McDougall Ian, Brown Francis H, Fleagle John G
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia.
Nature. 2005 Feb 17;433(7027):733-6. doi: 10.1038/nature03258.
In 1967 the Kibish Formation in southern Ethiopia yielded hominid cranial remains identified as early anatomically modern humans, assigned to Homo sapiens. However, the provenance and age of the fossils have been much debated. Here we confirm that the Omo I and Omo II hominid fossils are from similar stratigraphic levels in Member I of the Kibish Formation, despite the view that Omo I is more modern in appearance than Omo II. 40Ar/39Ar ages on feldspar crystals from pumice clasts within a tuff in Member I below the hominid levels place an older limit of 198 +/- 14 kyr (weighted mean age 196 +/- 2 kyr) on the hominids. A younger age limit of 104 +/- 7 kyr is provided by feldspars from pumice clasts in a Member III tuff. Geological evidence indicates rapid deposition of each member of the Kibish Formation. Isotopic ages on the Kibish Formation correspond to ages of Mediterranean sapropels, which reflect increased flow of the Nile River, and necessarily increased flow of the Omo River. Thus the 40Ar/39Ar age measurements, together with the sapropel correlations, indicate that the hominid fossils have an age close to the older limit. Our preferred estimate of the age of the Kibish hominids is 195 +/- 5 kyr, making them the earliest well-dated anatomically modern humans yet described.
1967年，埃塞俄比亚南部的基比什组出土了被鉴定为早期解剖学意义上的现代人类的原始人类颅骨遗骸，归为智人。然而，这些化石的来源和年代一直备受争议。在这里，我们证实奥莫I号和奥莫II号原始人类化石来自基比什组第一成员层位相近的地层，尽管有人认为奥莫I号在外观上比奥莫II号更具现代特征。在原始人类层位之下的第一成员层位凝灰岩中的浮石碎屑中的长石晶体上进行的40Ar/39Ar测年，为原始人类确定了一个较老的年龄界限，即198±14 kyr（加权平均年龄196±2 kyr）。第三成员层位凝灰岩中的浮石碎屑中的长石提供了一个较年轻的年龄界限，即104±7 kyr。地质证据表明基比什组的每个成员层位都是快速沉积的。基比什组的同位素年龄与地中海腐泥层的年龄相对应，这反映了尼罗河流量的增加，必然也意味着奥莫河流量的增加。因此，40Ar/39Ar年龄测定结果，再加上与腐泥层的对比，表明原始人类化石的年龄接近较老的界限。我们对基比什原始人类年龄的最佳估计是195±5 kyr，这使得他们成为迄今所描述的最早有确切年代的解剖学意义上的现代人类。
