What Ancient DNA Reveals About the Deep Past of Human Wandering

On David Reich’s book 'Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past.'

Sep 20, 2024

“We scientists are conditioned by the system of research funding to justify what we do in terms of practical application to health or technology. But shouldn’t intrinsic curiosity be valued for itself? Shouldn’t fundamental inquiry into who we are be the pinnacle of what we as a species hope to achieve? Isn’t an attribute of an enlightened society that it values intellectual activity that may not have immediate economic or other practical impact? The study of the human past—as of art, music, literature, or cosmology—is vital because it makes us aware of aspects of our common condition that are profoundly important and that we heretofore never imagined.”

-David Reich, Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past

Since about the middle of the 19^th century, our collective quest—usually at the behest of only a few intrepid scientists and scholars—to understand who we are as a species and how we got here has been an extraordinary one. Though Charles Darwin’s 1859 publication of On the Origin of Species was the monumental text for evolutionary theory, his 1871 publication of The Descent of Man is one of the first major instances where we begin to see modern evolutionary theory applied to humanity. For roughly the last 150 years, the primary fields of study to examine our shared human past have been through disciplines like archaeology, history, paleoanthropology, and linguistics. Studying artifacts, ancient texts, bones of extinct human relatives, and the interrelationship and spread of languages have been our main lines of evidence to interpret fundamental questions of who we are and how we got here.

As a scientific field, genetics has been around for nearly as long as the theory of evolution as presented by Darwin1 with its founding credited to the late 19^th-century Moravian monk Gregor Mendel. However, in only the last few decades a new science known as paleogenomics has emerged. Refined field and laboratory techniques and innovative technologies now allow for the analysis of ancient DNA—intact strands of the human genome preserved in and recovered from old bones and teeth. The oldest recovered ancient human DNA to date measures approximately 430,000 years ago from a significant archaeological cave site in Atapuerca, Spain known as Sima de los Huesos or “Pit of the Bones.”

Older DNA from the molars of mammoths have been reported in Siberia and dated to around one million years ago and sediments dating to around 2 million years ago have been reported in Greenland. However, there is a limit to how far back genetic material preserves and it likely doesn’t exceed much beyond the 2 million-year-old threshold—sorry to break the news, but there will likely be no Jurassic Park. Nonetheless, this nascent field of science has been providing revolutionary discoveries and insights about human movement across the globe during the last 100,000 years…and then some.

American geneticist David Reich’s 2018 popular publication Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past synthesizes an impressive amount of recent research on ancient DNA and what it reveals about our human story at a much higher resolution than previously known. To get to the punch line, “who we are” is a single species comprised of incredibly mixed and interrelated populations of humans in terms of our genomes. “How we got here” is through many more populations of humans wandering via migrations back and forth across the continents—far more than we could have realized just a few decades ago. As Reich sums up at different points throughout his work,

The story that is emerging differs from the one we learned as children, or from popular culture. It is full of surprises: massive mixtures of differentiated populations; sweeping population replacements and expansions; and population divisions in prehistoric times that did not fall along the same lines as population differences that exist today. It is a story about how our interconnected human family was formed, in myriad ways never imagined.2

More succinctly,

This means that our genomes hold within them a multitude of ancestors.3

More poetically,

The power of tracing this multitude of lineages to reveal the past is extraordinary. In my mind’s eye, when I think of a genome, I view it not as a thing of the present, but as deeply rooted in time, a tapestry of threads consisting of lines of descent and DNA sequences copied from parent to child winding back into the distant past. Tracing back, the threads wind themselves through ever more ancestors, providing information about population size and substructure in each generation.4

As to the implications,

We now know that nearly every group living today is the product of repeated population mixtures that have occurred over thousands and tens of thousands of years. Mixing is in human nature, and no one population is—or could be—“pure.”5

What is ultimately upended and tossed to the wayside in these revelations from ancient DNA are not only once reputable but now outdated scientific hypotheses on human relatedness and migrations but genuinely racist “theories” of once “pure” human lineages. One hopes paleogenomics will soon, as he states, “put the final nail in the coffin” of such retrograde and baseless ideas.

