The Improbable Primate: How Water Shaped Human Evolution. By Clive Finlayson. Oxford University Press; 202 pages; $27.95 and £16.99. Buy from Amazon.com, Amazon.co.uk

ACCORDING to the standard treatment in evolutionary biology, about 1.8m years ago man’s brain became larger, his gut became smaller and he started walking upright. No ape had done that before. It was an important milestone in the story of human evolution.

The ancestor in question, Homo erectus, could use simple tools and hunt. His diet was more meat-based than plant-based. Meat has more calories than food derived from plants. Humans had transformed themselves from tree-climbing apes that needed to spend a lot of time searching for food to upright, meat-consuming hunters that could roam large distances. So successful was this transformation, evolutionarily speaking, that in due course the descendants of Homo erectus, modern-day Homo sapiens, had no problems colonising the far reaches of the globe.

A few years ago Richard Wrangham, a British primatologist at Harvard University, challenged this accepted wisdom by arguing that learning to cook had made apes human. People cannot easily digest raw meat, he said. Cooking food increases its nutritional value. Mr Wrangham showed that Homo erectus learned to cook with fire about 1.8m years ago. This development conferred evolutionary benefits that ultimately led to the dominance of Homo sapiens today.

In a new book, Clive Finlayson, a zoologist and palaeontologist, who is the director of the Gibraltar Museum, offers another view of 7m years of human evolution. Instead of food, he focuses on water, advancing the theory that the spread of Homo sapiens across the globe was driven largely by changes in climate and access to fresh water. Man’s early ancestors made the move from the tropical rainforest to open spaces tentatively at first, then with increasing boldness. These creatures stayed close to the forest and lived at the edges of lakes and rivers, returning to trees for shelter. Gradually they extended their range, taking refuge in caves when they could not find trees.

Meanwhile Earth’s climate was changing. In the Middle Pleistocene era, which started about 800,000 years ago, there were repeated warm and cold cycles, producing severe droughts and ice ages. Lush rainforests gave way to steppes and savannah, sometimes even deserts. Arid times drove man’s ancestors farther afield in search of water sources. Evolutionary pressures at such times of stress resulted in people developing longer limbs, losing weight and becoming more agile. (Those humans who were taller, lighter and faster covered more ground and had a better chance of finding food and water.) As Mr Finlayson puts it, “Homo sapiens was an evolutionary response to the scattered distribution of water in space and time…Improved terrestrial mobility was a response, first and foremost, to the need to quickly locate water sources in a drying world.”

Mr Finlayson disputes the belief that humans migrated along the coasts as they spread across the world. Modern man’s ancestors, he says, were “rain chasers”, who moved north in the northern hemisphere when the climate warmed, and then across temperate zones. When temperatures dropped, they moved towards the equator.

Among the book’s most interesting examples is how the Mardu people adapted to the Great Western Desert of Australia. Water was all-important to the Mardu. They would move to find rain, which they were able to detect across vast distances using visual cues. Once it rained, they would first collect rainwater from clay pans and pools in the desert, moving to permanent water sources only when these ephemeral pools evaporated. It is a tale of smart water husbandry—and one that people would do well to heed today. Did water make people human? Mr Finlayson certainly makes a convincing case.