|In 2001 a team working in the Republic of Georgia uncovered a 1.7-million-year-old hominid skull that once housed a brain just half the size of a modern human's. No hominid skull so primitive had ever before been found outside Africa. |
Photograph by Gouram Tsibakhashvili.
Three hundred feet below the ash gray ramparts of a crumbling fortress, two rivers merge in a verdant valley and sweep eastward to the Caspian Sea. It's easy to see why Georgian royalty would have established a stronghold on this basalt bluff along the old Silk Road in southwestern Asia 1,000 years ago. It's also clear how this mild region, sheltered by mountains from the cold steppes to the north and from arid plains to the south, would have appealed to a band of human ancestors who may well have been the first to venture out of Africa nearly 2 million years ago. What is not at all clear, muses paleoanthropologist David Lordkipanidze, is how those primitive wanderers could have traveled here in the first place.
Lordkipanidze, the deputy director of the Georgian State Museum in the Republic of Georgia, has reason to wonder. In the last three years, he and his team at Dmanisi have unearthed four hominid skulls ancient human forebears at least 1.7 million years old. No hominids so ancient have ever been found outside Africa. Stranger still are their features: apelike brows, humanlike teeth, and most amazing of all, a brain roughly half the size of our own.
Which leads to the biggest puzzle for Lordkipanidze: Anthropologists have always assumed small-brained hominids lacked enough intelligence to create the tools they'd need to fan out from their African homeland and survive in new habitats. The remains of these ancient wanderers should not be here unless our conception of the origin of modern humans is wrong.
The skulls from Georgia are not the only hominid remains rewriting our history. Last July, researchers in central Africa unveiled an emissary from the time when our ancestors split from chimpanzees some 7 million years ago. That skull poses even more questions. First, it was found in Chad, 1,500 miles from the East African Rift Valley, long the presumed center of hominid evolution. Second, the area where the skull was found was once forest, not the primordial wide-open savanna where hominids and their hallmark bipedalism were thought to have evolved. Third, and perhaps most surprising, the skull is nearly twice as old as any other hominid skull ever uncovered. It is 2 million to 3 million years older than Ardipithecus ramidus,
the Ethiopian fossil thought to be the earliest exemplar of the transition from ape to hominid. "It's a revelation," says Fred Spoor, professor of evolutionary anatomy at University College London. "The blueprint for our common ancestor is gone."
The Chad and the Dmanisi skulls, says Bernard Wood, a paleoanthropologist at George Washington University in Washington, D.C., "have upset traditional views about the origins of hominids and about the origins of Homo,"
the genus to which humans belong. The story of human evolution no longer looks like a smooth, gradual transition from ape to hominid. Instead it resembles an eons-old game of Survivor,
with multifarious contenders popping in and out of the fossil record.
"These hominids were coming off the production line in a mix-and-match sort of way," says Wood, which forces researchers to revisit a fundamental question: What is a hominid? Or to phrase it another way: Which of these creatures are our ancestors, and which are also-rans?
One conviction has held firm since the days of Charles Darwin: Hominids descended from apes. Starting around 11 million years ago in the subtropical forests of Eurasia and, most likely, Africa, the great apes effloresced in an array of shapes and sizes. But within a couple of million years, the vast inland Tethys Sea, which extended across southern Europe, dried up. The Plateau of Tibet thrust up, and forests began a steady retreat toward the equator. That was bad news for the great apes. "They started to bite the dust," says Rick Potts, director of the human origins program at the National Museum of Natural History in Washington, D.C. Although a dearth of great-ape fossils makes it hard to reach conclusions, it appears that, beginning about 7 million years ago, the range of the last of the great apes was limited to Africa and Southeast Asia.
Not long afterward, some enterprising individuals evolved the ability to walk upright. How and why remains one of evolution's great mysteries. A long-standing theory held that bipedalism evolved only when early hominids needed to survive in a changing habitat. As the forests of Africa shrank, the theory suggested, bipedal hominids emerged who could move more freely while foraging on the expanding savanna.
Another defining hominid feature is the presence of small canine teeth. Anthropologists guess that their emergence accompanied a change in foraging habits that led to hominids walking upright.
This story was pieced together based on hominid finds in Africa's Rift Valley. But the skull found in Chad hints at a different, far more complicated history. "Could Chad represent its own little cauldron of experimentation in human evolution?" asks Potts. "Yeah. And that's really exciting."
Chad: 6 million years old
Dmanisi: 1.7 million years old
|The Chad skull has the thick browridge of a gorilla, but the rest of its face resembles hominids one-third its age. The Dmanisi skull, which belonged to a young person, is one of the most primitive skulls ever attributed to the genus Homo. By contrast, some paleontologists note that a child resembling the 12-year-old Turkana boy an H. erectus skeleton found in Kenya might pass unnoticed among us if a cap covered his low forehead and heavy brows. |
Photographs copyright M.P.F.T.; copyright Science.
