Climate, technology, and glaciers: the settlement of the Western Hemisphere

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Climate, technology, and glaciers: the settlement of the Western Hemisphere

J.F Hoffecker

PhD in Anthropology, Research Fellow, Institute of Arctic and Alpine Research, University of Colorado, Discovery Drive, Boulder, Colorado, USA;

V.V. Pitulko

PhD in History, Senior Research Fellow, Institute of the History of Material Culture, Russian Academy of Sciences, St. Petersburg, Russian Federation

E.Y. Pavlova

Research Fellow, Arctic & Antarctic Research Institute, Russian Federal Service for Hydrometeorology and Environmental Monitoring, St. Petersburg, Russian Federation

A major problem in human prehistory is the late settlement of the Americas, which were not occupied until after 15,000 years ago from Beringia. The most likely barriers to earlier settlement are: 1) high-latitude environments (characterized by low biological productivity and extreme winter temperatures); and 2) the North American ice sheet complex, which probably blocked both coastal and interior migration routes between Beringia and NW North America at various times before 15,000 years ago. Here we argue that each was a barrier to earlier settlement, but to different human taxa. Neanderthals, Denisovans, and other nonmodern representatives of Homo did not occupy latitudes above 60° North, which precluded occupation of Beringia and migration to the Western Hemisphere. The critical variables probably were: 1) resource scarcity, reflecting low plant and animal productivity; and 2) low winter temperatures. Homo sapiens, expanding out of Africa after 75,000 years ago, occupied virtually all terrestrial habitats in Eurasia no later than 32,000 years ago, including year-round settlement of the Arctic. Adaptation to high-latitude environments was achieved primarily with technologies of functional and structural complexity comparable to those of recent hunter-gatherers in similar settings. There is evidence of human settlement in Arctic Beringia before and during the Last Glacial Maximum (28,000-16,000 cal BP), and we conclude that the North American ice sheet complex is the only credible barrier to modern human occupation of the Western Hemisphere before 15,000 years ago.

Keywords: settlement of the Americas, Neanderthals, modern humans, paleoecology, Northern Asia, Beringia.

Климат, технология и ледники: заселение человеком Западного полушария

Д.Ф. Хоффекер

д-р антропологии, науч. сотр., Институт Арктических и Альпийских исследований, Университет Колорадо, Болдер, Колорадо, США;

В.В. Питулько

канд. ист. наук, ст. науч. сотр., Институт истории материальной культуры Российской Академии наук, Российская Федерация, Санкт-Петербург

Е.Ю. Павлова

науч. сотр., Арктический и антарктический научно-исследовательский институт Федеральной Службы по гидрометеорологии и мониторингу окружающей среды, Российская Федерация, Санкт-Петербург

Главной проблемой в истории расселения человека является его позднее проникновение в Новый Свет, который был заселен не ранее 15 000 лет назад через Берингию. Наиболее вероятным препятствием в предшествующие эпохи были 1) высокоширотные природные обстановки, характерными чертами которых являются низкая биологическая продуктивность и низкие зимние температуры и 2) оледение Североамериканского континента, которое в различные эпохи, вероятно, блокировало как прибрежный, так и внутриконтинентальный миграционный пути между Берингией и северо-западом Северной Америки ранее 15 000 л. н. Показано, что оба указанных фактора служили препятствием для раннего заселения человеком Америки, однако по-разному ограничивали расселение различных видов. Неандертальцы, денисовцы и другие ранние представители рода Homo не расселялись севернее 60° с.ш., что предопределило отсутствие человека в Берингии и миграцию в Западное полушарие. Критическими обстоятельствами, скорее всего, были 1) бедная ресурсная база, связанная с низкой продуктивностью растительных и животных сообществ, и 2) низкие зимние температуры. Homo sapiens, расселявшиеся из Африки после 75 000 л. н., не позднее 32 000 л. н. освоили практически все пригодные местообитания Евразии, включая арктические области, где были способны обитать круглогодично. Высокая степень адаптации к высокоширотной природной обстановке была достигнута преимущественно благодаря функционально и структурно сложным технологиям, сравнимым с теми, которые используют современные охотники-собиратели, обитающие в сходных условиях. В арктической Берингии известны свидетельства пребывания человека, возраст которых указывает на непрерывную заселенность территории в интервале 28 000-16 000 л. н. Присутствие человека в Берингии установлено и для самого холодного интервала последнего ледникового максимума. На этом основании мы заключаем, что Североамериканский ледниковый комплекс служил единственным препятствием для заселения современным человеком Западного полушария ранее 15 000 л.н. climate glacier western hemisphere

Ключевые слова: заселение Америки, неандертальцы, анатомически современные люди, палеоэкология, Северная Азия, Берингия.

1. Why was the Western Hemisphere settled after 15,000 years ago?

The genus Homo evolved in sub-Saharan Africa more than 2.5 million years ago. Its appearance in the fossil record is at least broadly correlated with significant changes in anatomy and behavior linked to foraging in less biologically productive environments. By roughly 2.0 million years ago, representatives of Homo had expanded into the middle latitudes (up to about 40° North), occupying habitats in North Africa and southern Asia, as well as the biologically richer areas of mid-latitude Eurasia. The latter eventually included the warmest and wettest parts of Europe (up to latitude 52° North) and mid-latitude areas in Asia (northern China).

