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A gradually dry climate, punctuated with variable moisture episodes, may have precipitated the transition from our hominid ancestors to anatomically modern humans, according to research published in the Proceedings of the National Academy of Sciences (PNAS).
Since the discovery of a rich assemblage of human fossils as well as stone tools and other archaeological evidence in the rift valley of East Africa, often referred to as the cradle of humanity, scientists have attempted to piece together the complex puzzle that is the history of our human origins, including the environmental context of this story.
This study, based on lake sediment cores, is the first to provide an ongoing environmental context for the various archaeological evidence collected in localities near the Rift Valley basins in southern Kenya. The cores were sampled in Lake Magadi as part of the Hominin and Paleolakes site drilling project, or HSPDP, led by Professor Andrew Cohen of the University of Arizona.
Lake Magadi, a shallow, periodically drained lake, is near the Olorgesailie basin in Kenya, one of the most productive sites in terms of archaeological evidence of human evolution in Africa. The authors suggest that deep climate change may have been the driving force behind the evolution of hominids, the origins of modernity. Homo sapiens and the beginning of the middle stone age.
While previous assumptions related hominin evolution to climate change, most previous studies lack evidence at the regional scale of a link between the environment and evolution hominins. The authors write in the article, "The progressive aridification in East Africa over the last 500,000 years and the human evolution. According to the study, a trend of intense aridification in the region began 575,000 years ago. This change, which was not documented in the continuous continental cores of East Africa, corresponds to extinctions of wildlife and a major transformation of stone tool technology documented in the Olorgesailie region.
"Many evidences of human evolution have been collected in the region, but linking these recordings to detailed environmental records was lacking until now," said the senior author of the 39, study, Richard Owen, of the Hong Kong Baptist University. "There is a big gap between the last tools dating from the beginning of the Stone Age dating back 500,000 years and the appearance of these tools around 320,000 years ago. Our results have filled this gap with a continuous environmental record. "
A critical transition period occurred during this period, a period during which archaeologists uncovered evidence of a leap forward in the ability of early humans to make, use and trade Pierre.
The Magadi Lake cores are the first detailed link between climate change and events from the region's archeological archives.
"We had known for a long time that the climate was very varied at the time, but the bottom line is that the documents are close to the archaeological evidence of this transition," says Cohen, a professor in the Department of Geosciences of Canada. University of Arizona. . "The older stone tools found at Olorgesailie have hardly changed between 1.2 million and a half million years ago. And suddenly, after 500,000 years and before 320,000 years ago – we do not know exactly when, but in this period – archeology has gone through a critical transition as tools have become more sophisticated and have been transported on more long distances. "
At the same time, the archives of the lake core indicate that the climate is becoming drier and more variable, there is evidence elsewhere in Africa of the appearance of modernity. Homo sapiens, prompting a lot of speculation as to whether the two are connected, said Cohen.
"That the evolution of the biggest brains goes hand in hand with the new tool kits is not entirely clear," he said. "But the oldest modern Homo sapiens The fossils of Morocco date back to 325,000 years, just as we are witnessing this transition of tools. And both occurred at about the same time that our main file indicates a significant drying up very close to the archaeological sites. "
The deepest core drilled at Lake Magadi reached 200 meters (650 feet), penetrating all sedimentary layers to the volcanic rock of the lake. The core samples, each about 10 feet long and 2 1/2 inches in diameter, are cut into manageable 5 foot segments, packaged and airlifted to the Lake Lake National Center at the University of Minnesota for preservation. , analyzed and stored.
On the assumption of variability selection, a rapidly changing environment creates selective pressure that forces species to adapt to rapid change, Owen said. In this scenario, the larger brain of anatomically modern humans would have allowed our ancestors to adapt quickly to an increasingly unpredictable world.
"We now have evidence that toolkits are changing, that mammalian wildlife and climate are becoming more arid," Owen said. "So you have a series of coincidences that makes you think:" This could be real. "Now we can tell when the environment has changed and then compare that to the archaeological evidence of the area."
HSPDP drilled at other nearby sites, with researchers collecting more climate data from the region to continue to study the significance of environmental variability in human evolution.
UNIVERSITY OF ARIZONA
Header Image – Study co-authors Veronica Muiruri (left), Anthony Mbuthia (blue shirt) and Andrew Cohen label a sample of freshly sealed sediment collected from Lake Magadi, Kenya. Credit: Anne Billingsley
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