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Who We Are And How We Got Here
Ancient DNA and the New Science of the Human Past
More books on Evolution
For over a decade, National Geographic's Genographic Project has been collecting saliva samples from willing participants, analyzing small pieces of their mother's and father's DNA (so-called mitochondrial DNA and Y chromosome DNA, respectively). In return, there is fascinating personal information to be had - about individual ancestries, and about close relatives whose existence had been previously unknown.
But geneticists can now achieve far more than those limited analyses could, and can approach their dream of using genetic evidence to reconstruct past human migrations. One reason is that methods for extracting DNA from bones of ancient humans who lived tens of thousands of years ago have improved. It's possible now to separate their DNA from all the bacterial and modern human DNA that contaminates those bones. Another reason is that, due especially to the Harvard geneticist David Reich and his colleagues, we now have efficient methods for analyzing whole ancient human genomes, not just the few percent contributed by mitochondrial DNA and the Y chromosome.
In Who We Are and How We Got Here, Reich summarizes this rapidly advancing field. He begins with a crash course on genetics and DNA sequencing, then discusses the Neanderthals and 'ghost populations' whose existences are inferred from genetic evidence although they no longer exist. Most of the book then consists of chapters reconstructing the histories of modern Europeans, Indians, Native Americans, East Asians and Africans. Concluding chapters probe the controversial subject of race and identity, and prospects for new discoveries. I'll illustrate Reich's chapters with three examples: Neanderthals, Europeans and Polynesians.
First, let's take the quintessential 'cave men,' the Neanderthals, who thrived in Europe for several hundred thousand years until modern Homo sapiens (our own species) arrived around 44,000 years ago. Within a few thousand years, the Neanderthals were extinct. That strongly suggests that our ancestors had much to do with it; murderers have been convicted on weaker circumstantial evidence. What happened when Neanderthals met our ancestors? Did they have sex and produce viable hybrids?
Genetics now says: Yes they did, but apparently without enthusiasm or happy outcomes. All modern humans outside Africa, but no modern Africans, carry about 2 percent of Neanderthal genes. That means that, as modern humans expanded from their African origins into the Near East between 50,000 and 100,000 years ago, they met and occasionally had sex with Neanderthals: perhaps mainly when a lonely modern human found few other moderns with whom to mate. Their hybrid descendants then spread around the world, carrying their 2 percent of Neanderthal genes to everywhere outside Africa.
A Romanian skeleton from 40,000 years ago, already recognized by anatomists as a probable hybrid, proves to have somewhere between 6 percent and 9 percent Neanderthal genes, implying further hybridization when modern humans reached Europe. The skeleton may have had a Neanderthal relative just a few generations back. But that extra dose of Neanderthal genes didn’t propagate itself, because modern Europeans carry no more Neanderthal genes than do modern Chinese or New Guineans. Apparently the hybrids had low fertility. Still unanswered is the question: What advantage let our ancestors replace the Neanderthals? Did we possess grammatical language where they didn’t, bringing huge survival benefits? Were we more inventive? Did we live in large groups that overwhelmed small families of Neanderthals? Geneticists are already examining ancient human skeletons for genes associated with speech.
My second example: When and from where did the ancestors of today's Europeans arrive in Europe? Archaeology shows that farming reached Greece from Anatolia (modern Turkey) 9,000 years ago, and spread northwest to Britain. The prevalent view has thus been that invading Anatolian farmers intermarried with Europe's original hunter-gatherers, and their genes became diluted with hunter-gatherers' genes as the farmers spread northwest. According to that view, the modern Europeans most closely approximating Europe’s original hunter-gatherers would be the red-haired blue-eyed Irish on the west coast of Ireland.
But genetic discoveries have revealed a third ancient group, whose genes dominate modern Northern Europeans. That group consisted of herders from the Asian steppes, whose skeletons and genes are known from their burial mounds called kurgans. It turns out that those herders contributed about half the genes of Northern European and British skeletons beginning around 5,000 years ago. Evidently, the herders somehow outbred or exterminated most of Europe’s original farmers. How on earth could small numbers of herders have overwhelmed dense farmer populations?
A clue is that 7 percent of ancient DNA samples from Europe and the steppes contain DNA of the plague microbe that caused medieval Europe’s Black Death. Perhaps steppe peoples acquired plague, developed some immunity with exposure, and transmitted it to unexposed European farmers, thereby decimating them. It would be ironic if Europe’s first farmers succumbed to an introduced disease, because other diseases introduced by European colonists of the New World after 1492 equally decimated Native American populations.
My last example: What was the origin of Polynesian people, who colonized every habitable Pacific island from Tonga to Hawaii within the last few thousand years? Archaeological and linguistic evidence had already shown that Polynesians can be traced back to the island of Taiwan. But what happened to them between their leaving Taiwan and their reaching Tonga?
The astonishing answer is: genetically, nothing! The first proto-Polynesian culture located archaeologically on islands east of the Solomons is called the Lapita. DNA of early Lapita skeletons from Vanuatu and Tonga is mostly Taiwanese DNA, with little or no contribution from the Papuan peoples inhabiting the New Guinea region through which Taiwanese farmers passed en route to Vanuatu and Tonga. That sexual abstemiousness is unexpected. But Papuans subsequently reached those Pacific islands and mixed with the Lapita first arrivals, because the islands’ modern inhabitants do carry Papuan genes.
As with any new scientific methodology, new genetic methods tend to alter previously held beliefs about the course of human history. And some readers will certainly find Reich's conclusions unpalatable. Japanese may not like hearing that they share 80 percent of their DNA with Koreans. Reich's collaborators from India didn't like hearing that their own DNA samples attested to massive past migrations into the Indian subcontinent. Native Americans may not like hearing that 10,000-year-old Native American skeletons whose repatriation and reburial they seek have no demonstrable connection to tribes living in the same geographic area today. Those reactions are inevitable, but Reich bends over backward to present both the evidence and the uncertainties.
What should you expect when you read this book? Don't expect an oversimplified story. Population genetics is a complicated, fast-moving field with many uncertainties of interpretation. To tell that story to the broad public, and not just to scientists reading specialty journals, is a big challenge. Reich explains these complications as well as any geneticist could; others rarely even try. Some of Reich’s conclusions will surely be superseded within a few years. But few subjects fascinate us as much as human origins. Humans have been on the move not just since Columbus's voyage of 1492, but throughout our long history. The ancestors of modern Japanese, Indians and Native Americans didn't become fixed in their modern locations in ancient times and simply stop moving. If you want to understand our origins over the course of the last 100,000 years, this book will be the best up-to-date account for you.
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