A large-brained form of Homo evolved in Africa less than 1.0 million years ago and dispersed into the middle latitudes about 0.5 million years ago. Its Eurasian descendants included both the Neanderthals and Denisovans, who occupied most, if not all, of the places in Eurasia inhabited by earlier forms of Homo (although the Denisovans appear to have been confined to Asia). They also settled in places in northern Eurasia that were significantly colder and probably less productive than the environments populated by earlier forms of Homo. These places included parts of Europe during periods of extreme cold (i. e., glacial periods) and -- most notably -- the Altai region of southwest Siberia (above 50° North in Asia), which was occupied by both Neanderthals and Denisovans during the late Pleistocene.

Anatomically, modern humans (H. sapiens) evolved in Africa no later than 300 years ago, and it is becoming increasingly apparent that their movements into Eurasia took place over an extended period of time, and may have involved multiple migration events. Before 75,000 years ago, at least some -- and possibly all -- of these movements out of Africa were undertaken by people who possessed “archaic” anatomical features, and sometimes are described as “near modern” humans. At least, some of the remains assigned to H. sapiens in Southeast Asia that antedate 75,000 years may represent one or more of these migration events. Thus far, the latitudinal range of “near modern” humans in Eurasia is confined to less than 45° North.

After 75,000 years ago (or possibly slightly earlier), the direct ancestors of the living maternal and paternal lineages expanded out of Africa. They occupied southern Asia and Australia/New Guinea before 55,000 years ago, but do not seem to have expanded into

Europe and Northern Asia until after 50,000 years ago4. Significantly, they invaded habitats and climate zones never previously inhabited by humans -- most notably, the Arctic, which was settled on a year-round basis no later than 32,000 cal BP5. Well-dated sites in this time range are found in the Yana-Indighirka Lowland, which is widely considered to lie within the boundaries of Beringia. Both the movement to Australia/New Guinea and dispersal in the Arctic may be attributed to a significant increase in the complexity of computation underlying their technologies6.

Despite their long history of habitation in Eurasia and occupation of the Arctic and Beringia more than 30,000 years ago (and at least tentative evidence for continued occupation of Beringia during the Last Glacial Maximum7) there is no compelling evidence for settlement of the Americas until about 15,000 years ago. Between 15,000 and 13,000 cal BP, well dated sites appear throughout unglaciated North America and South America8. The number of sites proliferates after 13,000 cal BP, reflecting rapid growth of the Native American population from a relatively diverse source9. Why did movement into the Western Hemisphere occur so late?

There are two likely barriers to settlement of the Americas before 15,000 years ago: (1) high-latitude environments (i. e., places above 60° North), which are characterized by low biological productivity and extreme climates; and (2) physical barriers to population movements, which include both large bodies of water and large masses of ice. Here we argue that both were barriers to migration, but to different representatives of the genus Homo.

High-latitude environments most likely are the reason that the Neanderthals, Deniso-vans, and other nonmodern humans never occupied Beringia, which lies above 60° North; both low biological productivity (i. e., scarce resources) and low winter temperatures probably were factors. High-latitude environments conceivably were a barrier to the “near modern” humans who moved into parts of Eurasia before 75,000 year ago, although they may not have been present in Asia. (Strictly speaking, physical barriers in the form of the Atlantic and Pacific Oceans also prevented occupation of the Western Hemisphere by nonmodern humans.)

The direct ancestors of the living human lineages successfully occupied the Arctic Zone, including Beringia, on a year-round basis, however. At the time that people were living in the Yana-Indighirka Lowland in northwestern Beringia (32,000 cal BP), falling sea level had exposed a broad plain between Chukotka and Alaska (i. e., central Beringia)10.

Although it is possible that some currently unidentified environmental variable was a barrier to occupation of eastern Beringia, the simplest explanation for the absence of compelling evidence for dispersal from Beringia into the Americas is that the coalesced Laurentide and Cordilleran glaciers were a physical barrier to migration via both the interior and coastal routes; coalescence of the two ice sheets is estimated to have occurred before 50,000 years ago.

2. Non-modern humans in Northern Asia

It is not possible to prove that non-modern representatives of the genus Homo (i. e., all species other than H. sapiens) did not inhabit latitudes above 60° North, because any traces of such habitation might remain unknown due to poor preservation conditions and low visibility in the fossil and archaeological records. This latter consideration applies particularly to the higher latitudes in Northern Asia, where intensive settlement and industrial development have been extremely limited. It is possible, however, to review the known distribution of non-modern human fossils (including remains assigned to non-modern taxa based on aDNA) and archaeological data that probably -- or at least possibly -- indicates the presence of one or more of these taxa in a specific geographic and stratigraphic setting. It also is possible to reconstruct the ecology of non-modern humans from multiple lines of evidence (e. g., stable isotope analysis of bone, population genetics, technology) and draw inferences about their range with respect to plant and animal productivity and temperature tolerance, and -- given the afore-mentioned limitations of the fossil and archaeological records -- this is an important component of the analysis.

