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Human evolution
From
tree to tumbleweed
The recent discovery of a skull in Kenya belonging to a new species
and perhaps even a new genus in the human genealogy marks the first time
anthropologists have found at least two, if not more, hominid species coexisting
in the Pliocene.
A team led by paleontologist Meave Leakey found Kenyanthropus platyops
—Kenyan flat-face — on the western side of Lake Turkana in northern Kenya
in 1999. Leakey saw that K. platyops was different enough to not be classified
in the hodge-podge genus Australopithecus. Among the many species grouped
under this genus is “Lucy” — the Australopithecus afarensis hominid
Donald Johanson and Tom Gray found in 1974 while working in Hadar, Ethiopia
— previously thought to be the only antecedent of modern humans from the
Pliocene. Leakey and colleagues reported their find in the March 22 Nature.
[At
right: Paleontologist Meave Leakey of the National Museums of Kenya examines
the skull of a new genus of hominid in the laboratory in Nairobi. Meave
and daughter Louise Leakey co-led the expedition that found the skull in
sediment on Kenya’s Lomekwi River. The Leakeys’ research was supported
by the National Geographic Society. Photo by Fred Spoor © National
Geographic Society.]
“The greatest significance [of this discovery] is that there were many
more species than previously suspected,” says physical anthropologist Geoffrey
Pope of William Paterson University in New Jersey. The new hominid is Lucy’s
age, but may be a better candidate for a human ancestor because its morphology
is more similar to modern people, he says.
Multiple species coexisted from 6 million years ago to about 35,000
years ago, when the last known species of Homo neanderthalensis went extinct;
however, until now, Lucy was the only known hominid skeleton from 3.5 to
3.0 million years ago.
“If you look at the evolution of any other mammal, there’s usually
a radiation of species and just a few survive,” Leakey said on the TV show
National Geographic Today. “It didn’t seem right that there was only one
line of evolution [for this time period]. There should have been other
species around.”
The 1999 find includes a complete cranium, dental fragments and roughly
30 other skulls and fragments that have not yet been assigned a genus or
species. The size of K. platyops’ brain and ear hole is similar to that
of a modern chimpanzee. Its flat nasal margin and thickly enameled teeth
are similar to those of Lucy and other primitive hominids. However, the
hominid also has features not found in any other known ancestors, a fact
that supports Leakey’s new genus theory.
The skull has the unusual combination of a large flat face, high cheeks
and small molars. Every previously known hominid with a similar cranial
structure had large teeth; small teeth corresponded with larger brains
and smaller faces.
“Smaller teeth are usually associated with modernity,” Pope asserts.
This hominid’s features make determining its lifestyle more complicated.
Larger teeth and smaller brains indicated that the hominid lived more primitively,
primarily grazing. “Associating small teeth and a small brain is anyone’s
guess,” Pope says.
K. platyops has the earliest known flat face. The only other skeletons
with flat faces dated to 2 million years ago, with the emergence of Homo
rudolfensis, seemingly the closest relative to K. platyops. Other
than brain size, which is much larger in the younger Homo rudolfensis,
most features are similar between the two species.
Leakey’s team discovered Kenyanthropus platyops in a mudstone that
was potassium-argon dated to between 3.6 million and 3.2 million years
old. The dark mudstone is underlain and overlain by volcanic tuff and was
deposited along the northern margin of a shallow lake. Small streams drained
into the lake from nearby hills and brought other mammal and hominid remains
into the lakebed, making it a treasure trove of bones. “Many specimens
at similar, but not identical stratigraphic levels in nearby areas, were
deposited on the floodplain of the ancestral Omo River, evincing that these
ancients inhabited floodplains of major rivers and lake-margin environments,”
says geologist Frank Brown of the University of Utah.
Where K. platyops was found, faunal assemblages show that the environment
was well watered and well vegetated. The wide variety of mammalian skeletons
found at the discovery site proves that the area was a mosaic of habitats
that included open grasslands and woodlands. The varied habitats allowed
species to adapt to different ecological niches. Because K. platyops subsisted
on a different diet than its contemporaries, it is plausible that Lucy
and the new discovery coexisted without being in competition for food.
The discovery of K. platyops confuses the picture of the human evolutionary
tree. As George Washington University paleoanthropologist Daniel Lieberman
points out, diagrams of human evolution are becoming more complex. Brown
adds: “Simplicity itself is not necessarily virtuous, particularly if it
obscures true relationships between entities in the natural world.”
Meg Rudolph
