A study of rocks from Canada’s Arctic has given scientists from Montreal’s McGill University a new clue as to how long Earth’s plant life has been using sunlight to feed.

A type of algae called Bangiomorpha pubescens — fossils of which were first found in rocks in Arctic Canada in 1990 — is believed to be the oldest known direct ancestor of modern plants and animals, but scientists until now could only roughly peg its age at between 720 million and 1.2 billion years.

To better pinpoint that time frame, researchers set up camp at a spot on Baffin Island where Bangiomorpha pubescens fossils have been found and collected samples of black shale from rock layers that sandwiched the rock unit containing fossils of the alga.

Using the Rhenium-Osmium dating technique, which in recent years has been used more often to date sedimentary rocks, the researchers determined those rocks to be 1.047 billion years old.

“That’s 150 million years younger than commonly held estimates, and confirms that this fossil is spectacular,” McGill associate professor Galen Halverson, a senior author of the new study, said in a release.

This finding, Halverson said, will help scientists to make “more precise assessments of the early evolution of eukaryotes” — the celled organisms that include plants and animals.

Also, with this information, McGill researchers now estimate the basis for photosynthesis in today’s plants was set in place 1.25 billion years ago.

Bangiomorpha pubescens, being nearly identical to modern red algae, is believed to have behaved like green plants, using sunlight to synthesize its nutrients from carbon dioxide and water.

Scientists have also established that the chloroplast — the structure in plant cells where photosynthesis takes place — was created when a eukaryote long ago engulfed a simple bacterium that was photosynthetic. The eukaryote then passed that DNA to its descendants — which include contemporary plants and trees.

With the Baffin Island fossils’ age at pegged at around 1.047 billion years, scientists ran that figure through a “molecular clock” computer model that calculates evolutionary events based on rates of genetic mutations.

From that model, the scientists conclude, the chloroplast must have been incorporated into eukaryotes about 1.25 billion years ago.

With this information in hand, the new study’s lead author, McGill PhD student Timothy Gibson, said evidence “is beginning to build to suggest that Earth’s biosphere and its environment in the latter portion of the ‘Boring Billion’ may actually have been more dynamic than previously thought.”

The “Boring Billion” is a term used to describe a stretch of Earth’s history from 1.8 to 0.8 billion years ago, when archaea, bacteria and some complex organisms that have since gone extinct hung around Earth’s oceans with little biological or environmental change to show for it.

With the new information in hand, however, it may be that era set the stage for more complex life forms, peaking with the so-called Cambrian Explosion around 541 million 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,” Gibson said in McGill’s release.

“If other scientists envision a better way to calculate when the chloroplast emerged, the scientific community will eventually decide which estimate seems more reasonable and find new ways to test it.”

The McGill researchers’ study was published online Dec. 8 in the journal Geology.

(Photo courtesy McGill University)




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