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, say they have discovered evidence deep within rocks that show 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 died out.
The ideas are a big departure from conventional thinking, and not all scientists agree.
Most experts believe that animal life began about 635 million years ago.
The research adds to an ongoing debate over whether the hitherto unexplained formations found in Franceville, Gabon are actually fossils or not.
Scientists looked at the rock around the formations to see if they showed evidence of containing nutrients such as 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 mold – a single-celled, brainless organism that reproduces by 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 validate ideas about the processes that create life on Earth.
“We’re saying, look, here are fossils, there’s oxygen, it stimulated the emergence of the first complex living organisms,” he said.
“We see the same process as in the Cambrian period, 635 million years ago – it helps us finally understand where we all 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.
Prof Chi Fru and his colleagues said the formation was composed of fossils which showed evidence of life that could “rock” and move on its own.
The findings were not accepted by all scientists.
To find more evidence for their theories, Prof Chi Fru and his team have now analyzed sediment cores drilled from rock in Gabon.
The chemistry of the rock showed evidence that a “laboratory” for life was created shortly before the formation appeared.
They believe the high levels of oxygen and phosphorus were created by two continental plates colliding underwater, creating volcanic activity.
The collision cut off some of the water from the oceans, creating a “shallow nutrient-rich marine inland sea.”
Prof 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 drive the increased body size and more complex behaviors seen in primitive, simple animal life forms such as those found in fossils from this period,” he said.
But he says the isolated environment also led to the extinction of life forms because there weren’t enough new nutrients to sustain a food supply.
PhD student Elias Rugen at the Natural History Museum, who was not involved in the research, agreed with some of the findings, saying it was clear that “the oceanic cycles of carbon, nitrogen, iron and phosphorus were all doing something somewhat unprecedented at this point in Earth’s history.”
“There is nothing to say that complex biological life could not have emerged and flourished up to 2 billion years ago,” he said, but added that more evidence was needed to support the theories.
The findings are published in scientific journal Precambrian Research.