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Alexander Joe/Agence France-Presse — Getty Images
Lee Berger with his son, Matthew, and fossils of Australopithecus
sediba. Matthew found one of the fossils while chasing his dog.
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By Nicholas Wade, The New York Times, September 8, 2011
An apelike creature
with human features, whose fossil bones were discovered recently in a South
African cave, is being greeted by paleoanthropologists as a likely watershed in
the understanding of human evolution.
The discoverer of
the fossils, Lee Berger of the University of Witwatersrand in Johannesburg,
says the new species, known as Australopithecus sediba, is the most plausible
known ancestor of archaic and modern humans. Several other paleoanthropologists,
while disagreeing with that interpretation, say the fossils are of great
importance anyway, because they elucidate the mix-and-match process by which
human evolution was shaped.
Dr. Berger’s claim,
if accepted, would radically redraw the present version of the human family
tree, placing the new fossils in the center. The new species, in his view,
should dislodge Homo habilis, the famous tool-making fossil found by Louis and
Mary Leakey, as the most likely bridge between the australopithecenes and the
human lineage. Australopithecenes were apelike creatures that walked upright,
like people, but had still not forsaken the trees.
Dr. Berger and his
colleagues present this claim in five articles in the current issue of Science
that describe various aspects of the new fossils. As is common in the field of
paleoanthropology, the discoverer of a new fossil is seeking to place it as
close as possible to the direct line of human descent, while others are
resisting that interpretation.
In this particular
case, there are many uncertainties regarding the fossil record from that time,
including when the human lineage first emerged and how Homo habilis fits in the
picture.
The principal
significance of the new fossils is not that Australopithecus sediba is
necessarily the direct ancestor of the human genus, other scientists said, but
rather that the fossils emphasize the richness of evolutionary experimentation
within the australopithecine group.
“This is really
exciting new material,” said Ian Tattersall, a paleoanthropologist at the
American Museum of Natural History in New York. “I think it holds the
possibility of flinging wide open the question of what Homo is.”
Besides two skulls
reported last year, researchers led by Dr. Berger have since retrieved an
almost complete right hand, a foot and a pelvis. The bones are especially well
preserved because their owners apparently fell into a deep cave and a few weeks
later were swept into a sediment that quickly fossilized their bones. The rocks
above the cave have gradually eroded away, bringing the fossils to the surface,
where one was found by Dr. Berger’s 9-year-old son,
Matthew, in 2008, while chasing his dog.
That fall into the
cave happened 1.977 million years ago, according to dating based on the rate of
decay of uranium in the rock layer that holds the fossils.
In the articles in
Science, Dr. Berger’s team describes novel combinations of apelike and
humanlike features in the hand, foot and pelvis of the new species. The hand,
for instance, is apelike because it has long, strong fingers suitable for
climbing trees, yet is also humanlike in having a long thumb that in
combination with the fingers could have held tools in a precision grip. A cast
of the inside of the skull shows an apelike brain, but one that had taken the
first step toward being reorganized on human lines.
This mixture of
apelike and humanlike features suggests that the new species was transitional
between the australopithecines and humans, the researchers said at a news
conference on Wednesday. Given its age, Australopithecus sediba is just old
enough to be the ancestor of Homo erectus, the first species that
paleoanthropologists agree belonged to the human ancestry and which existed 1.9
million years ago.
But the fossils are
significant even if sediba is not a direct human ancestor. They are evidence
that a ferment of evolutionary experimentation was going on at the time, out of
which the human lineage somehow emerged. “If you take sediba as a metaphor for
evolutionary change, it is a whole lot more powerful than the claim for direct
ancestry,” Dr. Tattersall said.
A
similar view was taken by Bernard Wood, a paleoanthropologist at George
Washington University. “I think these are some of the most interesting papers
that have been published in recent years,” Dr. Wood said. “But these are
probably not the reasons the authors think they are interesting.”
Dr. Wood gave
little credence to Dr. Berger’s arguments that Australopithecus is a direct
ancestor of the human group, saying there was too little time for the
small-brained, tree-climbing ape to evolve into the large-brained Homo erectus.
More interesting, in his view, are the strange combinations of apelike and
humanlike features that Dr. Berger’s team has described. The new fossils
display the modular way in which evolution operates: they have mostly known
features but in novel combinations that have never been seen before.
“It’s clear that though the hand has to
be an integrated whole, the parts of the hand evolve as separate modules,” Dr.
Wood said. “You can pick a No. 3 thumb from five possible options, say, and
pair it with a No. 2 medial wrist and a No. 4 lateral wrist.”
Dr. Wood said that
although he had read the five papers quickly “late at night over a glass of
whisky,” he believed they would prove to be “a watershed in our understanding
of human evolution, even if only to demonstrate that things are pretty complex,
and because of this it will be very difficult to link different fossils in an
evolutionary sequence.”
In arguing that
Australopithecus is the ancestor of the human lineage, Dr. Berger dismisses all
earlier fossils held to be human, including a jawbone 2.33 million years old
discovered in 1994 in Hadar, Ethiopia, by the paleoanthropologist Donald
Johanson. The jawbone has some human-like features, Dr. Berger and colleagues
write, yet that does not mean the owner of the jawbone was necessarily human.
But Dr. Johanson said in an e-mail that the Hadar jawbone “possesses all the
hallmarks of Homo,” the human lineage, and “places the origins of Homo firmly
in eastern Africa, at least 400,000 years prior to the dating of A. sediba.”
Both Dr. Wood and
Dr. Tattersall see Dr. Berger’s discovery as pointing to the great variety of
australopithecine apes, from which it will be very difficult to select the
particular species that gave rise to humans. Dr. Tattersall believes the leap
to humans may have been brought about very suddenly, perhaps by a few critical
genetic changes, which is why the transition is so hard to trace in the fossil
record.
Dr. Wood praised
Dr. Berger for describing the new fossils so quickly. Dr. Tattersall said that
Dr. Berger had been “incredibly forthcoming” in giving other researchers access
to his fossils instead of hoarding them for his private use, as other
paleoanthropologists have been known to do.
“Unlike
anything else I can remember in 40 years, they have already sent us full casts
of the material they have described, and they will be available to any researchers
who want to see them,” Dr. Tattersall said.
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