London: The most primitive animals may have thrived in water that contained almost zero or little oxygen, a new study suggests, challenging a long held theory about the evolution of life on Earth.
One of science's strongest dogmas is that complex life on Earth could only evolve when oxygen levels in the atmosphere rose close to modern levels.
But now studies of a small sea sponge fished out of a Danish fjord - the westernmost inlet of the Baltic Sea - shows that complex life does not need high levels of oxygen in order to live and grow.
The origin of complex life is one of science's greatest mysteries. Researchers wondered how the first small primitive cells evolve into the diversity of advanced life forms that exists on Earth today.
The explanation in all textbooks is: Oxygen. Complex life evolved because the atmospheric levels of oxygen began to rise about 630-635 million years ago, researchers said.
However, new studies of a common sea sponge from Denmark shows that this explanation needs to be reconsidered. The sponge studies show that animals can live and grow even with very limited oxygen supplies.
In fact animals can live and grow when the atmosphere contains only 0.5 per cent of the oxygen levels in today's atmosphere.
"Our studies suggest that the origin of animals was not prevented by low oxygen levels," said Daniel Mills, PhD at the Nordic Centre for Earth Evolution at the University of Southern Denmark.
"But nobody has ever tested how much oxygen animals need? At least not to my knowledge. Therefore we decided to find out", said Mr Mills who worked with lead author Lewis M Ward from the California Institute of Technology.
The living animals that most closely resemble the first animals on Earth are sea sponges.
"When we placed the sponges in our lab, they continued to breathe and grow even when the oxygen levels reached 0.5 per cent of present day atmospheric levels," said Mr Mills.
This is lower than the oxygen levels we thought were necessary for animal life, researchers said.
The research was published in the journal PNAS (Proceedings of the National Academy of Sciences).