Over the last 150,000 years the world’s climate has switched constantly from warm interglacial to cold glacial conditions, marked by advances and retreats of ice sheets. From 75 – 60,000 years ago the climate was glacial and cold. The huge volume of water locked up in the ice caps of northern Europe and North America took the world’s sea levels to over 250 feet lower than today and made the climate very dry. Even in areas not directly affected by ice sheets, dry conditions saw forests give way to dry grassland, then to deserts.
The effect on the natural resources available to the world’s very small modern human population, at that time mainly confined to their African homeland, was devastating and put their viability as a species on a knife edge.
The low sea level around 60,000 years ago exposed vast land masses and extended coastlines around the world. A land bridge at the southern end of the Red Sea offered a route out of Africa into the Arabian Peninsula.
Migration to Australia
From the Red Sea crossing, the first migrants likely followed a southern route eastward along the coast of the Arabian Peninsula; a path that did not require adaptations to new climate or diet as it kept them in warm weather and close to the sea, a familiar source of food. They spread around the enlarged coastal regions of India and Sri Lanka, then on to the Andaman Islands and modern-day Indonesia. At this time most of the East Indies, or Maritime Southeast Asia, was one land mass, known today as the continent of Sunda.
The migrants probably continued on the coastal route southeast until they reached the straits between Sunda and Sahul, the continental land mass that was made up of present-day Australia and New Guinea (the land bridge connecting New Guinea and Australia became submerged around 8,000 years ago).
The shores of Australia extended out several hundred miles further than they do today and the low sea level between Timor and the north coast of Australia made the difference between success and failure in making the sea crossing. The present day distance of 400 miles between the two points shrank to just over 100 miles at around 250 feet below current sea levels. They would have travelled the open sea with no view of land and may have crossed in rafts made of bamboo logs. Archaeological evidence suggests that the first human settlers made it to Australia by around 50,000 years ago.
Migration to Europe
It was not the first time they had been in the Levant; archaeological evidence suggests they had settled there around 90,000 years ago. However, the rapid onset of glacial conditions 75,000 years ago caused Neanderthal populations to migrate south to reoccupy the territory.Modern human Europeans entered Eurasia by the Arabian Peninsula around 50,000 years ago, using the coastlines and rivers of the south-eastern Mediterranean (the Levant) as a gateway.
The migration patterns of the Neanderthals were also driven by the climate. Over the previous 100,000 years the European climate had swung, usually every few centuries, from glacial conditions to about as warm as at present. Having been in Europe throughout this period the Neanderthals were physically adapted to live through intense cold, but they fled the most extreme conditions for warmer refuges in the south such as Iberia, the Balkans and the Levant. The Neanderthals arriving back in the Levant 75,000 years ago had presumably seen off (either directly or indirectly) this modern human ‘advance party’.
This time round, however, modern humans out-competed the Neanderthals in the Levant. As the climate in Europe rapidly improved from 50,000 years ago, they pushed north and continued to replace (and occasionally interbreed with) the Neanderthals along the way.
By 45,000 years ago modern humans had reached the Black Sea area. They crossed a land bridge, where the Bosphorus runs now, and entered the Balkans to become the first modern humans in Europe. As they spread around the continent, bolstered by other modern human migrants from Central Asia, they further displaced the Neanderthals.
The emergence of modern humans as the sole human species on the planet was not a foregone conclusion. It may have simply come down to different methods of hunting. Neanderthals probably hunted from fixed settlements, depleting the resources of a particular area before moving on. Modern humans were more nomadic and followed game herds over greater distances from temporary settlements. One strategy is not necessarily superior to the other in terms of survival, but if the Neanderthals had still been static in their southern refuges following the intense cold period, then modern humans may have caught them ‘on the hop’ as the climate improved. More mobile modern humans may have been able to colonise a central and northern Europe that was still largely deserted by the Neanderthals, swinging the competitive balance in their favour.