Not since the advent of radiocarbon dating in the late 1940s—which allowed researchers to date organic materials and essentially put a range of time stamps on archaeological sites and cultural material—has a scientific method radically opened up vast potential for human insight into our shared past. And yet, ancient DNA, as Reich argues, is even more groundbreaking than radiocarbon dating,

It is tempting to view ancient DNA as just one more new scientific technology that became available to archaeologists after the radiocarbon revolution, but that would be underestimating it. Prior to ancient DNA, archaeologists had hints of population movements based on the changes in the shapes of ancient skeletons and the types of artifacts people made, but these data were hard to interpret. But by sequencing whole genomes from ancient people, it is now possible to understand in exquisite detail how everyone is related.

The measure of a revolutionary technology is the rate at which it reveals surprises, and in this sense, ancient DNA is more revolutionary than any previous scientific technology for studying the past, including radiocarbon dating. A more apt analogy is the seventeenth-century invention of the light microscope, which made it possible to visualize the world of microbes and cells that no one before had even imagined. When a new instrument opens up vistas onto a world that has not previously been explored, everything it shows is new, and everything is a surprise. This is what is happening now with ancient DNA. It is providing definitive answers to questions about whether changes in the archaeological record reflect movements of people or cultural communication. Again and again, it is revealing findings that almost no one expected.6

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Many academic researchers and scholars once assumed the past to be a far simpler and more isolated time. While people may have moved regularly, they were thought to conservatively try to guard and maintain their presence in a region after they had discovered virgin territory and were believed not to be in contact or connected over vast distances by long-distance social networks. One can be forgiven when you are looking at tens or hundreds of thousands of years and all you have to go on are fragments of bones, pottery, and stone tools. Inferring the past from archaeological data from such minimal scraps of evidence can only get you so far.

Nevertheless, these presumptions are steadily being challenged through genomics and other academic fields that attempt to explore social interaction and exchange networks over large distances in the past (something my archaeological thesis focused on). My takeaway from all this, and what I hope every one of us can come to appreciate and keep in mind about who we are and how we got here is that humans have always been drawn to movement, for we are a wandering species. While there are many motivations for humans to move, e.g., acquiring food, meeting other mates, gathering resources, averting conflict, and many other risks, they are not just out of necessity but because we are also an exploratory species interested in maintaining connections with our world and others over vast distances—that too is in our DNA. As the famed astronomer Carl Sagan once opined about the importance of becoming a multi-planetary species,

It is important for us, humans, to be out there…we are an exploratory species, the last ten thousand years we’ve been sitting around in civilization, before that, for the last 100,000 years we were wanders, explorers, nomads, and that is in our blood.

While the general outline and paths of human migration have been known for some time–something I poetically map out in my first post Our Ancient Narrative–this new way of tracing ancestry is giving us a far more refined and higher-resolution look into the frequency of those movements. In nearly every corner of the world, we see not just single migrations by single populations setting up camp and staying put, but multiple migrations by multiple populations moving in, sometimes staying, pivoting after a few thousand years, and then moving elsewhere before starting the process again. The results are truly astonishing, and reveal other fascinating insights that strengthen arguments in long-fought debates about human origins, e.g., that the ancestry between some groups that were once thought to share little are much more closely related and that phenotypes of skeletons–the physical manifestation of genes–are not always adequate at inferring ancestry.

Reich’s work also provides us with a powerful snapshot into an imminent future where we come to understand the resolution of human history in far more detail which ought to give us a calm reflective pause followed by a reorientation in our collective understanding but will, one worries, simultaneously challenge many peoples’ deeply entrenched views of their origins. What is presently being revealed by paleogenomics substantially builds upon what many anthropologists, archaeologists, and linguists have been meticulously trying to piece together with pottery sherds, stone implements, languages, fragments of bones, and ethnographic stories for centuries. It is one of the more enticing books I’ve come across lately and one that demonstrates why a multipronged approach to understanding the past is vitally important.

