A recent finding in the dating of Chinese hominid fossils has challenged the prevailing "out-of-Africa" theory regarding the origin of modern man.
With a new dating method, scientists determined that Liujiang Hominid roamed south China approximately 70,000 to 130,000 years ago, rather than 30,000 years ago or less as it was previously believed. This new finding supports the theory that modern Chineseman originated in what is present-day Chinese territory rather than the mainstream "out of Africa" hypothesis which held that modern humans evolved from African ancestors alone.
This "out of Africa" theory, developed in 1987, is based on mitochondria DNA -- scraps of genetic tissue inherited exclusively from the maternal side -- that were found in ancient fossils.
DNA is a kind of genetic fingerprint unique to every individual that transmits hereditary characteristics, and mitochondria can be described as "energy packs" within cells.
The out-of-Africa theory contends that anatomically modern man first arose in eastern Africa about 150,000 years ago, then migrated out on a relentless push in which the species eventually conquered the planet.
This suggests that waves of African wanderers helped Man to evolve smoothly along a single path rather than branch out into starkly different genetic lines, and so Homo erectus in Asia was replaced by Homo sapiens out of Africa about 100,000 to 200,000 years ago.
This theory now faces a serious challenge.
The Liujiang Hominid fossils were discovered in 1958 in a cave in Liujiang County in south China's Guangxi Zhuang Autonomous Region. The fossilized bones, which include the skull and partial body and limbs, comprise one of the most complete and intact skeletons for a single modern man of this kind ever discovered in east Asia.
The accurate dating of the fossils will contribute to understanding man's origin or origins, a heatedly debated issue among the international academic community.
There is scientific consensus with respect to the theory that the oldest human genus appeared on the earth at least 5 million years ago, then evolved into Homo habilitus, Homo erectus and early Homo sapien about 100,000 years ago, and then finally into Homo sapien, to which we, or anatomically modern man, belong. Although few disputes remain over the origin of mankind in Africa, the origin of modern man continues to spark intense debate.
Paleontologist Wang Wei said, ?The 8-meter thick sediment at the site where the fossils were found can be divided into three layers. We took samples from the three layers respectively and sent for U-series testing in the world's two leading laboratories, Australian Queens land University and Nanjing Normal University."
"The result showed the middle layer in which the skull was embedded dated back to between 70,000 and 130,000 years ago or even earlier," Wang added.
The limited capacity of the commonly-used radiocarbon, or carbon-14 method, which could date back no further than 50,000 years, had previously rendered it impossible to achieve an accurate dating of the human fossils.
Through the use of the new thermal ionization mass spectrometer,which measures the rate of decay in radioactive uranium (TIMS U-series dating), a more accurate and larger dating range can be achieved, said Wang, who is also a senior researcher with the Guangxi Museum of Nature.
The scientists dated "Liujiang Man" by measuring the rate of decay of uranium by counting the number of thorium atoms, but instead of dating the fossils, which are porous, they dated the sediments, which better retain uranium, above and below the fossils.
"Referring to the excavation record in 1958, we confirmed that the Liujiang Hominid was discovered in sedimentary breccia fragments in the middle layer, whose age is the same as that of the fossils," Wang said.
The estimated age of "Liujiang Man" challenges the 15-year-old "out of Africa" theory that holds that modern humans first appeared in eastern Africa about 150,000 years ago, migrated out of the continent between 35,000 and 89,000 years ago, and moved across the globe to sweep aside populations, with no inter-breeding.
There are still dissident scientists who insist on the multi-regional evolution model which holds that modern man descended from several indigenous archaic human populations in the Old World, such as the Neanderthals who resided in Europe or from the so-called Java man or from the Peking man in Asia.
This alternative theory, called multi-regionalism, also holds that our ancestors emigrated from Africa 1.5 million years ago, but differs in that it holds that different branches in several different regions -- what is now Africa, Europe, east Asia and west Asia -- evolved simultaneously into modern humans through interbreeding between the regions.
The out-of-Africa theory seemed to have genetic support, Wang said, referring to genetic research carried out by a Sino-American team.
The geneticists screened more than 12,000 genetic samples (blood) of men in 163 populations from different regions in Asia --in such places as Iran, China, New Guinea and Siberia -- for three specific Y chromosome mutations that are derived from a single earlier mutation seen in African populations.
