南方古猿Taun幼儿头骨:改变对人类起源认识的化石

The Australopithecus Taun child skull: A fossil that changed human evolution history

人类的出现是地球和生命发展史上的一个重大事件.尽管目前对人类起源与演化的许多问题尚未阐明,经过一个多世纪几代人的探索与努力,古人类学界已经建立了人类起源与演化的大致框架.根据迄今发现的古人类化石证据,人类起源与演化的历史至少可以追溯到大约700万年前,经历了撒海尔乍得人、原初人、地猿、南方古猿、能人、直立人、古老型智人、海德堡人、尼安德特人。

In 1859,Charles Darwin published The Origin of Species in which he proposed the principle of natural selection.Later,Thomas Henry Huxley emphasized similarities in anatomical structure between human and apes to argue that man and ape have a common ancestry.Darwin further proposed that Africa may be the place for human origins.At their time,the evidence for human origins highlighted by Darwin and Huxley was mainly derived from the fields of comparative anatomy and embryology with no relevant fossils yet available as direct evidence.Starting from late nineteenth century,studies of human origins gradually began to benefit from the discovery of fossils of early human ancestors.By the early twentieth century,only a few human fossils had been discovered,like Neanderthals,Pithecanthropus from Java,Broken Hill man from Rhodesia.Based on assessments of this fossil evidence,human history was regarded as originating only about 0.5 mya.In 1924,a fossil infant skull was found from the lime quarry at Taun about 320 km southwest of Johannesburg,South Africa that would eventually change the trajectory of human origins theory.The skull was among two boxes of fossiliferous breccia sent by a geologist colleague to Raymond Dart,a British trained anatomist who in 1922 was appointed Professor of Anatomy at the University of the Witwatersrand in Johannesburg.With the stratigraphic evidence of this district concerning the succession of stone ages in South Africa and that of Broken Hill in Rhodesia,Dart soon realized that Taun and the surrounding area might contain fossil remains of primitive man.Fossils in the crates mainly represented additional baboons,but one crate contained a natural endocast(mold of the inner surface of a skull)that Dart immediately realized came from a hominoid primate.Among the breccia blocks in the crates,Dart found one with a depression into which the endocast perfectly fit.Inside this block,he saw traces of bone that he hoped was the front of the skull associated with the natural endocast.Although he eventually determined that the front was present,it took Dart several weeks to fully expose it with a hammer,chisels,and most famously,a sharpened knitting needle.The dentition of the skull indicated that it represented a juvenile whose first molars were just erupting,and various other features showed that in important respects the creature was intermediate between apes and human.For example,it was obviously ape-like in the small size of its brain—or more precisely in its endocranial(internal skull)volume,which Dart estimated would have reached 525 cc at adulthood.Although subsequent re-estimates have reduced this adult brain size to between 404 and 440 cc,even Dart’s figure was much closer to the average endocranial volume for common chimpanzees(ca.400 cc)and gorillas(ca.500 cc)than it was to the average for modern humans(1350–1375 cc).At the same time,the deciduous canines were much smaller and less projecting than those in apes,and the foramen magnum,through which the spinal cord and brainstem passed to connect to the brain,was more anteriorly positioned on the base of the skull.This suggested that the skull was balanced on top of the spinal column,as in people,where this position is a requirement of efficient upright,bipedal locomotion.In short,the position of the foramen magnum implied that the Taun individual was bipedal like people rather than quadrupedal like apes.On February 7,1925,Dart published his initial description of the Taun child skull and natural endocast in Nature and concluded that it came from a previously unknown species"intermediate between living anthropoids and man".He named the species Australopithecus africanus("African southern ape")and stressed both the skull’s intermediate or transitional morphology and its discovery far outside the geographical ranges of living apes.Taun occupied a non-forested region where living apes could not survive,and from geological studies Dart argued that the regional environment today was probably similar to the environment when the fossils were accumulated.The ancient vegetation of the Taun area was thus inappropriate for living apes in the narrow sense.It was,however,clearly suitable for an ape that was developing human traits and a different diet.Rare bone fragments that may have come from a nyala antelope(Tragelaphus angasi)now suggest that the ancient environment may have been more wooded than Dart believed,but baboon and rock hyrax fossils dominate the Taun fossil collection,continuing to imply a reconstructed environment that living apes would generally avoid.The discovery of the Taun Child skull extended human origins back to about 1 million years ago at that time and led the way for further human fossil discoveries and research in South Africa,East Africa and Central Africa beginning in the 1930 s until present day.It had an integral role in making Africa the center of human origins and evolution research and also providing solid fossil evidence for Darwin’s prediction of Africa as the birthplace of human origins.Now,Australopithecus is widely recognized as an important stage of human evolution by the scientific community.