How a fossil discovery could simplify humankind's family tree
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The earliest, now-extinct human lineages, once thought to be multiple species, may actually have been one species, researchers now controversially suggest.
Modern humans,听Homo sapiens, are the only living member of the human lineage,听Homo, which is thought to have arisen in Africa about 2 million years ago at the beginning of the ice age, also referred to as the聽. Many extinct human species were thought to once roam the Earth, such as聽Homo habilis, suspected to be among the first stone-tool makers; the relatively larger-brained聽Homo rudolfensis; the relatively slender聽Homo ergaster; and聽Homo erectus, the first to regularly keep tools it made.
To learn more about the roots of the聽, scientists investigated a completely intact, approximately 1.8-million-year-old skull excavated from the medieval hilltop town of Dmanisi in the Republic of Georgia. Archaeological excavations there about 30 years ago unexpectedly revealed that Dmanisi is one of the oldest-known sites for ancient human species out of Africa and the most complete collection of聽Homo erectus聽skulls and jaws found so far. The聽聽once lived in the area, and scientists cannot rule out whether it fed on these early humans.
This fossil, the most massively built skull ever found at Dmanisi, is the best-preserved fossil of an early human species discovered yet. It probably belonged to a male, and its right cheekbone has signs that it healed from a fracture. []
"We can only guess how the fracture was inflicted on the individual 鈥 it could be that it had an argument with another member of the group it lived in, or it could be that it fell down," study co-author Christoph Zollikofer, a neurobiologist at the Anthropological Institute and Museum in Zurich, Switzerland, told LiveScience.
Unique skull
This new skull, called Skull 5, was discovered alongside the remains of four other skulls of ancient humans, all of them associated with the same location and period of time, which back 1.8 million years ago was a relatively temperate mix of forest and steppe near a river. The fossil is unlike any other聽Homo聽remains on record 鈥 it combines a long face, massive jaw and large teeth with a small braincase, just about a third the size of that found in modern humans and no larger than those of much more primitive African fossils and even modern gorillas. Scientists hadn't observed such a combination of features in an early聽Homo聽fossil until now. []
"Had the braincase and the face of Skull 5 been found as separate fossils at different sites in Africa, they might have been attributed to different species," Zollikofer said in a statement.
The level of variation seen in聽Homo聽fossils is typically used to define separate species. However, the scientists found the level of diversity now seen between the five sets of fossils at Dmanisi 鈥 Skull 5 and the four other specimens 鈥 is no greater than any seen between five modern humans or five chimpanzees.
"If you take the biggest skull there and compare it to the smallest, the smallest one is 75 percent the size of the bigger one, and that's absolutely standard in what you would see in聽," Zollikofer said.
This discovery is supported by recent findings from these researchers showing that differences seen in Dmanisi jawbones that might seem to suggest they came from different species actually are聽.
"The Dmanisi individuals all belong to a population of a single early聽Homo聽species," Zollikofer said. At the same time, "the five Dmanisi individuals are conspicuously different from each other, but not more different than any five modern human individuals, or five chimpanzee individuals from a given population."
The scientists also investigated the level of variation seen among ancient African聽Homo聽fossils. They found their level of diversity was similar to that seen in Dmanisi. As such, the researchers suggest that early, diverse聽Homo聽fossils may not represent several human species each specialized for their habitat, but rather variants of a single lineage that emerged from Africa capable of coping with a variety of habitats.
In other words, instead of Africa once being home to multiple human species such as聽, Homo habilis, Homo ergaster聽andHomo rudolfensis, "we think it is sensible to attribute all specimens to聽Homo erectus," Zollikofer told LiveScience.
"We now have one global human species," Zollikofer said. "What we can infer from our study at Dmanisi is that at 1.8 million years ago, there was another single global human species."
Controversial human find
The researchers caution they do not propose all fossil human specimens, including聽聽and modern humans, are lumped into聽Homo erectus. "We only refer to the time between 2 million and 1.8 million years ago," Zollikofer said. "We are not compulsive lumpers."
The researchers expect their interpretation of their findings to prove controversial among some other scientists. In rebuttal to any who suggest early聽聽all belong to multiple species and not one, Zollikofer asks "opponents to present us the magic-wand methods that they use to recognize species from single specimens."
Although paleoanthropologist Ian Tattersall, who did not participate in this research, told LiveScience from Laos in Southeast Asia that "this is a most amazing and important specimen," he added, "I believe it is a mistake to force it intoHomo erectus聽in the interests of maintaining a linear picture of聽. It is actually very distinctive morphologically, and it dramatically underscores that human evolution involved vigorous diversification and experimentation with the hominid potential."
"I think they will be proved right that some of those early African fossils can reasonably join a variable聽Homo erectusspecies," paleoanthropologist Chris Stringer at the Natural History Museum in London, who did not take part in this study, told LiveScience. "But Africa is a huge continent with a deep record of the earliest stages of human evolution, and there certainly seems to have been species-level diversity there prior to 2 million years ago, so I still doubt that all of the 'early Homo' fossils can reasonably be lumped into an evolving聽Homo erectus聽lineage. We need similarly complete African fossils from 2 million to 2.5 million years ago to test that idea properly."
The scientists detailed their findings in the Oct. 18 issue of the journal Science.
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