Early oceans had oxygen-loving life
Simple, photosynthesising life forms created an excess of oxygen in the oceans 700 million years earlier than previous estimates suggest, an international team of geologists claim.
The research, published today in the journal Nature Geosciences, pushes back the earliest appearance of photosynthesising organisms from 2.7 to 3.46 billion years ago.
Microscopic organisms such as cyanobacteria create oxygen as a by-product of photosynthesis.
The timing of their first appearance is hotly debated as it provides clues as to how early life on earth evolved.
Crystallised in time
Until now, the earliest evidence of photosynthesis was microscopic fossils found in shale rocks in Western Australia dating from 2.7 billion years ago.
Now a team of Japanese, US and Australian scientists, led by Dr Masamichi Hoashi of the Kagoshima University, Japan, have found evidence for oxygen in ancient sea water from marine sedimentary rocks in the Pilbara region of Western Australia.
The evidence comes from tiny crystals of the iron-oxide mineral haematite in a 160-metre-long core section that forms part of the Marble Bar Chert.
Haematite can form in the presence of aerobic (oxygen-loving) bacteria in the water, or by photo-electric processes in the upper 10 metres of seawater.
The researchers say haematite crystals in the Marble Bar Chert formed in water at least 200 metres deep, because microscopic analysis of the rocks show no sign of wave action or other structures characteristic of shallow-water sediments.
The orientation and nature of the grains of haematite also show that it precipitated directly from the seawater, rather than forming later from other processes, such as the movement of groundwater, they add.
"These data strongly suggest that oxygenic photoautotrophs flourished in the photic zone of the 3.46 [billion-year-old] oceans and supplied molecular oxygen to the deep water," the researchers write.
Billion year gap
Geologist Professor Malcolm Walter, from the University of New South Wales in Sydney, says the research represents a "significant development", but presents a dilemma as previous research suggests a much later date for the evolution of photosynthesis.
Evidence from uranium deposits and iron-rich rocks in the nearby Hammersley region of Western Australia point to the earth's atmosphere and oceans first becoming oxygenated around 2.4 billion years ago.
"[The researchers] want to suggest that photosynthesis must have evolved before 3.5 billion years ago, and that despite that, it took one billion years to oxygenate the surface of the earth. That's hard to reconcile with what we know about how this sort of bacteria would have spread," says Walter.
But researcher Professor Hiroshi Ohmoto from the NASA Astrobiology Institute and Department of Geosciences at the Pennsylvania State University says other data backs their claim for an early development of photosynthesising life.
"Recently accumulated massive amounts of geochemical and biochemical data can be better explained by a theory postulating the emergence of oxygenic photosynthesis and the development of a fully oxygenated atmosphere in the very early evolutionary stage," says Ohmoto.
"Once cyanobacteria appeared in one area of the ocean, it probably took less than 10 million years to fully oxygenate the atmosphere and oceans."