Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived
Page 12
There were no census takers fifty thousand years ago, so we don’t know how many humans were living on the planet, counting members of every species, though genetic studies may soon illuminate this; a few hundred thousand, perhaps, certainly less than a million. The generally accepted view is that we Homo sapiens bided our time in Africa until we launched a concerted worldwide migration beyond the Dark Continent beginning about this time. This is called, not surprisingly, the Out of Africa theory. According to this hypothesis Homo sapiens displaced and then eventually replaced all other human species that had arisen over the long epochs that preceded the post–African travels of their ancestors, whomever they might have been.
The Newest Members of the Human Family
As I was writing this book, various teams of scientists around the world announced the discovery of four entirely new species of humans, an indication of exactly how quickly the field, and the human family tree that reflects it, is changing. (See The Human Family Tree, page 12.) Three of these were discovered the old–fashioned way—fossilized bones stubbornly excavated from their hiding places in the ground. Of those three, two lived some time ago—Australopithecus sediba and Ardipithecus kadabba—species that roamed Africa two and four and a half million years ago, respectively. From these remains paleoanthropologists have been able to develop some fairly deep insights into these creatures’ anatomies and lifestyles.
Based on four partial skeletons found in South Africa, sediba illustrated an emerging theme in paleoanthropology: there was a good deal more variation in ancestral humans than previously thought, and therefore lots of room to debate where they fall in the family tree. Sediba seems to have combined some old australopithecine traits and some traits of early Homo species. Its brain wasn’t terribly large (about 450 cc), but hand, pelvis, and leg bones indicate it may have been an early tool user and well on the way to walking upright more often than not. Yet fossilized plants found with some specimens tell scientists that sediba lived in forested areas as well as open ones and often ate fruits like their chimpanzee cousins.
Researchers read these clues in different ways. Some argue that sediba was a precursor to the Homo species of humans that followed. Others didn’t believe this was possible because that branch had already sprouted on the human family tree a half million years earlier with the emergence of Homo rudolfensis.
Ardipithecus kadabba is an ancestral human, and lived as many as three and a half million years before sediba in Ethiopia. (Some debate this age and set it one million seven hundred thousand years earlier.) He is ancient enough that his big toe was still designed for grasping tree branches, though other aspects of his anatomy indicate he moved on two feet on open ground. His brain was about the size of a modern bonobo at 300 to 350 cc, but his smaller incisors indicate, at least to some paleoanthropologists, that he and his fellow creatures were more socially cooperative than chimps. Male chimps have large incisors often used when battling for the attentions of the troop’s females. This makes sense if he was spending more time in the more dangerous open grasslands where he would have to rely more on others in the troop for survival.
The third and fourth species come from a different time and different parts of the world than the first two. Both dramatically reinforce the emerging reality that our direct ancestors coexisted with a variety of other extremely sophisticated humans throughout the world until very recently. Just a few years ago an assertion of this kind would have been considered heresy in the field of human evolution. Each of these species lived when we Homo sapiens did, and DNA evidence indicates that at least one also mated with us, and with Neanderthals. Human species embraced one another, it seems, in more than a metaphorical way when they had the chance.
Of these two the most recent discovery was announced in March of 2012, and because of its novelty remains controversial. The fossils haven’t even yet acquired a scientific classification. Instead researchers call their find the Red Deer Cave people, humans, but not likely Homo sapiens, that lived in south central China, north of Vietnam, as recently as 11,500 years ago. That these people were setting up camp not long before Homo sapiens had made their shattering transition from hunting and gathering to agriculture is one of the aspects of this discovery that has anthropologists both giddy and astounded.
The astonishment, however, only begins there. The fossils reveal that these people looked a little bit like us but also like more ancient humans. They have our rounded brain cases, less sloped than Neanderthals, but still retained thick, simian–style brow ridges. Like ours their faces were flat and tucked under their brain, but their chin, though the jaw juts forward, isn’t squared off like ours. And strangest of all, scans of their brain cases indicate that they had modern frontal lobes, but archaic parietal lobes, which sit farther back in our brain. It makes one wonder if their reality was different from ours, and if it was, how?
