The researchers compared environmental data and fossil records of large mammals that lived across Africa during the last four million years. The results contradict the long-standing theory that climate shifts repeatedly prompted evolutionary changes in early humans and other mammals.
The Plio-Pleistocene is a period in Earth’s history that spans the last five million years, including the last ice age about 20,000 years ago.
Over this time period, the researchers observed a long-term trend of increasing environmental variability across Africa. However, this variation did not strongly correspond with rates of species origination or extinction, indicating that environmental variability and species turnover are not closely related.
According to study first author Professor Andrew Cohen, the idea that long-term trends toward a wetter or drier climate may have been a driver of human evolution goes back to the time of Charles Darwin. In the late 1990s, a new theory was introduced – the influential variability selection hypothesis.
“The idea here is that it’s not just the direction of climate change that was important as a driver for evolutionary novelty in the hominin lineage, but the variability in the environmental and climate conditions,” explained Professor Cohen. “As our ancestors faced rapidly shifting conditions, this hypothesis suggests they had to be more resourceful and capable of dealing with many different contingencies, which, in turn, led to new species appearing while others went extinct.”
While the study authors acknowledge that the variability selection hypothesis could still be correct but operating at different scales, they hope to encourage the scientific community to think about the variability selection hypothesis in a more critical way – “rather than just accepting it as an underlying principle of how we look at the fossil record in Africa, and especially the human fossil record,” said Professor Cohen.
“We don’t say that environmental variability is not important for human evolution, but the data we have currently compiled is very inconsistent with that idea.”
“If environmental variability was as important as it has been made out to be, we would expect to see that long-term trend of increasing variability mirrored in evolutionary turnover in all kinds of species, including hominins, but we just don’t see that.”
The study is published in the journal Proceedings of the National Academy of Sciences