Investigating evolution in the birthplace of humanity
A study of fossils in Africa seems to rule out one driver of evolution in mammals – potentially including humans.
The study, co-authored by Professor Bernhart Owen from Hong Kong Baptist University, casts doubt on the theory that climate variability acted as a major driver of evolution in mammals, including our human ancestors. The research was led by researchers at the University of Arizona and published in Proceedings of the National Academy of Sciences.
Modern humans, Homo sapiens, evolved around 300,000 years ago in Africa alongside several other species of hominins that are now all extinct. But what drove this human variety, causing new species to appear while others went extinct?
There are many theories. One popular one is the variability selection hypothesis which suggests that frequent environmental changes lead to more extinction and speciation. This means a climate that changes rapidly and often would require animals – and humans – to adapt, driving evolution.
Although this makes some intuitive sense, it has been difficult to test. To do so requires two massive sets of data: how climate changed over millions of years and how many species have emerged or gone extinct.
One place these two records can coexist is in basins: landscapes defined by huge depressions that collect water, often containing lakes at their bottom. Basins can be rich in fossils and human archaeology such as caches of tools. Taking cores of the sediments from lake beds can also provide a wealth of climate records. These can include pollen, fossil algae, and deposits with distinctive geochemical clues to past environments.
But a single basin can give a skewed view. For example, says Owen, one basin in Kenya he has studied for decades shows a correlation between a shift towards a more arid climate and a change in the types of human tools present. But this could just be coincidence. Or one population of humans moving away and another moving in; there doesn’t need to be an evolutionary explanation.
More than one basin is needed then. The team used climate data from 17 sites, including some land-based settings and cores from ocean beds surrounding the African continent. Lining up all the various records gave the team a picture of climate variability and its possible impact on evolution over the past four million years.
Finding human fossils
The climate dataset was in pretty good shape. It showed two main patterns. One is a predictable variability driven by Milankovitch cycles – climate changes caused by several natural phenomena to do with the Earth’s position relative to the Sun. These include the way the Earth ‘wobbles’ on its axis and the shape of the planet’s orbit, which act over timescales of tens to hundreds of thousands of years. Then, there is a general trend of increasing climate variability across Africa, driven by changes in global ice volume and ocean temperatures.
But what about the fossil record? While many of the basin sites were chosen because they are near caches of human fossils or archaeology, there is a problem.
“The fact is, the hominin fossil record is still not very good,” says Owen. “We tend to take the first and the last appearance of a species as the speciation and extinction moments, but a single fossil can change that picture, sometimes by tens of thousands of years.
“The mammal fossil record is much more complete, giving us the data to avoid more of these kinds of gaps. Using a statistical method borrowed from present-day ecological studies, we were able to quantify the likelihood an appearance really was a speciation, and a disappearance really was an extinction.”
The future of past evolutions
The team used bovid and other large mammal fossils to track trends in evolution, comparing these to their climatic record. Significantly, their analysis showed no correlation between the two, leading the team to conclude that climate variability was not a main driver of evolution, at least on the long timescales they considered.
So is there still a place for the variability selection hypothesis, and if it was not the cause of major evolutionary changes in Africa, what was?
Owen says: “Climate and environmental variability could still be operating on shorter timescales than we have good data for. Other factors may also drive evolution. Trait changes that do lead to new species also don’t have to be driven by factors like climate variability; new traits can simply make an organism more successful in the environment just as it is. Alternatively, populations may have migrated into or out of a basin leading to only an apparent change in the fossil record that does not reflect true extinction or speciation.”
And the team’s work on large mammals doesn’t necessarily mean the same is true for humans. The findings do however provide a hypothetical framework which, as human fossil record improves, can be used to test our understanding of evolution.