2.1 Spatial-temporal distribution of non-modern humans in Northern Asia

Our review of non-modern humans in Northern Asia is primarily focused on the most recent known representatives of Homo in this part of the world -- the Neanderthals and Denisovans. Both taxa have been identified in the Altai region of southwestern Siberia, based reliably on the analysis of aDNA extracted from skeletal remains recovered from caves located near Gorno-Altaisk (latitude 52° North): Okladnikov (315 m asl), Chagyrskaya (398 m asl), and Denisova (~700 m asl). Neanderthal remains are found at all three locations, while multiple individual Denisovans are present at Denisova Cave.

The human remains are associated with a local Middle Paleolithic industry. At Denisova Cave (Layers 20-12), this industry is characterized as Typical Mousterian (i. e., non-Levallois flake technology), although some Levallois products are present. A similar industry is associated with most of the Neanderthal remains at Chagyrskaya Cave and Okladnikov Cave, with the exception of the lowest level (Layer 7), which contains a Levallois assemblage.

A Levallois industry associated with non-stone artifacts and ornaments is found in Layer 11 at Denisova Cave, and has been attributed to non-modern humans. A new set of dates indicates, however, that the non-modern skeletal remains may antedate most of the artifacts in Layer 11 (i. e., materials of different age may have been mixed by post-dep- ositional disturbance in the cave). More generally, a Levallois blade and point industry in Western Asia (“Initial Upper Paleolithic”) and Central Europe (Bohunician) probably was produced by modern humans, and may have spread widely across northern Eurasia after 50,000 years ago.

Non-modern humans, possibly both Neanderthals and Denisovans, are represented at higher latitudes in Northern Asia by artifacts that appear too old to have been produced by modern humans. An example is Ust'-Izhul (~300 m asl) in the Upper Yenisei Valley (55° North), where flake tools associated with large mammal remains and former hearths are dated to the Last Interglacial climatic optimum (MIS 5e). Farther north (61° North) on the Lena River, the open-air site of Diring Yuriakh represents a potential non-modern human occupation possibly dating as early as the late Middle Pleistocene, although uncertainties regarding the stratigraphic context of the artifacts make it difficult to evaluate the significance of the finds (Fig. 1).

Establishing the spatial-temporal context of non-modern humans in Northern Asia is complicated by ambiguities about the authorship of many stone artifacts. As already noted, assemblages containing Levallois blades and points, which are found at a number of major sites such as Kara-Bom in the Altai, may have been produced by modern humans; similar assemblages of comparable age are found in Central Europe and Western Asia. Some sites contain flake tool assemblages like those of the typical Mousterian (or other Middle and even Lower Paleolithic industries) that conceivably represent expedient tools made and used by modern humans for processing large mammal carcasses or other functions that do not require typical Upper Paleolithic forms (e. g., burins, microblades). An example is Priiskovoe in the Transbaikal, now dated to less than 30,000 cal BP. The same phenomenon has been encountered among open-air sites of the East European Plain (e. g., Betovo on the Middle Desna River, where the artifacts are assigned to the Mousteri- an, but the context recently was dated to ~32,000 cal BP).

Fig. 1. Location of sites mentioned in the text (A) and (B) present-day winter temperatures (combined December, January, and February mean temperatures [C]) (base map adapted from Robinson and Henderson-Sellers 1986: 84. Fig. 2.32a)

Dating a non-modern human presence in Northern Asia poses the same problems as dating sites and skeletal remains more than 40,000 years ago in other parts of the world. Even with application of the most rigorous pretreatment methods, most of the record of non-modern settlement is beyond the range of effective radiocarbon dating. Luminescence and ESR dates have been obtained on several sites, but at least some of the results appear to be problematic. At Denisova Cave, RTL dates of as much as 280,000 years (i. e., late Middle Pleistocene) were obtained on the lowest layer (Layer 22), while the small-mammal biostratigraphy indicates that the entire sequence probably is of Late Pleistocene age. Non-modern human remains in the Altai region are associated with evidence (palynological and faunal) for a variety of habitats, and many of these reflect climate conditions similar to those of today (or even somewhat warmer).

A goal of Neanderthal/Denisovan paleoecology is to determine the environmental tolerance limits of these non-modern human taxa in quantitative terms. What was their lower threshold with respect to levels of plant and animal productivity? How cold were the winter temperatures they could tolerate? The spatial-temporal distribution of their skeletal remains and artifacts suggests that at least one and perhaps both these taxa could survive in places where winter temperatures fell as low as -20 °C (suggested by the location of the Last Interglacial occupation at Ust'-Izhul in the Minusinsk Basin). The spatial distribution of their sites is confined to areas where plant productivity today is above 200 g/m2/year.

