4.II: Ice Subsides, Americans Arrive

first Americans, native Americans, paleo Indians, Beringia
The first humans to reach the American continents, 15 – 30 thousand years ago, shared the New World with megafauna such as the wooly mammoth and saber-toothed cat. 1

A. The Last Glacial Maximum and the Present Interglacial

B. The First Americans

C. Worldwide Human Evolution After 30 TYA

D. Citations

A. The Last Glacial Maximum and the Present Interglacial

Modern humans emerged in the middle of the last ice age, a 100,000-year glacial that probably postponed agricultural lifestyles.  This ice age peaked in the last glacial maximum about 20,000 years ago.  Canada and Northern Eurasia were almost entirely covered in glaciers three to four kilometers thick.  The Antarctic ice sheet was larger than it is today.  With a sizeable fraction of Earth’s water locked up in land ice, sea levels were up to 100 meters lower than at present.  This exposed a large land bridge in Beringia, between Russia and Alaska, which persisted from 30 to 11 TYA. 2 Significantly, Beringia had dry climate.  Alaska did not experience enough snowfall to form glaciers. 3 It remained an Arctic desert, a steppe with grassland ecosystems.  Animals such as camels, elk, and horses traversed Beringia regularly; it was all one continent to them. 

The last glacial maximum impacted more than just the highest latitudes.  The entire planet became colder and drier.  The habitable zone was more restricted than usual, even by ice age standards.  Most northern plants and animals, including humans, were forced to migrate southward, making populations denser and more diverse.  There was a time when reindeer lived alongside armadillos in the southern United States, 4 and plants that normally exist only high in the mountains crept down toward sea level. 5 Wildlife was confused for thousands of years.

Milankovitch Cycles brought increasing sunlight back to the planet after 16,000 years ago. 6 Northern hemisphere glaciers began to subside from their maximum extent, while the Antarctic ice sheet shrank considerably.  As these continental ice sheets melted, their waters returned to the global ocean, ultimately flooding Beringia and once again separating North America from Asia.  The flow of glacial water to the seas was a dramatic event on land, too.  As glaciers melted in North America, they filled thousands of lakes including the Great Lakes.  The current course of the Mississippi River was shaped by glacial water gushing toward the sea in torrents. 7   

By about 13,000 years ago, the northern hemisphere was well on its way into an interglacial.  Then cold climate momentarily resurged in an event known as the Younger Dryas.  It was probably caused by a sudden influx of fresh melt water into the Atlantic Ocean, which disrupted the normal flow of the Gulf Stream and its heat content. 8 Temperatures returned to ice age levels for a full millennium, with abrupt changes at both the beginning 9 and the end 10 of this interval. The Younger Dryas was only a northern hemisphere phenomenon, and was apparently unique to the last interglacial, making this one different from earlier ice ages. 11 This anomaly could help explain a tragic and mysterious circumstance surrounding the end of the last ice age – the mass extinction of many large mammals, megafauna, from North America.  For millions of years, North America had been a veritable wild animal park, roamed by museum favorites including woolly mammoths and mastodons, giant sloths, saber-toothed cats, camels, horses, woolly rhinos, dire wolves, and short-faced bears.  They survived every ice age except the last one, when they all suddenly vanished.  The Younger Dryas is one frequently cited factor behind these extinctions.  After all, the Dryas event itself is named after its drastic ecological shift, when woods were replaced by prairies of arctic Dryas flowers. 

The end of the Younger Dryas event around 11,500 years ago is used as a convenient marker to end the ice age.  Glaciers retreated almost everywhere, with holdouts remaining only in Antarctica, Greenland, and the highest mountain ranges.  The most recent geological epoch is technically called the Holocene but is more informally known as the present interglacial.  As this name suggests, we would expect today’s balmy climate to be just one lucky warm period in a continuing cycle of (long) ice ages and (short) interglacials – provided that human influences do not overwhelm the natural cycle. 1 12    

B. The First Americans

The details of exactly where, when, and how the first human beings reached America are fiercely debated questions in today’s archaeological community.  Whoever they were, the first Americans are now called Paleo-Indians. 2 They were nomadic and sparsely populated, leaving behind few traces of their existence.  Almost all experts now agree that Paleo-Indians walked across the most recent land bridge in Beringia, from Siberia to Alaska, less than 30 TYA.  

