Australopithecus sediba had human-like face and could walk well upright but was apelike in other ways.
An Australopithecus sediba skull
bears both human and ape traits.
Photograph courtesy Brett Eloff
Published April 8, 2010
Identified via two-million-year-old fossils, a
new human ancestor dubbed Australopithecus sediba may be the
"key transitional species" between the apelike australopithecines—and
the first Homo, or human,
species, according to a new study.
(See pictures
of Australopithecus sediba fossils.)
"We've never seen this combination of traits in any one
[early human species]," study leader Lee Berger told the journal Science, where the new study
is published today.
Found in the remnants of an underground cave network in South
Africa, the partial Australopithecus sediba skeletons are
believed to be from a roughly 30-year-old woman and an 8- to 13-year-old
boy.
The pre-human pair, who may or may not have been
related, apparently fell to their deaths into a chasm littered with
corpses of saber-toothed cats and other predators.
The new species may be the wellspring—"sediba"
in the local Sotho tribal language—from which our ancestors flowed, the
report suggests.
Berger, of the University of the Witwatersrand in
Johannesburg, conjures a different metaphor.
"It's the opinion of my colleagues and I that [Australopithecus
sediba] may very well be the Rosetta stone that unlocks our
understanding of the genus Homo," Berger said in a statement,
referring to the artifact that helped decipher ancient Egyptian
hieroglyphics.
(Also see "Oldest
Skeleton of Human Ancestor Found.")
A. Sediba Fossils
Suggest Human-Like Ape
Growing to just over 4 feet (1.2 meters) tall, A.
sediba has a number of key traits that some would say mark it as an
early human, like Homo habilis, which many consider the first
human species.
A. sediba, for example, had long legs and
certain humanlike characteristics in its pelvis, which would have made
it the first human ancestor to walk—perhaps even run—in an
energy-efficient manner, the study says. (Related: "Did
Early Humans Start Walking for Sex?")
Also, A. sediba's face had small teeth and a
modern—rather than chimpanzee-like—nose,
the study says.
And as in humans, the shapes of A. sediba's
left and right brain halves—discernible from indentations on a
remarkably preserved skull—appear to have been uneven.
A facial reconstruction is in the works, and many people will be surprised by how human the new fossil species looks, Berger predicted in a press conference Wednesday. "What you'll see, I suspect, is something surprisingly more modern than we would expect in ... other things that have been called Australopithecus," which translates to "southern ape."
So if our newest evolutionary ancestor is so human-like,
why doesn't the new study classify it as human?
Berger's team believes that certain apelike traits force
the new species into the Australopithecus genus, or group of
species.
For one thing, unlike human species but like other
australopithecines, A. sediba had a very small brain. The
fossil species also had long ape-like arms with primitive wrists that
were well suited for climbing trees.
Australopithecus Sediba's
World
In what's now South Africa, A. sediba lived in a
patchwork of grasslands and woods, where the fossil species likely ate
fleshy fruits, young leaves, and perhaps small animals.
The generally flat landscape was broken up by small
hills and cliffs, some of which contained caves, which could apparently
be treacherous.
Scientists speculate that a harsh drought may have
driven two desperately thirsty members of A. sediba to enter
one of these caves in an attempt to find an underground source of water.
The pair may have clambered partway down into the cave,
only to slip and fall several yards to their deaths. The deathtrap also
contained fossils of 25 species that lived alongside A. sediba,
including potential predators such as saber-toothed cats, hyenas, and
wild dogs.
A. Sediba Only Human
After All?
Other anthropologists seem to be unanimously excited
about the new human-ancestor fossils. But not everyone is so sure the
new species is the "key transitional species" between prehistoric apes
and humans suggested by the study.
"I don't think there's a lot of compelling evidence to
suggest that [A. sediba] lies between Australopithecus
and Homo," said anthropologist Bernard Wood of George
Washington University.
A. sediba "doesn't fit what our preconceptions
would be about the ancestor of Homo," said Wood, who wasn't
involved in the study.
For example, A. sediba's arms are too long—too
apelike—and the species isn't as well adapted for upright walking as
some scientists expect the direct ancestor to the first humans to be,
Wood said.
Also, at 1.95 to 1.78 million years old, the A.
sediba fossils simply aren't old enough to represent an ancestor to
Homo, said anthropologist Brian Richmond, also of George
Washington University. (Explore a prehistoric
time line.)
"It's hard to argue this is the ancestor of Homo
when it's occurring much later than the earliest members of the genus Homo
by half a million years," Richmond said, referring to an early fossil
of H. habilis that dates back to 2.3 million years ago.
Anthropologist William
Kimbel thinks this chronological conundrum could be resolved by
calling the new specimens Homo instead of Australopithecus.
"By putting it in Australopithecus and saying
it's ancestral to Homo, you're left with having to wonder how
to accommodate earlier Homo [species]," Kimbel said.
"If you put it in Homo, that problem falls
away," he said. "It's then just one of several species around two
million years ago that are near the base of the Homo lineage."
Susan Anton,
an anthropologist at New York University and a joint editor of the Journal
of Human Evolution, agreed.
A. sediba has so many similarities with Homo
that "I think they might have been better off including it in Homo,"
Anton said.
"If you do that, then this is really no longer a
transitional species between Australopithecus and Homo.
It is Homo"—and just an evolutionary dead end in human
ancestry.
With Fossils, Timing Is Everything
Berger, who has been funded in the past by the National
Geographic Society, maintains that A. sediba belongs with other
australopithecines because its anatomy suggests it was still climbing
trees. (The National Geographic Society owns National Geographic News.)
"It hasn't made that grade-level shift to the genus Homo"
yet, he said.
As for questions about its timing, Berger believes
future discoveries could turn up A. sediba fossils that are
hundreds of thousands of years older, which would make them old enough
to be the ancestors of early Homo species.
"This [discovery] site is only a point in time. It
doesn't represent the first appearance of this species, nor will it
probably represent the last," he said.
Regardless of where A. sediba ends up in the
human family tree, it's already an important fossil precisely because of
all the questions that it raises, said paleontologist Scott
Simpson of Case Western Reserve University in Cleveland, Ohio.
"This fossil is not one that resoundingly answers any
specific questions," Simpson said. "What it does is reinforce the idea
that we haven't even asked all the appropriate questions yet.
"People are going to be discussing this for a long, long
time."
• Fossil Ape: Human Ancestor?
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• Human Evolutionary Highway
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