So where did these remarkable people come from? Scientists have speculated along three lines: They might have been descended from a group of Homo sapiens that departed Africa earlier than generally thought and survived and evolved in isolation. They may truly be an entirely different human species, like Neanderthals, people who evolved from an earlier branch of the human family tree, Homo heidelbergensis or Homo erectus, perhaps. Or they could be hybrids: Homo sapiens that mated with archaic humans who were also living in south China, something that might help explain their unusual mix of features.
The fourth and perhaps the most intriguing species recently discovered left behind almost no evidence of its existence; no clues about how it looked, what tools it used, or where it came from; hardly even a bone. Like the Red Deer Cave people it has also not yet been assigned a scientific name. Instead researchers refer to this species as the Denisovans because the two tiny fossils they did leave behind—a wisdom tooth and the tip of a pinkie finger—were found in Denisova Cave in the remote Altai Mountains of Siberia. You could hardly imagine more meager leavings. Yet, after scanning the mitochondrial DNA within these tiny specimens, scientists at the Max Planck Institute for Evolutionary Anthropology managed to decode the creature’s entire genome. And when they had they realized that the juvenile to whom these paltry fossils had once belonged represented an entirely new human species that had hunted and settled in these mountains forty thousand years ago. Amazingly, Neanderthals, Homo sapiens, and Denisovans each lived in the very same cave, though probably not at the same time. The DNA analysis also revealed that the peoples who became Homo sapiens, Neanderthals, and Denisovans all shared a common ancestor a million years earlier. It’s not yet known exactly which species that was, possibly Homo ergaster.
It turns out that we share a genetic link with Denisovans in another remarkable way. In analyzing Denisovan DNA the scientific team compared it with living humans from six groups: the !Kung people of South Africa, Nigerians, the French, Papua New Guineans, Pacific Bougainville Islanders, and the Han Chinese. They were electrified when they found that between 4 percent and 6 percent of the genomes of the people of Papua New Guinea and Bougainville Island contain Denisovan DNA. Scientists surmise the genes were introduced to the islands when the hybrid descendants of Homo sapiens and Denisovans migrated into Southeast Asia and later Melanesia. There is even some evidence that these descendants made their way to Australia and the Philippines.
It’s difficult to not be transfixed by these discoveries when you really take the time to think about it. Like Neanderthals and Homo floresiensis they are species who fought and struggled and lived sophisticated lives for tens, even hundreds of thousands of years alongside our direct ancestors on the same planet we inhabit today. And if that isn’t astounding enough, some even mated with our kind, contributing forever to our DNA. Were these aberrations or the norm? How many more species and hybrids might we find now that DNA analysis has opened so many genetic doors?
If that’s true (and there’s little debate that it is, though it’s becoming clear it wasn’t quite this simple), most of the different varieties of hum
ans, given their nomadic ways, must have crossed paths from time to time as they wandered into the edges of one another’s territories.
There is evidence of this in the rocky hills of Galilee, not far from Nazareth, the birthplace of Jesus Christ. In 1929, in caves that pock the hills of Qafzeh, Israel, two scientists found an ancient burial ground and, remarkably, the bodies of eleven anatomically modern humans. At first scientists thought the bones were no more than fifty thousand years old, but later, improved dating technology revealed that they were nearly twice that age, making them the oldest modern human fossils to be found outside Africa. As researchers continued rummaging through the site, they realized the bodies retained some of the more archaic features of their ancestors, but that they were culturally advanced. The ornamental shells and red, yellow, and black ocher paints they left behind indicated as much. So did the hearth and the burials of the bodies themselves, one of which included a mother and her child. Their tools, however, weren’t as advanced as later Homo sapiens’.
