Scatter, Adapt, and Remember: How Humans Will Survive a Mass Extinction

by Newitz, Annalee

  Our capacity for symbolism evolved quickly, partly because our mating choices would have been shaped by our needs as creatures who evolved to survive by founding new communities. Over the past million years, humans bred themselves to be the ultimate survivors, capable of both exploring the world and adapting to it by sharing stories about what we found there.

  How Can We Possibly Know All This?

  A lot of the evidence we have for the routes that humans took out of Africa comes from objects and places you can see with your own eyes. Paleontologists have found our ancestors’ ancient bones, as well as their tools. To figure out the ages of these tools and skeletons, we use the same kinds of dating techniques that geologists use to discover the history of rocks. In fact, when an anthropologist talks about “dating the age of fossils,” he or she isn’t actually talking about the bones themselves—to date old bones, anthropologists carefully excavate them and take samples of the rock surrounding them. Then they pin a date on those rocks, under the assumption that the bones come from roughly the same era as the rocks or sand that covered them up. Basically, we date fossils by association, which is why you’ll often hear scientists suggesting that a particular fossil might be between 100,000 and 80,000 years old. Though we can’t pin an exact month or year on each fossil discovery, we do have ample evidence that certain humans like H. ergaster came before other humans like H. erectus in evolutionary and geological time.

  Over the past decade, however, the study of ancient bones has been revolutionized by new technologies for sequencing genomes, including DNA extracted from the fossils of Neanderthals and other hominins who lived in the past 50,000 years (sadly, we don’t have the ability to sequence DNA from Australopithecus or H. ergaster bones—their DNA is too decayed). At the Max Planck Institute in Leipzig, Germany, an evolutionary geneticist named Svante Pääbo and his team have developed technology to extract nearly intact genomes from Neanderthal bones. First they grind the bones to dust and chemically amplify whatever DNA molecules they can find, then analyze this genetic material using the same kinds of sequencers that decode the DNA of living creatures today. We’ll deal with the Neanderthal genome more in the next chapter, but suffice it to say that we have pretty solid evidence about the genetic relationships between H. sapiens and its sibling species H. neanderthalensis.

  A lot of the evidence for humans’ low genetic diversity has been made possible by DNA-reading technologies developed since the first human genome was sequenced, in the early 1990s. Though that first human genome took over a decade to sequence, we now have machines capable of reading the entire set of letters making up one genome in just a few hours. As a result, population geneticists are accumulating a diverse sampling of sequenced human genomes, from people all over the world. Many of these genomes are collected into data sets that scientists can feed into software that does everything from make very simple comparisons between two genomes (literally analyzing the similarities and differences between one long string of letters and another), to extremely complex simulations of how these genomes might have evolved over time.

  One of the first pieces of genetic evidence for the serial-founder theory emerged when scientists had collected DNA sequences from enough people that we could start to analyze genetic diversity in different regions all over the world. Geneticists discovered a telltale pattern: People born in Africa and India tend to have much greater genetic diversity than people born elsewhere. This is precisely the kind of pattern you’d expect to see in a world population that grew out of founder groups originating in Africa. Remember, each successive founder group has less and less genetic diversity. So people descended from groups that stayed in Africa or India are from early founder groups. People in Europe, Australia, Asia, and the Americas were the result of hundreds of generations of founder effects—so we’d expect them to have less genetic diversity. When you add this genetic evidence to the physical evidence from fossils and tools left behind by people leaving Africa, you wind up with a fairly solid theory that founder effects created our genetic bottleneck.

  An Eruption That Launched Humanity

  Though it’s likely that the genetic bottlenecks we observe in the human population were caused mostly by founder effects and sexual selection, there is some evidence that the final human radiation out of Africa was precipitated by a catastrophe. Ancient humans had been crossing the Sinai out of Africa and into the rest of the world for over a million years, but roughly 80,000 years ago there was an extremely large migration that changed the world and every human on it. H. sapiens, a human with language, clothing, and sophisticated tools, took over Africa, then migrated beyond its borders. Certainly it’s possible that this wave of human immigrants was spurred by mass deaths in the wake of the Toba eruption. But that’s debatable.

