A 6 million-year-old creature that lacked sharp canines for fighting may be the first prehuman to have branched off from the ape line, researchers said Thursday.
The short, small-brained creature may provide a good hint of what the common ancestor of chimpanzees and humans looked like, the researchers said.
Fossil remains of the early hominid were found in Ethiopia three years ago, and it seemed to be a subspecies of a known prehuman, Ardipithecus ramidus.
But the scientists have found more teeth from a group of the hominid, re-classified it as a distinct species and named it Ardipithecus kadabba.
“Ardipithecus kadabba may also represent the first species on the human branch of the family tree just after the evolutionary split between lines leading to modern chimpanzees and humans,” said Yohannes Haile-Selassie, curator and head of physical anthropology at the Cleveland Museum of Natural History in Ohio, who led the study.
Long canines
His team’s report, published in Friday’s issue of the journal Science, suggests that the last common ancestor of chimps and humans had long canines used to fight — something chimps have today, but not humans.
The researchers dug up fossils from at least five individuals who once lived in a wooded environment, now a dry, rocky area in the Afar rift of Ethiopia’s Middle Awash region — a rich source of pre-human remains.
They had enough to determine that it was an upright-standing hominid about the size of a chimpanzee that lived between 5.2 and 5.8 million years ago.
Six new teeth were found at the site in 2002 and included an upper canine, premolars from both jaws, and upper molars.
“We see a lot of primitiveness in the teeth,” Haile-Selaisse said in a telephone interview.
One key characteristic is a self-sharpening function.
“The canine tooth comes across the outside face of the lower premolar, and it sharpens that way,” said Tim White of the University of California Berkeley, who worked on the report. “It is like honing a knife on a stone. Almost all of the monkeys and all of the apes, they have all very long and projecting canines (with this mechanism).”
Peaceable primates
In modern apes these sharp teeth are used by males for fighting, or to frighten off an aggressor. The theory is that hominids evolved more peaceable behavior, said White, with females choosing males who could stand upright and help raise young over males who were busy fighting and showing off.
Fossil remains of similar creatures found in Chad and Kenya are similar enough to suggest they are closely related — even in the same genus as Ardipithecus, the researchers said.
“We now have an assemblage or set of early canines, and none of them are big and slashing,” White said. “What this indicates is that earliest hominids had these small canines that were in the same animal as a small brain — we know that from the skull in Chad — and that head was attached to a bipedal body.”
One of the most famous prehumans, “Lucy” or Australopithecus afarensis, dates back 3 million years. “This doubles the time period all the way back to 6 million years that a small-brained, small-canine bipedal early hominid existed,” White said.
Genetics tells scientists that chimpanzees and hominids diverged from a common ancestor around 7 million years ago. “But genetics can’t tell us what this animal was like,” White said.
It is also becoming clear that looking at chimpanzees, who evolved as much as humans did if not more over this period, do not provide a good model of the ancestor either.
