How humans might have migrated into the Americas

When and how humans first migrated into the Americas are issues that have exercised anthropologists for the last two decades, often sparking off acrimonious debate. In the 20th century both seemed to well established: hunters using the celebrated Clovis fluted spear blades arrived first, no earlier than 13 ka ago. The Beringia land bridge across what is now the Bering Strait was exposed by falling sea level as early as 30 thousand years ago in the lead-up to the last glacial maximum (LGM) to link eastern Siberia and Alaska. However, ice sheets expanding to the south-west of the main area of glaciation on the Canadian Shield barred passage through Interior Alaska and NW Canada. Only around 13 ka had a N-S ice-free corridor opened through the mountains during glacial retreat. Nevertheless, humans had entered Alaska at least ten thousand years earlier, during the LGM, to occupy caves in its western extremity: Alaska was habitable but they were stuck there.

In the early 21st century, it became clear that the ‘Clovis First’ hypothesis was mistaken. Sediments in Texas that contained Clovis blades were found to be underlain by those of an older culture, reliably dated to about 15.5 ka. Furthermore, analysis of the DNA of all groups of native Americans (north and south) indicated a last common ancestor in Siberia more than 30 ka ago: they descended from that ancestor outside of Asia. More recently excavated sites in Mexico and Chile point to human populations having reached there as early at 33 ka (see: Earliest Americans, and plenty of them; July 2020), and there is a host of pre-Clovis sites in North and Central America dating back to 18.2 ka. Such ancient groups could not have walked from the Beringia land bridge because the present topographic grain in the Western Cordillera would have been blocked by ice since about 25 thousand years ago. The only viable possibility was that they followed the Alaskan coast to move southwards, either in boats or over sea ice.

Dated pre-Clovis sites in Mexico and North America and possible expanding distribution of people from 31.3 to 14.2 ka (Credit; Becerra-Valdivia and Higham; Extended Data Fig. 4)

A new focus on when such journeys would have been feasible was published in February 2023 (Praetorius, S.K et al. 2023. Ice and ocean constraints on early human migrations into North America along the Pacific coast. Proceedings of the National Academy of Science, v. 120, article e2208738120; DOI: 10.1073/pnas.2208738120). One advantage of moving along the coast is that, though it would be pretty cold, the warming effect of the Pacific Ocean would make it more bearable than travelling inland, where winter temperatures even today regularly reach -50°C. More important, there would be no shortage of food; fish, marine mammals and shellfish abound at the ice margin or onshore, at any season. But a coastal route may not have been possible at all times during the period either side of the LGM. Large glaciers still reach the ocean from Alaska and there is little more perilous than crossing the huge crevasse fields that they present. Boating would have been highly dangerous because of continual calving of icebergs from extensive ice shelves. Moreover, the Alaska Coastal Current flows northwards and would likely have sped up during episodes of glacial melting as the current is affected by fresh water influx. Yet there may sometimes have been episodes of open water at the ice front frozen to relatively flat sea ice in winter. That would making boat- or foot travel relatively safe. Sea ice would also make glacier-free islands accessible for encampments over the harsh winters or even for hundreds of years, with plenty of marine food resources.

Summer Praetorius of the US Geological Survey and colleagues from Woods Hole Oceanographic Institution, Oregon State University, and the Universities of California (Santa Cruz) and Oregon have attempted to model conditions since 32.5 ka ago in coastal waters off Northwest America. They used simulations of the behaviour of the Alaska Coastal Current during varying climate conditions before and during the LGM, while glaciers were in  retreat that followed and during the Holocene. Their modelling is based on the effects of changing sea level and water salinity on general circulation in the Northern Pacific. The relative abundance of sea ice can be tracked using variation in an alkenone produced by phytoplankton that wax and wane according to sea-surface temperature and sea-ice cover. The other input is the well-documented changing extent of continental glaciation in Alaska and the Yukon Territory. Based on their model they estimate that the most favourable environmental conditions for coastal migration occurred just before the LGM (24.5 to 22 ka) and between 16.4 and 14.8 ka during the initial stages of warming and extensive melting of ice sheets. The Alaskan Coastal Current probably doubled in intensity during the LGM making the use of boats highly dangerous

