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Who Was Homo Habilis--And Was It Really Homo?

by Ann Gibbons
http://www.sciencemag.org/content/332/6036/1370.fullScience 
17 June 2011
Science Vol. 332 no. 6036 pp. 1370-1371
DOI: 10.1126/science.332.6036.1370

Ever since he received a cryptic telegram in 1959,
Phillip Tobias has pondered the identity of a certain
human ancestor. The telegram came from his colleagues
Louis and Mary Leakey in Kenya. "We've found the man,"
it said. "Come quickly." As soon as he could book a
flight, Tobias rushed from Johannesburg, South Africa,
to Nairobi to examine the partial skull, teeth, and hand
and foot bones of several individuals that the Leakey
team had found in Olduvai Gorge, Tanzania. Because the
fossils were found near stone tools, Tobias and the
Leakeys named the new hominin Homo habilis, or "handy
man." As new H. habilis fossils emerged over the
decades, the researchers and others came to consider the
species the first member of our own genus, a crucial
ancestor that gave rise to H. erectus in an unbroken
lineage that led to us.

But in the past decade, the handyman's status has been
undermined. Newer analytical methods suggested that H.
habilis matured and moved less like a human and more
like an australopithecine, such as the famous partial
skeleton of Lucy. Now, a report in press in the Journal
of Human Evolution finds that H. habilis's dietary range
was also more like Lucy's than that of H. erectus, which
many consider the first fully human species to walk the
earth. That suggests the handyman had yet to make the
key adaptations associated with our genus, such as the
ability to exploit a variety of foods in many
environments, says lead author Peter Ungar of the
University of Arkansas, Fayetteville. In a separate
commentary this week in the Proceedings of the National
Academy of Sciences (PNAS), paleoanthropologist Bernard
Wood of George Washington University in Washington,
D.C., writes that today, if one considers all the
evidence, "There are grounds for excluding H. habilis
from Homo."


View larger version:
In this page In a new window
Food processors. Microscopic images show pits and
diverse patterns of wear on two H. erectus teeth
(right), as compared with H. habilis teeth, suggesting
that H. erectus individuals ate a broader range of tough
foods that gouged their teeth.
CREDIT: PETER UNGAR

View larger version:
In this page In a new window
All in the family? Researchers question whether Homo
habilis (left) belongs in the genus Homo with the more
modern H. erectus.
CREDIT: W. KIMBEL/C. LOCKWOOD
When Tobias, now an emeritus professor at the University
of the Witwatersrand, Johannesburg, first looked at the
1.75-million-year-old fossils that the Leakeys had
found, he recognized features in the teeth and skull,
such as a narrowing of the premolars and molars, that
were "decidedly humanlike," he recalls. The body and
brain were smaller than those of H. erectus, but the
hand and foot had modern traits suggesting that H.
habilis walked upright and could make stone tools. Some
researchers pointed out that the brain of the type
specimen, OH 7-which Tobias had estimated at 647 cubic
centimeters (cc)-didn't meet what was then considered
the "cerebral Rubicon" for Homo of at least 700 cc to
800 cc. But H. habilis's brain is significantly larger
than that of Lucy's species, Australopithecus afarensis,
which checks in at about 450 cc. And recently unearthed
H. erectus specimens have brain sizes of only 600 cc to
700 cc.

At the time of its discovery, H. habilis was the most
humanlike hominin known to be on the African landscape
at the crucial time before H. erectus appeared. "Even
with its small brain and body, [H. habilis] was
essentially humanlike in its function-a striding biped,
capable of having the dexterity in its hand to make the
tools," Wood says. Back then, "the whole thing made
sense," he says.

