Humans may have conquered the world, but not without a big helping hand from climate change. A major study of the last 120,000 years of history reminds us that, while we are adaptable, our species is ultimately at the mercy of the climate. Homo sapiens evolved in Africa around 200,000 years ago, but only left the continent about 70,000 years ago. After that our species rapidly went global, colonising first Europe and Asia, and then Australasia and the Americas.
But why did early humans linger so long in Africa, and what spurred them to finally move? Several theories have been proposed, but according to a large effort to reconstruct the last 120,000 years of human history – including the climate we lived in and the vegetation we fed on – the current population spread around the planet would not be as it is without key changes in the climate.
The new climate model revealed that climate changes probably had a key role in lifting four major roadblocks to humanity's global takeover. The first and most important roadblock was the Arabian peninsula, an impassable desert that trapped humans in Africa for tens of thousands of years. Then, 70,000 years ago it began receiving more rain. The coastal areas became more fertile, allowing humans out of Africa. One group expanded east into Asia, spreading south-east into Indonesia. There, they hit a second roadblock: high sea levels meant that wide stretches of open water separated the many islands. Manica assumed that crossings of 100 kilometres were a bridge too far, leaving pioneers no way to reach Australia. That meant people could only go further once sea levels fell, exposing more patches of low-lying land and making for shorter sea journeys. The waters fell 60,000 years ago and then again 15,000 years later, as successive glaciations trapped more of the world's water at the poles.
Further north, humans reached Siberia by 30,000 years ago, where they were met by a vast ice sheet which prevented them from entering North America – the third roadblock. Not until 15,000 years ago did it shrink, allowing them into the Americas. Once in, they spread rapidly. Back in Europe and Asia, populations faced one last roadblock: their local ice sheets. During warm periods humans went north into Scandinavia and northern Asia, but they were forced south when the ice advanced again.
Via Dr. Stefan Gruenwald