By 35 ka ocean-going boats are known to have been used by people in northern Japan. Traversing sea-ice was the way in which Inuit people occupied all the Arctic coastal areas of North America and Greenland during the last five thousand years, and is the form of travel favoured by the authors. It is not yet possible to prove and date such coastal journeys because campsites or settlements along the coast would now be inundated by 100 m of post-glacial sea-level rise. Yet such migration was necessary to establish settlements at lower latitudes in North America and Mexico in the period when overland routes from Beringia were blocked by ice sheets. By 32.5 ka falling sea level probably made it possible to cross the Bering Strait for the first time and for the next 7.5 ka an ice-free corridor made it possible for the rest of North America and points further south to be reached on-foot from Alaska. That window of opportunity might have allowed humans to have reached Mexico and South America, where the earliest dates of occupation have been found. But many of the early sites across North America date to the period (25 to 13 ka) when overland access was blocked. Of course, those sites might have been established by expansion from the very earliest migrants who crossed the Beringia land bridge and took advantage of overland passage before 25 ka. But if later migrants from Asia could follow the coastal route, then it seems likely that they did. Later Inuit spread along  the shores of the Arctic Ocean since 5000 years ago probably with a material culture little different from that of the earlier migrants from Siberia.

Earliest Americans, and plenty of them

Who the first Americans were is barely known outside of the tools that they left in the archaeological record. For most of the late 20th century US researchers claimed that the first people to migrate into the Americas produced stone tools of the Clovis culture that first appear just before the Younger Dryas cold period, around 13.2 to 12.9 thousand years (ka) ago. The hallmark of Clovis culture is the finely-worked stone spear point, and its association with butchered large mammals: the Clovis people were apparently big-game hunters  Despite other, albeit less convincing, signs of earlier human habitation, this notion ossified for a seemingly irrefutable reason. To reach the Americas from NE Asia on foot, these people would have had to cross the Bering Straits via the Beringia land bridge exposed as sea level fell during the Last Glacial Maximum (LGM). That would have taken them to Alaska, but an exit to the south remained blocked by the huge Laurentian ice sheet until around 13 ka. Once an ice-free route had opened, the Clovis people migrated quickly to reach the site from which they take their name in New Mexico. But other archaeological sites discovered in the last couple of decades, extending as far south as Chile, have yielded ages that clearly predate the Clovis culture (see: Clovis First hypothesis dumped, May 2008). Beneath a Clovis-bearing layer at a site in Texas excavators unearthed thousands of totally different tools reliably dated to as far back as 15.5 ka (see: Clovis first hypothesis refuted, May 2011). This opened the realistic possibility that the earliest migrants had not necessarily walked from Asia, but may have followed a marine route along the Pacific coast and spread eastwards as opportunities presented themselves.

Now Mesoamerica has convincingly verified migration more than twice as long ago as that which littered North America with Clovis tools. It emerged from the Chiquihuite Cave 2.7 km high in the Astillero Mountains of northern Mexico. Almost 2000 stone artefacts were found throughout a 3 m thick layer of sediment beneath the cave floor that spans 27 to 13  ka, (Ardelean, C.F. and 27 others 2020. Evidence of human occupation in Mexico around the Last Glacial Maximum. Nature, v. 584 p. 87–92; DOI: 10.1038/s41586-020-2509-0). The technology revealed by the tools is more primitive than that of the Clovis culture. Artefacts occur throughout the layer, which extends back in time from the Younger Dryas, through the preceding period of warming and the LGM itself. Although colder than the present equitable climate of the high mountain valleys of Northern Mexico environmental data obtained from the layer show that it was viable for occupation through the LGM. Of the 42 highly precise and accurate radiocarbon dates those from some of the stratigraphically deepest part of the layer exceed 33 ka, which the authors suggest may establish the initial human occupation of the cave. Incidentally, although the paper was published online in July 2020 it was submitted to Nature in October 2018. That is a very long time in the editorial and review process. There is no indication as to why there was such a delay: maybe an indication of some continuing defence of the Clovis First hypothesis among the reviewers …

Dated pre-Clovis sites in Mexico and North America and possible expanding distribution of people from 31.3 to 14.2 ka (Credit; Becerra-Valdivia and Higham; Extended Data Fig. 4)

The radiocarbon dating in the paper was carried out at the state-of-the-art accelerator mass spectrometer unit at the University of Oxford, UK, by two of the co-authors (Lorena Becerra-Valdivia and Thomas Higham). They too published a Nature paper in late July 2020, which discusses their new dating of 42 archaeological sites in North America and Siberia (Becerra-Valdivia, L. & Higham, T. 2020. The timing and effect of the earliest human arrivals in North America. Nature, v. 584, p. 93-97; DOI: 10.1038/s41586-020-2491-6). In Mesoamerica and North America (the Clovis heartland) their results suggest that, as in Chiquihuite Cave, ‘people were present in different settings before, during and immediately following the LGM’, their ranges increasing over time. These people would likely not have followed the same route suggested for the later Clovis people, i.e. across Beringia and then parallel to the topographic grain in the Western Cordillera, ice-cap melting permitting. An interesting suggestion by Becerra-Valdivia and Higham is that post-LGM expansion in numbers and range of these early American contributed to the famous extinction of the North American Pleistocene megafauna. Dating the extinctions of different genera suggests that disappearance of the megafauna may not have been a single event during the Younger Dryas, but seems to have been during at least two other episodes peaking at about 40 and 24 ka. Both the ecological devastation supposedly associated with the Clovis people and the impact theory for its cause depend on a single event.

