Image caption Wonderwerk Cave is in the Northern Cape and was occupied by ancient human species

Scientists say they have new evidence that our ancestors were using fire as early as a million years ago.

It takes the form of ash and bone fragments recovered from Wonderwerk Cave in South Africa.

Theteam tells the journal PNASthat the sediments suggest frequent, controlled fires were lit on the site.

The ability to use fire is regarded as a key step in human development because itgave us access to cooked foodsand new technologies.

Stone tools found at Wonderwerk Cave indicate the ancestor in question may have beenHomo erectus, a species whose existence has been documented as far back as 1.8 million years ago.

Establishing precisely when humans first acquired the ability to control fire has been very difficult.

There have been several claims that the skill was in existence even earlier than at Wonderwerk.

But they have all been challenged, with sceptics arguing the fire remains from open sites could have been the result of natural blazes ignited by lightning.

In contrast, the PNAS team, which consists of scientists based at US, Israeli, German and South African institutions, says statements about the Northern Cape cave are far more secure.

If correct, the Wonderwerk discovery would push the earliest indisputable controlled use of fire back by about 300,000 years.

In their paper, the researchers describe burnt remains of grasses, brushes, leaves and even bones in the cave's sediments some 30m back from the entrance.

This makes it far less likely that what they are viewing is material from wildfires that was simply blown into the cave by wind, they argue.

The depth of the sediments also suggests fires were lit on the same spot over and over again.