The odd thing about that was that their tools instead resembled Neanderthal implements, yet they themselves were not Neanderthal. The best guess is that somehow they, or earlier generations, had crossed paths with their northern cousins and borrowed some of their technology because it was better than their own.
For all we know, these were early Homo sapiens explorers, the Marco Polos and Vasco da Gamas of their day, wandering the Arabian Peninsula while less adventurous Homo sapiens tribes remained on the home continent. By all accounts, their expedition wasn’t terribly successful. The skeletons of red and fallow deer, small animals, aurochs, and some seafood shells show they gave colonizing the area a game effort, but their excursions never made it beyond the hills of Qafzeh and barely beyond the borderlands of Africa. There is no evidence that any of their kind ever made it north of this sector of the Middle East, not this far back in human prehistory. Maybe the explorers retreated home across the Red Sea straits, played out and tired; or maybe those eleven buried were put to rest by a last few survivors, or maybe they hung on for years in a small group like a prehistoric version of the settlers at Plymouth Rock or Jamestown, until disaster or disease at last carried them off. No one knows.
In the same area, paleoanthropologists recently discovered that Neanderthal explorers likewise found their way south into Galilee thirty thousand years later (December 26 or so in the HEC), but they came from the north rather than the south. Did the fair–haired, bulky colonists run into lithe, dark–skinned people from across the Arabian straits? If so, did the Neanderthals do them in or run them off the peninsula back to Africa?
“At that point,” says paleoanthropologist Nicholas J. Conard of the University of Tübingen in Germany, “the two species are on pretty equal footing.” The tools of both Homo sapiens and Neanderthals would have been about equally advanced, and given what the Neanderthals had been facing in the wilds and weather of Europe the past 130,000 years, they would have been an extremely tough breed. Modern humans may not have been their match, not yet. Or perhaps they mated and their offspring dissolved into the continent and disappeared from the map. Either way, it seems that for another twenty thousand years or so Homo sapiens ceded Asia to their barrel–chested cousins.
Whatever happened, we do know that Neanderthals and Homo sapiens eventually encountered one another in Europe sometime after our long–lost ancestors finally made their big push out of the Dark Continent. But what about the East and the erectus bands that had begun heading off toward India and China and Southeast Asia two million years earlier? What became of them and their ancestors, and did ours make contact with them?
Scientists haven’t uncovered direct fossil evidence of even a single meeting—no burial sites, artifacts, or bones—but in 2004 a team that included biologist Dale Clayton and anthropologist Alan Rogers, both working at the University of Utah, proved that our ancestors did indisputably have a close encounter with another human species in the Far East sometime around twenty-five thousand years ago. How could they know if there was no fossil evidence?
Head lice.
Like every other living thing on Earth, head lice have DNA. And like humans or finches or predatory big cats, different species of lice have different DNA. Anytime we find head lice on ourselves—breakouts among schoolchildren are more common than parents would prefer—we find two kinds that are rarely separated. Despite nearly always being in one another’s company, however, each initially evolved separately while dining on two different species of early humans. One of those species led to us. The other is extinct. For those two species of lice to coexist today, both had to have come into close contact sometime in the past.
By studying their DNA and then time–stamping the evolution of both strains, the Utah study concluded that at least one meeting took place sometime between thirty thousand and twenty-five thousand years ago in Asia. “We’ve discovered the ‘smoking louse,’” Clayton wryly observed. “The record of our past is written in our parasites,” added Rogers.
What makes this discovery especially surprising, aside from its creative use of parasites to track human behavior, is that most paleoanthropologists believe that Homo erectus met his end seventy thousand years ago, long before this encounter could possibly have taken place. Nevertheless it’s difficult to dispute the evidence. Parasites reflect the evolution of their hosts. They rely on them for their livelihood after all, and their fortunes and survival are inextricably bound. So some direct descendant of Homo erectus must have survived forty–five thousand years longer than previously believed. Whoever this species was, the genetic history of the head lice that colonized it shows that it split into two species around 1.18 million years ago, about the same time that Homo erectus and our direct ancestors in Africa, possibly Homo ergaster, parted ways. That explains why the lice themselves also parted company and eventually evolved into two species in the first place.
