"There's no way modern humans could be direct descendants of Homo erectus," said Kenneth Mowbray, a paleoanthropologist at the American Museum of Natural History in New York City.
"The dating is tricky, but the Java material suggests that H. sapiens and H. erectus overlapped in time. H. erectus can't stay the same and be an ancestor at the same time," he said. "It's possible that there's a side branch in H. erectus, but there's no fossil evidence that can lead us in that direction."
Raising the Question of Different Species
As Mowbray suggests, the geological complexity of the island makes precise dating of the fossil material difficult and controversial.
"Fossils found at Trinil and Sangiran range in age from about 1.8 million years old to maybe as young as 780,000 years old," said Carl Swisher, a geochronologist at Rutgers University, New Jersey. "Fossils found at Ngandong have been dated at about 50,000 years old."
Sm 4 is thought to fit somewhere between these two groups in age.
"Depending on who you talk to it could be half a million years old or less than 100,000, possibly making it, along with Ngandong, contemporary with Homo sapiens," said Swisher.
"The uncertainty of Sm 4's age lies in part with current disagreement as to whether or not all fossils from Sambungmacan represent a single fauna or are composites being derived from various age strata," he said.
Whether there is enough difference between the early fossils and the later fossils that they should be considered two separate species or a sub-species is also controversial.
The authors conclude, based on variations in skull shape and a lack of diversity among Javanese populations living 25,000 to 50,000 years ago, that Sm 4 is a transitional form, an evolutionary step taking the later Javanese populations farther away from classical Homo erectus remains found at Trinil and Sangiran.
Other researchers disagree with this conclusion.
"There's an argument that the larger brain sizes of later materials [fossils dated at 25,000 to 50,000 years ago] are different enough that they should be considered a different species or at least sub-species," said Susan Antón, a paleoanthropologist at Rutgers University. "Sm 4 looks like a lot of the other material found in Indonesia. The material is morphologically very consistent, and shows continuity within Indonesian Homo erectus."
"There are some features, particularly around the jaw joint that may be unique to the Ngandong fossils," she said. "But it's not clear whether the features are taxonomically significant or useful as species indicators."
Mowbray concurs, arguing that the differences in the skulls seen in Indonesia are a function of normal variability in any species. Keep in mind that Los Angeles Lakers basketball player Shaquille O'Neill, and the Mini-Me character in the Austin Powers films represent polar extremes in terms of size, but are both Homo sapiens.
"If you look at modern human populations, you see people with skulls that are short and round, and skulls that are long and narrow; these are normal variances within any population," said Mowbray. "Sm 4 falls neatly within the Indonesian H. erectus clade. It has a relatively complete cranial base that helps us understand overall cranial growth and allows us to see normal population variation in how a skull grows with increased brain size.
"But I think they're grasping at straws to suggest that Sm 4 is an intermediate form."
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