During the last glacial maximum, Beringia was a rare “refuge” from northern glaciation.  For perhaps thousands of years, Paleo-Indians were isolated in arid Alaska, blocked by glaciers from entering Canada.  As the world warmed, this obstacle melted away.  The Pacific coast was clear 16,000 years ago. 13 A few millennia later, an ice-free corridor opened up through inland Canada. 14 

Once they broke out of their ice trap, the Paleo-Indians settled the Americas rapidly.  The famed Monte Verde site in Chile – almost at the southern tip of South America – is dated conservatively to 14,500 years ago.  That would mean that it took humans less than two millennia to cross all of the Americas, ten times their pace in Asia.   

Various hypotheses speculate about this blazing fast settlement.  Some Paleo-Indians could have boated down the coast.  This would have given them a mode of transportation faster than walking.  Hunting and fishing implements found on the Channel Islands, 30 km off the coast of California, lend credence to the boating hypothesis. 15 The shoreline is difficult to study because sea levels are much higher now, and the sites where Paleo-Indians would have set up camp on the beach are now submerged.  The oldest human remains in the Americas, possibly up to 14,000 years old, are found in marine caves 30 meters underwater in Mexico. 16   

Another possible explanation for fast American colonization is the migrating-herd hypothesis.  Paleo-Indians hunted large herd animals like bison and mammoths.  If those animals migrated extensively and / or quickly resettled America as it de-glaciated, then the humans would follow.  In fact, this is another commonly cited factor behind the extinction of the megafauna.  Some scientists believe that mammoths and mastodons, giant beavers, and sloths were overhunted and / or crowded out by humans, leading to the demise of the carnivorous cats, bears, and wolves. 17 

These questions are far from satisfactorily answered.  Too many mysteries remain.  Could Paleo-Indians really have settled the Americas at such breakneck speed, or did some actually sneak into North America before the glacial maximum? 18 Neither climate change nor human activity seems sufficient to account for the megafauna’s mass extinction.  Was it a deadly one-two punch, or were there other crucial factors?  The peopling of the Americas is a high-profile niche of archaeology, so these questions are heavily researched, and the scientific consensus is sure to shift rapidly.       

The first culture to be widespread in the Americas was the Clovis people.  Clovis artifacts are found in a broad region (concentrated in the southeastern US) within a curiously narrow time span at precisely 13 TYA.  The Clovis people were genetically traceable to Siberian origins, and they were more closely related to today’s Native Americans (especially South – Central Americans) than to any other populations. 19           

The remnants that Paleo-Indians left behind at their sites include stone tools and hunting weapons, fire pits, and butchered animals.  Monte Verde is a unique site where marshy conditions have preserved organic artifacts.  Wooden hut foundations, tent stakes, ropes, and even scraps of clothing have been recovered there, alongside a human footprint over 14 TYO. 20  

The gate from Asia closed 11 TYA.  It is certainly possible that multiple waves of migrants colonized America during Beringia’s 20,000-year exposure.  In the Holocene, American migration was strictly intracontinental.  By 2000 BCE, even the Canadian arctic was settled by Paleo-Eskimos, who followed eastward migrations of the bowhead whale.       

C. Worldwide Human Evolution After 30 TYA

1. Grandparents

2. Settler-cultivators

3. Toward today’s racial spectrum

1. Grandparents

Human fossils reveal a striking demographic trend 30,000 years ago.  For the first time, humans were routinely living to more than twice the age of sexual maturity – old enough to become grandparents! 21 Longevity is valuable in an environment where knowledge, transmitted through the generations, benefits the survival of the clan. 22

Many species have grandparents.  What is unique to humans is the life phase of menopause:  as women age, they stop having children of their own.  At first glance, menopause might seem like a biological mystery.  How could older women have any evolutionary advantage after reproductive age? 

One plausible answer is that post-menopausal women generally focus on feeding and caring for their grandchildren, especially their daughters’ children. 23 A grandmother’s care not only nourishes her grandchildren but shortens nursing time, allowing her daughter to have more children.  Grandma’s genes succeed, including her genes for longevity, even when she is no longer having children of her own! 24

By remembering and transmitting cultural knowledge and caring for the young, grandparents revolutionized the family unit.  Clans became larger and cousins remained closer.  This was a huge step in making hunter-gatherer cultures more complex. 