The question that remains is whether the Neanderthals and/or Denisovans ever occupied places in Northern Asia when climates were significantly colder than those of the present interglacial. Did they inhabit areas such as the Altai or the Upper Yenisei River during cold intervals of the Late Pleistocene, such as MIS 4 or stadial periods of MIS 3, such as GS 13 (HE5)? In addition to lower annual and winter temperatures, these periods were probably characterized by reduced plant and animal productivity. In our view, this question has not been resolved (although there is no question that Neanderthals were present during periods of extreme cold in parts of western Eurasia, including MIS 4 in Western Europe).

2.2 Neanderthal/Denisovan ecology and Northern Asia

The Neanderthals were famously adapted anatomically -- and probably physiologically -- to cold climates, as proposed more than half a century ago by Coon. Coon became a controversial figure in American anthropology, and some of his suggested Neanderthal cold adaptations (e. g., “radiator nose”) were subject to severe critiques. More recent research also found a number of anatomical features (e. g., brachial and crural indices, thorax dimensions, cranial volume) that appear to be adaptations to low temperature, however, and the Neanderthals have been described as “hyperpolar” on a scale with modern humans (Fig. 2).

Evidence for anatomical adaptation to cold climate among the Neanderthals is based entirely on data from western Eurasia (Europe and Western Asia), but the pattern presumably applies to Neanderthals in Northern Asia. Owing to the scarcity of skeletal remains, nothing currently is known about anatomical adaptations to climate among the Denisovans, although it seems safe to assume that the northern (i. e., Siberian) Denisovans evolved features similar to those of the Neanderthals. It should be emphasized that anatomical features alone do not provide a basis for estimating low-temperature tolerance in Homo; they must be assessed in conjunction with technological adaptations, such as insulated clothing.

Fig. 2. Anatomical adaptation to climate: brachial index (ratio of humerus to radioulna) in selected living human populations, Neanderthals, and modern humans; and mean annual temperature (modified after [Trinkaus 1981, p. 211, table 7])

The size and shape of Neanderthal bodies suggests that -- like living human populations in cold environments such as the Inuit -- they had developed various physiological adaptations to low temperature, such as vasoconstriction in extremities and elevated metabolic heat production. Coon observed that their large infraorbital foramina indicated increased blood flow to the face. Their elevated basal metabolic rate (BMR) would have created high caloric demands, and it should be noted that the daily caloric needs of the Inuit are roughly 50 % higher than those of living people in the equatorial zone39. Among the Inuit and other modern human populations that occupy cold environments, those high caloric demands are met with a diet heavy in protein and fat40. The size and shape of Neanderthal bodies (i. e., large body mass combined with shortened limbs) would have placed added energetic costs on locomotion relative to humans in an equatorial setting41.

The analysis of large quantities of genomic data obtained from ancient DNA provides insights to Neanderthal demography, which, in turn, provides further insight to their ecology42. Estimates of effective population size (Ne), based on genetic diversity and calculated mutation rates, vary widely, but are generally low43. The Ne estimates generate low total or census population sizes (although the relationship between effective and census size is problematic) and -- applied to the assumed distribution of sites -- yield low density estimates (0.01-0.04 persons per sq. km44) in comparison to modern humans (i. e., recent hunter-gatherers). The genomic data also indicate a relatively low diversity and a high inbreeding coefficient45. The overall picture is one of small and relatively isolated populations, possibly subject to periodic local extinction (followed by subsequent local repopulation), especially in areas where resources were less abundant46.

The diet of the Neanderthals was high in animal protein and fat, as indicated by stable isotope analyses of their skeletal remains47. Neanderthal S15N and S13C values are higher than those of living carnivores, including hyena and bear48. Significantly, these values remain high even in warm interglacial settings, where digestible plant foods were more common49. And both stable isotope analyses and the analyses of faunal remains from Neanderthal sites indicate a reliance on large mammals and relatively little consumption of small mammals, birds, and fish50.

The key constraint on the spatial-temporal distribution of Neanderthal/Denisovan sites appears to have been their technology. Both taxa seem to have lacked the ability to produce the structurally and functionally complex technologies of recent hunter-gatherers who occupied places where plant and animal productivity is low (and food resources are scarce and widely dispersed) and winter temperatures are very low. In cold climate settings, the need for these food-getting technologies is increased by higher caloric demands, combined with the scarcity of available plant foods, which force hunter-gatherers to extract protein and fat from small mammals, birds, and fish. Efficient harvesting of small vertebrates requires a suite of complex technologies, including mechanical instruments and facilities (or “multiple-state artifacts”), some of which function independently (i. e., automata). Protection from extreme low temperatures also requires complex technology, most notably insulated clothing (which must be tailored, so that foraging activities can be pursued) composed of many varied parts and potentially mechanical (i. e., drawstrings).

Most of the structurally and functionally complex technologies of hunter-gatherers are composed of perishable materials (e. g., wood, hide) that do not preserve in the archaeological record, except under highly unusual conditions. The presence of these technologies in Paleolithic sites is deduced or inferred from various indirect sources. For example, tailored clothing has been inferred from the recovery of eyed needles and footwear from the anatomy of the foot. The use of snares or traps has been inferred from the taxonomy and taphonomy of faunal remains in archaeological sites.