See also:  Gruhn, R. 2020. Evidence grows for early peopling of the Americas. Nature, v. 584, p. 47-48; DOI: 10.1038/d41586-020-02137-3; Rincon, P. 2020. Earliest evidence for humans in the Americas (BBC News, 22 July 2020); Keys, D. 2020. Humans reached the Americas 11,000 years earlier than previously thought, archaeologists discover (Independent, 22 July 2020)

The Great Barrier Reef and the Last Glacial Maximum (LGM)

The 2,300 km stretch of coral reefs and islands in the Coral Sea off the coast of Queensland, Australia is the largest single structure on Earth built by living organisms. The dominant reef builders are four hundred species of coral, most of which are a symbiosis that conjoins marine invertebrates in the class Anthozoa – part of the phylum Cnidaria – and photosynthesising single-celled eukaryotes known as dinoflagellates. These algae are mainly free-living marine plankton, some species of which evolved to be co-opted by corals. Their role in the symbiosis is complex; on the one hand providing energy in the form of sugars, glycerol and amino acids; on the other consuming the coral polyps’ carbon dioxide output. The latter is fixed, in the case of hard corals, by the secretion of calcium carbonate: the key to reef formation.

Marine photosynthesisers demand clear water in the upper few tens of metres of the sea, together with sunlight least affected by the atmosphere, as in the tropics where the sun rises to the zenith year round. The coral animal-algae connection limits reef growth to shallow seas, the top of the reef being close to mean sea level, sometimes rising above it at low tide. Hence the formation of fringing and barrier reefs. In the case of atoll reefs, a connection with sea-floor volcanoes that rose from hotspots on the oceanic abyssal plains to form active volcanic islands that began to sink once they became extinct. The pace at which reefs can grow is generally able to match that of crustal subsidence so that atolls remain throughout the Western Pacific. Reef growth is also capable of coping with global sea-level changes, so that the present top level of the Great Barrier Reef has been in balance with the generally static sea level of the Holocene since the ice caps of the last glaciation melted back to roughly their present extent about seven thousand years ago.

There are many cases of different reef levels on and around islands that match the sea-level fluctuations during the last Ice Age.  High-resolution bathymetry produced by multi-beam sonar across the eastern edge of parts of the Great Barrier Reef reveals a series of submerged terraces down to almost 120 m below modern sea-level (Yokoyama, Y. and 17 others 2018. Rapid glaciation and a two-step sea level plunge in the Last Glacial Maximum. Nature, v. 559, p. 603-607; doi:10.1038/s41586-018-0335-4). Globally, the LGM began at around 31 ka when sea level fell by about 40 metres, thanks to massive accumulation of glacial ice at high latitudes. Previous studies to chart the changes in global mean sea level during the LGM suggested a steady fall until about 20 ka, followed by rapid rise as ice caps melted back. The multinational team led by Yusuke Yokoyama of the University of Tokyo, obtained precise ages of coral samples from different depths in drill cores through the coral terraces. These data revealed a more complex pattern of sea-level change, in particular a hitherto unsuspected plunge between 21.9 and 20.5 ka of 20 m to reach -118 m. This immediately preceded the warming-related rise that continued to Holocene levels.

GBR Bathymetry
High-resolution sonar images of the sea floor at two sites on the eastern edge of Australia’s Great Barrier Reef. They show terraces associated with, the lowest of which corresponds to the Last Glacial Maximum. (Credit: Yokoyama et al. 2018, Figure 1)

Curiously, this massive phenomenon is not shown by sea-level estimates derived from the records of changing oxygen isotopes in ocean-floor sediments and ice cores. The team’s complex modelling incorporated global changes in land and sea-bed levels, and thus changes in the volume of the ocean basins, due to the changing isostatic effects of both ice-cap and ocean masses. From these it is possible to reach an interesting conclusion (Whitehouse, P. 2018. Ancient ice sheet had a growth spurt. Nature, v. 603, p. 487-488; doi:10.1038/d41586-018-05760-3). Rather than an increase in snowfall onto ice-caps, their retreat may have been hindered by thickening of marginal floating ice shelves that created buttresses around Antarctica and the northern ice sheets. Slowed glacial flow to the oceans could have promoted ice sheet growth for a time as melting of calved icebergs was hindered, especially in the case of the ice sheet over northern North America. Certainly, this crucial climatic turning point was a lot more complex than previously believed.