The mutations are characteristic DNA sequences called alleles, or genetic markers located in the Y chromosome (the male chromosome), one of the two sex chromosomes (X and Y) which only men carry (women carry two X chromosomes).
The Y chromosome is considered one of the most powerful molecular tools for tracing human evolutionary history because it remains unchanged over eons when passed from father to son.
The test showed all of them carried one of the three mutations, suggesting that every one of the men could trace his ancestry to forefathers who lived in Africa. So they concluded modern people in east Asia could trace their roots back to a common ancestor who lived in Africa maybe 100,000 years ago and migrated to Asia about 35,000 to 89,000 years ago.
The findings, appearing in the May 2001 issue of the respected US Journal Science, also claimed that there was no genetic evidence that modern people (Homo sapiens) mated with archaic humans (Homo erectus) that already inhabited east Asia, who had migrated from Africa about 1 million years ago. Homo erectus and Homo sapiens fossils have been found in abundance in east Asia.
"Genetic testing is an indirect method of assessing the human evolutionary process, while the dating of fossils is direct hard evidence. The dating of the Liujiang Hominid proved that He lived in south China 70,000 to 130,000 years ago or even earlier, ruling out the possibility of migration from Africa," Wang Wei said.
The oldest ancient human fossils found in China are those of the 1.7-million-year old Yuanmou Hominid. And all ancient human fossils unearthed in China share a common morphology: shovel-shaped fore-teeth, a rectangular eye pit and a flat face, which indicate that ancient man living in China had evolved continuously along an uninterrupted evolutionary chain for 1.7 million years, Wang noted.
The absence of fossils dating between 50,000 to 100,000 years ago shed doubt on this continuity, however, and geneticists therefore presumed that aborigines in East Asia became extinct and Home sapien, or the modern human, rose out of Africa and migrated to the region.
"Our finds have validated the continuity of the evolutionary chain. Further comprehensive research, including for example, chronology, on archaic human relics in south China will hopefully reveal the time when modern man first appeared in east Asia as well as shed light on the evolutionary model," Wang said.
Early last year, Australian scientists analyzed DNA taken from remains unearthed in 1974 at Lake Mungo in the state of New South Wales.
The analysis astonishingly revealed that neither "Mungo Man's" completely modern skeleton nor its DNA had any links with modern human ancestors from Africa found in other parts of the world.
The Australian researchers said that because Mungo Man is modern anatomically, yet has a vanished DNA line, it means at least one group of Homo erectus's descendants evolved outside of Africa.
Dating in May 1999 put the age of the skeletal remains at between 56,000 and 68,000 years, The previously oldest human DNA tested came from Neanderthal remains -- a 45,000-year-old specimen in western Germany and 28,000-year-old remains from Croatia.
Following the Mungo Man finding in January 2001, the next month's Science issue said Australian and Chinese scientists had made another important breakthrough.
The scientists dated China's "Nanjing Man" as 580,000 to 620,000 years old, further supporting a multi-regional theory. "These ages along with those from other sites in China imply that most of the Homo erectus specimens there are older than previously thought and perhaps do not overlap significantly with younger Homo sapiens," Science said. Homo erectus "Nanjing Man", a skull discovered in 1993 in Tangshan Cave near Shanghai, showed that humans evolved in isolation in China and much earlier than previously thought.
The scientists believed that "Nanjing Man" and the more famous "Peking Man" family, which had been estimated at 230,000 years old, evolved in small communities in eastern Asia. Under the multi-regional evolutionary model, these small communities of Homo erectus probably developed in widely separate locations, but had some communication, which explains the DNA similarities which developed.
Apart from the fossil evidence, Paleolithic tools excavated in China, created according to ancestral techniques, also support the theory that modern Chinese man is more likely to have originated in China, said Huang Weiwen, a senior research fellow with the Institute of Vertebrate Paleontology and Pale anthropology (IVPP) of the Chinese Academy of Sciences.
According to Hou Yamei, also an IVPP expert, up until recently, the lack of modern man's fossils in east Asia had led experts to presume that the out-of-Africa theory was accurate, but the recent dating of the Liujiang Hominid fossils is causing the international academic community to once again focus its attention on east Asia.
(Xinhua News Agency October 11, 2002)
