Early globetrotters: how humans first spread out of Africa, across Europe and Asia

With the Earth's continents arranged more or less as they are today, it was possible then, as now, to travel overland from Africa across the Middle East into south Asia, India and China. Could Stone Age man really have made it all that way without roads and tracks, let alone cars, boats or planes?

Unlike most of us, these people had one huge advantage: they were not in a hurry.

Homo erectus lived on average for about 30 years. Even at a supremely leisurely pace of travel, say 10 miles a year, it still would have taken them only 600 years to wander across the 6,000 miles of land from Africa to China. That's about 30 generations. The earliest Homo erectus fossils found in Africa date back 1.8 million years. People had more than enough time to make the journey. In fact, they could have walked there and back dozens of times.

Early humans walked their way around the habitable continents of Africa, Europe and Asia from the time of Homo erectus. The first evidence of humans in Britain dates from 700,000 years ago. Boxgrove Man, a human skull found in Sussex, is a Homo erectus descendant, known as Homo heidelbergensis, dating back 500,000 years. Did he build a raft or wade across to Britain when the sea was shallow, or simply walk across when there was no English Channel? Any of these is possible.

While Homo erectus was populating Asia, the climate took a turn for the worse, as the beginning of a deep Ice Age chill caused glaciers to swoop down over the continents. For Homo erectus, who had wandered so far and wide, such cold conditions were a big problem. Even the miracle of fire wasn't always enough to ensure survival in the bitterly cold temperatures that sometimes descended on much of the European and Asian continents for thousands of years at a time. Nature had its own answer, of course, in the form of Neanderthals – the most recent human species apart from you and me.

In 1856, quarry workers near Düsseldorf, north Germany, found what looked like human bones in the Neander Valley. Their discoveries first alerted naturalists to the possibility that there may have been several species of humans that lived before our own Homo sapiens, and that man was almost certainly descended from apes. (What they didn't know then was that, genetically, humans were actually a branch of the ape family itself.)

Since then, bones from many Neanderthal sites have been found. The oldest date back about 350,000 years, which means that several species of humans must have lived simultaneously for a very long time – at least until about 70,000 years ago, when the Homo erectus line disappeared, probably as a result of climate change and the arrival of a yet more powerful species. Experts believe that during this period at least five different species of humans were living on the planet, including: Homo erectus, Homo ergaster (the African form of erectus), Homo neanderthalensis, Homo heidelbergensis and Homo rhodesiensis. There may have been more, and scientists are still unsure if some of these are subspecies rather than separate species.

Did they fight? Did they live together or in their own separate communities? Did they interbreed? Could they talk? There are still a lot of unanswered questions. What does seem clear is that once Homo erectus migrated out of Africa from about 1.7 million years ago, several different species of humans evolved in separate parts of the world, and that geographical and climatic differences caused small but significant evolutionary changes. Chances are that these species didn't mix much, because back then there were very few humans around – maybe a million or so spread out widely across the Europe and Asia, continents which hold more than 4 billion people today.