The climate behind human evolution
Climate change is not new. During the last few million years, the earth has frozen and thawed and frozen again, and these severe climatic changes had a major impact on plants and animals, and also on humans. At the height of the last glacial period, around 18 000 years ago, Australia’s giant kangaroos and giant wombats went extinct; in southern Africa, the giant long-horned buffalo and the giant hartebeest followed suit.
These climatic factors are likely to have influenced how humans evolved, and a better understanding of past climate change and the environments in which humans were living, may provide clues to our own evolution.
We cannot directly study historic climates, so researchers use various proxies, such as ocean sediments, fossil pollen, tree-ring analysis and animal remains to figure out what the climate was like thousands of years ago. Bones and teeth can survive for a long time and they record biologically meaningful “clues”, particularly about the environment in which they formed. The carbon and oxygen molecules left in bones and teeth occur in different forms known as isotopes, which have different atomic weights but the same chemical properties.
Trees and shrubs photosynthesise (consume light and carbon dioxide and turn it into plant matter) in a different way to tropical grass, absorbing the carbon isotopes in varying amounts. Tropical grasses have more of the heavy isotope, while leaves from trees and shrubs as well as temperate grasses have lower carbon isotope ratios.
By studying these ratios, we can get a glimpse of what the environment was like, where and when these long-dead animals were alive.
Since prehistoric animals ate these leafy greens, they have these same ratios, with some known alteration, in their bones and teeth. And so the carbon isotope value of the animal sampled can tell us two things: whether the animal was eating grass or leaves and, as the type of grass is restricted by the temperature of the growing season, whether it was growing in summer or winter rainfall areas.
What we do not understand well enough is the effect of environmental factors — such as differences in temperature, amount of rainfall and humidity — on these isotopic ratios left in animals. This is why we are working in contemporary ecosystems at the same time as delving into the past. Luckily, many parts of South Africa are covered by national parks, and this makes it possible to collect samples of modern animals living in relatively undisturbed natural environments, and check their isotope ratios now.
By figuring out what plants the ancient animals consumed, we can reconstruct the landscape and piece together what the environment looked like.
For example, if the fossils in an area are mainly of browsing species (which only eat leaves from trees or shrubs, such as kudu or bushbuck), then we can assume that there were trees and shrubs in that area once upon a time. If fossils of mainly grazers (animals that eat only grass, such as buffalos or hartebeest) are found, then we can deduce how much grass was available to them.
Oxygen isotopes in animals also contain information about what the climate was like millions of years ago: the oxygen isotope ratio left in a fossil’s tooth enamel is linked to the animal’s source of water. Animals that get most of their water through drinking (called water-dependent animals) will have different oxygen isotope ratios to those that get most of their water from the moisture in plants (called water-independent animals). If the environment is particularly arid, there will be a large difference in the oxygen isotope ratios of animals that drink water regularly and those of animals that get their water from plants.
My research, conducted through the University of Cape Town, has found that the natural variation within a species in the same biome is higher than previously thought. Understanding natural variation within ecosystems is important: when we are looking at archaeological data sets we need to be equipped with the knowledge of what is simply natural variation in isotopes within a species under the same climatic conditions and what might be a real climatic shift.
So, while we can compare the isotopes of water-dependent and water-independent animals within an ecosystem, and link this back to the water availability in the air, we can’t yet make a definitive prediction based on meteorological factors within biomes.
Collecting more evidence will produce a clearer picture and put us in a better position to investigate past climates and how humans adapted to them. The hope is that this research will lay the groundwork to supplement the investigation into the factors that may have influenced human evolution, particularly in southern Africa.
Julie Luyt is a PhD cndidate at the University of Cape Town.