“Modern” humanity emerged in the middle of the last ice age, a 100,000-year deep freeze that severely restricted human potential. 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 also 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 KYA. 1 Significantly, Beringia had dry climate. Alaska did not experience enough snowfall to form glaciers. 2 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, 3 and plants that normally exist only high in the mountains crept down toward sea level. Wildlife was very confused for thousands of years.
Milankovitch Cycles brought increasing sunlight back to the planet after 16,000 years ago. 4 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. 5
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. 6 Temperatures returned to ice age levels for a full millennium, with very abrupt changes at both the beginning 7 and the end 8 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. 9 This anomaly could help explain another 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 all suddenly vanished. The Younger Dryas is one of the two most frequently cited factors 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 tops of high 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 10
The details of exactly where, when, and how the first human beings reached America are some of the most fiercely debated questions in today’s archaeological community. Whoever they were, the first Americans are now called paleo-Indians. 2 They were nomadic and initially very sparsely populated (a few thousand persons in all of the Americas), leaving behind very few traces of their existence. Almost all archaeologists now agree that paleo-Indians walked across the most recent land bridge in Beringia, from Siberia to Alaska, less than 30 KYA. 3 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. 11 A few millennia later, an ice-free corridor opened up through inland Canada. 12
Once they broke out of their ice trap, the paleo-Indians apparently settled the Americas very 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. 13 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. 14
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. 15
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 they actually sneak into North America before the glacial maximum? 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, named after the site of its discovery in Clovis, NM. This culture is defined by a particular arrowhead known as the fluted point, with grooves centered in the faces like the fluting of some swords. Fluted points are found in a broad region (concentrated in the southeastern US) within a curiously narrow time span at precisely 13 KYA. Scientists have been fortunate enough to not only find one Clovis skeleton, a baby boy in Montana named Anzick-1, but to extract a sample of his DNA. Genetic analysis confirmed that the Clovis people were traceable to Siberian origins, and that they were more closely related to today’s Native Americans (especially South – Central Americans) than to any other populations. 16
Paleo-Indians were hunter-gatherers who traveled in large extended families like everyone else in the world. The evidence left at their sites includes 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 15,000 year old human footprint. 17
The gate from Asia closed 11 KYA. It is certainly possible that multiple waves of migrants colonized America within this 20,000 year window. In the Holocene, American migration was strictly intracontinental as paleo-Indians occupied all available land. By -2000, even the Canadian arctic was settled by paleo-Eskimos, who followed eastward migrations of the bowhead whale.
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! 18 Longevity might have been fostered in an environment where knowledge transmitted through the generations was valuable for the survival of the clan. 19
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?
The plausible answer is that post-menopausal women generally focus on feeding and caring for their grandchildren, especially their daughters’ children. 20 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! 21
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!
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 KYA. 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. 22
Hunter-gatherers are 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. 23
To this point in the book, it has been fair to use the term “our” ancestors. Far enough back in time, my genealogical ancestors were identical to yours and to those of every human alive today. The point of identical ancestry dates back to about 15,000 years. 24 Since that time, the human family tree has diverged into isolated lines of descent. The “races” as we know them today have evolved by virtue of isolation and pressures from their separate environments.
The most visible and well-known form of racial differentiation is coloration. The dark skin of equatorial peoples is an adaptation to reduce sunburn, and was probably the species’ original hue. Lighter skin is an adaptation to higher latitudes, making the skin more sensitive to sunlight for the production of vitamin D. A general reduction of pigment gave Europeans light-colored hair and eyes. Asians evolved their characteristic single-fold eyelid, which probably does not serve any purpose but was simply a neutral mutation in that part of the world. There are also internal differences local to certain regions. One of the best-documented is Africans’ resistance to malaria at the cost of sickle-cell anemia. “Racial identity” was never much of an issue in human affairs until very recently, in the modern age of global migration and mixing. Today’s perspective gives us an inflated sense of race’s significance in the past.
