For generations, scientists have believed Africa was the cradle of mankind.

Now a stunning archaeological discovery suggests our primitive ancestors left Africa to explore the world around 800,000 years earlier than was previously thought before returning to their home continent.

It was there – hundreds of thousands of years later – that they evolved into modern humans and embarked on a second mass migration, researchers say.

Archaeologists have unearthed six ancient skeletons dating back 1.8 million years in the hills of Georgia which threaten to overturn the theory of human evolution.

The Georgian bones – which include incredibly well preserved skulls and teeth – are the earliest humans ever found outside Africa.

The remains belong to a race of short early humans with small primitive brains who walked and ran like modern people.

They were found alongside stone tools, animal remains and plants – suggesting that they hunted and butchered meat.

Prof David Lordkipanidze, the direct of the Georgian National Museum, said: ‘Before our findings, the prevailing view was that humans came out of Africa almost 1 million years ago, that they already had sophisticated stone tools, and that their body anatomy was quite advanced in terms of brain capacity and limb proportions. But what we are finding is quite different’.

He said Africa was still the unchallenged cradle of mankind. But he added: ‘Georgia may have been the cradle of the first Europeans.’

Their discovery muddies the already complicated history of mankind.

Archaeologists believe that the first true humans – a race of squat people called Homo habilis – evolved in Africa around 2.5 million years ago. The were followed by a taller athletic species called Homo erectus who migrated out of Africa to colonise Europe and Asia.

Outside Africa their descendents are thought to have died out. But in Africa, they turned into modern man who began a second wave of migration around 120,000 years ago.

The new finds suggest Homo erectus left Africa far earlier than was previously estimated and lived for a while in Eurasia.

The new ancestors – found in Dmanisi – were around 150cm tall, and had brains half the size of modern people’s.

‘While the Dmanisi people were almost modern in their body proportions, and were highly efficient walkers and runners, their arms moved in a different way and their brains were tiny compared to ours,’ he told the British Science Festival at Surrey University.

‘Their brain capacity is about 600 cubic centimetres. The prevailing view before this discovery was that the humans who first left Africa had a brain size of about 1,000 cubic centimetres.

‘Nevertheless they were sophisticated tool makers with high social and cognitive skills.’

The first Dmanisi fossils were found in 2001. The most recent has only just been unearthed and its details have yet to be published in a scientific journal.

Prof Lordkipanidze said the Dmanisi bones may have belonged to an early Homo erectus which lived in Georgia before moving on to the rest of Europe.

Or the early humans may then have returned to Africa, eventually giving rise to our own species, Homoe sapiens, he said.

‘The question is whether Homo erectus orginated in Africa or Eurasia, and if in Eurasia, did we have vice-versa migrations? This idea looked very stupid a few years ago, but not today,’ he told the British Science Festival.