Newly discovered fossils
from China shed light on common ancestry of monkeys, apes and humans
DEKALB, Ill.
For the first time, scientists have discovered skeletal parts of an extinct
primate that documents an early phase in the evolution of monkeys, apes,
and humans.
In an article published today in the prestigious British journal Nature,
the team of American and Chinese paleontologists describe fossilized foot
bones of Eosimias, an early higher primate that lived about 45 million
years ago in China.
"We have the first unambiguous evidence that is able to bridge the
anatomical gap between lower and higher primates," said paleontologist
Dan Gebo, a professor of anthropology at Northern Illinois University
and lead author of the Nature article.
Gebo's co-authors are Marian Dagosto of Northwestern University Medical
School in Chicago; K. Christopher Beard of Carnegie Museum of Natural
History in Pittsburgh; and Qi Tao and Wang Jingwen of the Institute of
Vertebrate Paleontology and Paleoanthropology in Beijing.
 |
Previously, paleontologists had found only jaws and
teeth of Eosimias, a primitive tree-dwelling primate about the size of
the smallest living monkeys. Because of the scanty anatomical evidence
available for Eosimias prior to today¡¦s report, leading
paleoanthropologists had been divided on the issue of exactly where Eosimias
fits on the primate family tree.
Some prominent scientists even doubted that Eosimias was a primate at
all. But the new evidence, consisting of multiple ankle bones from sites
in central and eastern China, confirms that Eosimias is a very primitive
member of the lineage that today includes monkeys, apes and humans.
"The most interesting aspect of these new foot bones is that they
represent a mosaic," Gebo said. "They possess primitive lower-primate
features as well as several advanced or higher-primate characteristics.
No other fossil primate in the Eocene has this interesting combination."
Co-author Christopher Beard said the latest discovery is important because
it helps fill a major gap in the fossil record of humans and their nearest
relatives.
"I hate to use the term "missing link" because it's such
a cliche, but these fossils really do fill a wide gap that previously
separated higher primates, also known as anthropoids, from their prosimian
relatives," said Beard, who coordinates the American side of the
joint Sino-American expeditions that resulted in new fossil discoveries.
Living anthropoids include monkeys, apes and humans. Living prosimians
include lemurs, bush babies, lorises and tarsiers. The evolutionary origin
of higher primates has stymied paleontologists and primatologists for
decades, because so little was known regarding the ancestral anthropoid
lineage until recently.
Modern primates possess a variety of anatomical adaptations for moving
through their environment--usually the trunks and branches of trees in
tropical and subtropical forests. Many prosimians are renowned for their
ability to leap and cling to vertical tree trunks, while monkeys tend
to walk on all fours on the tops of branches.
The anatomy of the fossilized ankle bones of Eosimias show that this animal
already preferred walking quadrupedally on the tops of branches like living
monkeys.
In addition to verifying that Eosimias is an early higher primate, the
new fossils help settle a longstanding debate about where the anthropoid
lineage arose on the primate family tree.
 |
Previously, there were three main hypotheses regarding
the nearest relatives of anthropoids. Based on similarities in the anatomy
of their teeth, some scientists have argued that anthropoids evolved from
the lemur-like adapids. Genetic similarities and the anatomy of living
primates lead other scientists to believe that living and fossil tarsiers
are the nearest evolutionary cousins of anthropoids.
A third hypothesis accepts an evolutionary relationship between anthropoids
and tarsiers, but posits that the split between these two lineages is
very ancient, dating to at least 55 million years ago. The new ankle bones
of Eosimias are similar to those of anthropoids and fossil omomyids, a
group widely believed to be extinct relatives of tarsiers.
"The oldest known skeletal remains of a higher primate are inconsistent
with the view that monkeys, apes and humans evolved from the lemur-like
adapids," Beard said, "but they support a close evolutionary
relationship between anthropoids and tarsiers."
Scientists recovered the fossils from a commercial limestone quarry about
100 miles west of Shanghai and from a locality in Shanxi Province (China),
along the Yellow River, about 350 miles southeast of Beijing. The location
of the discovery also is significant, the researchers say.
"Most scientists in my field believe that if the ancestor of anthropoid
primates is to be found then it should come from Africa," Gebo said.
"Thus, the bones of Eosimias are important, as is its unusual location
(Asia)."
The new fossils were recovered during a series of expeditions organized
by scientists from Carnegie Museum of Natural History in Pittsburgh, Pennsylvania
and the Institute of Vertebrate Paleontology & Paleoanthropology in
Beijing.
|
