Plants initiated evolutionary drama of Earth’s oxygenation

WASHINGTON - An international team of scientists has taken a major step toward unlocking the secrets of oxygenation of the Earth’s oceans and atmosphere.

The new research indicated that the appearance of large predatory fish as well as vascular plants approximately 400 million years ago coincided with an increase in oxygen, to levels comparable to those we experience today.

If so, then animals from before that time appeared and evolved under markedly lower oxygen conditions than previously thought.

The method can be used to estimate global oxygen levels in ancient oceans from the chemical composition of ancient seafloor sediments.

“There has been a lot of speculation over the years about whether or not oxygen in the atmosphere was steady or variable over the last 500 million years,” explained Ariel Anbar, who led ASU’s Astrobiology Program.

“These new findings not only suggest that oxygen levels varied, but also that the variation had direct consequences for the evolution of complex life,” said Anbar.

The study involved measuring the relative amounts of different isotopes of the element molybdenum in rocks called “black shales”.

These rocks are formed from ancient ocean sediments.

The data Dahl obtained at ASU revealed that there were at least two stages of oxygenation during the Phanerozoic, separated by the oxygenation event 400 million years ago.

This inference from molybdenum isotopes is corroborated by the appearance of large (up to 30 feet long) predatory fish in the fossil record 400 million years ago, coincident with the rise in oxygen.

Anbar pointed to the fact that vascular plants also appear in the fossil record around 400 million years ago. The bodies of such plants decompose with difficulty, making it easier for organic carbon to be buried in sediments.

When that happens, the organic carbon - produced by photosynthesis - is not available for reaction with oxygen. The consequence is a rise in the amount of oxygen in the environment.

The biological innovation of vascular plants led to more carbon burial, and therefore to more oxygen. Then, the rise in oxygen made it possible for larger animals to evolve.

The finding were published Proceedings of the National Academy of Sciences (PNAS). (ANI)