A whiff of oxygen enough to survive and grow!

Believe it! Even complex form of life can grow with very limited oxygen supplies.

A study of a small sea sponge fished out of a Danish fjord, a long, narrow inlet with steep sides or cliffs, shows that animals need extremely little oxygen to live.

In fact, animals can live and grow when the atmosphere contains only 0.5 percent of the oxygen levels in today's atmosphere.

"The findings suggest that the origin of animals was not prevented by low oxygen levels," said Daniel Mills from the Nordic Centre for Earth Evolution at the University of Southern Denmark.

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," added Mills.

This is lower than the oxygen levels we thought were necessary for animal life.

The big question now is: If low oxygen levels did not prevent animals from evolving - then what did?

There must 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, said the study.

Perhaps the ancient earth lacked animals because complex, many-celled bodies are simply hard to evolve, said the research paper published in the journal Proceedings of the National Academy of Sciences.