Genetics and the Human Revolution
Can advances in DNA sequencing help explain what makes us human?
Before ancient humans put pen to paper, stylus to tablet, or even brush to cave wall, their comings and goings were noted in another record, within their very cells. The human genome consists of chunks of DNA passed forward from countless ancestors, so by comparing modern humans’ genetic material with that gleaned from ancient remains, it’s possible to reach into prehistory and learn about where people came from, and who they were.
David Reich, professor of genetics at Harvard Medical School, has spent the last decade extracting these stories from ancient DNA, using genetic evidence to overturn established theories and conventional wisdom about humanity’s past. But, so far, at least, the DNA has provided little clarity on the more fundamental question of our origin—what makes us human in the first place? In yesterday’s Distinguished Harvard Lecture in Mind Brain Behavior, Reich highlighted the transformative power—and tantalizing limitations—of ancient DNA in reshaping understanding of how Homo sapiens came to be and to act like modern humans.
The Mind Brain Behavior (MBB) interfaculty initiative, which celebrated its twenty-fifth birthday earlier this year, is one of several initiatives at Harvard that provide funding and programming for collaboration across the University’s different schools, with the aim of bringing researchers together to better understand an interdisciplinary research topic—in this case, the biology driving human behavior. The biannual Distinguished Harvard Lectures give students and researchers involved in the initiative a venue to hear about research in other fields that directly impacts their own. The room was packed with faculty members from different schools and departments, a reflection of the profound influence Reich’s research has had on many fields studying human behavior.
“David is an absolute superstar. He’s also my hero,” said Andrew J. Berry, a lecturer on organismic and evolutionary biology, introducing Reich. “His work on ancient DNA has changed the way that we evolutionary biologists think about evolution, and how we think about ourselves as human beings.”
That scientists can glean any information from ancient DNA is itself an incredible story, and one in which Reich has played no small part. He explained that his work builds on breathtaking advances in the affordability and sensitivity of DNA sequencing technology. Since 2002, at the conclusion of the Human Genome Project, the cost of sequencing one genome has been slashed to one hundred-thousandth of its former price. “This has made it possible to ask questions about the past that were simply not possible to ask before,” he said.
Reich spoke, especially about his own research, with an excited precision, equally eager to showcase his discoveries and the baffling questions his work has left unanswered. The first part of his talk focused on how research into prehistoric DNA—much of it by his group—has led to a new understanding of the migrations and intermingling that brought about modern humanity. By comparing the frequency with which naturally occurring genetic variations appear in various ancient and modern populations, it’s possible to reconstruct a tangled family tree of humanity, uncovering more details of early humans’ constant migration than the archaeological record alone can show (see “Who Killed the Men of England,” July-August 2009).
The story that emerges—both in the origin of hominids and in the history of humanity—is not of a cleanly branching phylogenetic tree, but one in which seemingly firm categories like species and ancestral group overlap and blur at the edges. Reich’s work has shown, for instance, that the ancestors of humans and modern chimpanzees interbred for perhaps millions of years after the two lineages had split into different species, and that, thanks to interbreeding with Neanderthals in Eurasia, most people outside of Africa are part Neanderthal (see “Human-Family Reunions,” July-August 2014.)
DNA is a remarkable record of the human family tree—but that is not its only role, Reich went on to point out. It is a functional molecule, with great influence on our development and behavior. This leads to a tantalizing question—can scientists pinpoint the genes that enabled humans, unlike other species, to create art, tools, and eventually civilization? The second part of Reich’s talk focused on the search for the genetic basis of modern human behavior, and why it has come up short, perhaps fatally.
Archaeologists once thought that many of humanity’s behavioral calling cards—including evidence of fishing, bone tools, and elaborate burial rituals—became rapidly widespread about 50,000 years ago. One hypothesis, popularized in the 1990s, is that some genetic change which occurred at about this time allowed the explosive spread of these behaviors, defining modern humanity.
The gene or genes that would have caused this “human revolution” have, however, proved elusive—if they exist at all. Reich detailed a number of reasons that such a switch would be hard to find: for one, rich genetic data are still scarce outside of European population groups (although this is changing due in part to concerted data collection efforts by Reich’s lab). What’s more, it’s far easier to track a genetic variation through its ancient migrations than to actually know what the variation means for someone’s biology and behavior. “We know a lot about human history,” said Reich, “but have a very poor ability to read the genome about biology.”
More damning for the theory of a 50,000-year-old genetic switch is a fairly simple factor: the lineages of modern humans probably diverged about 300,000 years ago, well before this “human revolution.” Some “modern” behaviors are spread even wider, since Neanderthals—who diverged from Homo sapiens some 700,000 years ago—show a surprising knack for art and ornamentation. And a growing body of archaeological evidence indicates that these behaviors did not arise as rapidly as once thought, with evidence of dispersed modern behavior showing 100,000 years ago or earlier. Given this, said Reich, “I’m doubtful that genetics is the key driver of any ‘human revolution.’”
Finding the genetic basis for modern humanity would require reaching deeper into the past, searching for many different genes that appeared long before they enabled the tool use and culture now regarded as hallmarks of modern behavior. This is an even greater challenge, but one that Reich is eager to probe, as his lab collects more samples of ancient DNA. And if human behavior didn’t arise from rapid genetic change, but rather from cultural transmission, it indicates something more profound than any “human” gene: modern humanity was not evolved, but invented.
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