New ancient human tests evolution 'chain'

The discovery of a new hominid species in a South African cave suggests human ancestry is more a big, sprawling bush than a simple family tree.

That's according to one of the scientists who recently helped unearth the so-called Homo naledi from its isolated resting place.

James Cook University's Dr Eric Roberts says the remarkable discovery came after two recreational cave explorers stumbled upon the bones deep in the Rising Star system.

It's thought the partial skeletons could belong to up to 15 different bodies - a smorgasbord of new material for scientists, Dr Roberts says.

"Until now, it has been a bone here, a tooth there," he told AAP.

"What's really spectacular is that now we have lots and lots of bones."

While the examination of these relics has only just begun, Dr Roberts says Homo naledi's composition has already challenged existing notions about linear human evolution.

This stems largely from the species' "half-half" body structure - curved fingers and shoulders are closer to an ape, but human-like feet suggest Homo naledi may have walked upright.

"Instead of a simple branching family tree or just a linear chain, (human evolution) may be a big bush or a meandering river," he said, paraphrasing a recent study's observations.

But that's not the only mystery surrounding the new species.

Scientists are also stumped as to how the bodies got to their remote gravesite in the first place, given there's no evidence to suggest predators or flooding dragged them there.

Some have speculated Homo naledi - with its brain the size of an orange - deliberately disposed of the bodies in a ritualistic fashion.

But critics have slammed that theory as little more than a headline-grabbing tactic to disguise the real scientific state of play: no one knows.

For his part, Dr Roberts concedes it's a conundrum.

"It's puzzling," he said.

"We've got a lot of work to do."