The lice reveal something else fascinating (who knew the little buggers could be so informative?). The Homo sapiens strain corroborates evidence that our direct ancestors had been reduced to extremely small numbers between one hundred thousand and fifty thousand years ago before rebounding and rapidly expanding, with their head lice, to colonize the rest of the world. This supports mitochondrial genetic evidence that our kind nearly met an early and tragic (at least for us) end around seventy thousand years ago before recovering to spend the next fifty thousand years becoming the planets dominant species on Earth.
Strangely enough, the archaic lice, the ones that made their homes on the heads of the species no longer with us, show not an iota of evidence that they went through either a similar bottleneck or population explosion. During those years, when Homo sapiens had nearly been rubbed out, perhaps by the Olympian–scale eruption at Lake Toba in Indonesia, these other humans were apparently getting along just fine. One theory is that they were safely upwind of the explosion and didn’t feel the immediate, violent effects, though this doesn’t explain how they managed to survive the subsequent global volcanic winter some scientists feel resulted from the gargantuan blast. All indications are that this line of humanity did just fine, at least until they crossed paths again with the Homo sapiens descendants of the species that they had split off from more than a million years earlier.
It is not as crazy as it might once have been thought that a more modern descendant of Homo erectus was still alive as recently as twenty-five thousand years ago. The more scientists examine the past, the more surprises they find. They found a particularly big one when the remnants of an entirely new human species that no one had had the slightest inkling had ever existed came to light in 2004 at Liang Bua, a cave on the island of Flores, 388 miles east of Java in Indonesia. After much debate and head scratching, most paleoanthropologists agreed that Homo floresiensis, as these remarkable creatures came to be known, was a bright, toolmaking human. The big surprise, beyond the discovery that these people existed at all, was their startlingly Lilliputan stature. The press and even astound
ed scientists took to calling them “hobbits.” One three–foot–three–inch–tall adult–woman skeleton that was discovered turned out to be even shorter than Lucy.
Their brain size at 420 cc was also not much larger than Lucy’s, a hominin that had walked the earth more than 3 million years earlier. Yet these creatures could control fire, make sophisticated tools, and hunt game, though it’s still an open question as to whether they could speak or used any advanced language. How, scientists have wondered, could a species with a brain less than one third the size of ours pull off these sorts of impressive feats?
Our best evidence indicates that the Flores hobbits lived between ninety–five thousand and seventeen thousand years ago, the descendants of earlier Homo erectus settlers who were eventually reduced in size by an odd evolutionary phenomenon scientists call island dwarfing. Island dwarfing happens when natural forces cause species to shrink in size over time in isolated locations, presumably because resources are severely limited. The theory is that in a kind of ecological bargain, animals grow smaller rather than starve. By reducing their size, both resources and diversity are both preserved, and life goes on with predator, prey, and the entire ecological niche surviving in a sort of pygmy state. Dwarfing can have other advantages under these circumstances. It’s easier to stay warm or cool when you are smaller, which saves energy and requires less food. On Flores, in addition to the hobbits themselves, scientists have found examples of a small, elephant–like creature called Stegodon, an animal the hobbits apparently hunted with some enthusiasm.
Because of Homo floresiensis’ size, especially the size of its brain, scientists have enjoyed some spirited debate about whether it came to the island in the form of a lean and tall Homo erectus (remains of erectus have been found on nearby Java), then shrank over time due to island dwarfing, or whether it may have been the descendant of smaller, Lucy–size creatures who came out of Africa before erectus and then made their way somehow to the islands of Indonesia.