Climate change gave man’s ancestors bigger brains, according to a new study.
Researchers analyzing the impact of climate change on early humans say that it likely led to “dramatic” increases in brain size and intelligence nearly a million years ago.
They found that, during severe glacial phases, ‘positive assortative mating’ (PAM) – the tendency for animals to choose mates more similar to themselves – increased in the ancestors of homo-sapiens due to the rising importance of commodities like fire, food and shelter.
Using basic assumptions and simulations based on what is already known about ancient hominins – members of the zoological tribe hominini of which humans are the last surviving species – the American research team concluded that changes in their climates led to successive growths in the size of the brain.
The researchers, from Washington University in St Louis, sought to explore how climate change may have led to an increase in hominin brain size.
The paper focused on hominin evolution between 700,000 years and 300,000 years ago — a period of sharp increases in both absolute and relative brain size.
The period also experienced significant behavioral developments in areas such as language capacity.
Study lead author Professor Bruce Petersen said: “It has long been argued that climate change was an important driver of hominin evolution, with considerable attention given to glacial phases.
“For example, William H Calvin emphasizes that ice ages likely greatly sped up gains in brain size and intelligence.
“Sexual selection is a key component of the model. In The Descent of Man, and Selection in Relation to Sex, Charles Darwin placed great emphasis on sexual selection for hominin evolution.
“However, its role as an evolutionary force was then largely ignored for over a century.
“This paper considers how climate change, together with assortative mating and parental cooperation, could have sped up the evolution of complex cognition in Middle Pleistocene humans.”
The researchers explained that there existed a set of ‘home-produced family public goods’ which were demanded to produce and included fire, shelter, conversation and child training.
The more severe the glacial phase hominins lived in, the more critical these goods became in preventing cold-related deaths caused by hypothermia.
The research team considered three types of hominin: a first which is the most intelligent but weakest, a second ‘intermediate’ set and a third which is the strongest but least intelligent.
During the inter-glacial period – when the value of public goods is relatively low – the study team argued the first type of hominin would have been the least desirable mates.
However, with a sufficiently deteriorating climate, public goods become more and more valuable and would have been best achieved by pairing with the intelligent, first type of hominin.
Simulations concluded that positive assortative mating of type-one pairs would not only produce offspring with the highest fitness but may also be the only pairing with high enough child production to survive a severe glacial period.
Petersen explained that these pairings of more intelligent hominins, for necessary survival during harsh glacial periods, may have contributed to an acceleration in language and the control of fire.
He said: “The model indicates that periods of severe climate change, beginning with MIS 16, would have led to positive assortative mating.
“This means that mates are less specialized, in part because complementarities arise only when mates work together.
“An efficient mating system surely became ever more important with the lengthening of offspring dependency and the start of the severe glacial phases.
“Second, many scientists have argued that the enormous advantages of both language and fire would have placed strong selective pressures on these behaviors.
“The present paper provides a model of how a selection process, operating through assortative mating, may have accelerated the acquisition of language and the control of fire.”
“Finally, this paper suggests that the economics of the family, particularly the focus on assortative mating, can be useful for future research on the evolution of sexual dimorphism in Homo.”
Petersen added: “A prediction of this paper’s model is that the decline in body size dimorphism in Homo may have continued well into the time period of Homo heidelbergensis – an extinct species of human known from fossils dating from 600,000 to 200,000 years ago in Africa, Europe, and possibly Asia.
“To conclude, climate change is a leading explanation for the evolution of hominin intelligence, yet there are no formal models of how this might work.
“In addition, sexual selection, while heavily emphasized by Charles Darwin, has been given little attention in the literature on the evolution of human intelligence.
“This paper argues that sexual selection and parental cooperation, in conjunction with severe glacial phases, helped drive hominin intelligence in the Middle Pleistocene.
“The paper applies core economic principles, rarely used to explain human evolution prior to Homo sapiens.
“The model shows that negative assortative mating can arise during non-glacial phases because of gains from specialization.
“Sufficiently severe glacial phases, however, cause family public goods – and therefore intelligence – to become valuable enough to drive sexual selection from negative to positive assortative mating.
“Once this switch occurs, the incentives for positive assortative mating, and the reproductive advantage of the most intelligent pairings, become ever greater the more severe the glacial phase.
“Evidence on speciation and stasis and increases in brain size is consistent with key testable predictions of the mode.”