Sites occupied by Neanderthals and Denisovans lack any evidence -- direct or indirect -- for mechanical technologies, including automata such as snares and traps, or for structurally complex technologies composed of three or more levels of organization (i. e., parts comprising sub-parts), such as tailored clothing. It is more than simply an “absence of evidence,” moreover. Comparative analysis of stone points from Middle and Upper Paleolithic sites in the Levant indicates that only the latter exhibit the morphometric characteristics of points used with mechanical projectile weaponry. Comparative analysis of foot anatomy reveals that non-modern human remains lack the reduced robusticity of the middle pedal phalanges found in modern humans. Sandgathe et al. inferred an absence of fire-making technology among West European Neanderthals from the lack of hearths in sites occupied during periods of intense cold.

3. Modern humans in Northern Asia

Modern humans became the first representatives of the genus Homo-and probably the first primates -- to occupy the Arctic Zone (above latitude 66° North). Since 2000, evidence has accumulated that modern humans colonized subarctic and arctic environments much earlier than previously thought, possibly in conjunction with their initial dispersal in Northern Asia. They were present, at least on a seasonal basis, in the subarctic zone of Siberia by 45,000 cal BP and, on a year-round basis, above the Arctic Circle no later than 32 cal BP. Significantly, these conclusions are based not on the general distribution of sites but on isolated data points, reflecting the limited archaeological visibility of Pleistocene sites at high latitudes.

3.1 Spatial-temporal distribution of modern humans in Northern Asia

The modern humans who occupied Northern Asia after 50,000 years ago represent the living paternal and maternal lineages, which dispersed out of Africa about 60,000 years ago (or somewhat earlier). They were a subset of Homo sapiens, which now is recognized in Africa as early as 300,000 years ago. Other representatives of H. sapiens (some of which have been described as “near modern" and might be considered a subspecies) also dispersed out of Africa during earlier periods, possibly as early as 220,000 years ago, and later between 120,000 and 90,000 years ago. The earlier dispersals were limited to the lower and middle latitudes (below 40° North).

The earliest dated remains of modern humans in Northern Asia are found at latitude 57° North near the southern margin of the subarctic zone in western Siberia (local January mean of -15 °C). An adult male femur shaft recovered from Ust'-Ishim on the Irtysh River is dated to 45,000 cal BP and assigned to Y-DNA haplogroup (paternal lineage) K (xLT) and mtDNA haplogroup (maternal lineage) R, both of which emerged more than 50 years ago and are closely linked to parent groups in Africa. It may be noted that mtDNA haplogroup B, which is a daughter lineage of R and an ancestral Native American lineage, has been identified from skeletal remains in North China dated to 40,000 cal BP.

The Ust'-Ishim femur dates to a pronounced and protracted warm interval (Greenland Interstadial 12 [GI 12]), when climate conditions in Northern Asia seem to have been similar to those of the present day. Both the GI 12 interstadial (~47,500-44,500 years ago) and succeeding GI 11 interstadial (~43,500-42,500 years ago) may have played an important role in the dispersal of the living lineages into the higher latitudes; the earliest reliably dated traces of modern humans in Europe (including 51° North on the East European Plain) also date to this time range.

The initial phase of the dispersal may have extended into the Arctic Zone, although the supporting evidence remains tentative (human remains and/or artifacts in dated context are lacking). The Sopochnaya Karga mammoth carcass, excavated from frozen sediment near the mouth of the Yenisei River on the Taimyr Peninsula (71°54' North), exhibits possible traces of a projectile point, as well as damage that may have been caused by butchering tools; the tibia yielded a date of 49,886-46,257 cal BP, which is generally consistent with the stratigraphic context.

East of the Verkhoyansk Mountains -- in northwest Beringia -- an accumulation of large mammal remains at Bunge-Toll 1885 in the Lower Yana River region at 68°55' North yielded a wolf bone dating to 49,930-46,355 cal BP and exhibiting possible traces of a projectile weapon. And in eastern Beringia, on the Seward Peninsula near the mouth of the Inmachuk River, indisputable traces of human working were identified on a piece of mammoth ivory dating to 40,949-38,635 cal BP . The tusk fragment was not found in late Pleistocene stratigraphic context, however, and conceivably was scavenged from ancient deposits and worked by recent inhabitants of the area.

In contrast to the problematic traces of non-modern humans in areas above latitude 55° North, there is relatively good evidence -- in addition to the Ust'-Ishim femur -- for occupations at roughly 58° North on the Angara (Ust'-Kova), Lower Tunguska (Nepa 1), and Upper Lena River (Alekseevsk)70. All of these sites appear to contain levels dating to late MIS 3, although Alekseevsk has yielded a slightly younger radiocarbon date. Sites of late MIS 3 age at higher latitudes (between 59° and 63° N) on the Aldan River (a tributary of the Lena) were reported decades ago, but there are uncertainties about the relationship between the materials dated and the artifacts; with the possible exception of Ust'-Mil 2 (59° N), all of these sites probably date to late MIS 2 or post-MIS 2 (i. e., post-LGM).

