The presence of oxygen in rocks that created Earth's ocean floors 3.2 billions years ago suggests the earliest known photosynthesis by living organisms, U.S. researchers say.

The sole logical source of the oxygen that made its way into the rock is cyanobacteria, primitive organisms living in ancient oceans that were capable of photosynthesis, geoscientists at the University of Wisconsin-Madison say.

"Rock from 3.4 billion years ago showed that the ocean contained basically no free oxygen," says researcher Clark Johnson.

However, he says, an examination of well-preserved rocks dated to 3.2 billion years ago revealed positive signs of oxygen.

That would put the presence of oxygen in Earth's oceans at a much earlier period than previous work has suggested, he says.

The oldest evidence of life suggests it arose 3.5 billion years ago, so the discovery of oxygen at 3.23 billion years means oxygenic photosynthesis may have evolved fairly soon after the emergence of life itself, the researchers say.

Until recently, it had been assumed by many geoscientists that oxygen on Earth was quite rare until an occurrence known as a "great oxygenation event" between 2.4 billion and 2.2 billion years ago.

For their NASA-funded study appearing in Earth and Planetary Science Letters, the researchers studied samples of rocks provided by collaborator Nicolas Beukes of the University of Johannesburg, who collected them in a geologically stable area of South Africa.

Because the rocks were collected from a single site, it cannot be proved that photosynthesis was in fact widespread 3.23 billion years ago, the researchers acknowledge, but they suggest that once developed, it most likely spread rapidly.

"There was evolutionary pressure to develop oxygenic photosynthesis," says Johnson. "Once you make cellular machinery that is complicated enough to do that, your energy supply is inexhaustible. You only need sun, water and carbon dioxide to live."

Early cyanobacteria would have lived in shallow water, where they could generate oxygen by photosynthesis, although there may not have been any oxygen in deeper water or in the atmosphere above for some time, he says.

Still, he suggests, an evolutionary "nifty trick" like photosynthesis was almost certain to spread sooner or later.

"Once life gets oxygenic photosynthesis, the sky is the limit," he says. "There is no reason to expect that it would not go everywhere."