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A scientist at Columbia University's Lamont-Doherty Earth Observatory today reported the first detailed evidence that profound climate shifts on earth may have changed the course of human evolution.
Analyzing ocean sediments off the African coast, Peter deMenocal, a Lamont-Doherty paleoclimatologist, reported in the Oct. 6 issue of Science that the continent suffered cycles of colder, drier climate about 2.8 million, 1.7 million and 1 million years ago. The three periods coincide with major steps in human evolution as documented by the fossil record. The evidence strongly suggests that shifting environmental conditions contributed to the extinction of some human ancestors while other, more adaptable, species survived.
At 2.8 million years ago, the human family tree split into at least two major branches, Paranthropus and Homo. At 1.7 million years ago, humans' most immediate ancestor, Homo erectus, first appeared. At 1 million years ago, Paranthropus had died out and great numbers of Homo erectus began to migrate out of Africa into a variety of regions and habitats in Europe and Asia.
The report in Science expands on findings first presented by Dr. deMenocal at the American Geophysical Union Meeting in December of 1993, sharpening and further confirming the connection between climate change and human evolution. The new research, for example, newly identifies the second period at about 1.7 million when cold, dry cycles further intensified in Africa.
Dr. deMenocal's research for the first time links these African climate shifts--and probably the evolutionary fate of humans--with the growth of ice sheets in North America and Europe.
Anthropologists have long speculated that climate changes influenced human evolution, but Dr. deMenocal's research provides the first detailed, continuous record of African climate to document it. Because climate records from land were sparse, he studied ocean sediment cores drilled by the Ocean Drilling Program--in the Atlantic Ocean off the coasts of Sierra Leone and Liberia, in the Arabian Sea off Oman, and in the Gulf of Aden off Somalia. The sediments, accumulating over the past 8 million years, contained soil particles and tiny remnants of grasses that had been blown off Africa and out to sea by winds. Greater amounts of these materials, collectively called eolian dust, blew out to sea when Africa was drier, resulting in thicker layers in the sediments.
The ocean sediment record showed that until 2.8 million years ago, Africa had oscillating cycles of cool, dry climates and warm, wet ones. The climate changes were relatively brief and mild and would not have permanently altered the prevailing habitat of rain forests and fruit trees. But 2.8 million years ago, thicker, more abundant layers of eolian dust revealed that the cold, dry periods had become longer and more severe. Those conditions would have progressively killed off the rain forests and set the stage for grasslands to take hold. Species that could not adapt to more open and arid condition would have become extinct. Others that could more easily adapt to different habitats and food sources would have thrived.
Fossil evidence has shown that the early bipedal human ancestor, Australopithecus afarensis, often known as "Lucy," evolved sometime after 2.8 million years ago into at least two separate lineages, the genus Paranthropus and the genus, Homo. The appearance of the Homo lineage reflects an increase in brain size, while members of the Paranthropus lineage developed broader teeth for processing coarse vegetable matter. The first stone tools appeared at the same time that these two lineages evolved.
Dr. deMenocal's sediment record showed another period of colder, drier climate cycles in Africa at about 1.7 million years ago. During the same time period, the fossil record shows that Homo habilus died out and the larger-brained Homo erectus first appeared. Shortly after, tools used by human ancestors became more sophisticated.
About 1 million years ago, thicker dust layers in the sediments began to appear, indicating the cold, dry periods had become even more extreme. By that time, the genus Paranthropus had become extinct. Homo erectus became the sole survivor of the hominid line. It began to migrate from Africa and eventually evolved into modern humans, Homo sapiens.
The African climate changes documented by Dr. deMenocal coincided with the first appearance of large North American and European ice sheets about 2.8 million years ago. The ice sheets' cooling effects on high-latitude climate have been known for some time, but the Columbia scientist showed for the first time that they had a great impact on the climate in low latitudes as well.
Dr. deMenocal used the NASA/Goddard Institute for Space Studies' global circulation model to determine how growing high-latitude glaciers might have affected African climate. The computer model is based on physical laws that simulate the complex interplay of winds, temperatures, ice sheets and other factors that affect climate.
The models showed that expanding North American ice sheets would have chilled the Atlantic Ocean, sending cold, dry air blowing across Africa. As the ice sheets grew, cold and dry periods in Africa intensified.
Dr. deMenocal's research was supported by the Ocean Sciences Division of the National Science Foundation.