Sunday, 9 March 2014

Life's Great Leap Forward

Oxygen in its liquid form is a beautiful blue colour.
The history of life, for billions of years, has been intimately connected with changes in oxygen levels. The appearance of the gas in the atmosphere caused the first great extinction event the planet experienced, the so-called oxygen holocaust.

A rise in oxygen levels around 500 million years ago resulted in the greatest proliferation of species ever - the Cambrian Explosion. A drop in oxygen levels can also kill. Indeed some of the greatest extinction events in natural history have coincided with periods of extreme anoxia.

The mechanisms underpinning oxygen and these tumultuous changes in the biological world are well understood. A rise in oxygen levels has long been associated with the appearance of the first animals in the fossil record.
Cyanobacteria are common in the fossil record.

A reasonably high concentration of the gas is needed for cells to manufacture collagen, the protein which allows for the creation of tissues and organs. This explains why animals evolved 800 millions years ago. What was less clear was why the rise in Oxygen levels occurred in the first place?

Cyanobacteria would have been the only viable source of oxygen, yet what could have caused their proliferation on a scale capable of altering the chemistry of the atmosphere?

A recent study suggests that it was the ability to directly process nitrogen from the atmosphere which facilitated this great leap forward. Researchers, led by Dr Patricia Sanchez-Baracaldo from the University of Bristol, used genomic data to show that the genes which allow some species of cyanobacteria to directly combine atmospheric nitrogen into organic molecules, which can be used to form amino acids, evolved 800 million years ago.

A cyanobacterial bloom in Lake Eyrie
The group as a whole evolved at least three billion years ago. Their primary metabolic pathway used water and carbon dioxide to manufacture sugars. The atmosphere three billion years ago contained an abundance of these gases.

The limit on cyanobacterial growth therefore was not the reagent supply to synthesise a food source. Instead it was the nitrogen needed in protein synthesis, in the form of nitrates or ammonia. The evolution of nitrogen-fixing genes 800 million years ago removed this constraint on growth, allowing these photosynthetic micro-organisms to proliferate and inject vast quantities of oxygen into the atmosphere, resulting in the increase in the level of the gas.

This increase would result in complex organisms bound together by collagen proteins. 'The timing of the spread in nitrogen fixers in the open ocean occurs just prior to global glaciations and the appearance of animals,' said Professor Andy Ridgewell, co author of the study. 'Although further work is required, these evolutionary changes may well have been related to, and perhaps provided a trigger for, the occurrence of extreme glaciation around this time as carbon was now being buried in the sediments on a much larger scale.'

An artist's impression of the Snowball Earth glaciation 800 million years ago
Carbon burial would have depleted carbon dioxide in the atmosphere, lowering the global temperature and resulting in the so-called Snowball Earth glaciation 800 million years ago. 'It's very exciting to have been able to use state of the art genetic techniques to help solve an age-old mystery concerning one of the most important and pivotal moments in the evolution of life on Earth,' concluded Dr Sanchez-Baracaldo.

The Cambrian Explosion is one of the most studied events in evolutionary history. Yet what came just before it is often overlooked. This new study explains the tumultuous series of events which occurred at the very end of the Precambrian and links together a rise in oxygen, the Snowball Earth and the origin of the animals.