FOR the first time, we have had a detailed look at how our climate has changed throughout prehistory, thanks to a surprisingly detailed computer model. And it could shed light on how ecosystem changes shaped our evolution and intelligence.
Thanks to ice cores and other natural records, we already knew that, for the past 2.5 million years, Earth has been in an ice age, with permanent ice at both poles. The extent of this ice has often waxed and waned during this time, and we are currently in a warmer, “interglacial” period.
But this doesn’t explain why these climate changes happened or how they affected wildlife, says Mark Maslin of University College London, who wasn’t involved in the modelling work. “An ice core in Antarctica just tells you what’s happened in Antarctica,” he says. “Only by using computer models can you actually connect the dots.”
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Mario Krapp at the University of Cambridge, UK, and his colleagues have now done this, simulating global climate changes over the past 800,000 years.
They did it by using models of the past 120,000 years to develop an algorithm, which they then used to reconstruct an outline of the past 800,000 years. This was fleshed out by simulating detailed “snapshots” at intervals throughout the 800,000 years. Running a detailed model for the whole period would have taken too much computer time.
The model successfully reconstructed known changes in average global temperature, as well as the different patterns over sea and land. “They seem to be approximating what’s going on extremely well,” says Eleanor Scerri of the Max Planck Institute for the Science of Human History in Jena, Germany.
To see what the changing climate meant for life, Krapp’s team simulated variation in vegetation. They found that many habitats were unstable 90 per cent of the time. Of the stable habitats, many were inhospitable, including the deserts of northern Africa, Arabia and southern Asia (EarthArXiv, ).
This variability may have helped to drive our evolution in Africa’s habitable regions, Krapp’s team says. Homo sapiens arose in Africa perhaps 350,000 years ago, so the 800,000-year study spans our entire existence.
Fossil evidence suggests that our species arose in east Africa, and later thrived around the continent, especially in the south. Many suspect that it was the changeability of conditions in these areas that drove the evolution of our big brains, as our ancestors grappled to adapt to their varying environment.
Constant variability
Maslin says the model supports this idea. “If we look at east Africa and south Africa, there is no ecosystem stability there whatsoever,” he says. “Over 800,000 years there is constant variability and changes in those areas.”
But the simulation suggests that some of Africa was stable and hospitable. The forests of west Africa, for example, seem to have changed little over this time. While a changeable climate was important for our evolution, we don’t know what role these stable habitats may have played in our evolution, says Scerri. “We only have fossils for about 10 per cent of Africa,” she says.
“Over 800,000 years, there has been constant change in the places our ancestors evolved”
The climate patterns may also help explain why so many different hominins once existed. Our species overlapped in time with the Neanderthals in Europe and western Asia (see “Cosy up with the Neanderthals, the first humans to make a house a home”), the Denisovans in east Asia, the “hobbits” on Flores island in Indonesia and possibly also Homo naledi in southern Africa. Why didn’t these hominins interbreed extensively and merge?
The stable deserts may have proved impassable, says Scerri. “It seems very reasonable that the deserts of the Sahara, Arabia, Mongolia and Gobi probably played an important role in keeping different populations apart for long periods of time.”
The instability of so many of Earth’s ecosystems over the past 800,000 years is a worry, because it implies many will be drastically altered by modern climate change, says Maslin.
“On the majority of the land, vegetation is clearly sensitive to climate change,” he says.
