Java Skull Raises Questions on Human Family Tree
for National Geographic News
|February 27, 2003|
Scientists have been unraveling the mysteries of when early hominids first left Africa, where they went, how many hominid species there were, and how they relate to modern humans, for more than a century.
A skull recently found in Indonesia adds a valuable piece to the fossil record, but scientists differ about where it fits in the human family tree.
The skull, which scientists call Sambungmacan 4 (Sm 4), was found in the Sambungmacan district of central Java, Indonesia. It is that of a middle-aged or slightly younger male Homo erectus who had probably suffered and recovered from head wounds. Two partial skulls and the fragment of a tibia had previously been discovered in the area.
Homo erectus, and perhaps other early hominid [see side bar for definition] species, began leaving Africa sometime around 2 million years ago. Fossil remains have been found in Asia, the Middle East, and Europe.
Indonesia, an island nation in southeast Asia, is the site of some of the earliest Homo erectus remains yet found. The relatively abundant fossil material provides scientists with an opportunity to study the evolution of the species and how it relates to modern humans.
Scientists led by Hisao Baba, an anthropologist at the National Science Museum in Tokyo, analyzed the Sm 4 skull using digital visualization techniques, and compared it with other skulls found in Java.
Writing in the February 28 issue of the journal Science, Baba and colleagues argue that morphological characteristics of early H. erectus in Java, represented by fossil finds from Trinil/Sangiran, more closely resemble those of modern humans. Fossil material from Ngandong, which has been dated to anywhere between 25,000 to 50,000 years old, suggests that Java H. erectus had gone off on an evolutionary tangent of its own, developing distinct features that are not shared by modern humans.
They conclude that Javanese populations became progressively more isolated from other Asian H. erectus populations, and made minimal contributions to the ancestry of modern humans.
At one time scientists considered it possible that modern humans were the direct descendants of Asian Homo erectus. That idea has been discarded by many scientists who now think that while African H. erectus may be ancestral to H. sapiens, Asian H. erectus was an evolutionary dead end, rather than the immediate precursor to modern humans.
Still other scientists believe that the African version of H. erectus is different enough that it belongs in a separate species category called Homo ergaster.
"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."
More Information About Human Origins
1.8 Million-Year-Old Hominid Jaw Found
When Did "Modern" Behavior Emerge in Humans?
Documentary Redraws Human's Family Tree
Fossil Implies Our Early Kin Lived in Trees, Study Says
Controversy Over Famed Ancient Skull: Ape or Human?
Skull Fossil Opens Window Into Early Period of Human Origins
Skull Fossil Challenges Out-of-Africa Theory
New Study Supports Idea That Primates, Dinosaurs Coexisted
Human Fossil Adds Fuel to Evolution Debate
Did Our Species Mate With Other Human Species?
Did Humans and Neandertals Battle for Control of the Middle East?
Killer Cats Hunted Human Ancestors
Adolescence Came Late in Human Evolution, Study Shows
Viewpoint: Is It Time to Revise the System of Scientific Naming?
African Bone Tools Dispute Key Idea About Human Evolution
Africa's Imperiled Rock Art Documented Before it Disappears
Bones, Tools Push Back Human Settlement in Arctic Region
Oldest Asian Tools Show Early Human Tolerance of Variable Climate
Telltale Face Betrays Neandertals as Non-Human
Fossils From Ethiopia May Be Earliest Human Ancestor
New Face Added to Humankind's Family Tree
Discoveries Breathe New Life into Human Origins Debate
Additional National Geographic Resources
Interactive Feature: Outpost: In Search of Human Origins
National Geographic magazine online: Who Were the First Americans?
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