A tall, cheerful Swede, Pääbo is the main engine behind a breathtaking scientific tour de force: the attempt, expected to be completed next month, to read out not just single Neanderthal genes, but the entire three-billion-letter sequence of the Neanderthal genome. Traces of DNA in fossils are vanishingly faint, and because Neanderthal DNA is ever so close to that of living people, one of the biggest hurdles in sequencing it is the ever present threat of contamination by modern human DNA—especially by the scientists handling the specimens. The precautions taken in excavating at El Sidrón are now becoming standard practice at other Neanderthal sites. Most of the DNA for Pääbo's genome project, however, has come from the Croatian specimen, a 38,000-year-old fragment of leg bone found almost 30 years ago in the Vindija cave. Originally deemed unimportant, it sat in a drawer in Zagreb, largely untouched and thus uncontaminated, for most of its museum life.
Now it is the equivalent of a gold mine for prehistoric human DNA, albeit an extremely difficult mine to work. After the DNA is extracted in a sterile laboratory in the basement of the Max Planck Institute, it is shipped overnight to Branford, Connecticut, where collaborators at 454 Life Sciences have invented machines that can rapidly decipher the sequence of DNA's chemical letters. The vast majority of those letters spell out bacterial contaminants or other non-Neanderthal genetic information. But in the fall of 2006, Pääbo and his colleagues announced they had deciphered approximately one million letters of Neanderthal DNA. (At the same time, a second group, headed by Edward Rubin at the Department of Energy Joint Genome Institute in Walnut Creek, California, used DNA provided by Pääbo to read out snippets of genetic code using a different approach.) By last year, dogged by claims that their work had serious contamination problems, the Leipzig group claimed to have improved accuracy and identified about 70 million letters of DNA—roughly 2 percent of the total.
"We know that the human and chimpanzee sequences are 98.7 percent the same, and Neanderthals are much closer to us than chimps," said Ed Green, head of biomathematics in Pääbo's group in Leipzig, "so the reality is that for most of the sequence, there's no difference between Neanderthals and [modern] humans." But the differences—less than a half percent of the sequence—are enough to confirm that the two lineages had begun to diverge around 700,000 years ago. The Leipzig group also managed to extract mitochondrial DNA from two fossils of uncertain origin that had been excavated in Uzbekistan and southern Siberia; both had a uniquely Neanderthal genetic signature. While the Uzbekistan specimen, a young boy, had long been considered a Neanderthal, the Siberian specimen was a huge surprise, extending the known Neanderthal range some 1,200 miles east of their European stronghold.
So, while the new genetic evidence appears to confirm that Neanderthals were a separate species from us, it also suggests that they may have possessed human language and were successful over a far larger sweep of Eurasia than previously thought. Which brings us back to the same hauntingly persistent question that has shadowed them from the beginning: Why did they disappear?
To coax a Neanderthal fossil to reveal its secrets, you can measure it with calipers, probe it with a CT scan, or try to capture the ghost of its genetic code. Or if you happen to have at your disposal a type of particle accelerator called a synchrotron, you can put it in a lead-lined room and blast it with a 50,000-volt x-ray beam, without disturbing so much as a single molecule.
Over a sleep-deprived week in October 2007, a team of scientists gathered at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, for an unprecedented "convention of jawbones." The goal was to explore a crucial question in the life history of the Neanderthals: Did they reach maturity at an earlier age than their modern human counterparts? If so, it might have implications for their brain development, which in turn might help explain why they disappeared. The place to look for answers was deep inside the structure of Neanderthal teeth.
"When I was young, I thought that teeth were not so useful in assessing recent human evolution, but now I think they are the most important thing," said Jean-Jacques Hublin, who had accompanied his Max Planck Institute colleague Tanya Smith to Grenoble.
Along with Paul Tafforeau of the ESRF, Hublin and Smith were squeezed into a computer-filled hutch at the facility—one of the three largest synchrotrons in the world, with a storage ring for energized electrons half a mile in circumference—watching on a video monitor as the x-ray beam zipped through the right upper canine of an adolescent Neanderthal from the site of Le Moustier in southwestern France, creating arguably the most detailed dental x-ray in human history. Meanwhile, a dream team of other fossils sat on a shelf nearby, awaiting their turn in the synchrotron's spotlight: two jawbones of Neanderthal juveniles recovered in Krapina, Croatia, dating back 130,000 to 120,000 years; the so-called La Quina skull from a Neanderthal youth, discovered in France and dating from between 75,000 to 40,000 years ago; and two striking 90,000-year-old modern human specimens, teeth intact, found in a rock shelter called Qafzeh in Israel.
