Thursday, July 19, 2007

Ancient Jawbone Could Shake Up Fossil Record

An ancient jawbone, above, was recently found in Africa's fossil-rich Afar region, just 20 miles (32 kilometers) north of the spot where the famed skeleton of "Lucy," an early human ancestor, was unearthed in 1974. The bone, which is at least 3.5 million years old, may spark further debate about how many types of human ancestors roamed East Africa at once. Photograph courtesy Cleveland Museum of Natural History/Handout/REUTERS

Ancient Jawbone Could Shake Up Fossil Record
Nick Wadhams in Nairobi, Kenya
for National Geographic News
July 13, 2007

Jawbones from an early human ancestor, found recently in northeast Ethiopia, could shine light on a murky period of human evolution, paleontologists say.
The bones were found in the fossil-rich Afar region, just 20 miles (32 kilometers) north of the spot where the famed skeleton of "Lucy"—early human ancestor who lived 3.2 million years ago—was unearthed in 1974. (What was Lucy?)
The new bones are believed to date from 3.8 million to 3.5 million years ago.
Bridging the Gap
Though more research needs to be done, the group says the bones could bridge the gap between two known human ancestor species.
Australopithecus anamensis lived some 4.2 million to 3.9 million years ago, and Australopithecus afarensis—the species to which Lucy belonged—thrived from 3.6 million to 3 million years ago.
Some researchers believe that Lucy and others of her species were descendants of A. anamensis—and these new Ethiopian jawbones could end that speculation.
"This will help us test this very hypothesis and see if we can falsify it or prove it," said Yohannes Haile-Selassie, one of the lead researchers on the project and head of physical anthropology at the Cleveland Museum of Natural History in Ohio.
"We have had isolated teeth and [other skeleton parts] from previous years. What we didn't have was a complete jaw, which we have now," he said.
Along with the jawbone, the team has also uncovered more than 30 or 40 specimens to further test the hypothesis, Haile-Selassie noted.
Teeth Truths
Finding a complete jawbone is crucial in determining how a human ancestor developed. (Related: "Dental Detectives Reveal Diet of Ancient Human Ancestors" [November 9, 2006].)
The earlier species, A. anamensis, had large canine teeth and a narrow jaw. When Lucy appeared, compared to A. anamensis, the jaw had widened, the canines had become smaller, and the molars had grown. Such changes suggest that the A. afarensis chewed, not tore, its food.
Previously, the researchers had found teeth dating from about 3.5 million years ago, but their 2007 field work yielded more complete jaws.
Generally, the bones could help clarify a long-standing debate in human evolution: How many branches of human ancestor existed millions of years ago? Did some evolve into new forms only to die out and disappear?
"People are prepared to accept that there's diversity," said Chris Stringer, a research leader at London's Natural History Museum, who was not involved in the latest work. "But of course what is uncertain is how widespread it was through the last six million years."
"If we had a complete fossil record, would we see branching events right through that period of time ... or were there only specific times of branching events?" Stringer said.
Fertile Ground
The Afar Depression—a sunken area in the Horn of Africa where the new jawbone was found— has produced some of the most significant fossil finds in history.
For example, the region has yielded some of the earliest examples of human ancestors ever, which lived 5.8 million years ago, as well as an early anatomically modern human from 160,000 years ago.
(Related: "Fossil Find Is Missing Link in Human Evolution, Scientists Say" [April 13, 2006].)
Three years of research in the area where the latest bones were found have yielded more than 1,900 vertebrate specimens. They include human ancestor species from many time periods, as well as other animals such as mice, elephants, monkeys, rhinoceros, primitive horses, and fish.
A partial skeleton of a human ancestor was discovered in 2005, and Haile-Selassie's team continue to excavate what's left of it. Haile-Selassie said his team would need some time to study the bones and come up with conclusions about the relationship between A. anamensis and A. afarensis.
"Two years down the line we may be able to say something about it," Haile-Selassie said. "Patience is needed."

The fossil skull of early human ancestor Paranthropus robustus, which died out 1.5 to 1 million years ago, is seen here. High-tech dental detectives have revealed that the species had a surprisingly diverse diet, countering the theory that it went extinct because it couldn't cope with food scarcity in Africa's changing environment. Photograph © Science

Dental Detectives Reveal Diet of Ancient Human Ancestors
Sean Markey
for National Geographic News
November 9, 2006

Paranthropus robustus, a dead-end branch of the early human family tree, has been described as a "chewing machine" that was mostly jaws and not much brains.
While the label may still apply, pioneering dental detective work has revealed unexpected news about the species' dietary variety.
Using lasers to vaporize tiny particles of tooth enamel, researchers in the United States and Great Britain analyzed the chemical makeup of 1.8-million-year-old fossil teeth from four individuals unearthed in the Swartkrans cave site in South Africa.
Different types of edible plants leave unique chemical signatures in living tissue, including teeth.
Based on the types of carbon isotopes preserved in the P. robustus fossils, the team concludes that the diminutive primates had a surprisingly varied and flexible menu.
Their seasonally adapted diet may have included fruits, seeds, roots, tubers, and even insects.
The findings contradict the long-held theory that P. robustus was a dietary specialist that chomped solely on low-quality plants, the researchers report in tomorrow's issue of the journal Science.
Scientists had long cited this theory to explain why the bipedal primate went extinct 1.5 to 1 million years ago, arguing that the human ancestor couldn't cope with food scarcity in Africa's changing environment.