  What’s certain is another explosion that nobody denies: the one in human symbolic communication. Our capacity for culture is what allowed us to survive in the perilous lands beyond the warm, fecund West African regions where Australopithecus first stood up. We never stayed in any one place for long. We moved into new places, founding new communities. And when we evolved complex symbolic intelligence, our growing facility with tools and language made these migrations easier. We could take advantage of many kinds of environments, teaching each other about their bounties and dangers in advance.

  As H. sapiens poured off the continent of our birth, we discovered lands inhabited by our sibling hominins. We had to adapt to a world that already had humans in it. What came next will take us into one of the most controversial areas of population genetics and human evolutionary history.

  7. MEETING THE NEANDERTHALS

  NEANDERTHALS WERE HUMANS who went extinct between 20,000 and 30,000 years ago. Though there is some debate about who these people were, there is no question that there are none left. All that remains of the hundreds of Neanderthal groups that roved across Europe and Central Asia are a handful of ambiguous funeral sites, bones, tools, and pieces of art—along with some DNA that modern humans inherited from them. How can we avoid meeting the Neanderthals’ fate? That depends on what you think wiped out these early humans in the millennia after they met H. sapiens.

  By 40,000 years ago, humans had spread in waves across most of the world, from Africa to Europe, Asia, and even Australia. But these humans were not all perfectly alike. When some groups of H. sapiens poured out of Africa, they walked north, then west. In this thickly forested land, they came face-to-face with other humans, stockier and lighter skinned than themselves, who had been living for thousands of years in the cold wilds of Europe, Russia, and Central Asia. Today we call these humans Neanderthals, a name derived from the Neander Valley caves in Germany where the first Neanderthal skull was identified in the nineteenth century.

  Neanderthals were not one unified group. They had spread far enough across Europe, Asia, and the Middle East that they formed regional groups, something like modern human tribes or races, who probably looked fairly different from each other. Neanderthals used tools and fire, just as H. sapiens did, and the different Neanderthal groups probably had a variety of languages and cultural traditions. But in many ways they were dramatically unlike H. sapiens, leading isolated lives in small bands of 10 to 15 people, with few resources. They had several tools, including spears for hunting and sharpened flints for scraping hides, cutting meat, and cracking bones. Unlike H. sapiens, who ate a wide range of vegetables and meat, Neanderthals were mostly meat-eaters who endured often horrifically difficult seasons with very little food. Still, there is evidence that they cared for each other through hardship: fossils retrieved from a cave in Iraq include the skeleton of a Neanderthal who had been terribly injured, with a smashed eye socket and severed arm, whose bones had nevertheless healed over time. Like humans today, these hominins nursed each other back to health after life-threatening injuries.

  Roughly 10,000 years after their first meeting with H. sapiens, all the Neanderthal groups were extinct and H. sapiens was the dominant hominin on Earth.
What happened during those millennia when H. sapiens lived alongside creatures who must have looked to them like humanoid aliens?

  A few decades ago, most scientists would have answered that it was a nightmare. Stanford’s Richard Klein, who spent years in France comparing the tools of Neanderthals and early H. sapiens, lowered his voice a register when I recently asked him to describe the meeting between these hominin groups. “You don’t like to think about a holocaust, but it’s quite possible,” he said. He referred to the long-standing belief among many anthropologists that H. sapiens exterminated Neanderthals with superior weapons and intellect. For a long time, there seemed to be no other explanation for the rapid disappearance of Neanderthals after H. sapiens arrived in their territories.

