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New finds ignite controversy over ape and human evolution

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Jack Linthicum

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Nov 3, 2007, 1:18:52 PM11/3/07

Pictures at the cites, did gorillas separate from humans much earlier
than DNA would indicate?

Week of Nov. 3, 2007; Vol. 172, No. 18
Fossil Sparks
New finds ignite controversy over ape and human evolution

Bruce Bower

Fifty years ago, British anatomist Wilfrid Le Gros Clark explained in
a lecture why evolutionary scientists argue so vehemently about how
ancient apelike and humanlike creatures eventually gave way to modern
humans. "Every fossil relic which appears to throw light on connecting
links in man's ancestry always has, and always will, arouse
controversy," he stated, "and it is right that this should be so, for
it is very true that the sparks of controversy often illuminate the
way to truth."

a8985_1846.jpg

DENTAL PLAN. Three Chororapithecus teeth (top) are shown aligned with
the corresponding teeth of a female gorilla.
Suwa

Le Gros Clark was no stranger to wringing the truth out of bits of
fossilized skeleton. In 1953, he assisted in unmasking the infamous
Piltdown hoax. For more than 40 years, researchers had assumed that
skull and jaw fragments collected from a British gravel pit came from
a previously unknown early human species. The finds actually consisted
of an orangutan's lower jaw and a modern man's skull.

But Le Gros Clark knew that genuine fossil discoveries ignite brighter
sparks of controversy than any cranial con job ever could. Given
limited evidence about long-gone populations of our predecessors,
researchers devise competing evolutionary scenarios that are often
difficult to disprove and that can easily accommodate whatever ancient
bones turn up next.

Scientific reactions to the latest fossil finds and analyses
underscore Le Gros Clark's point. Consider a handful of 10-million-
year-old teeth recently unearthed in Ethiopia and attributed by their
discoverers to a direct ancestor or close relative of the gorilla. If
the scientists are right, ancient gorillas initially diverged from
human ancestors more than 10 million years ago, several million years
before DNA-based analyses date the split. However, some researchers
regard the ancient teeth as remnants of an extinct ape that probably
bore no relation at all to gorillas.

Further along evolution's path lie new fossil finds in Kenya that tell
a disputed story about the emergence of direct human ancestors.
Scientists who uncovered the ancient braincase and partial upper jaw
say that this evidence, combined with prior fossils, indicates that
two Homo species lived simultaneously in eastern Africa from about 1.9
million to 1.4 million years ago. In this scenario, one species died
out and the other led to modern humans. But one prominent
anthropologist rejects that conclusion, placing both new fossils in a
single species that preceded Homo sapiens.

Finally, a research team recently argued that its new analysis of
fossil teeth from sites in and beyond Africa supports the
controversial notion that human ancestors trekked from Africa into
Asia well before 2 million years ago and then colonized Europe from
Asia. Critics of the work say that more fossil evidence is needed to
overturn this team's conclusion that Africans migrated into Asia no
more than 1.8 million years ago and eventually settled Europe as well.

"It's possible that hominids [the fossil ancestors of people] left
Africa as early as 2 million years ago," says anthropologist Tim White
of the University of California, Berkeley, "but it's hard to untangle
the geographic patterns of their movements."

Pieces of ape

In February 2006, a field assistant working with fossil hunters in
Ethiopia's Chorora Formation, a series of sediment layers dated at
between 10 million and 11 million years old, found an ape's canine
tooth. One year later, the researchers returned to the site and found
eight more teeth from the same ancient-ape species, which they dubbed
Chororapithecus abyssinicus.

a8985_2108.jpg

SHRUNKEN HEAD? A small, newly discovered Homo erectus cranium, shown
from above, contrasts with a large, previously unearthed skull from
the same species.
Spoor and J. Reader/National Museums of Kenya

Anthropologist Gen Suwa of the University of Tokyo and his coworkers
see signs of gorilla ancestry in the fossils. Computerized tomography
scans show that the gorilla-size teeth contain thick enamel suitable
for shredding foods such as stems and leaves, the scientists report in
the Aug. 23 Nature. Modern gorillas display slightly thinner dental
enamel but eat the same types of vegetation. Crests on the chewing
surfaces of the ancient teeth look like early versions of the more-
pronounced crests in present-day gorillas, the researchers note.

