Many of the most fascinating advances in DNA research have been in the extraction of readable genetic material from ever older biological sources. When those sources are ancient humans, the analysis of DNA fragments takes us back to our very origins as a species, analogous to the Hubble Telescope peering back through time to the beginnings of the universe. But sometimes that rewind of time takes people to a place where they do not want to go. Sometimes your ancestors are not who you think they were.
Modern politics and religion still fight Charles Darwin and his successors tooth and nail (teeth, by the way, coincidentally being great sources of your ancestors’ genes). This ancient DNA tells us that we are all closer worldwide genetic cousins than the differences our prejudices invent.
This is also a story of disagreements between archaeologists, who use sophisticated and well-tested dating techniques on remnants of prehistoric human culture such as tools, and a new breed of anthropologist-biologists who use ancient DNA samples to mathematically walk backwards through time. Often the calendars between the two disciplines are remarkably congruent. But sometimes they clash, because fossils, rudimentary tools, and ancient DNA each provide a narrow window on one but piece of the past.
Jennifer Raff, a researcher and teaching professor at the University of Kansas, has written a fascinating new book that not only covers the current state of genetic research regarding the first inhabitants of the Americas, but also weaves in stories of the careful political dance that must be navigated in obtaining, analyzing, and publishing about the earliest biological records of America’s First Nations. Origin: A Genetic History of the Americas (Hachette, 2022) even starts out acknowledging that her current home university is on land stolen from the Kaw, Osage, and Shawnee nations.
If you think Raff is being too careful with the sensitivities of America’s first inhabitants, you might be surprised to learn that the current governor of Florida is so sensitive about letting students know that within my own lifetime his state was heavily racially segregated that he has made teaching that fact illegal, and now bans math textbooks for even mentioning race, so as to not hurt the fragile feelings of his constituents of European descent. Leaving the politics until later, first we need to review some of the basics of genetic science (which are perhaps the next set of textbooks to be banned in Florida):
The science and math of ancient DNA
The famous “helix” of every DNA strand in your body is what is called a polymer, a coiled chemical chain made up of over three million “base pairs” selected from four different linked compounds, commonly called A, G, C, and T for short. We receive half of our primary DNA set from our mothers and half from our fathers. But importantly, we also have in each of our cells tiny fragments of mitochondrial DNA that we inherit only from our mothers. Along with the DNA “Y” chromosome that males inherit from their fathers, that means we all have mathematical DNA “signatures,” unique numerical representations of those base pair sequences, that can be tracked back tens of thousands of years to specific places on the planet from whence our ancestors came.
These mathematical signatures have evolved slightly over time due to genetic mutations, some favorable, some fatal, but most just uniquely numerical and without any known physical impact on you. Importantly, these genetic mutations appear at an impressively predictable rate over the long haul, which means that researchers can “time” the age of many ancient DNA specimens, give or take a couple thousand years, even in the absence of other archeological evidence. In short, DNA is combinatorial math (the DNA chemical sequences) plus the probabilities that mutated your progenitors’ DNA into you, via a couple hundred thousand years of human sex.
As a result, researchers do not need to obtain a complete DNA sequence from some ancient bones. A single piece of mitochondrial DNA from a lone stray tooth may well contain enough “math” to peg its deceased owner to a specific human lineage and era in time. And those random lost teeth show up more than once in Raff’s account as critical finds.
The archaeologists versus the biologists
Origin presents several potential scenarios regarding the sequence of events and timeframe that brought humans to North and South America. The author focuses most strongly on two of these, where the older techniques of archaeological dating stand in partial conflict with the newer technology of DNA dating. I say partial conflict because the archaeological evidence can typically go no further back than the introduction of stone tools or pottery in human culture. But what if, the biologists suggest, humans migrated into the Americas at an earlier time from which no recognizable tools have been found (or found yet), or even before certain tool-making skills existed in the population? Human DNA preserved from an earlier period may indicate a human presence in a timeframe preceding the typical archaeologist’s calendar.