  Today, however, there is a growing body of evidence from the field of population genetics that tells a very different story about what happened when the two groups of early humans lived together, sharing the same caves and hearths. Anthropologists like Milford Wolpoff, of the University of Michigan, and John Hawks have suggested that the two groups formed a new, hybrid human culture. Instead of exterminating Neanderthals, their theory goes, H. sapiens had children with them until Neanderthals’ genetic uniqueness slowly dissolved into H. sapiens over the generations. This idea is supported by compelling evidence that modern humans carry Neanderthal genes in our DNA.

  Regardless of whether H. sapiens murdered or married the Neanderthals they met in the frozen forests of Europe and Russia, the fact remains that our barrel-chested cousins no longer walk among us. They are a group of humans who went extinct. The story of how that happened is as much about survival as it is about destruction.

  The Neanderthal Way of Life

  We have only fragmentary evidence of what Neanderthal life was like before the arrival of H. sapiens. Though they would have looked different from H. sapiens, they were not another species. Some anthropologists call Neanderthals a “subspecies” to indicate their evolutionary divergence from us, but there is strong evidence that Neanderthals could and did interbreed with H. sapiens. Contrary to popular belief, Neanderthals probably weren’t swarthy; it’s likely that these early humans were pale-skinned, possibly with red hair. We know that they used their spears to hunt mammoths and other big game. Many Neanderthal skeletons are distorted by broken bones that healed, often crookedly; this suggests that they killed game in close combat with it, sustaining many injuries in the process. They struggled with dramatic climate changes too. The European and Asian climates swung between little ice ages and warmer periods during the height of Neanderthal life, and these temperature changes would have constantly pushed the Neanderthals out of familiar hunting grounds. Many of them took shelter from the weather in roomy caves overlooking forested valleys or coastal cliffs.

  Though their range extended from Western Europe to Central Asia, the Neanderthal population was probably quite small—a generous estimate would put it at 100,000 individuals total at its apex, and many scientists believe it could have been under 10,000. By examining the growth of enamel on Neanderthal teeth, anthropologists have determined that many suffered periods of extreme hunger while they were young. This problem may have been exacerbated by their meat-heavy diets. When mammoth hunting didn’t go well, or a particularly cold season left their favored game skinny or sick, the Neanderthals would have gone through months of malnutrition. Though Neanderthals buried their dead, made tools, and (at least in one case) built houses out of mammoth bones, we have no traditional evidence that they had language or culture as we know them. Usually such evidence comes in the form of art or symbolic items left behind. Neanderthals did make art and complex tools after meeting H. sapiens, but we have yet to find any art that is unambiguously Neanderthal in origin.

  Still, there are intriguing hints. A 60,000-year-old Neanderthal grave recently discovered in Spain suggests that Neanderthals may have had symbolic communication before H. sapiens arrived. Researchers discovered the remains of three Neanderthals who appeared to have been gently laid in identical positions, their arms raised over their heads, then covered in rocks. The severed paws of a panther were found with the bodies, heightening the impression that the discovery represented a funeral ritual complete with “burial goods,” or symbolic items placed in the graves. Erik Trinkhaus, an anthropologist at Washington University in St. Louis, says this site shows that Neanderthals might have had symbolic intelligence like modern humans.

  Gravesites like these have led many scientists, including Trinkhaus, to believe that Neanderthals talked or even sang. But we haven’t found enough archaeological evidence to sway the entire scientific community one way or the other.

  By contrast, the H. sapiens groups who lived at the time of first contact with Neanderthals left behind ample evidence of symbolic thought. Bone needles attest to the fact that H. sapiens sewed clothing, and pierced shells suggest jewelry. There are even traces of red-ochre mixtures found in many H. sapiens campsites, which could have been used for anything from paint or dye to makeup. Added together, these bits of evidence suggest that H. sapiens groups weren’t just using tools for survival; they were using them for adornment. And culture as we know it probably started with those simple adornments.

  Looked at from the perspective of Neanderthals, then, there might have been a vast gulf between themselves and the newly arrived H. sapiens. The newcomers not only looked different—they were taller, slimmer, and had smaller skulls—but they probably chattered in an incomprehensibly complex language and wore bizarre garments. Would Neanderthals have tried to communicate with these people, or invited them to a dinner of mammoth meat?