- Hu et al, “Influence of Bering Strait flow and North Atlantic circulation on glacial sea-level changes”, Nature Geoscience 3, 118 – 121 (2010), abstract and paid access at http://www.nature.com/ngeo/journal/v3/n2/full/ngeo729.html , as cited by Meiri et al, “Faunal record identifies Bering isthmus conditions as constraint to end-Pleistocene migration to the New World”, Proc Biol Sci. 2014 Feb 7; 281(1776), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3871309/ (accessed and saved 7/02/17). ↩
- 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). ↩
- Sharon Levy, Once and Future Giants, Oxford University Press (2011), p. 15. ↩
- David Meltzer, First Peoples in a New World: Colonizing Ice Age America, University of California Press (2009), Kindle ebook version, location 1205. ↩
- Pat Middleton et al, “Glaciers Left Their Mark on the Mississippi River”, http://greatriver.com/Ice_Age/glacier.htm (accessed and saved 7/09/2017). ↩
- National Oceanic and Atmospheric Administration, “The Younger Dryas”, 2011+, https://www.ncdc.noaa.gov/abrupt-climate-change/The%20Younger%20Dryas (accessed and saved 7/09/2017). ↩
- William Patterson quoted by Kate Ravilious, “Mini ice age took hold of Europe in months”, New Scientist, 11/11/2009, https://www.newscientist.com/article/mg20427344-800-mini-ice-age-took-hold-of-europe-in-months/ (accessed and saved 7/09/2017). ↩
- Alley et al, “Abrupt increase in Greenland snow accumulation at the end of the Dryas event”, Nature 362, 527 – 529 (08 April 1993), doi:10.1038/362527a0, abstract accessed and saved 7/09/2017. ↩
- Wallace Broecker, “Was the Younger Dryas Triggered by a Flood?” Science 5/26/2006: vol. 312, Issue 5777, pp. 1146 – 1148, at 1147. http://ocp.ldeo.columbia.edu/res/div/ocp/gs/pubs/broecker_science.pdf (accessed and saved 7/09/2017). ↩
- 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). Summarized by Owen Gaffney in “The Anthropocene equation” slide show, 4/13/2017, https://www.slideshare.net/owengaffney/anthropocene-equation (accessed 12/17/2017) and (less technical) Melissa Davey, “Humans causing climate to change 170 times faster than natural forces”, The Guardian, 2/12/2017, https://www.theguardian.com/environment/2017/feb/12/humans-causing-climate-to-change-170-times-faster-than-natural-forces (accessed and saved 7/09/2017). ↩
- 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). ↩
- 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 and paysite accessed 7/23/2017). Article summarized by Emily Chung, “Popular theory on how humans populated North America can’t be right, study shows”, CBC News (8/10/2016), http://www.cbc.ca/news/technology/ice-free-corridor-north-americans-1.3715397 (accessed and saved 7/23/2017). ↩
- JM 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 (abstract and paysite accessed 7/23/2017). ↩
- 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, http://www.latinamericanstudies.org/maya/Maya_caves.pdf (accessed and saved 7/23/2017). ↩
- 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. ↩
- 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). ↩
- Tom 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). ↩
- Rachel Caspari and Sang-Hee Lee, “Older age becomes common late in human evolution”, PNAS vol. 101, no. 30 (July 27, 2004), DOI: 10.1073/pnas.0402857101 (accessed and saved 7/23/2017). ↩
- 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). ↩
- 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://collections.lib.utah.edu/details?id=702745 (accessed 7/30/2017). ↩
- Mirkka Lahdenperä et al, “Fitness benefits of prolonged post-reproductive lifespan in women”, Nature vol. 428 (3/11/2004) 178 – 181, DOI: 10.1038/nature02367, free copy at https://www.researchgate.net/publication/5391472_Fitness_Benefits_of_Prolonged_Post-Reproductive_Lifespan_in_Women (accessed and saved 7/30/2017). ↩
- Nicholas Wade, Before the Dawn: Recovering the Lost History of Our Ancestors, Penguin Press (2006), Kindle ebook edition, location 2234. ↩
- Nicholas Wade, Before the Dawn: Recovering the Lost History of Our Ancestors, Penguin Press (2006), Kindle ebook edition, location 2264. ↩
- Douglas Rohde, “On the Common Ancestors of All Living Humans”, apparently unpublished and marked with the caveat “Work in progress; do not cite”. https://www.researchgate.net/publication/250314367_On_the_Common_Ancestors_of_All_Living_Humans (accessed and saved 2017). Rohde’s computer model indicates that humanity’s identical ancestry point could have occurred 5 – 15 KYA. Bearing in mind the realities of American settlement, it would have occurred at the earlier end of this range. ↩
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