Dr Stewart has studied ancient ecosystems and the evolution of humans and other organisms over the past 100,000 years, using existing knowledge of the spread of plant and animal species throughout the warming and cooling of the Ice Ages to provide insights into human origins, including the evolution and extinction of Neanderthals.
     Stewart has also examined the rise of the 'first Europeans', along with the Denisovans who occupied a realm stretching from Siberia to Indonesia, and the important role of refuges taken by species to escape harsher conditions. As a result of genetic mutations, time spent apart in refuge generally serves to splinter a once unified species. Even once an Ice Age ends and the different populations start intermingling again, they never really merge back together. This explains why Homo sapiens are still here and our archaic cousins went extinct some 30,000 years ago; our ancestors chose the right refuge.
     Stewart's claim that climate change caused the Neanderthals' demise is supported by work by Love Dalen at the Swedish Museum of Natural History in Stockholm, who has looked at the genes in 13 Neanderthal fossils found in southern Europe and western Asia. All Neanderthal fossils more than 48,000 years old, and those found in Asia, had a higher level of genetic diversity than later European fossils, suggesting the Neanderthals went through an evolutionary 'bottleneck' where a significant percentage perished. When a bottleneck occurs, the remaining individuals are often a much less diverse group, which makes it more difficult for them to evolve and adapt to a changing environment.
     Stewart is now focusing on how animal populations changed as a result of Ice Age, and evolutionary processes over the last 50,000 years, but his work is not confined to the past. A recent proposal to eradicate the Eagle Owl because it killed other birds, and was not thought to be native to the UK, was abandoned after Dr Stewart's studies revealed the bird, or something like it, has been present in Britain for up to 700,000 years.
     Stewart's research can also help predict the future. "By studying how organisms have reacted to past climate change," he explains, "we can learn lessons about what may take place due to human-caused global warming."

Edited from ScienceDaily (26 February 2013)