日期:2011-09-21 12:07


Human evolution

You look familiar

Another piece of humanity’s family tree is fitted into place

Sep 10th 2011 | from the print edition

THE opening scene of Mel Brooks’s film “History of the World: Part One” dispenses with human origins in one line: “And the ape stood, and became man.” Would that it were that easy for palaeontologists to sort out. The transition to humanity is generally agreed to have occurred between Australopithecus, a genus of small-brained, bipedal primates whose most famous member is a fossil nicknamed “Lucy”, and the big-brained species Homo erectus. But pinning down when precisely this took place, and which of the various australopithecine species were involved, has been challenging. Now the most human-like australopithecine found to date is clarifying things—and staking a claim to be the species from which early humans evolved.

梅尔·布鲁克斯执导的电影“世界史:序幕”开场仅用一句话就概括了人类起源:“类人猿直立起来,变成了人类。” 要是古生物学家也能如此轻而易举地阐释人类进化的奥秘就好了。人们普遍认为人类是由南方古猿和直立人之间的某个物种演化而来。南方古猿头部较小,属双足灵长类动物,其最有名的家族成员是一具被称做“露西”的化石,比较而言,直立人的头部就大多了。不过,要想确定确切的人类演化时间和哪些南方古猿物种参与了演化,仍是极具挑战性的课题。目前,迄今为止发现的与人类最为相似的南方古猿正逐渐解开这团迷雾——它阐明了早期人类是由哪个物种演化而来的。

Fossils of the new species, Australopithecus sediba, were discovered in 2008 in a cave in South Africa. Initial research, led by Lee Berger of the University of the Witwatersrand, in Johannesburg, concluded that the species came too late in the fossil record to be the ancestor of the Homo lineage. This week, however, a range of new research into sediba, again led by Dr Berger, has been published in Science. These studies conclude that sediba did in fact predate Homo erectus and, moreover, that parts of its anatomy are surprisingly similar to modern man.


The fossils examined in the Science papers are of an adolescent boy and an adult woman. They are well preserved, and encased in sediments that allow uncommonly precise dating. They lived 1.977m years ago, predating the appearance of Homo erectus by 77,000 years. The period is an especially muddled one for palaeontology, being full of fragmentary fossils that are difficult to assign either to Homo or to Australopithecus. The sediba fossils, by contrast, have some of the most complete features in the early human record.


The new studies centre on the most telling bits of anatomy in the story of human evolution: the brain, pelvis, hands and feet. The brain itself does not fossilise, but the inside of the cranium retains an impression of its contours. The researchers mapped these with high-powered X-ray beams to create a three-dimensional model of the surface of sediba’s brain. They found that its size was on a par with other australopithecines, but its shape was more like that of a human brain. Specifically, the frontal lobes, which are the seat in modern humans of higher cognitive functions such as abstract reasoning, looked more humanlike in sediba’s brain than they do in the brains of other australopithecines. That suggests the neurological changes which gave rise to humanity may have predated the brain’s expansion—an event that had, hitherto, been regarded as crucial to the emergence of humans.


The hands, feet and pelvis of sediba indicate that it both climbed trees and walked upright, though with a different gait from that of humans or chimpanzees. The species may have been a toolmaker, as its hand allowed for a human-like grip. Sediba’s pelvis, an upright butterfly shape, is likewise reminiscent of the human one. It therefore sheds light on a longstanding debate: whether it was bipedalism or giving birth to babies with large heads that drove changes in the shape of the human pelvis. Adult sediba (and therefore, presumably, their babies) had small heads, which indicates that walking upright was the advantage brought by the modern pelvis.


All of which makes sediba more similar to modern humans than are other australopithecines—and more similar, even, than Homo habilis, until now seen as one of the earliest humans. The consensus had been that habilis was a transitional form between Australopithecus and Homo erectus. Dr Berger posits that sediba may have evolved directly into Homo erectus, leaving habilis as an evolutionary sideline, and not even part of the genus Homo. Slowly, then, the origin of the strange assemblage of characters that makes a human being human is emerging. As the oracle said, the beginning of wisdom is: know thyself.


  • claimn. 要求,要求权;主张,断言,声称;要求物 vt. 要
  • appearancen. 外表,外貌,出现,出场,露面
  • consensusn. 共识,一致,合意 n. [生理]交感
  • evolutionaryadj. 进化的,发展的,演变的
  • crucialadj. 关键的,决定性的
  • fragmentaryadj. 碎片的,碎块的,碎屑状的,零碎的,不完全的
  • initialn. (词)首字母 adj. 开始的,最初的,字首的 v
  • cognitiveadj. 认知的,认识的,有认识力的
  • challengingadj. 大胆的(复杂的,有前途的,挑战的) n. 复杂
  • advantagen. 优势,有利条件 vt. 有利于