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

The team working in Gabon says it has found evidence deep in the rocks showing the environmental conditions for animal life 2.1 billion years ago.

But they say the organisms were confined to an inland sea, did not spread globally and eventually went extinct.

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

Most experts believe that animal life arose about 635 million years ago.

The research adds to the ongoing debate over whether or not the previously unexplained formations found in Franceville, Gabon, are actually fossils.

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

Professor Ernest Chi Frew of Cardiff University is working with an international team of scientists.

He told BBC News that if his theory were correct, these life forms would be similar to slime mold – a brainless single-celled organism that reproduces by spores.

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

“I’m not opposed to the idea that there were higher nutrients 2.1 billion years ago, but I’m not convinced that this could have led to the diversification to form complex life,” he said, suggesting that more is needed evidence.

Professor 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, that’s what fueled the first complex living organisms,” he said.

“We see the same process as in the Cambrian period, 635 million years ago – this helps us to confirm it. It helps us understand ultimately where we came from,” he added.

The first hint that complex life might have started earlier than previously thought came about 10 years ago with the discovery of something called the Francevillian Formation.

Professor Chi Fru and his colleagues said the formation was made up of fossils that pointed to evidence of life that could “wiggle” and move of its own accord.

The findings are not accepted by all scientists.

In order to find more evidence for their theories, Prof. Chi Fru and his team have now analyzed sediment cores drilled from the rock in Gabon.

The chemical composition of the rock showed evidence that a “laboratory” for life was created just before the formation appeared.

They believe the high levels of oxygen and phosphorus are caused by two continental plates colliding underwater, creating volcanic activity.

The collision cut off some of the water from the oceans, creating a “nutrient-rich shallow marine inland sea.”

Professor Chi Fru says that this protected environment had the conditions to allow photosynthesis, leading to significant amounts of oxygen in the water.

“This would have provided enough energy to promote increased body size and more complex behavior seen in primitive, simple animal forms such as those found in fossils from this period,” he said.

But he says the isolated environment also led to the death of life forms because there weren’t enough new nutrients to sustain the food supply.

PhD student Elias Rugen of the Natural History Museum, who was not involved in the research, agreed with some of the findings, saying it was clear that “ocean cycles of carbon, nitrogen, iron and phosphorus were doing something a little unprecedented at this point in history on the ground.”

“There is nothing to say that complex biological life could not have emerged and flourished as early as 2 billion years ago,” he said, but added that more evidence is needed to support the theories.