Chororapithecus represents either an early, direct ancestor of
gorillas or a dead-end primate that happened to evolve gorillalike
teeth, in their view.

Precious few African-ape fossils from between 12 million and 7 million
years ago have been recovered. Some scientists have speculated that
the line of ancestral apes from which chimpanzees, gorillas, and
people emerged came from Asia and Europe and later spread into Africa.
Chororapithecus suggests instead that this evolutionary process began
in Africa, Suwa's team holds.

Moreover, the new finds indicate that an evolutionary split of direct
gorilla ancestors from apelike precursors of people occurred more than
10 million years ago, the investigators say. In contrast, analyses of
modern human and ape DNA place that split at about 8 million years
ago.

DNA studies also estimate that the split of chimp from human ancestors
happened 6 million years ago, and that the human-orangutan split
occurred about 14 million years ago.

Given the age of Chororapithecus, Suwa's group puts the human-
orangutan split at roughly 20 million years ago, the human-gorilla
split at about 12 million years ago, and the human-chimp split at 9
million years ago.

Suwa's conclusion that the Ethiopian fossils come from either a
gorilla ancestor or an evolutionary cousin of ancient gorillas makes
sense, remarks anthropologist Michel Brunet of the University of
Poitiers in France. The teeth of the oldest known hominid, which lived
about 7 million years ago, look "completely different" from those of
Chororapithecus, Brunet says.

The French researcher's team unearthed the ancient hominid's nearly
complete skull in central Africa (SN: 7/13/02, p. 19). Many
investigators accept that specimen as the oldest fossil ancestor of
people, but others regard it as an ancient ape.

Chororapithecus also has a disputed identity. The new find could
easily have come from an ancient ape that had nothing to do with
gorilla ancestors but evolved one or a few gorillalike dental traits
on its own, says anatomist John Kelley of the University of Illinois
at Chicago. The Ethiopian fossils have virtually nothing in common
with the teeth of modern gorillas, aside from crests on their chewing
surfaces that would have aided in grinding up vegetation, Kelley
asserts.

Suwa will keep looking for more pieces of ape. "There is no way to
predict future finds at Chorora," he says.

Separate paths

Scientists have long regarded Homo habilis and Homo erectus as the
first two links in an evolutionary chain that ended with the
appearance of modern humans. In this view, the relatively small-
brained H. habilis evolved about 2 million years ago from earlier
African hominids. It evolved into the larger-brained H. erectus by
around 1.6 million years ago.

New fossil finds challenge that portrait of our distant ancestors, say
anatomist Fred Spoor of University College London and his coworkers.
H. habilis and H. erectus evolved independently of each other, Spoor's
team contends. Rather than one species giving way to the next, both
species lived simultaneously in eastern Africa for roughly 500,000
years, the scientists report in the Aug. 9 Nature.

That conclusion rests on an analysis of two fossils unearthed in Kenya
in 2000. One fossil consists of a piece of upper jaw. Chemical studies
of volcanic-ash layers above and below the find place its age at 1.44
million years.

The jaw contains six teeth, running from a canine tooth in front to a
wisdom tooth in back. The size and shape of the teeth, as well as
evidence that the roof of the mouth was wide and shallow, align the
fossil with H. habilis, according to Spoor's group.

The second fossil consists of a small braincase with an estimated age
of 1.55 million years. This specimen bears several traits unique to H.
erectus, including a bony ridge running over the top of the head and a
delicate jaw joint.

Since the two species coexisted in the same region for such a long
time, each must have had separate origins between 3 million and 2
million years ago, the researchers contend. Few hominid fossils have
turned up from that period.

H. sapiens apparently evolved from H. erectus, possibly via an
intermediate species, in Spoor's view. H. habilis was a sister species
of H. erectus and eventually hit an evolutionary dead-end.

