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.

Carbon emissions: It’s an ill wind…

The original saying emerged in Shakespeare’s Henry IV Part 2 (Act 5, Scene 3) during a jocular exchange when Ancient Pistol brings news from Court to Sir John Falstaff and other old codgers at dinner in Gloucestershire. Falstaff: ‘What wind blew you hither, Pistol?’ Pistol: ‘Not the ill wind which blows no man to good’. In the present context it seems anthropogenic CO2 emissions have staved off the otherwise inevitable launch of another glacial epoch. Climate-change deniers will no doubt pounce on this in the manner of a leopard seizing a tasty young monkey.

Auyuittuq National Park: Penny Ice Cap
Penny Ice Cap on Baffin Island ( credit: Wikipedia)

Climatologists at the Institute for Climate Impact Research in Potsdam, Germany, Potsdam University and the Santa Fe Institute in New Mexico, USA set out to develop a means for predicting the onset of ice ages (Ganopolski, A. et al. 2016. Critical insolation-CO2 relation for diagnosing past and future glacial inception. Nature, v. 529, p. 200-203) Many researchers have concluded from the oxygen isotope data in marine sediments, which tracks changes in the volume of glacial ice on land, that the end of previous interglacial periods by inception of prolonged climatic cooling may be attributed to reduction of solar heating in summer at high northern latitudes. This conclusion stems from Milankovic’s predictions from the Earth’s astronomically controlled orbital parameters and fits most of previous interglacial to glacial transitions. But summer insolation at 65°N is now more or less at one of these minima, with no signs of drastic global cooling; rather the opposite, as part of 7 thousand years of constant global sea level during the Holocene interglacial.

The latest supercomputer model of the Earth System (CLIMBER-2) has successfully ‘predicted’ the last eight ice ages from astronomical and other data derived from a variety of climate proxies. It also forecasts the next to have already begun, if atmospheric CO2 concentration was 240 parts per million; the level during earlier interglacials most similar to that in which we live. But the pre-industrial level was 280 ppm and the model suggests that would have put off the return of huge ice caps in the Northern Hemisphere for another 50 thousand years – partly because the present insolation minimum is not deep enough to launch a new ice age with that CO2 concentration – making the Holocene likely to be by far the longest interglacial since ice-age cycles began about 2.5 Ma ago. Based on current, industrially contaminated CO2 levels and a rapid curtailment of carbon emissions the model suggests no return to full glacial conditions within the next 100 ka and possibly longer; a consequence of the sluggishness of natural processes that draw-down CO2 from the atmosphere.

English: Ice age Earth at glacial maximum. Bas...
Simulation of the Earth at a glacial maximum. (Photo credit: Wikipedia)

So, does this indicate that unwittingly the Industrial Revolution and subsequent growth in the use of fossil fuels tipped the balance away from global cooling that would eventually have made vast tracts of both hemispheres uninhabitable? At first sight, that’s the way it looks. But the atmospheric carbon content of the 17th century would have resulted in much the same long drawn out Holocene interglacial; an unprecedented skipping of an ice age in the period covering most of the history of human evolution. This raises a question first posed by Bill Ruddiman in 2003: did human agriculture and associated CO2 emission begin the destabilisation of the Earth system shortly after Holocene warming and human ingenuity made farming and herding possible since about 10 thousand years ago?

But, consider this, the CLIMBER-2 Earth System model is said to be one of ‘intermediate complexity’ which is shorthand for one that relies on the ages-old scientific method of reductionism or basing each modelled scenario on modifying one parameter at a time. Moreover, for many parameters of the Earth’s climate system – clouds, dust, the cooling effect of increased winter precipitation as snow, and much else – scientists are pretty much in the dark (Crucifix, M. 2016. Earth’s narrow escape from a big freeze. Nature, v. 529, p. 162-163). Indeed it is still not certain whether CO2 levels have a naturally active or passive role in glacial-interglacial cycles, or something more complex than the simple cause-effect paradigm that still dominates much of science.