The earliest indisputable evidence for settlement of the Arctic Zone of Northern Asia -- and NW Beringia -- dates to 32,000 cal BP and a cold interval (Greenland Stadial 5). Several of the sites near the mouth of the Yana River at 70°43' North (local January mean of -38 °C) are reliably dated to ~32,000-30,0000 cal BP. In addition to thousands of stone artifacts, the Yana River sites yielded eyed needles (sometimes decorated), possible needle cases, awls, pendants, mammoth-bone beads (possibly sewn onto clothing), ivory rods, and decorated ivory vessels. The profusion of household items such as these indicate that the Yana River sites probably (1) were occupied for extended periods of time, (2) by family groups, (3) during the winter months, and (4) contained dwelling structures. In sum, these sites probably indicate a year-round presence in the Arctic Zone.

It is difficult to understate the importance of the Yana River sites for explaining why the Western Hemisphere was not settled until after 15,000 years ago: they show that modern humans had successfully adapted to virtually every habitat and climate zone in northern Eurasia by late MIS 3 (if not earlier, when climate conditions probably were more favorable). The occupants of the Yana River sites lived in arctic steppe-tundra with woody shrubs and some trees (a modest quantity of wood likely was required for efficient use of bone fuel). They harvested a rich array of steppe-tundra mammals, including bison, horse, and reindeer, as well as small mammals, especially hare. During the warmer months, they may have fished and hunted migratory waterfowl. Although climates were somewhat cooler and drier in eastern Beringia, similar habitat was widespread in Alaska and Yukon--and readily accessible via the exposed Bering Platform or land bridge (sea level was ~75 meters below that of the present day).

Despite the settlement of NW Beringia before 30,000 years ago, there is no indisputable evidence for people in central or eastern Beringia during MIS 3 (other than the problematic tusk fragment from the Seward Peninsula noted above). Meiri et al. suggested that the mesic tundra zone located in the central Beringian lowland (i. e., the land bridge) was an ecological barrier to humans; in fact, they argue that the central Beringian tundra belt was the principal barrier to settlement of the Americas during MIS 3 and MIS 2. The argument is based on the observation that wapiti (Cervus elaphus canadensis) dispersed in North America simultaneously with humans (after 15,000 cal BP) and -- although present in NW Beringia during MIS 3 -- apparently was excluded from the central Beringian lowland. They suggest that the wapiti may be considered a proxy for modern humans.

The mesic tundra in the central lowland -- exposed by lower sea level during cold periods -- was the basis for the original concept of Beringia, later expanded to include a much larger area between the Verkhoyansk Mountains and Mackenzie River. The expanded definition of Beringia encompassed a more diverse landscape with respect to plants and animals: in addition to the mesic tundra zone in the central lowland (sustained by a supply of warmer and wetter air from the North Pacific Ocean), areas of drier steppe-tundra existed in both western and eastern Beringia. Poor drainage on the exposed Bering Platform, which was a relatively level plain, probably contributed to the mesic tundra environment in central Beringia; the BIOME4 model predicts a concentration of wetlands in this region during MIS 2. Significantly, a number of steppe-adapted species (including mammals and invertebrates) on both sides of Beringia were unable to cross the mesic tundra belt (Fig. 3).

Fig. 3. Map of Beringia, as currently defined, showing locations of the Yana River sites and Bluefish Caves (adapted from [Hoffecker 2017. Fig. 7.3])

In our view, however, the mesic tundra in central Beringia is unlikely to have been a barrier to humans during later MIS 3 and MIS 2. The environment probably was similar to postglacial shrub tundra in Chukotka and Alaska. Although the large mammalian biomass would have been reduced relative to the more steppic areas in western and eastern Beringia, other resources -- which conceivably included seasonal concentrations of waterfowl on the wetlands -- may have compensated for the scarcity of grazers like bison and horse. The south-central lowland probably experienced relatively mild winters, even during MIS 2, and is likely to have been a source of wood (and possibly peat) for fuel. New research at Bluefish Caves in NE Beringia (described below) indicates that people probably were present in eastern Beringia during MIS 2 at the latest (i. e., significantly earlier than 15,000 cal BP).

Climates became increasingly cold and dry after ~28,000 cal BP with the onset of the Last Glacial Maximum (LGM), and some large areas in the northern hemisphere were abandoned by human populations for an extended period of time, including northern Europe, North Africa, and the Arabian Peninsula. Increased LGM aridity in the southern hemisphere also forced abandonment of drier regions, most notably in Australia. The supporting evidence is based on indications of population bottlenecks in the genetic data and a local settlement hiatus in the archaeological data. In northern Europe, at least some of the habitat loss was due to expansion of the Scandinavian ice sheet, but in areas not directly affected by glaciation, the contraction of human populations probably was due to a significant decline in plant and animal productivity -- largely caused by the effects of decreased moisture on primary production with secondary impacts on animal populations. The LGM reveals the limits of modern human distribution in space and time.

