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How Climate Made Us Human

14 Nov 2016, 7:09 am

Author: Michon Scott,

Rick Potts is the director of the Smithsonian Institution’s Human Origins Program. He has researched human origins since the 1980s, and oversees the National Museum of Natural History’s What Does It Mean To Be Human? exhibition, which opened in 2010.

Potts developed the variability selection hypothesis of human origins, linking key human traits to a process of adaptation to climate variability and uncertainty. interviewed Dr. Potts by telephone on September 29, 2016. Here are his lightly edited responses.

C rick-potts-core-620Dr. Rick Potts examines sediment cores. Image courtesy Smithsonian Institution What is variability selection?

Potts: Variability selection is a form of natural selection that explains adaptation as a response to dramatically increased variability in the environment. When climate and other aspects of the environment vary dramatically, it can really affect the survival and success of an organism and its offspring over time. The effects can be evident in the gene pool and adaptations of an organism over time. Ultimately, organisms that can cope with widely varying conditions have a better chance of surviving novel and unpredictable environments. How does variability selection explain human evolution better than other hypotheses?

Potts: For many years, the tradition among paleoanthropologists was to try to find the selective environment that drove human evolution, key traits such as walking upright, tool use, larger brains, language, complex innovations.

For a long time, the favored explanation was the savanna hypothesis: the drying out of Africa meant that early humans found themselves in arid grassland, and generation after generation there was pressure to adapt to that drying trend.

Variability selection offers a different explanation. Over time and in different places where our ancestors lived, environments varied widely. Variability selection proposes that major features of human evolution were actually ways that our ancestors became more adaptable.

It’s a process of selection and adaptation to environmental variability, and it accounts for traits that cannot be explained by adaptation to any one environment or trend. For example, our large brains are useful for processing a wide range of information, our teeth and ability to make tools are useful for consuming a wide variety of foods, our sociability helps us team up with others when our survival is threatened.

C species-climate-milestones-620This graph shows milestones in human evolution correlated with climate variability. Milestones indicated along the bottom of the graph show hominin origins; habitual bipedality; first stone toolmaking, eating meat/marrow from large animals; onset of long-endurance mobility; onset of rapid brain enlargement; and expansion of symbolic expression, innovation, and cultural diversity. Adapted from Smithsonian Institution. How can an organism with a lifetime of 40 years “sense” or adapt to a change that is occurring over the span of thousands of years?

Potts: Individual animals do not evolve biologically. They can show a certain capacity to adjust to a changing environment. But the evolution of that capacity—the adaptability of the organism—evolves in a population over time. Sexual reproduction allows DNA to be remixed every generation, creating slightly different genetic makeups to be “tested” against the environment. If these new genomes allowed individuals greater flexibility in nutrition or behavior, they would, according to the variability section hypothesis, provide a survival advantage in new or variable environments. Even if the advantages were small, over many generations the genetic makeups that favored them would become widespread.

So, variability selection is a process where combinations of interacting genes, and the genes that enable flexible interactions with the environment, are favored. This process promotes adaptability. The idea or variability selection is that the evolution of adaptability can’t take place in an animal’s lifetime, or in a relatively stable or directionally changing environment. Rather the genetic changes that yield flexible environmental responses are built up in eras of instability in the surroundings.

C climate_brains_620This graph correlates changes in climate to changes in hominin brain size. Adapted from the Smithsonian Institution.

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