Life became difficult for warm-adapted plants, animals and humans when ice age conditions increasingly spread across Europe and Asia (Eurasia) after 100,000 years ago. But, some populations survived in isolated areas, and such refuges may have helped spur on the evolution of new species, including new human species, according to scientists reporting in the journal Science today.
Scientists studied how plant and animal populations moved as their environments changed dramatically during the ice ages and created models of this movement. They then applied the same models to data known about where ancient humans lived, and how they might have evolved.
The research was led by Dr John Stewart at Bournemouth University and co-authored by Prof Chris Stringer at the Natural History Museum.
The team found that important events in human evolution, such as the first appearance of Neanderthals, may have corresponded to these patterns of climate change, and especially the appearance of refuges, or refugia.
'No one has applied this knowledge to humans before," says Stewart. 'We're thinking about humans from the perspective of what we know about other species.'
Populations move to refuges when the environment becomes too harsh, and they either survive until conditions improve, or go extinct.
The team found that when populations moved to refuges in new areas, there was a much higher chance of a new species evolving. This is because over time, important evolutionary changes take place as the isolated populations adapt to their surroundings.
The team's research showed that the appearance of new human species in Eurasia corresponded to the locations of the refuges. Stringer says, 'These ideas may well explain how new human species such as Homo antecessor and Homo neanderthalensis evolved in Eurasia'.
For example the ancestor of Neanderthals (Homo neanderthalensis) and us (Homo sapiens) was Homo heidelbergensis who lived about 600,000 years ago.
The H. heidelbergensis population that stayed in Africa evolved into us.
But another population expanded across Eurasia, living through many ice age cycles. They expanded during the interglacial stages and retreated to refuges in glacial stages, ultimately evolving into Neanderthals in the west, and another group called Denisovans in the east.
Polar bears are an example of a species that evolved in a refuge. Recent genetic studies have shown that polar bears are in fact brown bears that became isolated in a northern coastal refuge. They adapted to the arctic maritime conditions and evolved into a new species, Ursus maritimus.
They would have had a much larger distribution in cold glacial times but are currently living in refuges, which are contracting as global warming takes effect.
Refuges were also places where species became extinct – for example the last places with evidence of Neanderthals were in the refuges of southern Europe such as Iberia and the Balkans, between 35-40,000 years ago.
Recent genetic studies have shown that modern humans outside Africa interbred with other human species.
'The concept of refugia may also explain why the hypothesised interbreeding events between modern humans and Neanderthals and Denisovans occurred in the south of Eurasia rather than further north,' says Stringer.
Using the models from this research, the interbreeding could be explained by the movements of populations who had retreated to refuges in the south when climate conditions were harsh. The previously separated populations then overlapped, providing an opportunity for interbreeding to occur.