Modern humans’ mobility and ability to adapt to extreme climate changes were their key to survival. Through innovations in technology and social organisation, they became expert colonisers. By 30,000 years ago, the Neanderthals were mainly confined to their few isolated southern refuges and, by 25,000 years ago, had became extinct.
By then, the Earth was starting to freeze again as it headed into the Last Glacial Maximum (LGM). Sea levels dropped by over 400 feet below current levels as ice sheets spread across the northern latitudes, covering nearly 10 million square miles of the planet. Iceland was completely covered, as was much of the area as far as the southern British Isles, Germany, Poland, the Baltic nations and deep into Russia. Ice caps covered the Alps, the Pyrenees and the Carpathian Mountains. Forest and woodland were almost non-existent, except for isolated pockets close to the mountain ranges of southern Europe. A polar desert covered the parts of northern Europe not occupied by ice sheets. In central Europe average summer temperatures were about 10°C and winter temperatures were as low as -20°C.
Modern humans survived along the fringes of the continental glaciers by wearing animal furs for warmth and sheltering in caves, then, as the Neanderthals had done before them, retreating south to milder climate refuges in the Iberian peninsula, the Balkans and the shores of the Black Sea. The Black Sea today connects to the Meditteranean, but was then a landlocked, freshwater lake; archaeologists have found man-made structures submerged under its deep waters. Similarly, the Adriatic Sea was a large, well-watered, coastal plain connecting northeastern Italy with the Balkans.
When the LGM finally started to abate around 19,000 years ago, the human populations emerged from their southern refuges to reoccupy central and northern Europe in the wake of the retreating ice sheets.
Migration to the Americas
As the earliest settlers crossed into Australia 50,000 years ago, others continued along the coast of Sunda, eventually turning northeast to China and finally reaching Japan by 35,000 years ago. At this time Japan was connected to the Asian continent through land bridges linking to the Korean peninsula and to the Siberian mainland.
The eastern coast of Asia was quite hospitable to humans with watercraft and a maritime adaptation, but their progress northward stalled as they adjusted to the colder climate of the LGM. The coastlines of the North Pacific finally warmed and deglaciated after about 16,500 years ago, enabling migration by sea from Northeast Asia into the Americas.
As well as journeying to the Americas by navigating the coastlines, a land route also became available to them. The Beringia land bridge, exposed by the lower sea levels, ran for around 1,000 miles north to south at its greatest extent and joined present-day eastern Siberia and Alaska, stretching for several hundred miles into both continents. It was a grassland steppe and was not glaciated because snowfall was extremely light due to weather systems from the Pacific Ocean.
A small population of humans, Paleo-Indians, survived the LGM in Beringia, isolated from their ancestor populations in Asia for at least 5,000 years, before expanding to populate the Americas after the ice sheets blocking the way southward melted and parted. The Paleo-Indian moved south from Alaska through the ice-free corridor between modern British Columbia and Alberta into North America, and then onwards to Central and South America.
The melting ice sheets also allowed maritime-adapted Paleo-Indians to move south down the coastal islands from Alaska by watercraft, settling as the ice receded, then moving up rivers to the interior. Further south, California’s Channel Islands have produced evidence for seafaring by Paleo-Indians.
Under the Sea
Whatever triggered the idea of migration in the minds of those few hundred individuals in Africa 60,000 years ago, and it was probably nothing more than a survival strategy to seek out new food sources, the climatic conditions were just right to make the move. The world was just starting to become warmer and the low sea levels presented a rare window of opportunity to ‘go global’ that had only last been available 80,000 years before, at the dawn of our species. They weren’t the first to do it, but the descendants of this group of migrants where the first successful colonisers of Asia, Australia, Europe and the Americas.
While the LGM peaked around 22,000 years ago, glacial melting did not effectively cease until around 6,000 years ago. By this time, our present mild interglacial climate had stabilised, humans had moved from hunting and gathering to agriculture and settlement, and the first civilizations were starting to flourish. The extended coastlines, land bridges and fertile, low-lying plains which had supported human migration had become submerged by the rising sea – taking most of the evidence of their journey with it.