A team of scientists say they have found new evidence to support their theory that complex life on Earth may have begun 1.5 billion years earlier than previously thought.
A team working in Gabon say they have discovered evidence deep inside rocks that suggests environmental conditions were suitable for animal life 2.1 billion years ago.
But they said the creatures were limited to inland seas, did not spread globally and eventually became extinct.
These ideas are completely different from conventional thinking and not all scientists agree.
Most experts believe that animal life began about 635 million years ago.
The study adds to an ongoing debate over whether the previously unexplained rock formations found in Franceville, Gabon, are actually fossils.
Scientists looked at the rocks around the formations to see if they showed evidence of containing nutrients like oxygen and phosphorus that could support life.
Professor Ernest Chi Fru at Cardiff University worked with an international team of scientists.
He told BBC News that if his theory was correct, these life forms would resemble slime molds – brainless single-celled organisms that reproduce by spores.
But Professor Graham Shields at University College London, who was not involved in the research, said he had some reservations.
“I don’t dispute the idea that there were more nutrients 2.1 billion years ago, but I don’t believe that this could have led to diversification to form complex life,” he said, adding that more evidence was needed.
Professor Chi Fru said his work helped prove ideas about the process of creating life on Earth.
“We want to say, look, there are fossils here, there is 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 – this helps prove that. Ultimately, it helps us understand the origins of all of us,” he added.
The first hint that complex life might have begun earlier than previously thought came about 10 years ago with the discovery of something called the Francevillian Formation.
Professor Chi Fru and colleagues say the formation includes fossils, showing evidence of life that could “wobble” and move at will.
These findings are not accepted by all scientists.
To find further evidence for their theory, Professor Chi Fru and his team have now analyzed sediment cores drilled from rocks in Gabon.
The chemical composition of the rock shows evidence that a “laboratory” for life was created shortly before this rock appeared.
They believe the high concentrations 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 ocean, creating a “shallow, nutrient-rich inland sea”.
Professor Chi Fru said this protected environment is conducive to photosynthesis, creating significant amounts of oxygen in the water.
“This would have provided enough energy to drive the increased body size and more complex behavior observed in primitive, simple animal-like life forms like those found in fossils from this period,” he said.
But he said the isolated environment also led to the extinction of life forms because there were not enough new nutrients to maintain the food supply.
Elias Rugen, a PhD student at the Natural History Museum, who was not involved in the study, agreed with some of the findings and said it was clear that “the carbon, nitrogen, iron and phosphorus cycles in the ocean are all doing something unprecedented at this point in Earth’s history”.
“There is no guarantee that complex biological life could not have emerged and evolved 2 billion years ago,” he said, but added that more evidence is needed to support these theories.
The findings were published in Journal of Precambrian Studies.