Grandma – she’s more than just a woman to love!

2. Settler-cultivators

For millions of years, humans were hunter-gatherers, traveling in family units.  It was necessary to be on the move.  Animals migrated, plants were seasonal, and bands of hostile humans were all around.  It was a big step to settle down in one spot year-round, especially for years on end.  Though they were literally sitting targets, settlements had the advantage of supporting larger populations for defensive strength.

The earliest evidence of settlements appears 15 TYA.  From eastern Europe to central Asia, there are numerous finds of huts made from mammoth bones, presumably covered with skin at the time.  The Natufians of the Levant built houses with stone foundations, probably covered with sticks.  Chile’s Monte Verdeans built wooden huts. 

Settlement required cultivation, the caretaking of local plants for ongoing food supplies.  Cultivation goes a step beyond gathering; it involves acts such as selective weeding and burning.  It does not go as far as agriculture, which is characterized by methodically isolating crops, selecting and planting seeds, breeding animals, and controlling water.  However, the settler-cultivator way of life would make an easy and natural transition to agriculture. 25          

Hunter-gatherers were characterized by egalitarianism.  The earliest settlements already showed signs of social inequality.  A minority of Natufians wore expensive seashell jewelry and were honored with special burial treatment. 26

3. Toward today’s racial spectrum

To this point in the book, it has been fair to use the term “our” ancestors.  More than 15,000 years ago, my genealogical ancestors were identical to yours and to those of every person alive today. 27 As we pass the point of worldwide identical ancestry and come closer to the present, the evolution of continents and then of nations becomes increasingly isolated. Your ancestors start to look, sound, and behave more like you and less like me. 

“Race” has recently taken on a social significance that far exceeds its biological basis.  That’s why many scientists now describe race as a social construct, something that matters only if we make it matter. Yet we can’t deny the obvious differences between people from, say, Nigeria and Japan. To escape this cognitive dissonance, we must properly characterize racial differences as they appear in the real and imagined worlds.  

When populations are isolated, they evolve in response to different environments, which may pressure them into assuming different appearances.  Coloration is the most visible and well-known form of racial differentiation.  The dark skin of equatorial peoples is an effective sunscreen; it was probably sapiens’ original hue.  Lighter skin is an adaptation to higher latitudes, making the skin more sensitive to sunlight for the photosynthesis of vitamin D.  Some populations have adapted to unique environmental challenges by evolving tall / lean or short / stout bodies, resistance to local pathogens, lactose tolerance, etc.  These adaptations have not made any groups of people “better” than anyone else, except when it comes to being healthy in their pre-industrial native environments.  Other characteristics are health-neutral, like the way that some Asians’ eyelids fold. 

In the short term, such distinctions will cluster locally. By and large, they now serve as visual markers associated with regions on the map.  That’s where the psychology takes over.  The skin tones and eyelid folds, which may or may not have served some biological function, now play an overinflated social role because people relate more to those who look like themselves.  On a grander scale, we still evolve multiregionally, and this means that we all partake of the same gene pool.  Some genes are denser in some parts of the pool than others, but there are no genes that are unique to one part of the world. 

Most people tend to mentally categorize humans into just a few physical types, 28 something like this palette (these colors are obviously metaphorical).

The socially constructed fiction comes in two parts: first, that people come in such fixed and discrete categories, and second that these physical differences are socially important. The underlying diversity is real, but it is continuous, like this spectrum. 29

It’s not surprising that our mind perceives a small number of categories, because the underlying psychological motivation is simply to distinguish “us” from “them”; we don’t care that much about the variation in outsiders. 

Real human diversity is more like a three-dimensional spectrum.  We can map it across Earth’s two-dimensional surface.

The third dimension is time, because this spectrum is constantly churning and mixing. 30 Unless you are reading this book in print, you can animate the full 3D spectrum by clicking the map above. We would probably recognize the racial spectrum of 3,000 years ago.  It was completely different 30,000 years ago and will be unrecognizable again in another 30,000 years. 3

Prehistorically, race was not much on people’s minds, because they interacted locally and hardly ever encountered people from different continents.  In fact, I include this discussion not because race was salient 10,000 years ago but because it is endlessly fascinating to present-day readers.  Racial politics are only an issue of post-colonial times, when intercontinental migration became common.  For today’s sake, it’s important to understand what racial diversity is, how it evolves, and how to think and talk about it.


