Contrary to previous beliefs, a study conducted on a small sea sponge found that complex life forms on Earth don't need high levels of oxygen to grow or survive.
The origin of complex life on Earth remains science's greatest mystery. Scientists are baffled about how the first small primitive cells evolved into the diversity of advanced life forms that exist on Earth today. According to most scientific theories, oxygen had a great role to play in this evolution. When the amount of this gas in the atmosphere reached levels similar to the one that exist in the modern world, complex life was formed.
However, the findings of a new study suggest otherwise. The study of a small sea sponge fished out of a Danish fjord by University of Southern Denmark researchers found that some complex life forms can thrive and survive even in the presence of very little oxygen. The researchers found that some animals can live and grow when the atmosphere contains only 0.5 per cent of the oxygen levels in today's atmosphere, according to a press release.
Researchers chose the species Halichondria panacea for the study because it closely resembles the first animals on Earth. This small sea sponge lives only a few meters from the University of Southern Denmark's Marine Biological Research Centre in Kerteminde.
"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 Daniel Mills, PhD at the Nordic Center for Earth Evolution at the University of Southern Denmark, in a statement.
Approximately 630 - 635 million years ago, the first complex life on the planet evolved in the form of animals. Prior to that, life only consisted of simple single-celled life forms. Coincidentally, the evolution of animals coincided with a sudden rise in atmosphere oxygen levels. An obvious link was created between the two occurrences and researchers conclude that the increased oxygen levels had led to the evolution of animals.
"But nobody has ever tested how much oxygen animals need -- at least not to my knowledge. Therefore we decided to find out," said Daniel Mills, PhD at the Nordic Center for Earth Evolution at the University of Southern Denmark in a statement.
The new study found that the animals were able to survive even at lower oxygen levels. The findings of this research give rise to another big scientific dilemma. If low levels of oxygen weren't preventing the evolution of complex life previously, what was? Why were there only simple single-celled life forms for a long time?
According to Mills, there may have been other ecological and evolutionary mechanisms at play.
"Maybe life remained microbial for so long because it took a while to develop the biological machinery required to construct an animal. Perhaps the ancient Earth lacked animals because complex, many-celled bodies are simply hard to evolve," he said.
A previous study conducted on the origin of oxygen in the atmosphere revealed that a prior rise in oxygen level did take place but didn't lead to the evolution of complex life on the planet.
Findings of the current study are published online in the journal PNAS.
