Thursday, June 18,
2009
New evidence underscores the theory of human origin that suggests
humans most likely share a common ancestor with orangutans,
according to research from the University of Pittsburgh and the
Buffalo Museum of Science. Reporting in the June 18 edition of the
Journal of Biogeography, the researchers reject as "problematic"
the popular suggestion, based on DNA analysis, that humans are most
closely related to chimpanzees, which they maintain is not
supported by fossil evidence.
Jeffrey H. Schwartz, professor of anthropology in Pitt's School of
Arts and Sciences and president of the World Academy of Art and
Science, and John Grehan, director of science at the Buffalo
Museum, conducted a detailed analysis of the physical features of
living and fossil apes that suggested humans, orangutans, and early
apes belong to a group separate from chimpanzees and gorillas. They
then constructed a scenario for how the human-orangutan common
ancestor migrated between Southeast Asia—where modern orangutans
are from—and other parts of the world and evolved into now-extinct
apes and early humans. The study provides further evidence of the
human-orangutan connection that Schwartz first proposed in his book
The Red Ape: Orangutans and Human Origins, Revised and Updated
(Westview Press, 2005).
Schwartz and Grehan scrutinized the hundreds of physical
characteristics often cited as evidence of evolutionary
relationships among humans and other great apes—chimps, gorillas,
and orangutans—and selected 63 that could be verified as unique
within this group (i.e., they do not appear in other primates). Of
these features, the analysis found that humans shared 28 unique
physical characteristics with orangutans, compared to only two
features with chimpanzees, seven with gorillas, and seven with all
three apes (chimpanzees, gorillas, and orangutans). Gorillas and
chimpanzees shared 11 unique characteristics.
Schwartz and Grehan then examined 56 features uniquely shared among
modern humans, fossil hominids—ancestral humans such as
Australopithecus—and fossil apes. They found that orangutans shared
eight features with early humans and Australopithecus and seven
with Australopithecus alone. The occurrence of orangutan features
in Australopithecus contradicts the expectation generated by DNA
analysis that ancestral humans should have chimpanzee similarities,
Schwartz and Grehan write. Chimpanzees and gorillas were found to
share only those features found in all great apes.
Schwartz and Grehan pooled humans, orangutans, and the fossil apes
into a new group called "dental hominoids," named for their
similarly thick-enameled teeth. They labeled chimpanzees and
gorillas as African apes and wrote in Biogeography that although
they are a sister group of dental hominoids, "the African apes are
not only less closely related to humans than are orangutans, but
also less closely related to humans than are many" fossil
apes.
The researchers acknowledge, however, that early human and ape
fossils are largely found in Africa, whereas modern orangutans are
found in Southeast Asia. To account for the separation, they
propose that the last common human-orangutan ancestor migrated
between Africa, Europe, and Asia at some point that ended at least
12 million to 13 million years ago. Plant fossils suggest that
forests once extended from southern Europe, through Central Asia,
and into China prior to the formation of the Himalayas, Schwartz
and Grehan write, proposing that the ancestral dental hominoid
lived and roamed throughout this vast area; as the Earth's surface
and local ecosystems changed, descendant dental hominoids became
geographically isolated from one another.
Schwartz and Grehan compare this theory of ancestral distribution
with one designed to accommodate a presumed human-chimpanzee
relationship. They write that in the absence of African ape fossils
more than 500,000 years old, a series of "complicated and
convoluted" scenarios were invented to suggest that African apes
had descended from earlier apes that migrated from Africa to
Europe. According to these scenarios, European apes then diverged
into apes that moved on to Asia and into apes that returned to
Africa to later become humans and modern apes. Schwartz and Grehan
challenge these theories as incompatible with the morphological and
biogeographic evidence.
Paleoanthropologist Peter Andrews, a past head of Human Origins at
the London Natural History Museum and coauthor of The Complete
World of Human Evolution (Thames & Hudson, 2005), said that
Schwartz and Grehan provide good evidence to support their theory.
Andrews had no part in the research, but is familiar with it.
"They have good morphological evidence in support of their
interpretation, so that it must be taken seriously, and if it
reopens the debate between molecular biologists and morphologists,
so much the better," Andrews said. "They are going against accepted
interpretations of human and ape relationships, and there's no
doubt their conclusions will be challenged. But I hope it will be
done in a constructive way, for science progresses by asking
questions and testing results."
Schwartz and Grehan contend in the Journal of Biogeography that the
clear physical similarities between humans and orangutans have long
been overshadowed by molecular analyses that link humans to
chimpanzees, but that those molecular comparisons are often flawed:
There is no theory holding that molecular similarity necessarily
implies an evolutionary relationship; molecular studies often
exclude orangutans and focus on a limited selection of primates
without an adequate "outgroup" for comparison; and molecular data
that contradict the idea that genetic similarity denotes relation
are often dismissed.
"They criticize molecular data where criticism is due," said Malte
Ebach, a researcher at Arizona State University's International
Institute for Species Exploration who also was not involved in the
project but is familiar with it.
"Palaeoanthropology is based solely on morphology, and there is no
scientific justification to favor DNA over morphological data. Yet
the human-chimp relationship, generated by molecular data, has been
accepted without any scrutiny. Grehan and Schwartz are not just
suggesting an orangutan–human relationship—they're reaffirming an
established scientific practice of questioning data."
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University of Pittsburgh
Thanks to University of Pittsburgh
for this article.