Up to Chapter 4

Back to Section 4.I:  Geological And Archaeological Ages

Top of this page

Continue to Section 4.III:  Agriculture

D. Citations

  1. Cover image in the public domain as the creative work of a US federal government agency. National Park Services, https://www.nps.gov/bela/learn/nature/the-ice-age.htm (accessed and saved 12/23/17, archived 5/02/20).
  2. Aixue Hu et al., “Influence of Bering Strait flow and North Atlantic circulation on glacial sea-level changes”, Nature Geoscience 3, 118 – 121 (1/10/2010), http://www.nature.com/ngeo/journal/v3/n2/full/ngeo729.html (accessed and saved 1/18/20).
  3. Ned Rozell, “Why was interior Alaska green during the last ice age?” University of Alaska Fairbanks Geophysical Institute, 9/25/2014, http://www.gi.alaska.edu/alaska-science-forum/why-was-interior-alaska-green-during-last-ice-age (accessed and saved 7/02/17, archived 1/18/20).
  4. Sharon Levy, Once and Future Giants, Oxford University Press (2011), p. 15.
  5. Craig D. Allen, Julio L. Betancourt, and Thomas W. Swetnam, “Land Use History of North America (LUHNA): The Paleobotanical Record”, USGS (12/09/2016), https://geochange.er.usgs.gov/sw/impacts/biology/paleobotany/  (accessed, saved, and archived 12/20/20).
  6. David Meltzer, First Peoples in a New World:  Colonizing Ice Age America, University of California Press (Kindle eBook edition, 2009),  location 1205.
  7. Pat Middleton et al., “Glaciers Left Their Mark on the Mississippi River” (c. 1997), http://greatriver.com/Ice_Age/glacier.htm (accessed and saved 7/09/2017, archived 1/18/20).
  8. National Oceanic and Atmospheric Administration, “The Younger Dryas” (c. 2016), https://www.ncdc.noaa.gov/abrupt-climate-change/The%20Younger%20Dryas (accessed and saved 7/09/2017).
  9. European Science Foundation, “Big freeze plunged Europe into ice age in months”, Science Daily (11/30/2009), https://www.sciencedaily.com/releases/2009/11/091130112421.htm (accessed, saved, and archived 1/18/20).
  10. R.B. Alley et al., “Abrupt increase in Greenland snow accumulation at the end of the Dryas event”, Nature 362, 527 – 529 (4/08/1993), https://www.nature.com/articles/362527a0 (accessed and saved 1/18/20).
  11. Wallace Broecker, “Was the Younger Dryas Triggered by a Flood?” Science 312(5777):1146-1148 at 1147 (5/26/2006), https://science.sciencemag.org/content/312/5777/1146/tab-figures-data (accessed and saved 7/09/2017).
  12. Owen Gaffney and Will Steffen, “The Anthropocene equation”, The Anthropocene Review, Vol. 4, Issue 1 (2/10/2017), http://journals.sagepub.com/doi/pdf/10.1177/2053019616688022 (official pay site), https://www.slideshare.net/owengaffney/the-anthropocene-equation-2017-gaffney-steffen (free low-res copy posted by author, accessed and saved 7/09/2017, archived 1/18/20).
  13. Lauriane Bourgeon, Ariane Burke, and Thomas Higham, “Earliest Human Presence in North America Dated to the Last Glacial Maximum: New Radiocarbon Dates from Bluefish Caves, Canada”, PLoS ONE 12(1): e0169486 (01/06/2017), https:doi.org/10.1371/journal.pone.0169486 (accessed and saved 7/16/2017).
  14. Mikkel Pedersen et al., “Postglacial viability and colonization in North America’s ice-free corridor”, Nature 537, 45-49 (9/01/2016), http://www.nature.com/nature/journal/v537/n7618/full/nature19085.html (abstract accessed and saved 7/23/2017).
  15. Jon M. Erlandson et al., “Paleoindian seafaring, maritime technologies, and coastal foraging on California’s Channel Islands”, Science Vol 331, issue 6021, pp. 1181-1185 (3/04/2011), http://science.sciencemag.org/content/331/6021/1181.long (accessed and saved 1/19/20).
  16. Arturo Gonzalez et al., “The Arrival of Humans on the Yucatan Peninsula: Evidence from Submerged Caves in the State of Quintana Roo, Mexico”, Current Research in the Pleistocene vol. 25, 2008, Special Report pp. 1 – 24, https://www.researchgate.net/publication/310750546_The_arrival_of_humans_on_the_Yucatan_Peninsula_Evidence_from_submerged_caves_in_the_state_of_Quintana_Roo_Mexico (accessed and saved 1/19/20).
