from north to south.

The peaks and subsequent decreases in radiocarbon age frequency occur hundreds of years prior to the onset of the Pacific climatic episode ( BP), which is thought to have contributed to dry conditions in eastern Colorado. The Pacific episode would have been a likely culprit for contributing to population movement and decline.

In fact, the peaks and subsequent decline in both basins occurs in the middle of the Neo-Atlantic episode ( ), which is hypothesized to have been a period of greater summer precipitation and higher carrying capacity.

Component Frequency Curves and Paleoclimatic Episodes

The drop in population in both the Platte and Arkansas basins does, however, correspond to the onset of the Medieval Climate Anomaly (remember?) mentioned above. The decrease in population in eastern Colorado also correlates to the drop in populations throughout the region and west into the Colorado Plateau, the Great Basin and California.

Introduction of new technologies such as the bow and arrow and ceramics to the prehistoric inhabitants of eastern Colorado in the first few centuries A.D. may have enhanced already established trends in prehistoric population growth by contributing to the more efficient exploitation of food resources.

The onset of dryer conditions associated with the Pacific paleoclimatic episode that occurred after 850 BP almost certainly contributed to already established trends of population decrease. The introduction of epidemic disease by Europeans some five to seven hundred years after population begins to decrease also undoubtedly contributed significantly to further decline in population.

Changes in subsistence, technology, economy and climate all contributed to prehistoric population dynamics during the past 3000 years… But…

The mechanisms providing the initial impetus that resulted in almost exponential growth in proxy population starting at around 2200 BP and the equally precipitous decrease in population that occurred after about BP in eastern Colorado still remain a mystery.

Conclusions

If the radiocarbon age and component frequency curves are accepted as a proxy for general population trends, and the spatial distributions of radiocarbon ages represent the spatial distributions of prehistoric people, what are the implications?

There is a dramatic increase in population in the region and beyond starting at approximately 2200 BP and peaking at approximately BP.

There is an equally dramatic decrease in population in the region that begins at approximately 1000 BP, or 500 years prior to European contact and the introduction of epidemic European diseases.

There is evidence that prehistoric population change in eastern Colorado can be partially explained by migration.

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Out of Africa (or not) The incredibly complex and mostly true story of the origin and dispersal of Homo sapiens throughout the World.

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Out of Africa (or not) The incredibly complex and mostly true story of the origin and dispersal of Homo sapiens throughout the World.

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