A groundbreaking study from Washington University in St Louis suggests that climate change played a significant role in the evolution of humanity’s ancient ancestors by contributing to the development of larger brains. Researchers found that increases in the brain sizes of ancient hominins aligned with glacial phases over 600,000 years ago.
To demonstrate this, the research team developed a computer simulation that mathematically illustrates how new mating habits and parental cooperation, crucial for survival during the inhospitable Ice Age, could have accelerated the evolution of the human brain. The simulation suggests that hominins sought mates who were similar to themselves due to the increasing importance of necessities like fire, food, and shelter in the face of deadly cold.
These new mating habits, referred to as positive assortative mating by the researchers, may have also contributed to the advancement of critical human abilities like language-based communication and the control of fire. Greater ingenuity and cooperative parenting between individuals would have significantly helped humanity’s ancestors avoid cold-related deaths, including hypothermia.
Lead author of the study, economist Bruce Petersen, highlights the long-ignored role of climate change as a driving force in hominin evolution. Petersen used anthropological and climate data to develop the simulation and found that severe climate change periods, starting with a major glacial freeze approximately 676,000 to 621,000 years ago, would have resulted in increased selectivity in mating.
Dubbed MIS 16, this “Ice Age within an Ice Age” led to positive assortative mating, where mates were less specialized and complemented each other’s abilities. Petersen notes that an efficient mating system became increasingly important as offspring dependency lengthened and severe glacial phases began.
The simulation conducted by the research team compared three categories of early men: a group characterized as intelligent but physically weak, an “intermediate” group, and a third group that was the strongest but least intelligent. The mathematical models demonstrated that positive assortative mating within the first category produced the fittest offspring and often proved to be the only pairing with enough children for their genes to survive the harsh glacial freeze.
Furthermore, Petersen suggests that the enormous advantages of language and fire placed strong selective pressures on these behaviors. These behaviors, which he describes as “home-produced family public goods” involving fire, language, shelter, and child training, were demanding to produce but crucial for survival.
The study also proposes that survival pressures resulting from climate change would have led to physical changes, reducing body differences between the sexes. Specifically, there may have been a decline in body size dimorphism, including differences in height, weight, and strength. These changes could have persisted during the time period of Homo heidelbergensis, an extinct human species from 600,000 to 200,000 years ago.
Petersen argues that the extreme hardships of the Ice Age demonstrated the significant impact of positive natural selection, manifested through co-parenting choices, on human evolution compared to negative natural selection resulting from death and competition. He emphasizes that the application of core economic principles, rarely employed in explaining human evolution before Homo sapiens, sheds light on the role of sexual selection and parental cooperation in driving hominin intelligence during the Middle Pleistocene.
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