The earliest known skull of the first human-like ancestors has been pieced together after researchers realised it wasn’t a baboon.

A team of international experts unearthed more than 150 pieces of the Homo erectus skull in the fossil-rich Drimolen cave system in South Africa over five years.

A report in Science on Friday has outlined how the two-million-year-old fossil was pieced back together to form the cranium of a two- or three-year-old Homo erectus (Latin for upright man).

Homo erectus is considered to be the first ancestor similar in anatomy and behaviour to humans.

The team originally thought it was piecing together a baboon’s skull until it became bigger and bigger, and then the penny dropped.

Skull fragments of Homo erectus from East Africa, the Middle East and Eastern Europe. Photo: Getty

La Trobe University lead researcher Andy Herries says it is extremely rare to find a human fossil, let alone this skull.

“The rarity of this fossil, it is two, three years old, it’s extremely delicate and so the chances of finding it are extremely small,” he told AAP.

“When we pulled it out of the ground in pieces and we looked at bits of the cranial vault, it was quite thin because it is quite young, so initially we were like ‘it’s a baboon’ – we find lots of baboon at the site.

“As we started putting the pieces back together in the sort of brain case, it got bigger, and bigger and bigger, it was like ‘ah, not a baboon’. A sort of eureka moment.”

The discovery proves that Homo erectus existed 100,000 to 200,000 years earlier than previously known, Professor Herries said.

“Homo erectus is a very iconic species … this is the beginning of what it means to be human,” he said.

“It is tall like us, it has human adaptations, it makes stone tools.

“Crucially, it is the first explorer. It is the first human species to get out of Africa – not that they would have known that.”

Other research has found stone tools were in China about two million years ago, which is about the same time period as this skull, he said.

The discovery proves Homo erectus walked when two other types of humans in South Africa, Paranthropus and Australopithecus, existed.

It’s possible that Australopithecus sediba may not be a direct ancestor of Homo erectus, or humans as first thought, he said.

Researchers hope to find the rest of the skeleton.

-AAP