A group of scientists say they have found new evidence to back up their theory that complex life on Earth may have begun 1.5 billion years earlier than thought.

The team, working in Gabon, say they discovered evidence deep within rocks showing environmental conditions for animal life 2.1 billion years ago.

But they say the organisms were restricted to an inland sea, did not spread globally and eventually died out.

The ideas are a big departure from conventional thinking and not all scientists agree.

Most experts believe animal life began around 635 million years ago.

The research adds to an ongoing debate over whether so-far unexplained formations found in Franceville, Gabon are actually fossils or not.

The scientists looked at the rock around the formations to see if they showed evidence of containing nutrients like oxygen and phosphorus that could have supported life.

Professor Ernest Chi Fru at Cardiff University worked with an international team of scientists.

He told BBC News that, if his theory is correct, these life forms would have been similar to slime mould - a brainless single-cell organism that reproduces with spores.

But Professor Graham Shields at University College London, who was not involved in the research, says he had some reservations.

"I'm not against the idea that there were higher nutrients 2.1 billion years ago but I'm not convinced that this could lead to diversification to form complex life," he said, suggesting more evidence was needed.

Prof Chi Fru said his work helped prove ideas about the processes that create life on Earth.

"We're saying, look, there's fossils here, there's oxygen, it's stimulated the appearance of the first complex living organisms," he said.

"We see the same process as in the Cambrian period, 635 million years ago - it helps back that up. It helps us understand ultimately where we have all come from," he added.