The newly discovered fossil brain case belonged to the smallest known
H. erectus individual. The find thus indicates a size range for H.
erectus fossils of eastern Africa that almost equals that for modern
gorillas. The gorilla pattern reflects males' large size advantage
over females, a condition that may also have applied to male and
female H. erectus, says study coauthor Susan C. Antón of New York
University.

If so, then H. erectus males may have mated with multiple females and
tried to monopolize access to them, as male gorillas do.

Such conclusions don't sit well with Berkeley's White. He classifies
both new fossil finds as H. erectus and as valuable additions to the
fossil record, but hardly the stuff of major evolutionary revisions.

White sees Spoor's paper as part of an ongoing scientific movement to
increase the number of species and evolutionary branches on the
hominid family tree. In contrast, White argues that early hominids
usually evolved from one species to the next, without branching into
multiple species (SN: 4/15/06, p. 227).

"What's interesting is how few hominid lineages there were," White
says. He regards early hominids, or australopithecines, as one such
lineage that evolved into a second, the Homo lineage, more than 2
million years ago. A group of species called robust
australopithecines, which died out 1.2 million years ago, qualifies as
a third hominid lineage, in his view.

Eastern origins

Teeth sometimes tell contested evolutionary tales. That adage applies
to a new analysis of hominid teeth conducted by researchers who have
found 400,000-year-old skeletons of Neandertal ancestors in Spain and
the 1.77-million-year-old remains of an early Homo species in central
Asia (SN: 9/22/07, p. 179). The scientists suspect that, perhaps 2
million to 3 million years ago, Asian hominids began to move west,
exerting a huge impact on the evolution of Neandertals and other Homo
species in western Asia and Europe.

That suggestion contrasts with the traditional view that hominids left
Africa around 1.8 million years ago and evolved into species such as
Neandertals after reaching Europe and other locales. Asian hominids of
the time evolved separately and eventually died out, according to this
perspective.

A team led by Maria Martinón-Torres of the National Center of Human
Evolution Studies in Burgos, Spain, has examined 51 anatomical traits
on more than 5,000 hominid teeth. Fossils came from African
australopithecines and from African, Asian, and European Homo species,
including H. sapiens.

Up to the appearance of Neandertals in Europe around 130,000 years ago
and modern H. sapiens in Africa 200,000 years ago, dental features
fall into two geographic categories, the researchers report in the
Aug. 14 Proceedings of the National Academy of Sciences. African teeth
display one suite of characteristics, while Asian and European teeth
share a different dental signature.

Asian hominids apparently made a substantial genetic contribution to
the evolution of later European species, the scientists say.

However, anthropologist David Frayer of the University of Kansas in
Lawrence says that Martinón-Torres and her coworkers need larger
numbers of fossils, especially of H. sapiens, to make their case.
Moreover, the researchers didn't consider several dental traits that
differ between Neandertals and Asian hominids, he argues.

To paraphrase Le Gros Clark, let the sparks of controversy fly. They
may burn brightly enough to illuminate a bit of evolutionary truth.

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References:

Dennell, R., and W. Roebroeks. 2005. An Asian perspective on early
human dispersal from Africa. Nature 438(Dec. 22/29):1099-1104.
Abstract available at http://dx.doi.org/10.1038/nature04259.

Martinón-Torres, M., et al. 2007. Dental evidence on the hominin
dispersals during the Pleistocene. Proceedings of the National Academy
of Sciences 104(Aug. 14):13279-13282. Abstract available at
http://www.pnas.org/cgi/content/abstract/104/33/13279.

Spoor, F., et al. 2007. Implications of new early Homo fossils from
Ileret,, east of Lake Turkana, Kenya. Nature 448(Aug. 9):688-691.
Abstract available at http://dx.doi.org/10.1038/nature05986.

Suwa, G., et al. 2007. A new species of great ape from the late
Miocene epoch in Ethiopia. Nature 448(Aug. 23):921-924. Abstract
available at http://dx.doi.org/10.1038/nature06113.