Although affected minimally by the expansion of glaciers (due to the lack of moisture), interior regions of Northern Asia (and Beringia) also experienced increased cold and aridity, and it is likely that some areas were abandoned by modern humans for some period of time during the LGM. As in the case of Neanderthals in the Arctic (see above), it is not possible to prove that modern humans were absent in any specific place during the LGM, and the issue has been controversial because some radiocarbon dates from sites in interior Northern Asia fall into the time range of the LGM. There now appears to be at least a brief hiatus in settlement during the LGM in southern Siberia.

The impact of the LGM climates on the distribution of human populations in Northern Asia is especially germane to the peopling of the Western Hemisphere, because it represents the most likely explanation for how and why the ancestral Native American population became isolated from its Asian source population before 20,000 years ago. Whole-genome analyses of living populations and of aDNA from Pleistocene samples indicate that gene flow from Asian to Native American groups ceased during the early phases of the LGM or roughly 25,000-23,000 years ago. The pattern of dental variation in living Asian and Native Americans, as well as of classic genetic markers (e. g., red blood cell antigens), supports a relatively early divergence of the two populations, as do the linguistic data.

Despite genetic isolation from their Asian parent groups during the LGM, Native Americans did not disperse in North and South America until after 15,000 years ago. Where was this population located between 25,000 and 15,000 years ago? At present, the most likely LGM refugium for ancestral Native American groups appears to be Beringia, where climate conditions were probably milder -- and plant and animal productivity higher -- than those in the interior of Northern Asia, owing to the moderating influence of the North Pacific Ocean. An LGM occupation in NE Beringia is at least tentatively documented at Bluefish Caves, where artifacts, including a microblade core, are buried in a loess unit dating to 24,000-21,000 cal BP. The associated vertebrate fauna is surprisingly rich and includes mammoth, horse, bison, saiga, and other mammals, as well as birds and fish. There also are possible indications of a refugium population in the Yana-Indighirka Lowland (NW Beringia) during the LGM.

3.2 Modern human ecology and their settlement of Northern Asia

Modern humans present a striking contrast to the Neanderthals (and probably Siberian Denisovans as well) with respect to anatomical climate adaptation. Recently derived from the lower latitudes, modern humans in northern Eurasia retained anatomical features characteristic of living peoples in the equatorial zone (e. g., elongated lower limb bones, small crania). In comparison to Neanderthals -- as well as to living peoples of the Arctic Zone -- they would have been particularly susceptible to hypothermia and frostbite in cold climates. A significant trend towards high-latitude anatomical adaptation is not evident until after the LGM.

As in the case of the Neanderthals, the supporting data are confined to Europe, but the pattern is assumed to apply to Northern Asia, given the pattern of rapid dispersal in northern Eurasia. Especially revealing are several modern human skeletons from the central East European Plain (e. g., Kostenki 14, Layer III; Sunghir') that exhibit remarkably high brachial indices (79-81), which exceed those of living people in tropical and subtropical climates (see Fig. 2). Modern humans living on the central plain of Eastern Europe probably were exposed to significantly colder winter temperatures than their counterparts in Western Europe (the current January mean for Sunghir' is -- 8.5 °C).

Modern humans in Northern Asia may have evolved a genetic adaptation to high-latitude, however, during or before the LGM. Hlusko et al. suggested that the EDAR V370A allele -- relatively common in Asian and Native American populations, apparently as the result of strong positive selection during the LGM -- represents an adaptation to low sunlight exposure and vitamin D deficiency. The allele is linked to several traits, including ductal branching of the mammary gland, which amplifies transfer of critical nutrients to infants via breast milk. Hlusko et al. tied the timing and intensity of selection for EDAR V370A to the late MIS 3 occupation of NW Beringia (Yana River sites) and subsequent genetic isolation of the ancestral Native American population at high latitude (probably in Beringia during the LGM).

Another sharp contrast between modern humans and their predecessors in northern Eurasia is the long-distance movement of raw materials. While Neanderthals and other non-modern humans rarely moved materials more than 100 km, modern humans in Eastern Europe and Northern Asia moved fossil shells, ostrich eggshell, and other items up to 500 km from their source areas . It is not clear whether the pattern reflects long-distance movement of people, exchange of goods, or a combination of both, but it probably indicates widespread networks comparable to those of recent hunter-gatherers in places where plant and animal productivity are low. Whole-genome analyses of the Kostenki and Sunghir' skeletons reveal a marriage network with low inbreeding coefficient similar to those of recent hunter-gatherers.

Modern humans also differ from the Neanderthals/Denisovans with respect to diet breadth, which was significantly greater than that of their predecessors in northern Eurasia. Most importantly in the context of adaptation to environments where plant and animal productivity was low, modern humans consumed a wider spectrum of vertebrates in the form of small mammals and possibly birds and fish. The supporting data are found in both Europe and Northern Asia. A pre-40,000-year-old occupation at Kostenki 14 on the central East European Plain contains a large concentration of fragmented hare bones, also found at the Yana River sites. Stable isotope analyses of human bone from an early Upper Paleolithic context in Crimea (Buran-Kaya III) also suggests a significant contribution to the diet from hare. The 45,000-year-old Ust'-Ishim femur from western Siberia yields high 15N values, which formerly was attributed to a diet rich in freshwater aquatic foods (as at other early and middle Upper Paleolithic sites in Europe and Northern Asia), but now appears equally likely to reflect other sources.