(Related news: "Tooth Study Reveals Diets of Early Humans" [August 3, 2005].)

But the new study suggests a different story.
"Lucy's Children"
Standing just four feet (about a meter) tall and weighing about a hundred pounds (45 kilograms), P. robustus appeared in East Africa about 2.5 million years ago.
The species had a brain that was slightly larger than a chimp's and about 60 percent smaller than a modern human's.
P. robustus is a descendent of Australopithecus afarensis, a group first chronicled by the discovery of the famous "Lucy" fossil in Ethiopia (get Lucy fast facts).
Lucy's kin also gave rise to the highly adaptable, tool-using genus Homo, which includes Homo sapiens, or modern humans.
Matt Sponheimer, an anthropologist at the University of Colorado at Boulder who led the study, says the very different fates of "Lucy's children" have always puzzled scientists.
"There's this question about this tale of two children, as it were, one of which flourishes [and] the other which doesn't," Sponheimer said. "And the question is, Why?
"What I think we've done in this study is show that traditional explanations as to why [P. robustus went extinct] are probably, at the very least, dramatically oversimplified and possibly just outright incorrect."
Sponheimer and his colleagues suggest that other biological, social, or cultural factors may explain the ultimate disappearance of P. robustus.
As for the success of the Homo species who lived at the same time as P. robustus, the paeloanthropologists says that any number of factors may explain why our ancestor survived and P. robustus did not.
"It would be very interesting if it turned out that the main difference was that we just were able to procreate more quickly—we were the rabbits to [P. robustus's] apes," Sponheimer said.
Laser Dentistry
The new findings are also significant because they showcase the powerful new dental investigative technique known as laser ablation.
The method was advanced by study co-authors Benjamin Passey and Thure Cerling at the University of Utah in Salt Lake City.
Sponheimer says the innovation enables scientists to uncover a trove of previously untapped data that lie preserved in the fossil teeth of early humans and other ancient and modern animals.
By studying the growth rings and chemical composition of teeth, for example, researchers can reveal dietary habits, detect changes in climate, and possibly track migrations.
Stanley H. Ambrose, an anthropologist at the University of Illinois, Urbana-Champaign, said the study's findings are "very exciting."
He says the new dental technique opens new possibilities to learn not only what early humans and ancient animals ate but also how they interacted within their communities and how their environments changed over time.
"It will be a great tool … because it will allow a fairly high-resolution monitoring" of dietary and seasonal climate changes within ancient animal communities, he said.
In a related perspective article in Science, Ambrose says the dental detective technique could also explore whether environmental changes helped drive the evolution of modern humans.

Fossil Find Is Missing Link in Human Evolution, Scientists Say
John Roach
for National Geographic News
April 13, 2006

When the famous skeleton of an early human ancestor known as Lucy was discovered in Africa in the 1970s, scientists asked: Where did she come from?
Now, fossils found in the same region are providing solid answers, researchers have announced.
Lucy is a 3.5-foot-tall (1.1-meter-tall) adult skeleton that belongs to an early human ancestor, or hominid, known as Australopithecus afarensis.
The species lived between 3 million and 3.6 million years ago and is widely considered an ancestor of modern humans.
The new fossils are from the most primitive species of Australopithecus, known as Australopithecus anamensis. The remains date to about 4.1 million years ago, according to Tim White, a biologist at the University of California, Berkeley.
White co-directed the team that discovered the new fossils in Ethiopia (map) in a region of the Afar desert known as the Middle Awash.
The team says the newly discovered fossils are a no-longer-missing link between early and later forms of Australopithecus and to a more primitive hominid known as Ardipithecus.
"What the new discovery does is very nicely fill this gap between the earliest of the Lucy species at 3.6 million years and the older [human ancestor] Ardipithecus ramidus, which is dated at 4.4 million years," White said.
The new fossil find consists mainly of jawbone fragments, upper and lower teeth, and a thigh bone.
The fossils are described in today's issue of the journal Nature.
Found Links
According to White, the discovery supports the hypothesis that Lucy was a direct descendent of Australopithecus anamensis.
Previously, Au. anamensis was known only from fossils discovered about 470 miles (750 kilometers) away in Kenya.
The new fossils also link the older Ardipithecus with the more recent Australopithecus.
In 1992 White's team discovered 4.4-million-year-old Ardipithecus ramidus fossils in the Middle Awash.
But the relationship between Australopithecus and Ardipithecus remained unclear because of the million-year gap in the fossil record.
The newfound fossils narrow this gap to 300,000 years, which is still long enough to keep the scientists from saying that Ardipithecus ramidus is the direct ancestor of Australopithecus anamensis.
However, White believes the two species are closely related.
"This discovery strengthens the ties between Ardipithecus and Australopithecus at the genus level, but what happened at the species level is going to require more fossils," he said.
Awash in Fossils
The discovery of the new fossils in the Middle Awash highlights the region as a treasure trove for the study of human origins.
Its mile-thick (1.6-kilometer-thick) column of sediment spans more than 6 million years, encompassing three major phases of human evolution.
"At 4.4 [million years] you have ramidus, at 4.1 you have anamensis, and at 3.4 in the same rock column—less than a day's walk away—you have the Lucy species," White said.
"That is unique," he added. "There is no other site like this."
In addition to the ancient hominids, the site includes fossils of more modern early-human species.
There are also thousands of fossils of animals, such as colobus monkeys, pigs, birds, rodents, and even carnivores like hyenas and big cats. Petrified wood has also been found in the area.
These fossils together "show that a closed wooded habitat type persisted over a long period in this part of the Afar," team member Giday WoldeGabriel, a geologist with the Los Alamos National Laboratory in New Mexico, said in a statement.

