Climate and Life: A Human Retrospective.

Abstract:

A renaissance scientist, Cesare Emiliani was also interested in climate change and its influence on human origins, ancient cultures, and our future. Climate shapes life across a range of time and space scales - seasons pace the cycle of death and renewal, and the diversity of all life is bounded by latitude. Each of the “big five” mass extinctions of the Phanerozoic is linked to environmental crisis.

Has climate change also shaped us? Analytical advances, new sediment archives, and heroic international collaborations have brought new light to this question. Gone is the dated view of our ancestors emerging from some ancient dark forest to assert dominion over the grassy plains. In its place is new evidence for rapid and large orbital-scale climate cycles that shifted, stepwise after 2.8 and then again after 1.8 million years ago to establish the African savannah we know today. These climate events are coincident with clusters of hominin speciation, extinction, and behavioral innovation milestones that came to define us as human.

The African Humid Period is one of the best and oldest examples of human cultural responses to climate change. Between 15,000-5,000 years ago the Saharan desert supported grassy, wooded plains, large lakes, and clusters of human settlements due to orbital increases in monsoonal rainfall. While there is an ongoing debate whether the end of this wet phase was fast (centuries) or slow (millennia), the rich archeological record shows that this region was depopulated and, within centuries, the first settlements appear along the Nile River near 5 ka BP. Many “firsts” are associated with these predynastic cultures of the Naqada III Period including the first named kings, pyramids, and hieroglyphs, resulting in political unification and Dynastic rule along the Nile.

As these diverse lines of evidence come together, it appears as if an answer to the age-old question “How did I get here?” is no longer beyond our reach. Climate has played an important role in our past, and a current far greater challenge is to understand our future.