A prehistoric graveyard has been unearthed in the Sahara desert, revealing Stone Age people who lived there when the land was wet and green. The site is in Niger in the Tenere desert, an intensely dry quarter of the Sahara. Millennia ago it was grassland, where big game roamed and fish swam in huge lakes and rivers. Animal bones scattered all over the area were from wildlife common today in the Serengeti in Tanzania, such as elephants, giraffes and warthogs. Stone Age people known as the Kiffian hunted and fished there 10,000 years ago, and their remains reveal big, powerful people who grew up to 1.8m (6ft) tall, and who must have had a good diet to grow so big.

But 8,000 years ago the Kiffian vanished during a dry period. When the monsoons returned the rains were less plentiful and another tribe, the Tenerians, colonised the area. These were much shorter and less muscular people, who seem to have herded cattle. Then 4,500 years ago the Tenerians also disappeared, possibly in only a few decades, marking the end of human settlement in the desert.

The dramatic swing in the Sahara’s climate, from grassland to desert, was due to a change in the Earth’s orbit. As the Earth spins on its axis, it slowly wobbles in a circle like a spinning top. Thousands of years ago, that wobble strengthened the solar energy during the summer in the northern hemisphere and helped to drag monsoon rains further north. As the Earth’s spin gradually tilted back thousands of years later, the monsoons weakened, and the Sahara became drier, until eventually it turned to desert and left the Stone Age tribes in dire trouble.