Tooth Study Reveals Diets of Early Humans
Stefan Lovgren
for National Geographic News
August 3, 2005

Researchers have used new microscopic technology to reconstruct the diets of two extinct human species that lived in what is now South Africa.
The technique involves scanning the tooth surfaces in extreme detail to learn what a species ate. Reconstructing the diet of extinct human species can help shed light on our evolutionary history.
Scientists studied two species of early humans, Australopithecus africanus, which lived in southern Africa around three million years ago, and Paranthropus robustus, which inhabited the same area about a million years later.
Researchers found that the diets of the two species were similar, though A. africanus may have eaten a greater share of tough foods, while P. robustus probably ate more brittle foods, such as seeds.
The key difference in their diets was in how they each adjusted in times of food scarcity, scientists found.
"The two species ate similar foods much of the time, but differed mostly at crunch times, when their anatomical differences allowed them to fall back on different less-preferred foods," said Peter Ungar, a study author and anthropology professor at the University of Arkansas in Fayetteville.
The research is described in tomorrow's issue of the academic journal Nature.
Overall Texture
Dental microwear analysis investigates the microscopic scratches and pits that form on a tooth's surface as a result of its use.
The method used in the new study provided a novel, three-dimensional look at microscopic surface wear.
The technique measures the height of tens of thousands, if not hundreds of thousands, of points on a surface, capturing its overall texture at the microscopic level.
"The closer you look, the more detail you see, and the larger the area appears," said Christopher Brown, a study co-author and professor of mechanical engineering at Worcester Polytechnic Institute in Massachusetts.
During their study, the researchers first examined the teeth of modern-day capuchins, howling monkeys, and other monkey species to discern the marks left by fruits, seeds, and other plants.
Knowledge of the monkeys' diets helped researchers link certain foods to different tooth-wear patterns.
"We take living species with known diets and see how they differ in their microwear patterning," Ungar explained. "Once we have a relationship between pattern of wear and diet, we can infer diet from pattern of wear in the fossils."
The researchers then turned their attention to the fossil teeth of two extinct human species.
One of these extinct humans, A. africanus, lived between about 2.3 and 3 million years ago in the Sterkfontein Valley of South Africa. At the time, vegetation in the region was probably more closed-in than it is today, and A. africanus probably spent some time in the trees.
Paranthropus robustus lived later—between 1.8 and 1.5 million years ago—in the same area but at a time when the landscape was more open.
The teeth and skulls of the two species differed dramatically: A. africanus had smaller cheek teeth; P. robustus had a heavier jawbone, larger teeth, and probably more powerful chewing muscles.
Scientists have long argued that the two species had markedly different diets.
Energy-Rich Foods
The new study by Ungar, Brown, and colleagues suggests that, on average, A. africanus probably ate a greater share of soft and tough foods than P. robustus, which probably ate more hard and brittle foods.
The researchers found, however, that there was substantial overlap between the two species in their dental microwear, and presumably, in their diets.
"This was actually quite surprising to us at first," Ungar said. "This suggests that much of the time [the two species] ate similar foods."
Ungar said the observation gelled after further reflection: Both species would probably have preferred to eat easy-to-consume, energy-rich foods, such as fruits, when they were available.
A similar phenomenon can be seen in modern chimpanzees and gorillas that live in the same geographical area. These so-called sympatric animals share food resources much of the year, but differ mostly during times of food scarcity.
At these times, gorillas fall back on tougher foods, such as leaves and stems, because their teeth and guts allow them to do so.
Ungar says reconstructing the diet of early humans is important to understanding our evolutionary lineage.
"First and foremost, understanding the evolution of human diet gives us important insights into hominin ecology and evolution," he said.
"Diet is a direct link between an animal and its environment," he added. "It is the single most important factor underlying behavioral differences among living primates, and the same was probably true of early hominins. After all, you are what you eat."

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