Changing climate, changing behaviour

We could say that climate is the average state of the weather; unlike weather it is fairly stable. It is by no means a modern thing - changes in global climate are natural and expected and have continued throughout the entire history of the Earth.

Climate change refers to the cyclical variations over time of the Earth’s atmosphere as well as its land surface, oceans and polar caps. Changes are measured over time scales ranging from seasons to millions of years. Indicators such as ice cores and tree rings reveal that the earth’s climate has in fact gone through dramatic hot and cold periods every 1,500 years or so. Changes have primarily been the result of events linked to the Earth’s geology, atmosphere, solar intensity and slight variations in its orbit.

At the most basic level, when climate changes significantly species migrate to landscapes that they have been adapted for. Of those surviving members of populations left behind many may become isolated enough to prompt some evolutionary changes. William Calvin, in his book A Brain for all Seasons states that ‘many times in the lives of our ancestors, the climate abruptly cooled, just within several years. Worse, there was much less rainfall in many places, together with high winds and severe dust storms. Many forests dried up in the ensuing decade. Animal populations crashed – and likely early human populations as well. Lightning strikes surely ignited giant forest fires, denuding large areas even in the tropics.’ [2002, Chicago, p.156].

Calvin goes on to say that had the climate changed gradually, the upright walking two legged hominids would have adapted quite well. But with abrupt climate change surviving the transition posed serious challenges for survival.

He looks at nutrition as a case in point. Since grasses are the first to grow after fires, this means that certain species would have had to learn to adapt to eating grasses. Before agriculture it would also mean that primates would have to learn to eat the animals that turned the grass into muscle. The act of catching a prey in turn requires specific sensory-motor skills, and in the case of catching a big animal, cooperation. Even the act of sharing the prey required cooperative behaviour for that period at least.

There were thus environmentally induced selection pressures on each species to develop strategies for survival. The strategy skills in turn could only become possible with certain modifications to the brain that had, of course, have the right genetic programming. Calvin thus argues that dramatic climate change is intricately tied up with evolutionary leaps in hominid – and other species – biologies.

Fossil records provide clues to evolutionary changes from one set of species to the next. These changes or ‘faunal turnovers’ occur regularly throughout the earth’s history albeit over very large time-scales. Similarly mammalian fossils provide evidence of how they were affected by climate change and evolutionary factors.

There is considerable evidence that an environmentally induced catastrophic event brought our species close to extinction about 70,000 years ago. Based on the results of a genetic survey, the palaeontologist Maeve Leaky is reported as having said that ‘who would have thought that as recently as 70,000 years ago extremes of climate had reduced our population to such small numbers that we were on the very edge of extinction’.

Scientists call such circumstances a genetic bottleneck caused by catastrophic climate change. Leakey writes that that early humans before the stone age suddenly started to separate and form smaller groups due to severe and unforgiving droughts that took place in Africa 100,000 years ago. By random chance, the people who survived happened to have the machinery that had us grow from a mere 2,000 individuals to 6.6 billion today.

The idea of a genetic bottleneck might explain why evidence of human art pops up at about 72,000 or so years ago, at the Blombos Cave site near Cape Agulhas. Art is about imagination and dreams, and that part of the brain principally responsible for abstract and metaphorical thinking had favourable developmental genetics conspire with a fish-based diet to witness unusual growth of the cerebral cortex, or least that is the theory.

Historically, climate acted on us. It shaped and reshaped the ecologies in which certain species and not others thrived. Still true today, civilisation has returned the compliment, affecting climate itself and with a vengeance.