Out of Africa
For the human journey to really get into its stride, our species had to leave the warm embrace of mother Africa. Researchers identify the Bab-el-Mandeb Strait on the Red Sea as the most likely departure point. This narrow stretch of water between the Horn of Africa and the Arabian Peninsula offered the shortest route to new continents. The strait would actually have been even less of a stretch than it is today (12 miles), because when Homo sapiens made the crossing some 60,000 to 70,000 years ago, sea levels were 230 feet lower due to the onset of ice age conditions that locked water up in vast polar ice caps. Given some kind of raft, and perhaps a few islands to hop between, such a crossing isn’t difficult to imagine.
There were probably earlier attempts. Modern human remains have been found at sites in the Middle East that are in excess of 100,000 years old. Yet these trailblazers likely left little or no genetic trace on humans living today, suggesting that either climate change forced them to double back or they died out.
Studies mapping human genetic diversity support the theory that modern humans emerged in Africa, and identify the Middle East as their gateway to the wider world. The so-called “multiregional theory,” which envisions Homo sapiens interbreeding with archaic human species already living outside Africa, is challenged by the finding that genetic variation in today’s populations decreases with increased distance from Africa. The Middle East has a unique mixture of African, Asian, and European DNA markers, which indicates the ancestors of all non-Africans passed that way.
The Horn of Africa also offers clues to how our species might have spread swiftly along the coasts of Arabia, India, Southeast Asia, and all the way to Australia. Sites with garbage dumps filled with clam and oyster shells reveal that local inhabitants were familiar with coastal living and exploiting the sea long before any Red Sea crossings.
Archaeological evidence for the remarkably quick passage of modern humans to Australia, perhaps just a few millennia after leaving Africa, is backed by genetic analysis which ties Australian Aborigines to that first migration wave.
They must have improved their seafaring skills on the way, because getting from Asia to the continental landmass of which Australia was then a part would have meant navigating across a series of straits. Australia shared its prehistoric continent with present-day New Guinea, explaining why, genetically, at least, the island’s indigenous population shares genetic markers with Australian Aborigines. The two landmasses lost touch due to rising sea levels only about 8,000 years ago.
That modern humans should reach down under before setting foot in Europe seems remarkable given Europe’s closeness to the cradle of humanity. But whereas this glacial period enabled the first Australians to walk most of the way without getting wet feet, its impact in Europe was much less welcoming. Europe’s earliest occupiers didn’t show up until about 40,000 years ago.
Genetic evidence points to Europeans originating from a second migration wave from Africa that took a circuitous path via the Middle East into the steppes of Central Asia before swinging west. The challenges faced by these frostbitten pioneers are illustrated by the start-stop colonization of Britain.
Britain’s first settlers were soon evicted by northern Europe’s fluctuating chills some 25,000 years ago. Evidence of more permanent occupancy isn’t found until around 12,000 years ago, when the retreating ice sheet and warmer conditions tempted back tribes from refuges in continental Europe, one in the southwestern and one in the southeastern part of the continent. Sea levels remained low enough for these hunter-gatherers to make the journey by land, lured by herds of reindeer and wild horse that had already made the crossing. Today, genetic patterns in European populations still retain traces of the time their ancestors overwintered the last ice age in the southern refugia.