The Chad skull was discovered in July 2001 in the Djourab Desert by Ahounta Djimdoumalbaye, a student from the University of N'Djamena in Chad who was working under paleontologist Michel Brunet of the University of Poitiers in France. "To my great surprise, it was nearly complete," says Brunet.
His team members contend that the skull's original owner nicknamed Toumaï, which means "hope of life" in the local language was a hominid, most likely a male. They make their case based on Toumaï's small canines, short lower face, and the forward position of an indentation in the cranium where the spinal column connects to the head, indicating a head held upright.
Bernard Wood imagines glimpsing Toumaï in the flesh. "From the back, you might have mistaken him for a chimpanzee," he says. Head on, however, he would have been far more interesting. Eyes set wide like a gorilla's and a heavy browridge level with the top of the skull suggest an ape. Suggesting a human are the modestly sized lower face, a short snout like an early Homo
from a couple of million years ago, and petite canines that fit snugly against the incisors, just as they do today. "If it's not a hominid, it's something that's damn near one," Wood says.
Yet Toumaï may not have been a biped. Another bit of paleo-phrenology suggests he possessed the huge neck muscles of a bodybuilder or, as some claim, a female gorilla. The skull "has a whopping big crest for the connection of neck muscles," Potts says. That brawny neck could have kept a head upright for an ape loping on all fours. "You get the sense that it's really close to the branching point [the last common ancestor between chimpanzees and hominids]."
Without more of the creature's skeleton particularly a crucial leg or foot bone, or remains of kin no one can be certain Toumaï walked upright. Brunet argues that the canines alone make it a strong candidate for a hominid. Until Toumaï was found, such dentition was thought to have originated in australopithecines, as many as 2 million to 3 million years after Toumaï lived. These heavy-jawed hominids, presumed to be human ancestors, arose in Africa about 4 million years ago and include the famous 3.5-million-year-old skeleton named Lucy.
Toumaï is not the only fossil vying for the title of earliest hominid. Two years ago, Martin Pickford and Brigitte Senut of the College of France unveiled a clutch of 6-million-year-old fossils unearthed in Kenya's Tugen Hills. The remains, named Orrorin tugenensis,
are just a few fragments of limbs and the lower jaw and some teeth. Nonetheless, the discoverers contend that Orrorin's
molars small and thickly enameled like our own look hominid. So does its femoral head, the rounded end of the leg bone that fits into the pelvis. The bone appears robust enough to hold a torso upright. The specimen's only shortcoming, though, is glaring: Unlike the teeth in the Chad skull, Orrorin's
canines are not only larger than our own but grooved like those of a chimpanzee.
Click on the image to enlarge. (32k)
|For their size, humans have larger brains than all other animals except dolphins and whales. The emergence of larger brains appears in the fossil record, summarized at right. The Chad skull (Sahelanthropus tchadensis) held a brain less than one-third the size of a modern human's. Over time, hominids lost the wide flat teeth for grinding plant food. Their brains got bigger as their teeth got smaller. Presumably, bigger brains allowed them to forage more effectively. |
Graphic by Matt Zang
Not surprisingly, some experts say Orrorin
was a chimp. The argument is fueled more by vitriol than evidence, as there is no fossil record for chimpanzees. "We badly need to find what people are desperately trying to say that they haven't found, which is a fossil chimpanzee," says Wood. Biological anthropologist Leslie Aiello, dean of the graduate school at University College London, urges caution as well. "How do we know that any of the recent finds are on the line to modern humans?" she asks.
Another fresh set of fossils from the northern Rift Valley, in Ethiopia, has only added to the confusion. Perhaps 5.8 million years old, the jawbone, teeth, and other fragments belong to a species of the ancient Ardipithecus
line. This candidate's canines look more like our own. The shape of a well-preserved foot bone hints at an upright posture. Until recently, one of this specimen's presumed descendants, 4.4-million-year-old Ardipithecus ramidus,
held the title of earliest known human ancestor. "A couple of years ago, quite a number of us were simply waiting for Ardipithecus
to tell us what it was all about. We thought that it would be the most primitive hominid," Potts says. "All this mixing and matching [of traits] suggests a lot of population isolation, independent evolution, and coalescence of populations again. It's going to be really difficult to figure all this out," he says.
Daniel Lieberman, a biological anthropologist at Harvard University, agrees. There are but a few dozen hominid skulls more than 1 million years old, and skulls don't reveal much about evolutionary relationships. "You don't inherit bones from your ancestors; you inherit their biochemistry," he says.
While scholars can easily dispute the meaning and classification of hominid finds in Africa, the discoveries in Georgia are provoking near-unanimous wonder and bewilderment. Almost a million years older than any hominid remains found in Europe, they are forcing scholars to rethink not only what constitutes an early human but how those early humans left Africa and peopled the globe.
|Paleontologists infer the evolution of various hominid species and the migration of Homo erectus out of Africa from remains in the fossil record. The 6-million-year-old skull found in Chad is now the oldest contender for first hominid. A few years ago, the oldest candidate was believed to be from Ethiopia. |
Graphic by Matt Zang