When teeth are imaged at high resolution, they reveal a complex, three-dimensional hatch of daily and longer periodic growth lines, like tree rings, along with stress lines that encode key moments in an individual's life history. The trauma of birth etches a sharp neonatal stress line on the enamel; the time of weaning and episodes of nutritional deprivation or other environmental stresses similarly leave distinct marks on developing teeth. "Teeth preserve a continuous, permanent record of growth, from before birth until they finish growing at the end of adolescence," Smith explained. Human beings take longer to reach puberty than chimpanzees, our nearest living relatives—which means more time spent learning and developing within the context of the social group. Early hominin species that lived on the savanna in Africa millions of years ago matured fast, more like chimps. So when in evolution did the longer modern pattern begin?
To address this question, Smith, Tafforeau, and colleagues had previously used the synchrotron to demonstrate that an early modern human child from a site called Jebel Irhoud in Morocco (dated to around 160,000 years ago) showed the modern human life history pattern. In contrast, the "growth rings" in the 100,000-year-old tooth of a young Neanderthal discovered in the Scladina cave in Belgium indicated that the child was eight years old when it died and appeared to be on track to reach puberty several years sooner than the average for modern humans. Another research team, using a single Neanderthal tooth, had found no such difference between its growth pattern and that of living humans. But while a full analysis from the "jawbone convention" would take time, preliminary results, Smith said, were "consistent with what we see in Scladina."
"This would certainly affect Neanderthal social organization, mating strategy, and parenting behavior," says Hublin. "Imagine a society where individuals start to reproduce four years earlier than in modern humans. It's a very different society. It could also mean the Neanderthals' cognitive abilities may have been different from modern humans'."
Neanderthal society may have differed in another way crucial to group survival: what archaeologists call cultural buffering. A buffer is something in a group's behavior—a technology, a form of social organization, a cultural tradition—that hedges its bets in the high-stakes game of natural selection. It's like having a small cache of extra chips at your elbow in a poker game, so you don't have to fold your hand quite as soon. For example, Mary Stiner and Steven Kuhn of the University of Arizona argue that early modern humans emerged from Africa with the buffer of an economically efficient approach to hunting and gathering that resulted in a more diverse diet. While men chased after large animals, women and children foraged for small game and plant foods. Stiner and Kuhn maintain that Neanderthals did not enjoy the benefits of such a marked division of labor. From southern Israel to northern Germany, the archaeological record shows that Neanderthals instead relied almost entirely on hunting big and medium-size mammals like horses, deer, bison, and wild cattle. No doubt they were eating some vegetable material and even shellfish near the Mediterranean, but the lack of milling stones or other evidence for processing plant foods suggests to Stiner and Kuhn that to a Neanderthal vegetables were supplementary foods, "more like salads, snacks, and desserts than energy-rich staple foods."
Their bodies' relentless demand for calories, especially in higher latitudes and during colder interludes, probably forced Neanderthal women and children to join in the hunt—a "rough and dangerous business," write Stiner and Kuhn, judging by the many healed fractures evident on Neanderthal upper limbs and skulls. The modern human bands that arrived on the landscape toward the end of the Neanderthals' time had other options.
"By diversifying diet and having personnel who [did different tasks], you have a formula for spreading risk, and that is ultimately good news for pregnant women and for kids," Stiner told me. "So if one thing falls through, there's something else." A Neanderthal woman would have been powerful and resilient. But without such cultural buffering, she and her young would have been at a disadvantage.
Of all possible cultural buffers, perhaps the most important was the cushion of society itself. According to Erik Trinkaus, a Neanderthal social unit would have been about the size of an extended family. But in early modern human sites in Europe, Trinkaus said, "we start getting sites that represent larger populations." Simply living in a larger group has biological as well as social repercussions. Larger groups inevitably demand more social interactions, which goads the brain into greater activity during childhood and adolescence, creates pressure to increase the sophistication of language, and indirectly increases the average life span of group members. Longevity, in turn, increases intergenerational transmission of knowledge and creates what Chris Stringer calls a "culture of innovation"—the passage of practical survival skills and toolmaking technology from one generation to the next, and later between one group and another.