Fig. 4. Relationship of latitude to technological complexity, as measured by number of parts (or “technounits”) of food-getting instruments and facilities among selected recent hunter-gatherers (redrawn from [Torrence, 1983. Fig. 3.2], based on data in [Oswalt, 1976])

The critical factor in modern human settlement of the Arctic Zone and other places where plant and animal productivity was low -- and winter temperatures were very low -- was the structural and functional complexity of their technology, which represented a quantum jump over their predecessors. Multiple lines of evidence indicate that by the time they dispersed into northern Eurasia (above latitude 45° N), modern humans had developed technologies with moving parts (e. g., mechanical projectiles), including selfacting devices or automata (e. g., snares), both of which appear to be present in southern Africa before 50,000 years ago. Functionally complex technologies were probably essential for increasing hunting efficiency and success rates, and expanding dietary breadth to include small vertebrates in environments where (1) caloric demands were high, (2) digestible plant foods were scarce, and (3) large-mammal biomass was low per unit area (Fig. 4).

The earliest evidence for insulated, tailored clothing (e. g., eyed needles) is associated with the dispersal of modern humans in Eastern Europe and Northern Asia. In this case, the technology, which is hierarchically complex with multiple sub-parts, and may include moving components (i. e., drawstrings), was developed in northern Eurasia and did not originate in Africa. Other structurally and functionally complex technologies related to cold climate adaptation include heated artificial shelters and mechanical fire drills.

4. What were the barriers to settlement of the Americas before 150 years ago?

The Neanderthals, Denisovans, and other non-modern humans probably did not settle the Americas because they never occupied areas above latitude 60° North, which is a prerequisite for movement into Beringia. Although it is not possible to prove that non-modern human taxa did not inhabit high-latitude environments in Northern Asia, the current distribution of their skeletal remains and artifacts and -- more importantly -- our current understanding of their biology and ecology indicate that this most likely was the case. Had one or more of these non-modern forms occupied Beringia, there would have been multiple opportunities (e. g., during later MIS 5) to move into North America when (a) sea level was sufficiently low to expose a land connection between Chukotka and Alaska, and (b) glaciation in northwestern North America was sufficiently limited to allow movements along coastal and interior corridors from Beringia to mid-latitude areas of North America.

The principal barriers to Neanderthal/Denisovan occupation of latitudes above 60° North in Asia probably were low plant and animal productivity (i. e., widely scattered food resources) and extreme winter temperatures. The presence of Neanderthals and Denisovans in the Altai Mountains, where winter temperatures are low but biological productivity is somewhat elevated, suggests that the latter was a tighter constraint than low temperature. Owing to their body mass and proportions, the Neanderthals (and presumably northern Denisovans) were forced to satisfy high daily caloric demands, further compounded by cool climates and relatively high energetic costs of locomotion. Their seeming dependence on large mammals probably was an insurmountable barrier to occupation of habitats, such as boreal forest and shrub tundra, where large mammals are widely dispersed; recent hunter-gatherers in these habitats depended on a broad diet comprising small mammals, birds, and fish, in addition to available large mammals. In places characterized by extreme winter temperature lows (e. g., interior Beringia), their apparent lack of insulated, tailored clothing, as well as other technologies for coping with cold climate, also would have been a major problem.

Modern humans, on the other hand, occupied most terrestrial habitats in northern Eurasia, including arctic steppe-tundra, before the LGM, and possibly as early as 45 years ago. They are firmly documented on a year-round basis in the Yana-In- dighirka Lowland (NW Beringia), at latitude 70° North, no later than 32,000 cal BP (during a cold interval), and also appear to be present in the northern Yukon (NE Beringia) at roughly 24,000-21,000 cal BP, during the LGM.

It now is apparent that the modern humans who dispersed out of Africa after 75,000 years ago (i. e., the living non-African maternal and paternal lineages) were equipped with most, if not all, of the more complex technologies of recent hunter-gatherers, including mechanical instruments and self-acting facilities (i. e., automata). Structurally and functionally complex technologies were essential for adaptation to habitats where plant and animal productivity was low (and caloric demands high); they also were required for coping with extreme winter climates, even during warm intervals.

Modern humans probably were denied access to North and South America until after 15,000 cal BP by the Laurentide and Cordilleran ice sheets, which apparently blocked both coastal and interior migration routes after 55,000 years ago (i. e., before modern humans were present in Northern Asia). In our view, the coalesced glaciers represent the simplest explanation of why settlement of the Western Hemisphere by modern humans was delayed until the final millennia of the Pleistocene. We do not find the mesic tundra zone in central Beringia a credible barrier to modern humans during MIS 3 and/or the LGM (as suggested a few years ago by Meiri et al.); it may have been more of a refugium -- especially during the winter months -- than a barrier during the LGM.

New dates on artifacts in buried context in NE Beringia appear to confirm that modern humans had extended their range across Beringia (and virtually to the margin of the Laurentide ice sheet) before 20,000 years ago.

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