Overall, the book is refreshingly honest, moderately accessible, and appropriately sensitive where necessary. It is a work that also revitalizes a thread that has been, to my mind, lost in recent years within the discipline of anthropology (at least within the United States). As with many other disciplines in the social sciences and humanities in countless departments across the nation’s most esteemed academic institutions, anthropology—unbeknownst to much of the lay public— has become embarrassingly sectarian, dogmatic, and, frankly, too exhausting to participate in at the moment—but if you know, you know…

However, the central theme of anthropology that has always kept me going, and remains kindled in Reich’s book, is that we are a single resilient species with an incredibly rich and diverse history and we all have a right and duty to take an interest in our shared past. This is what Reich labeled as our “common condition.” Our common condition of who we are and how we got here is a truth as real as gravity that every one of us either learns to grapple with and reconcile or, I fear, will only lead to sustained strife and division for the foreseeable future.

Grappling with history isn’t for the faint of heart for it is bleak and peppered with too many dark eras of violence, slavery, and inequality but it is also an eternal story of human resilience, cooperation, and progress. As I’ve written previously,

We should neither be in the business of romanticizing the past nor glorifying the present, but humanizing who we have been and who we are so that we have sound foundations to build better worlds and realistic expectations of who we can become. Human well-being and life satisfaction, I would argue, ought to be some of the more important metrics by which we measure “progress.”

The most salient underlying lesson demonstrated in a work like this, however, is that the learning and teaching of history need not be a morbid exercise. Just because history can be grim, unjust, and downright depressing doesn’t mean it needs to be taught as such. If we can make amends for the sins of the past, we obviously should, but recognizing our limits in that project is warranted too. You need not channel the worst threads of human history in every lecture, podcast, book, or protest for people to understand the gravity of inhumanity in the past and present. Bear in mind, that there are also many positive and uplifting threads in the tapestry of our human story to draw inspiration from and take refuge in as well. Perhaps those lessons may even draw more powerful changes than the guilt-ridden tactics of late.

There is a kind of cosmopolitanism embedded in our DNA—a deep story of interacting, mixing, and migrating through the millennia on this planet—in a word, we are a mosaic of many past populations and we should act accordingly with that knowledge in hand. We are a rich tapestry (another wonderful metaphor) of cultures and ideas that are only left begging to be discovered, understood, and appreciated by all of us—I can’t help but feel that’s the only way to approach a mature study of human history, one with an eternal sense of awe, wonder, humility, and intrinsic curiosity as our defining principles.

Whether we collectively come to understand this truth about ourselves and embrace it seems dubious at the moment as we witness intense factionalism all around us. Maybe it’ll take a few more centuries for us to wise up. Perhaps it’ll never happen and we’ll lamentably revert to evermore division and thinking our differences, rather than our shared humanity, are what define us. Nevertheless, while it remains debatable whether one can derive an ought from an is, I believe we should embrace this fact of cosmopolitanism in our DNA which is at the core of who we have been all along and who we will continue to be.

A synthesis of a synthesis can hardly do justice to anyone’s work and a topic as rich and full of nuance as genetics requires careful consideration as to its implications. Thus, there is no easy way to reduce the contents of Reich’s book to a mere Substack post and it deserves far more than time can allow here to unpack. Nevertheless, there is a whole host of wonderful graphs and illustrations to keep you wondering while you digest one remarkable discovery after another. So if you want to learn more about the latest insights into human-Neanderthal interbreeding; what a “ghost” population is; the spread of agriculture and diseases like that which caused the Black Death; the spread of the Indo-European language; the peopling of Eurasia and the Americas; and plenty more on the vast migrations of our humanity, I encourage anyone interested in this topic to seek out this work and take it to heart.

Additional Reading:

Two other fantastic books that can get you up to speed on some of the latest insights into past human migrations and paleogenomics are the American anthropologist David Anthony’s The Horse, the Wheel, and Language which focuses primarily on the linguistic and archaeological evidence of human migration as it relates to Indo-European migrations, and Swedish geneticist Svante Pääbo’s Neanderthal Man: In Search of Lost Genomes which reveals more details about the genome revolution and the author’s work sequencing the Neanderthal genome.

Also, I have plenty more anthropology and archaeology books and articles I could recommend, so for those interested please feel free to reach out for suggestions.

What Ancient DNA Reveals About the Deep Past of Human Wandering

On David Reich’s book 'Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past.'

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