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http://cul.news.tom.com 2005年04月15日07時07分 來源:北京科技報
■供圖/Imaginechina托邁無疑是當下古人類學研究最耀眼的明星。
這個距今600萬－700萬年、保存相當完整的頭蓋骨化石，在整個古人類學界引起了轟動。它的名字是乍得總統親自起的，意思是「生命的希望」。但另一個研究小組堅持認為，他們在肯尼亞發現的距今600萬年前的「千禧人」才是人類最早的祖先，托邁其實是一隻母猿。對這一問題的爭論仍在繼續。
據中科院古脊椎與古人類所院士吳新智介紹，2002年7月11日《自然》雜誌報道了在非洲中部的乍得出土的距今600萬－700萬年的一具頭骨化石，俗名托邁（Toumai），學名叫乍得撒海爾人。研究者認為，這是迄今所知最早的人類祖先遺骨。
媒體在介紹托邁時往往強調，他比著名的距今大約300萬年前的「露西」早300多萬年。實際上，1994年報道的發現於埃塞俄比亞的440萬年前的「地猿」，2000年報道的發現於肯尼亞的600萬年前的「千禧人」，都已經被多數古人類學家接受為早期的人類。
人類區別於猿類的一個主要特徵是人類能用兩條腿直立行走。但從托邁的化石只發現了與人比較相似，而與猿差別較大的顏面和腦顱的一部分，無法判斷是否具備那些足以判斷行動方式的骨骼構造，所以他是人是猿還有爭論。如果托邁是人，那就意味著人類的發源地區擴大了，目前多數古人類學家關於人類與猿類在大約500萬年前分道揚鑣的主張也將面臨挑戰。圖為距今約700萬年前的非洲乍得撒海爾人（即托邁）的復原圖。
[責編：辛堯]
Last Updated: Thursday, 18 May 2006, 11:00 GMT 12:00 UK
Evolution\’s human and chimp twist
<font color=\”gray\”>The new finding raises questions about the
Toumai fossil from Chad</font>ans and chimpanzees may have split away from a common ancestor far more recently than was previously thought.
A detailed analysis of human and chimp DNA suggests the lines finally diverged less than 5.4 million years ago.The finding, published in the journal Nature, is about 1-2 million years later than the fossils have indicated.
A US team says its results hint at the possibility that interbreeding occurred between the two lines for thousands, even millions, of years.
This hybridisation would have been important in swapping genes for traits that allowed the emerging species to survive in their environments, explain the scientists affiliated to the Broad Institute of MIT and Harvard and the Harvard Medical School.
And it underlines, they believe, just how complex human evolution has been.
\”This is a hypothesis; we haven\’t proved it but it would explain multiple features of our data,\” said David Reich, assistant professor of genetics at the Harvard Medical School and an author on the Nature paper.
\”The hypothesis is that there was gene flow between the ancestors of humans and chimpanzees after their original divergence.
\”So, there might have been an original divergence and a separation for long enough that the species became differentiated – for example, we might have adapted features such as upright walking – and then there was a re-mixture event quite a while after; a hybridisation event,\” he told the Science in Action programme on the BBC World Service.
Gene swapping
Humans and chimps contain DNA sequences that are very similar to each other; the differences are due to mutations, or errors, in the genetic code that have occurred since these animals diverged on to separate evolutionary paths.
By analysing where these differences occur in the animals\’ genomes, it is possible to get an insight into the two species\’ histories – the timing of key events in their evolution.
Scientists have been able to do this for some time but the recent projects to fully decode the two primates\’ genomes have provided details that have taken this type of study to a more advanced level.
The US investigation indicates the human and chimp lines split no more than 6.3 million years ago and probably less than 5.4 million years ago.
It is a problematic finding because of our current understanding of early fossils, such as the famous Toumai specimen uncovered in Chad.
Toumai (Sahelanthropus tchadensis) was thought to be right at the foot of the human family tree. It dates to between 6.5 and 7.4 million years ago. In other words, it is older than the point of human-chimp divergence seen in the genetic data.
\”It is possible that the Toumai fossil is more recent than previously thought,\” said Nick Patterson, a senior research scientist and statistician at the Broad Institute of MIT and Harvard, and lead author on the Nature paper.
\”But if the dating is correct, the Toumai fossil would precede the human-chimp split. The fact that it has human-like features suggests that human-chimp speciation may have occurred over a long period with episodes of hybridisation between the emerging species.\”
Commenting on the research, Daniel Lieberman, a professor of biological anthropology at Harvard, told the Associated Press: \”It\’s a totally cool and extremely clever analysis.
\”My problem is imagining what it would be like to have a bipedal hominid and a chimpanzee viewing each other as appropriate mates, not to put it too crudely.\”
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