Jawbone Fossil Fills a Gap in Early Human Evolution
By JOHN NOBLE WILFORD

On the morning of Jan. 29, 2013, Chalachew Seyoum was climbing a remote hill in the Afar region of his native Ethiopia, his head bent, eyes focused on the loose sediment. The site, known as Ledi-Geraru, was rich in fossils. Soon enough, he spotted a telltale shape on the surface — a premolar, as it turned out. It was attached to a piece of a mandible, or lower jawbone. He collected other pieces of a left mandible, and five teeth in all.

Mr. Seyoum, a graduate student in paleoanthropology at Arizona State University, had made a discovery that vaulted evolutionary science over a barren stretch of fossil record between two million and three million years ago. This was a time when the human genus, Homo, was getting underway. The 2.8-million-year-old jawbone of a Homo habilis predates by at least 400,000 years any previously known Homo fossils.

More significant, scientists say, is that this H. habilis lived only 200,000 years after the last known evidence of its more apelike predecessors, Australopithecus afarensis, the species made famous by “Lucy,” whose skeleton was found in the 1970s at the nearby Ethiopian site of Hadar.

William H. Kimbel, director of the Institute of Human Origins at Arizona State, said the Ledi-Geraru jaw “helps narrow the evolutionary gap between Australopithecus and early Homo,” adding that it was an excellent “transitional fossil in a critical time period in human evolution.”

The discovery was announced Wednesday in two reports for the journal Science by researchers at Arizona State, the University of Nevada, Las Vegas, and Pennsylvania State University. One paleoanthropologist not on the teams, Fred Spoor of University College London and the Max Planck Institute for Evolutionary Anthropology in Germany, endorsed the analysis.

Dr. Spoor said in an email that he agreed with the hypothesis that the new Ledi-Geraru mandible “derives from Australopithecus afarensis, and at 2.8 million years shows morphology that is ancestral to all early Homo.”

How could Dr. Spoor not agree with the interpretation of the findings in the new report by Brian A. Villmoare of the University of Nevada, Las Vegas, and colleagues on the Arizona State team? By coincidence, Dr. Spoor was ready to predict many of the findings in the journal Nature a day before his predictions would have been proved right in the journal Science. When the relationship between the studies became clear, the two journals agreed to simultaneous publication of the articles on Wednesday.

Dr. Spoor’s predictions were drawn from a digital reconstruction of the disturbed remains of the jaws of the original 1.8-million-year-old Homo habilis specimen found 50 years ago by the legendary fossil hunters Louis and Mary Leakey at Olduvai Gorge in Tanzania.

The reconstruction, suggesting a plausible evolutionary link between A. afarensis and H. habilis, yielded a remarkably primitive picture of a deep-rooted diversity of a species that emerged much earlier than the 2.3 million years ago suggested by some specimens. The teeth and jaws appeared to be more similar to A. afarensis than to subsequent Homo erectus or Homo sapiens, modern humans that emerged about 200,000 years ago.

Dr. Spoor’s analysis also seemed to put a new face on H. habilis. He said that individual species of early Homo were more easily recognizable by jaw structure and facial features than by differences in brain size, which tend to be highly variable. Dr. Villmoare and colleagues made similar observations in their article. Both the predictions and the mandible findings called attention to smaller teeth with the emergence of H. habilis and evidence suggesting that the species probably split in different evolutionary lines, only one of which might have been ancestral to later H. erectus and H. sapiens.

In an email, Dr. Spoor explained that the split occurred sometime before 2.3 million years ago. The lineage leading to H. habilis must have kept the primitive jaw morphology. The Ledi-Geraru specimen kept the primitive, sloping chin that links it to a Lucy-like ancestor. Other lineages must account for the fact that H. erectus and H. habilis existed together for a period more than a million years ago.

In a second report for the journal Science, Erin N. DiMaggio of Penn State and other geologists examined soil, vegetation and fossils at Ledi-Geraru. They determined that when the H. habilis left its jaw there, the habitat was dominated by mammals that lived in a more open landscape — grasslands and low shrubs — than the more wooded land often favored by A. afarensis.

But after about 2.8 million years ago, increased African aridity has been cited as a possible result of widespread climate change affecting species changes and extinctions. Kaye E. Reed, co-leader of the Arizona State team, noted that the “aridity signal” had been observed at the Ethiopian fossil site. However, she said, “it’s still too soon to say this means climate change is responsible for the origin of Homo.”

For that, Dr. Reed said, “we need a larger sample of hominin fossils, and that’s why we continued to come to the Ledi-Geraru area to search.” That, and to learn more about the evolution of our genus, Homo.

距今280萬年 衣索比亞 發現最早人類祖先化石

研究人員四日說，在衣索比亞發現的一塊連帶牙齒的下顎骨碎片，是已知人屬（Homo）的最早化石，出現在大約兩百八十萬年前，比過去以為的最早人類祖先早了四十萬年。

這項發現顯示，人類的祖先住在現今衣索比亞阿法爾州的勒迪─吉拉魯研究區，大約兩百八十萬年前，這裡是開放的大草原，靠近湖泊、河流和活火山。

內華達大學助理教授維爾摩爾說：「這是我們已發現的人類祖先族譜上的最早化石。」他帶領的研究發表在「科學」期刊上。

這塊下顎骨代號為「LD 350-1」，是左下顎，帶有五顆牙齒，2013年在勒迪-吉魯拉研究區靠近表面的沉積層發現。

這塊化石尚未分類為某一物種，但它細細的臼齒和牙齒弧形比例，是在類人猿「阿法南方古猿」化石上看不到的進步。「阿法南方古猿」的最著名化石代表是「露西」。

專家認為新發現的這塊化石雖不是現代人類「智人」（homo sapiens）的直接祖先，卻是演化為現代人類的一種物種的早期代表。

研究人員特別有興趣，因為這塊化石的年代，只有極少這類化石被發現，有可能在這個時期，早期的人科出現分支，「智人」的祖先從類人猿的「阿法南方古猿」分離出來，後者直立行走，但腦部較小。

「阿法南方古猿」的代表「露西」是1974年在勒迪-吉拉魯研究區附近的哈達爾區發現，距今約三百廿萬年。新發現的化石非常接近人類與類人猿分道揚鑣的時期。

在發現這塊化石前，已知最早的「人屬」化石據信距今約兩百卅萬年或兩百四十萬年。

原文參照：
http://www.nytimes.com/2015/03/05/world/jawbones-discovery-fills-barren-evolutionary-period.html

2015-03-06．聯合報．A17．國際．編譯田思怡