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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


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