New York: Scientists claim to have uncovered evidence that the evolution of animal life on Earth was delayed by almost two billion years.

According to them, the delay was due to a deficiency of oxygen and the heavy metal molybdenum, a key micronutrient for life that serves as a proxy for oceanic and atmospheric oxygen.

"Following the initial rise of oxygen in the Earth's atmosphere 2.4 billion years ago, oxygen was transferred to the surface ocean to support oxygen-demanding microorganims, they said.

"Yet the diversity of these single-celled life forms remained low, and their multicellular ancestors, the animals, did not appear until about 600 million years ago," said the study's lead author Prof Timothy Lyons of California University.

The researchers reached the conclusion by tracking molybdenum in black shales, which are a kind of sedimentary rock rich in organic matter and usually found in the deep ocean.

When they estimated how much of the metal had been dissolved in the seawater in which the sediments formed, the researchers found significant, firsthand evidence for a molybdenum-depleted ocean relative to the high levels measured in modern, oxygen-rich seawater.

Molybdenum is of particular interest as it is used by some bacteria to convert the element nitrogen from a gas in the atmosphere to a form useful for living things -- a process known as "nitrogen fixation."
Source : PTI