LONDON, Oct 31: There's been much debate about why animals took so long to evolve and thrive on Earth.
Now scientists say it was due to incredibly low levels of oxygen on Earth more than a billion years ago.
A team determined the chemical composition of ancient rocks to find there was about 0.1% of the oxygen levels present compared with today.
The researchers present their work in Science journal.
Why complex life took so long to appear on the scene has puzzled scientists for many years. It was only over 500 million years ago that life on Earth began to flourish in a period known as the Cambrian explosion.
For the one billion years before that, in an era called the mid-Proterozoic, though life was present it consisted of very simple organisms.
These creatures were able to survive on very low levels of oxygen, but more complex life could not.
The idea that oxygen was far too low for animals to evolve before this period had been around for a long time, explained one of the lead authors of the study, Noah Planavsky of Yale University's Department of Geology and Geophysics.
"Our research now shows empirical evidence for a surface oxygen level that would have inhibited animal evolution. ?BBC Online