Whatever the suite of cultural buffers, they may well have provided an extra, albeit thin, layer of insulation against the harsh climatic stresses that Stringer argues peaked right around the time the Neanderthals vanished. Ice core data suggest that from about 30,000 years ago until the last glacial maximum about 18,000 years ago, the Earth's climate fluctuated wildly, sometimes within the space of decades. A few more people in the social unit, with a few more skills, might have given modern humans an edge when conditions turned harsh. "Not a vast edge," Stringer said. "Neanderthals were obviously well adapted to a colder climate. But with the superimposition of these extreme changes in climate on the competition with modern humans, I think that made the difference."
Which leaves the final, delicate—and, as Jean-Jacques Hublin likes to say, politically incorrect—question that has bedeviled Neanderthal studies since the Out of Africa theory became generally accepted: Was the replacement by modern humans attenuated and peaceful, the Pleistocene version of kissing cousins, or was it relatively swift and hostile?
"Most Neanderthals and modern humans probably lived most of their lives without seeing each other," he said, carefully choosing his words. "The way I imagine it is that occasionally in these border areas, some of these guys would see each other at a distance…but I think the most likely thing is that they excluded each other from the landscape. Not just avoided, but excluded. We know from recent research on hunter-gatherers that they are much less peaceful than generally believed."
"Sometimes I just turn out the lights in here and think what it must have been like for them."
Evolutionary biologist Clive Finlayson, of the Gibraltar Museum, was standing in the vestibule of Gorham's Cave, a magnificent tabernacle of limestone opening to the sea on the Rock of Gibraltar. Inside, fantastic excretions of flowstone drooled from the ceiling of the massive nave. The stratigraphy in the cave is pocked with evidence of Neanderthal occupation going back 125,000 years, including stone spearpoints and scrapers, charred pine nuts, and the remains of ancient hearths. Two years ago, Finlayson and his colleagues used radiocarbon dating to determine that the embers in some of those fireplaces died out only 28,000 years ago—the last known trace of Neanderthals on Earth. (Other hearths in the cave may be as young as 24,000 years old, but their dating is controversial.)
From pollen and animal remains, Finlayson has reconstructed what the environment was like from 50,000 to 30,000 years ago. Back then, a narrow coastal shelf surrounded Gibraltar, the Mediterranean two or three miles distant. The landscape was scrub savanna scented with rosemary and thyme, its rolling sand dunes interrupted by the occasional cork oak and stone pine, with wild asparagus growing in the coastal flats. Prehistoric vultures, some with nine-foot wingspans, nested high up in the cliff face, scanning the dunes for meals. Finlayson imagines the Neanderthals watching the birds circle and descend, then racing them for food. Their diet was certainly more varied than the typical Neanderthal dependence on terrestrial game. His research team has found rabbit bones, tortoise shells, and mussels in the cave, along with dolphin bones and a seal skeleton with cut marks. "Except for rice, you've almost got a Mousterian paella!" Finlayson joked.
But then things changed. When the coldest fingers of the Ice Age finally reached southern Iberia in a series of abrupt fluctuations between 30,000 and 23,000 years ago, the landscape was transformed into a semiarid steppe. On this more open playing field, perhaps the tall, gracile modern humans moving into the region with projectile spears gained the advantage over the stumpy, muscle-bound Neanderthals. But Finlayson argues that it was not so much the arrival of modern humans as the dramatic shifts in climate that pushed the Iberian Neanderthals to the brink. "A three-year period of intense cold, or a landslide, when you're down to ten people, could be enough," he said. "Once you reach a certain level, you're the living dead."
The larger point may be that the demise of the Neanderthals is not a sprawling yet coherent paleoanthropological novel; rather, it is a collection of related, but unique, short stories of extinction. "Why did the Neanderthals disappear in Mongolia?" Stringer asked. "Why did they disappear in Israel? Why did they disappear in Italy, in Gibraltar, in Britain? Well, the answer could be different in different places, because it probably happened at different times. So we're talking about a large range, and a disappearance and retreat at different times, with pockets of Neanderthals no doubt surviving in different places at different times. Gibraltar is certainly one of their last outposts. It could be the last, but we don't know for sure."
Whatever happened, the denouement of all these stories had a signatory in Gorham's Cave. In a deep recess of the cavern, not far from that last Neanderthal hearth, Finlayson's team recently discovered several red handprints on the wall, a sign that modern humans had arrived in Gibraltar. Preliminary analysis of the pigments dates the handprints between 20,300 and 19,500 years ago. "It's like they were saying, Hey, it's a new world now," said Finlayson.
No hay comentarios:
Publicar un comentario