The longstanding more conservative, archaeology-based model traces both cross-continent genetic matches and archeological evidence indicating the movement of people across the Bering Strait between 16,000 and 14,000 years ago, after the “Last Glacial Maximum” left a land bridge between Asia and the Americas. The “Clovis Culture” archaeological sites, named after the first one found in New Mexico, and the rudimentary human-made tools found in these digs, support this dating.
It is no coincidence that remnants of human DNA can also be dated to this period using the “walk backwards” through the probability timeline of mutation of those mathematical DNA “signatures.” That longer human biological timeline traces the flow of Homo Sapiens even further back out of Africa at least 160,000 years ago, with descendants gradually migrating eastward over many generations, some eventually reaching northeast Siberia. Along the way, different lineages of humans picked up DNA from now-extinct populations of Neanderthal and Denisovan genetic “cousins” in different proportions through some limited interbreeding. Those genetic fingerprints are also subsequently found in the American DNA samples dated to the Clovis era.
A newer model of human migration, one advocated by Raff, places humans in the Bering Strait region even earlier, as far back as 24,000 years ago, living for some length of time in that land bridge area, now mostly under the sea, and moving deep into the Americas as far back as 18,000 years ago. That alternative timeline is based on genetic traces and DNA dating more so than archeological evidence, which is admittedly slim. In this model, a genetic split separating those who populated most of North American from those eventually reaching South America is estimated to have happened about 15,700 years ago, based on more recent differences sequenced in their DNA. However, the current Arctic indigenous population, ranging from Alaska eastward as far as Greenland, is placed genetically in a much later Asian migration, from about 5500 years ago. By comparison, this later date was about the time communal agriculture was developing in ancient Mesopotamia in the Middle East.
Neither model, by the way, has any genetic elbow room for theorized “ancient white civilizations” in America (aside from a brief and unsuccessful Viking attempt to establish a colony in Newfoundland in the 11th-century CE) or transplanted Middle Easterners from 3000 years ago whose descendants survive today, even though those stories are central to some religious traditions. As theologian Frederick Buechner once wrote about the Christian doctrine of the virgin birth (also a DNA problem), “Life is complicated enough without confusing theology and gynecology.” Add genetics and DNA mathematics to that list. The DNA math tells convincingly of that northwest Asian source of American indigenous DNA.
The political dance of ancient American DNA
I lived across the road for a period of time from an Iowa small town cemetery dating back nearly 150 years, and so I became quite comfortable walking my beagle among the mix of old and new grave plots late at night without any supernatural angst. Zombie movies neither entertain nor scare me. However, if I had run into some strangers digging up old relatives just to extract some DNA from their decaying bones, I admit that I would be a bit put out.
Origin’s author Jennifer Raff is among the new breed of researchers who have placed the sensitivities of the modern First Peoples at the forefront of ancient American DNA research. She blends into her Origin story how each new archeological find requires cultural sensitivity to the current indigenous population and actively seeks their involvement. In the best cases for the researchers, tribal leaders have been anxious to learn about their possible ancestors, as long as respect for the dead, and eventual protected reburial, were ensured. Sometimes that effort faces intractable opposition to these digs, and there have been times when local opposition, in the end, won the day and that wish was honored.
Numerous people have been surprised to learn family secrets when 23andMe or AncestryDNA sequence part of their DNA genome. The same also happens with ancient DNA and later indigenous populations on the same land. Human populations have, for much of their existence, been gradually on the move, leaving their DNA behind. And this is even more true today, with common intermarrying across continents and cultures. Perhaps some future researcher may dig you up, or find your lost tooth from the third grade, and then sequence your DNA to add a new thread to human genetic history.
Jennifer Raff’s story, told in Origin: A Genetic History of the Americas, reaches well beyond those ancient inhabitants to teach us how we are each the result of a long string of genetic recombinations and the sum of a million probabilities. We were all perhaps within a fraction of a second from being fertilized into a very different genetic sequence, even a different sex, or not being here at all. We all winners in a fascinating DNA lottery.