Over subsequent decades, however, H. habilis, and indeed
the genus Homo, lost its status as a unique toolmaker.
Ethiopian stone tools were dated to at least 2.5 million
years old, making it unclear whether Homo or
australopithecines made them. And researchers labeled a
number of diverse, fragmentary fossils from East Africa
and South Africa "H. habilis," making the taxon a "grab
bag, . a Homo waste bin," says paleoanthropologist Chris
Ruff of Johns Hopkins University in Baltimore, Maryland.
Today some researchers think some larger "H. habilis"
specimens from sites other than Olduvai actually are
members of a separate species called H. rudolfensis.
Wood wrote in 1999 that neither species belongs in Homo
(Science, 2 April 1999, p. 65).

He says later work supports his point: In 2001, studies
suggested that H. rudolfensis and H. habilis individuals
grew their teeth rapidly, like an African ape, in
contrast to the slow dental development of modern
humans. And in a 2009 study of the cross sections of
limb bones of H. habilis from Olduvai, Ruff found that
the force distribution on these bones suggests a strong-
armed hominin that spent a lot of time climbing in
trees. Similar studies suggest that H. erectus was
completely committed to upright walking and had lost
this upper arm strength.

Now the report on diet presents a new line of evidence
that suggests that H. habilis was quite different from
H. erectus in its eating habits. Using two sophisticated
microscopes, including one that allowed him to map the
three-dimensional surface of the teeth, Ungar
scrutinized microscopic wear patterns on the teeth of 10
H. habilis individuals (who were mostly from Olduvai),
nine members of a robust australopithecine species
called Paranthropus boisei, and eight H. erectus. He and
his colleagues found that the H. erectus teeth had
diverse patterns of wear, as well as many small pits
gouged out by eating a broad range of hard, brittle
foods, such as nuts, seeds, and fibrous meat. In
contrast, the enamel of both H. habilis and P. boisei
showed no such pits and was scratched by softer foods
such as fruits and tender leaves, indicating that they
ate a narrower range of foods. The detailed study "will
now make people look harder at H. habilis, which is
good," says anatomist Christopher Dean of University
College London.

Eating a broad range of foods fits with researchers'
ideas of the key Homo adaptation: the flexibility to
thrive in many different environments. "H. erectus was
able to eat a broader range of foods, which is a legacy
that continues to today," Ungar says. "It's a legacy
that explains why we've taken over the world."

Indeed, H. erectus was an early world traveler. It
appeared in East Africa at 1.9 million years ago, and
Dmanisi, Georgia, by 1.77 million years ago. And new
dates published last week in PNAS show that tools at
Dmanisi-presumably also made by H. erectus-go back to
1.85 million years ago. That means the species appears
almost simultaneously in Africa and Asia. This, coupled
with the potential dethroning of African H. habilis as
H. erectus's ancestor, opens the door to a once-
outrageous possibility: H. erectus, which researchers
have long assumed was born in Africa, "could have
originated in Asia," says paleoanthropologist David
Lordkipanidze of the Georgian National Museum in
Tbilisi. "It no longer seems like such a crazy idea."

Not everyone is ready to demote H. habilis to being a
member of Australopithecus, however. Diet can vary among
individuals and is not a reliable trait for classifying
taxa, says paleoanthropologist William Kimbel of Arizona
State University, Tempe. Also, a fossil jawbone from
Hadar, Ethiopia, has traits that put it squarely in
Homo, dates back to 2.3 million years ago, and is very
similar to H. habilis, he says. If so, early members of
H. habilis may have given rise to H. erectus in Africa.

Tobias, for one, still thinks there is no better
candidate out there on the African savanna to bridge
Australopithecus and Homo. Some researchers have
nominated other australopithecines, such as the newly
discovered A. sediba from South Africa (Science, 29
April, p. 534) as the intermediary, but dental and
cranial evidence suggests some unspecified Homo did
indeed live in East Africa before 2 million years ago,
Kimbel points out. The problem is that there are
precious few fossils of either H. habilis or H.
rudolfensis, especially from the neck down. So hominin
researchers are waiting for the next excited cable from
fossil finders-this time, presumably by text rather than
by telegram.

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