  17. This was the hypothesis of Paul Martin, who compiled a lifelong summary of his research as Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America, University of California Press (2005).  Most experts believe that human presence contributed to the mass extinction, but that overhunting alone could not have killed so many animals so quickly.
  18. David Reich, Who We Are and How We Got Here, Pantheon Books (Kindle eBook edition, 2018), locations 1271 – 1280.
  19. Morten Rasmussen et al., “The genome of a late Pleistocene human from a Clovis burial site in western Montana”, Nature 506, 225-229 (2/13/2014), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4878442/ (accessed and saved 7/16/17).
  20. Tom D. Dillehay et al., “New Archaeological Evidence for an Early Human Presence at Monte Verde, Chile”, PLOS ONE (11/18/2015), http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0141923 (accessed and saved 7/16/2017).
  21. Rachel Caspari and Sang-Hee Lee, “Older age becomes common late in human evolution”, PNAS vol. 101, no. 30 (July 27, 2004), https://www.pnas.org/content/101/30/10895 (accessed and saved 7/23/2017).  This paper describes a sudden increase in longevity in the “early Upper Paleolithic.”  In a follow-up article, Caspari specified that the Paleolithic sample was 20 – 30 TYO and that “grandparents … first became common around 30,000 years ago.”  “The Evolution of Grandparents”, Scientific American 22(1):38-43 (Dec., 2012), https://www.scientificamerican.com/article/the-evolution-of-grandparents-2012-12-07/ (accessed and saved 1/19/20).
  22. Rachel Caspari and Sang-Hee Lee, “Is Human Longevity a Consequence of Cultural Change or Modern Biology?” American Journal of Physical Anthropology 129:512-517 (2006), http://faculty.ucr.edu/~shlee/Publications/06%20OY%20W%20As%20(AJPA).pdf (accessed and saved 7/23/2017).
  23. Kristen Hawkes et al., “Hadza Women’s Time Allocation, Offspring Provisioning, and the Evolution of Long Postmenopausal Life Spans”, Current Anthropology vol. 38 no. 4 (August – October 1997), 551-577, https://www.journals.uchicago.edu/doi/10.1086/204646 (accessed and saved 1/20/20).
  24. Mirkka Lahdenperä et al., “Fitness benefits of prolonged post-reproductive lifespan in women”, Nature vol. 428 (3/11/2004) 178 – 181, https://www.nature.com/articles/nature02367 (accessed and saved 1/20/20).
  25. Nicholas Wade, Before the Dawn:  Recovering the Lost History of Our Ancestors, Penguin Press (Kindle eBook edition, 2006), location 2234.
  26. Ibid. at location 2264.
  27. Douglas Rohde, “Modelling the recent common ancestry of all living humans”, Nature 431:562-566 (9/30/2004), https://www.nature.com/articles/nature02842 (accessed and saved 1/20/20).  Supplement at https://pdfs.semanticscholar.org/48ac/fefc93b65bc7313ce072be946ad6132a33bc.pdf (accessed, saved, and archived 1/20/20).  Rohde’s computer model indicates that humanity’s identical ancestry point could have occurred 5 – 15 TYA.  Bearing in mind the realities of American isolation, it would have occurred at the earlier end of this range.
  28. The Hebrew bible described three human races.  18th century German historians invented the five-color system “white, black, yellow, red, brown.”  Even today, the United States census has five racial categories, corresponding roughly to the continents.
  29. Image: “Colours of the visible light spectrum” by Meganbeckett27, CC BY-SA 3.0 (accessed and saved 12/06/20).
  30. David Reich, Who We Are and How We Got Here, Pantheon Books (2018) maps out the details of this churning racial spectrum, an overview of 21st-century ancient-DNA genomics.  The terms “racial spectrum” and “3D spectrum” are my own.
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