Toronto: The process of photosynthesis in plants first took place 1.25 billion years ago, finds a study that identified an algae fossil, believed to be the world's oldest known direct ancestor of modern plants and animals.

The study could resolve a long-standing mystery over the age of the fossilised algae -- Bangiomorpha pubescens -- which were first discovered in rocks in Arctic Canada in 1990, the researchers said.

Earlier estimates had placed it somewhere between 720 million and 1.2 billion years.

However, to pinpoint the microscopic organism's exact age, the researchers from McGill University in Canada, collected samples of black shale from rock layers that sandwiched the rock unit containing fossils of the Bangiomorpha pubescens, from the rugged area of remote Baffin Island.

Using a dating technique applied increasingly to sedimentary rocks, they determined that the rocks are 1.047 billion years old.

"That's 150 million years younger than commonly held estimates, and confirms that this fossil is spectacular," said Galen Halverson, Associate Professor at McGill's Department of Earth and Planetary Sciences.

"This will enable scientists to make more precise assessments of the early evolution of eukaryotes, the celled organisms that include plants and animals," Halverson added, in the paper published in the journal Geology.

In addition, the scientists also established that the chloroplast, the structure in plant cells that is the site of photosynthesis, was created when a eukaryote long ago engulfed a simple bacterium that was photosynthetic.

The eukaryote then managed to pass that DNA along to its descendants, including the plants and trees that produce most of the world's biomass.

The researchers plugged the figure, 1.047 billion years, into a "molecular clock," a computer model used to calculate evolutionary events based on rates of genetic mutations.

They concluded that the chloroplast must have been incorporated into eukaryotes roughly 1.25 billion years ago.

"We expect and hope that other scientists will plug this age for Bangiomorpha pubescens into their own molecular clocks to calculate the timing of important evolutionary events and test our results," the researchers said. (IANS)