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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