Read Online Lone Survivors by Chris Stringer - Free Book Online Page B
Authors: Chris Stringer
Ads: Link
fossils has a long history, as I explained in relation to Olduvai Gorge earlier, and the mapping of outputs from successive volcanic eruptions has played an important role in refining the age of many important fossil sites in East Africa, including Omo Kibish in Ethiopia. The two most complete human fossils from there, the Omo 1 skeleton and the Omo 2 braincase, were found in 1967 by a team led by Richard Leakey and were important in early proposals for a recent African origin. But although there were initial estimates that the material was over 100,000 years old, some of these were based on the application of uranium-series dating to shells in the depositsânot the most reliable material for such determinationsâand so doubts remained. Over thirty years after the original discoveries, an international team led by the anthropologist John Fleagle returned to the Kibish region, relocated the 1967 find-spots, and found further fossils and stone tools. Both Omo 1 and Omo 2 were originally recovered from the lowermost portion of the massive Kibish Formation, a series of annual but episodic sediments laid down by the ancient Omo River when it periodically flooded, before it entered Lake Turkana. These deposits lie about one hundred kilometers farther north than its present delta, close to the Ethiopian border with Kenya. Occasionally, volcanic eruptions deposited volcanic ash and pumice over the river and lake sediments, and these can be dated through their contained argon. A layer of ash about three meters below the location of Omo 1 was placed at about 196,000 years old, while a second ash about fifty meters above the location was dated to about 104,000 years. Because there were also clear signs of geologic erosion (the removal of sediments when the river and lake level fell) between the level of Omo 1 and the higher ash, it seemed likely that the age of Omo 1 was much closer to the age of the 196,000-year ash than to the 104,000-year one.
Additional indirect support for this came from much farther afield, on the seabed of the Mediterranean. During ancient monsoon periods, rain and snowmelt in the Ethiopian highlands sent annual floods pouring into the sources of the River Nile, causing sapropels (dark layers of sediment) to be deposited when these waters eventually flowed out into the Mediterranean. A particularly strongly marked sapropel can be dated from its position in Mediterranean seabed cores to about 195,000 years, suggesting that it correlates perfectly with the major monsoon event that sent floods in the opposite direction down the Omo River, producing the vast deposits of the lower part of the Kibish Formation, in which the Omo 1 skeleton and the underlying volcanic ash were found. The Omo 2 braincase was a surface find rather than a fossil excavated from sediments (which was the case for the Omo 1 partial skeleton), but the surrounding location consisted of the lowermost part of the Kibish Formation. Thus the team that revisited the region and published the new dating work remains confident that Omo 1 and Omo 2 are very close in age, at about 195,000 years, despite some strong contrasts in their level of modernityâsomething to which I will return in chapter 9.
Another instance in which Mediterranean sapropels provided clues about events deep within the African continent concerns the âgreeningâ of the Sahara about 120,000 years ago. Today, the Sahara is the largest hyperarid region on Earth, with an annual recorded rainfall as low as one millimeter across much of its vast extent. But as is well known from archaeological finds and rock paintings of animals and people deep within the desert, only 6,000 years ago the Sahara was a wetter place of grasslands, lakes, and gallery forests, fringing extensive river systems. What is less well known is that 120,000 years ago the Sahara was even wetter than that and was able to support a widespread population of Middle Paleolithic hunters and gatherers.
Additional indirect support for this came from much farther afield, on the seabed of the Mediterranean. During ancient monsoon periods, rain and snowmelt in the Ethiopian highlands sent annual floods pouring into the sources of the River Nile, causing sapropels (dark layers of sediment) to be deposited when these waters eventually flowed out into the Mediterranean. A particularly strongly marked sapropel can be dated from its position in Mediterranean seabed cores to about 195,000 years, suggesting that it correlates perfectly with the major monsoon event that sent floods in the opposite direction down the Omo River, producing the vast deposits of the lower part of the Kibish Formation, in which the Omo 1 skeleton and the underlying volcanic ash were found. The Omo 2 braincase was a surface find rather than a fossil excavated from sediments (which was the case for the Omo 1 partial skeleton), but the surrounding location consisted of the lowermost part of the Kibish Formation. Thus the team that revisited the region and published the new dating work remains confident that Omo 1 and Omo 2 are very close in age, at about 195,000 years, despite some strong contrasts in their level of modernityâsomething to which I will return in chapter 9.
Another instance in which Mediterranean sapropels provided clues about events deep within the African continent concerns the âgreeningâ of the Sahara about 120,000 years ago. Today, the Sahara is the largest hyperarid region on Earth, with an annual recorded rainfall as low as one millimeter across much of its vast extent. But as is well known from archaeological finds and rock paintings of animals and people deep within the desert, only 6,000 years ago the Sahara was a wetter place of grasslands, lakes, and gallery forests, fringing extensive river systems. What is less well known is that 120,000 years ago the Sahara was even wetter than that and was able to support a widespread population of Middle Paleolithic hunters and gatherers.
Similar Books
