Human Fossil Adds Fuel to Evolution Debate
for National Geographic News
|March 25, 2002|
A one-million-year-old partial skull found in Ethiopia has added new fuel to the human origins debate among paleoanthropologists.
The skull cap and several other bones from seven individualsall Homo erectus were found in a one-million-year-old layer of sediments known as the Dakanihylo Member.
Reporting in the March 21 issue of the journal Nature, an international team of researchers says the skull provides yet another piece of evidence that a single human ancestor, Homo erectus, ranged across Europe, Asia, and Africa as long ago as 1.8 million years.
For the last two decades, the question of whether fossils discovered from between two million and one million years ago represent one species or numerous branches on the family tree, some of which died out, has been a hot button of debate.
Tim White, a paleoanthropologist at the University of California, Berkeley and co-author of the study in Nature, believes the partial skull found in Ethiopia resolves that question. "The matter of early hominid distribution and species count is solvedone [species] at a million [years], from Spain to China to Java to Africa," he said.
The skull, he said, represents an evolutionary intermediate step linking older, more primitive forms of the species with younger, more human-like forms.
Other experts, however, disagree with that conclusion, and the issue remains controversial.
Piecing Together Fossil Evidence
The partial skull generating all the excitement was found near the village of Bouri in Ethiopia in what is called the Middle Awash study area. Based on fossils discovered in earlier digs, hominids appear to have lived in the area for nearly six million years.
Proponents of the "bushy tree"/multiple-species view argue that African fossils dating to about two million years ago belong to Homo ergaster. Homo erectus, the thinking goes, split off about 1.6 million years ago, and existed only in Asia. The Asian branch was an evolutionary dead end, and the species Homo erectus died off.
Under this scenario, modern humans evolved from the original African branch of Homo ergaster.
The caves and volcanic soil of Africa are extremely conducive to fossil preservation, and scientists have been able to accurately date African fossils. Fossils found in Eurasia and Asia, however, are more difficult to date and until recently were thought to be much younger than those found in Africa. "Java man" of Indonesia, for instance, was originally placed in the 500,000-year-old range.
The nearly one-million-year difference between African and Asian fossils, along with the more primitive features of the early African fossils, contributed to the idea that Homo ergaster and Homo erectus were two species.
New technology has allowed for more precise dating of fossils, and recent reassessments put the age of Java man at about 1.5 million years old, contemporaneous with other fossil finds in Africa. The age of fossils found in China has similarly been revised upward.
In addition, the researchers found that even taking precise measurements, it was impossible to differentiate between the skulls from Asia, Africa, and Eurasia.
The Daka fossils show that as of one million years ago, Homo erectus was probably a single species with gene flow across its known range from Java to Italy to Ethiopia, concluded Henry Gilbert, one of the study's co-authors and a biologist at the University of California, Berkeley.
"Lumpers" and "Splitters"
The underlying definition of a species is a group of organisms with common attributes, capable of interbreeding. The question is, how different is acceptable?
Paleoanthropologists generally fall into one of two categories based on their views of how much variation can exist within species. "Lumpers," such as White and his team, believe there can be a wide range of variation within a species. "Splitters"the "bushy tree folk," in White's termregard the amount of variation seen in the known fossils as indicative of different species.
Susan Anton, a paleoanthropologist at Rutgers University, said human origins research is complicated because scientists look at fossils across large geographic ranges and spans of time, and try to reach conclusions based on morphological evidence from a small number of fossils.
The situation is comparable to a researcher, one million years from now, looking at a few fossil remains of an African pygmy and an NBA basketball player. Both are members of the same species, but their features represent a lot of variation within the species. Without genetic or other supporting evidence, it's easy to see how questions could arise among anthropologists of the future.
Anton takes a middle-of-the-road position on the single-species versus multiple-species debate, saying she's willing to consider "one species with some serious morphs."
Susan Anton, a paleoanthropologist at Rutgers University, said the Ethiopian skull is "a great specimen and shows some really neat things," but she is not convinced it bears out White's claim that the fossil points to a single ancestor one million to two million years ago.
Early African fossils, she explained, have morphological characteristics that are very different from those of island Southeast Asia. "The Daka fossil still shows very African features," she said. "I was expecting the specimen to show more of a mix of Asian and African morphology."
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