  For anthropologists like Klein, who spoke about a Neanderthal holocaust, the answer is an emphatic no. He’s part of a school of anthropological thought that holds that H. sapiens would have met the Neanderthals with nothing but hate, disgust, and indifference to their plight. After those Neanderthals watched H. sapiens arrive, the next chapter in their lives would have been marked by bloodshed and starvation as H. sapiens murdered and outhunted them with their superior weaponry. Neanderthals were so poor, and had such a small population, that their extinction was inevitable.

  This story might sound familiar to anyone versed in the colonial history of the Americas. It’s as if H. sapiens is playing the role of Europeans arriving in their ships, and Neanderthals are playing that of the soon-to-be-exterminated natives. But Klein sees a sharp contrast between Neanderthals and the natives that Europeans met in America. When H. sapiens arrived, he asserted, “there was no cultural exchange” because the Neanderthals had no culture. Imagine what might have happened if the Spanish had arrived in the Americas, but the locals had no wealth, science, sprawling cities, nor vast farms. The Neanderthals had nothing to trade with H. sapiens, and so the newcomers saw them as animals. Neanderthals may have had fleeting sexual relationships with H. sapiens here and there, admitted Klein, but “modern human males will mate with anything.” Tattersall agreed. “Maybe there was some Pleistocene hanky-panky,” he joked. But it wasn’t a sign of cultural bonding. For anthropologists like Klein and Tattersall, any noncombative relationships forged between the two human groups were more like fraternization than fraternity.

  But there is a counter-narrative told by a new generation of anthropologists. Bolstered by genetic discoveries that have revealed traces of Neanderthal genes in the modern human genome, these scientists argue that there was a lot more than hanky-panky going on. Indeed, there is evidence that the arrival of H. sapiens may have dramatically transformed the impoverished Neanderthal culture. Some Neanderthal cave sites hold a mixture of traditional Neanderthal tools and H. sapiens tools. It’s hard to say whether these remains demonstrate an evolving hybrid culture, or if H. sapiens simply took over Neanderthal caves and began leaving their garbage in the same pits that the Neanderthals once used. Still, many caves that housed Neanderthals shortly before the group went extinct are full of ornaments, tools, and even paints. Were they emulating their H. sapiens counterparts? Had t
hey become part of an early human melting pot, engaging in the very cultural exchange that Klein and Tattersall have dismissed?

  Extermination and Assimilation

  The complicated debate over what happened to Neanderthals can be boiled down to two dominant theories: Either H. sapiens destroyed the other humans, or joined up with them.

  The “African replacement” theory, sometimes called the recent African origins theory, holds that H. sapiens charged out of Africa and crushed H. neanderthalensis underfoot. This fits with Klein’s account of a Neanderthal holocaust. Basically, H. sapiens groups replaced their distant cousins, probably by making war on them and taking over their territories. This theory is simple, and has the virtue of matching the archaeological evidence we find in caves where Neanderthal remains are below those of H. sapiens, as if modern humans pushed their Neanderthal counterparts out into the cold to die.

  In the late 1980s, a University of Hawaii biochemist named Rebecca Cann and her colleagues found a way to support the African replacement theory with genetic evidence, too. Cann’s team published the results of an exhaustive study of mitochondrial DNA, small bits of genetic material that pass unchanged from mothers to children. They discovered that all humans on Earth could trace their genetic ancestry back to a single H. sapiens woman from Africa, nicknamed Mitochondrial Eve. If all of us can trace our roots back to one African woman, then how could we be the products of crossbreeding? We must have rolled triumphantly over the Neanderthals, spreading Mitochondrial Eve’s DNA everywhere we went. But mitochondrial DNA offers us only a small part of the genetic picture. When scientists sequenced the full genomes of Neanderthals, they discovered several DNA sequences shared by modern humans and their Neanderthal cousins.