Skull Is Bone of Evolutionary Contention : Science: Researchers say new fossil supports theory that a sudden event spurred human development.
Scientists today announced the discovery of the first essentially complete skull of what may be humanity’s earliest known ancestor--an agile ape-like creature whose descendants developed language, discovered fire and, eventually, invented power tools.
The 3-million-year-old skull, unearthed in Ethiopia by a team from the private Institute of Human Origins in Berkeley, is the strongest evidence yet that a single ancestral primate species--best represented by a famous fossil skeleton known as Lucy--thrived across Africa for almost 1 million years without significant physical change. The species, Australopithecus afarensis, is believed to have roamed the continent until something abruptly spurred the developments that led to modern humans.
Experts in human origins at Yale University, UC Berkeley and Johns Hopkins University said the new skull is a compelling argument for the theory that the evolution of human life on Earth proceeded in fits and starts, with long periods of stasis punctuated by relatively sudden periods of change.
The skull unveiled Thursday is from a large, adult male perhaps no more than four feet tall. His pugnacious jutting jaw, large canine teeth, tiny brain and thick, bony ridges above his eyes form the newest portrait of the ancient stock from which humanity apparently arose.
He is, institute researchers said, the root of the human family tree.
“This species was as widespread as baboons are today,” said Tim D. White of UC Berkeley’s Laboratory for Human Evolutionary Studies, who has discovered some of the oldest examples of the hominids, as pre-human species are known generally, in Tanzania.
Anatomist Yoel Rak of Tel Aviv University discovered the first few gravel-sized flakes of the fossilized skull near the bank of a dry riverbed in Ethiopia’s Afar badlands in 1992, shortly after delivering an impromptu lunchtime lecture on the features of the typical hominid skull. Eventually, the institute team unearthed 200 rock-encrusted fragments in all--enough to reconstruct almost the entire skull.
“For me, the skull is a dream come true,” said institute president Donald C. Johanson, who led the 1992 expedition that found the fossil. The expedition’s findings were published today in the British science journal Nature.
“There is something very captivating about being able to pick up the skull and look into those eye sockets, to realize that this part of the brain case (held) gray matter . . . to see the face,” Johanson said.
The skull is the centerpiece of a new collection of 53 pre-human fossils that the institute team discovered during three years of field work in one of the richest sources of early hominid fossils in the world.
“It is of immense interest,” said Elisabeth Vrba, an expert in paleobiology and mammalian evolution at Yale University. “It would be the best evidence we have” for the state of human development at a crucial juncture in evolution.
The skull is 10,000 generations, about 200,000 years, younger than the Lucy skeleton, which was found in the same corner of northeastern Ethiopia. The skull is the youngest example of the species discovered so far. Lucy, found in 1974 about a mile from the site where the skull turned up, is thought to be 3.18 million years old; the oldest known afarensis specimens are 3.9 million years old.
The traditional theory of evolution suggests that any species acquires new characteristics gradually at a measured, steady pace.
But after the long period of afarensis stability, researchers say, there was an explosion of new hominid species that included larger-brained hominids that had started using tools by about 2.5 million years ago. That species led directly to modern humans.
During the period of afarensis, “there is no obvious sign of evolution in this pre-human species for about a million years,” said William H. Kimbel, institute director of paleoanthropology, who shared in the discovery. “Yet later, within only a fraction of that time, it gave rise to a great branching of the family tree.”
Not everyone agrees with the idea that the bones attributed to afarensis all belong to a single species--some of the individuals represented by the fossils seem much larger than others. The larger fossils, which include the new skull, could instead belong to a separate “robust” species that coexisted with smaller hominids in East Africa, some experts have argued.
If there were two coexisting hominid species and not one, then human development may have been proceeding at a more stately, traditional pace without the need for a sudden change to speed up the evolution of Lucy and her kind.
“Some people have gone so far as to say the (afarensis) species has nothing to do with the line that led to genus Homo, “ said Andrew Hill, curator for anthropology at Yale’s Peabody Museum of Natural History. “If afarensis is a real ancestor of these later lineages, then something must have happened fairly quickly.”
Johanson and his colleagues say the size difference is simply a normal variation between male and female members of the species, like that observed among modern apes.
Although the newest additions to the human fossil record may help settle whether the bones from Ethiopia represent one species or two, they will do little to resolve an argument about how far these primitive, upright-walking primates ventured from the trees their own ancestors had called home.
Among the new fossils announced Thursday is an especially long forearm bone more like that of a chimpanzee than a human being, indicating that the hominids had powerful arms, especially suitable for climbing trees. That has led some experts to suggest that the species was less evolved than Johanson and his colleagues believe. It may have spent as much time in the trees as it did on the ground, sleeping or hiding from predators in high branches.
Although no one knows what may have spurred pre-humans to develop intelligence, many experts believe that a radical change in climate could have been the reason, by driving the creatures out of the trees or making them compete more fiercely for dwindling food and water supplies.
The single hominid species seems to branch into several new species at just about the same time--2.5 million years ago--that a major ice age set in around the world, forcing dramatic changes in climate and the local environment.
“Many (animal) lineages of substantial duration either become extinct or give rise to new species at that time,” Vrba said. “Perhaps climatic change did cause these events.”
Some experts believe that their arboreal habits could have delayed the evolution of intelligence, by making it harder for afarensis mothers to bear larger-brained infants. Newborn chimpanzees, for example, are agile almost from the moment of birth, but some experts theorize that more intelligent hominids were born with smaller brains, which developed fully after birth, like a human child. That means that as newborns they were helpless. In the trees, such creatures simply would have fallen to their deaths.
“My view is that the reason that these things didn’t go anywhere for so long is because these guys are happily climbing trees,” said Steven Stanley, an expert on early human development at Johns Hopkins in Baltimore.
“About 2.5 million years ago is just about when the ice age began. At that time, Africa became drier; forests shrank. It is exactly the kind of thing that would force creatures out of the trees,” he said.
Clues to Humanity
During three years of field work in Ethiopia, an expedition from the private Institute of Human Origins in Berkeley discovered dozens of new early primate fossils that shed new light on human origins.
* The discovery: The first complete skull of humankind’s earliest known ancestor, Australopithecus afarensis
* How the fossil was found: In a dry riverbed about one mile from where the famous “Lucy” skeleton was found in 1974. Both are the same species.
* About the fossil: The 3-million-year-old skull--with a protruding, ape-like jaw, jutting, bony eye ridges and a small brain case--is from a large, adult male of the species many scientists think is the common stock from which all later hominid species, including modern humans, arose.
What the discovery means: The skull is the strongest evidence yet that a single ancestral hominid species flourished unchanged in Africa for almost a million years before climate change or some other unknown pressure spurred the sudden development of several different hominid species, one of which became the line that led to modern humans.
