Climate dents in humankind´s family tree: new correlations discovered

Scientists of the Potsdam Institute for Climate Impact Research (PIK) and Potsdam University have now provided a novel view on human evolution during the past five million years. A nonlinear statistical analysis of sediments taken from the seafloor near Africa indicates that abrupt changes in climate variability could have had a significant impact on human evolution. In the first instance, the scientists have spotted three primeval tipping points.

“It has long been assumed that climate changes are significant for the history of humankind, but until now this has not been statistically proven,” says Jonathan Donges from PIK, lead author of the study published this week in the renowned Proceedings of the National Academy of Sciences. Now some evidence has been demonstrated. “For the first time we can show that the concurrence between changes in climate variability and those in early human evolution were probably not arbitrary,” says Donges. The transition between times of little and strong climatic fluctuations – “the changes within changes”– is crucial. Apparently they raise selection pressure.

Dust from the ocean bottom supplies data

Instead of long-term trends, the scientists examined comparably short-term changes that still cover a few thousand years. The mathematical analysis of time series spanning millions of years was based on data already published several years ago by marine geologists. The data is derived from drill cores from the seafloor of the Indian and Atlantic Oceans as well as the Mediterranean. The deposits of desert dusts contained in these sediments allow conclusions on what kind of climate prevailed at certain points in time.

In the process, the scientists were able to identify some of the mechanisms that possibly triggered climate changes. For example, the warm waters flowing from the Pacific towards Africa diminished due to shifting landmasses in the region of today´s Indonesia. Since ocean currents are conveyers of heat, as a result regional temperatures and rainfall over Africa changed. This in turn had impacts on the local vegetation and therefore also animal populations as well as early humans like Australopithecus, who became extinct about a million years ago. In contrast, other ancestors of man could suddenly flourish under the changed conditions, because they were more adaptive. “As an allrounder, Homo had better chances in a fluctuating climate than more specialized hominids,” says Donges.

Look into the past sharpens that into the future

To look into the past can help sharpen our view into the future. “Climate changes have impacts on the living conditions of humankind – and what took many hundreds of thousands of years in the past could now happen in fast motion because of the man-made greenhouse effect,” explains Jürgen Kurths, head of the research team and co-author of the study. Regarding temperatures, one of the three relevant periods about three million years ago can be considered a counterpart to a world with unabated CO2 emissions, such as may exist by the end of this century. “This is not a one-to-one equivalence, but about understanding essential mechanisms of climate changes,” says Kurths. “So-called paleoclimatology often serves to verify assumptions about the climate of today and tomorrow.”

“It is a big step ahead. Finally the methods of nonlinear physics are being utilized for research on the evolution of humankind,” says Hans Joachim Schellnhuber, PIK director and also one of the study´s co-authors. He is one of the pioneers in the application of this discipline – known as chaos theory – to Earth system research. “We are succeeding better and better in understanding complex dynamic systems,” says Schellnhuber. “It is becoming ever more clear that this is not a glass bead game, but an extraordinarily relevant field of research – especially in regard to climate change.”

Article: Donges, J., Donner, R.V., Trauth, M.H., Marwan, N., Schellnhuber, H.J., Kurths, J.: Nonlinear detection of paleoclimate-variability transitions possibly related to human evolution. Proceedings of the National Academy of Sciences [doi:10.1073/pnas.0709640104] (Early Edition)

The article is available in PNAS's Early Edition online the day it gets published this week. Until then it is available only on a protected website of PNAS/Eurekalert or via email: pnasnews@nas.edu.

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