One-billion-year-old seaweed fossils are believed to be the oldest green algae discovered.
They could also be related to the ancestor of the earliest land plants and trees that first developed 450 million years ago, researchers say.
The micro-fossil seaweeds, a form of algae known as Proterocladus antiquus, are hardly visible to the naked eye, at 2mm in length.
The specimens were imprinted in rock taken from an area of dry land near the city of Dalian in the Liaoning Province of northern China, which used to be ocean.
Before the discovery, published in the Nature Ecology and Evolution journal, the earliest convincing fossil records of green seaweeds were found in rock dated at about 800 million years old.
Professor Shuhai Xiao of Virginia Tech university in the US, said: “These new fossils suggest that green seaweeds were important players in the ocean long before their land-plant descendants moved and took control of dry land.
“The entire biosphere is largely dependent on plants and algae for food and oxygen, yet land plants did not evolve until about 450 million years ago.
“Our study shows that green seaweeds evolved no later than one billion years ago, pushing back the record of green seaweeds by about 200 million years.”
He added that the current hypothesis is that land plants like trees, grasses and food crops, evolved from green seaweeds – aquatic plants.
“These fossils are related to the ancestors of all the modern land plants we see today,” Prof Xiao said.
However, not all geobiologists agree on the origins of green plants, with some suggesting they started in rivers and lakes, before conquering the ocean.
The researchers suggest the tiny seaweeds once lived in a shallow ocean, died and then became “cooked” under a thick pile of sediment, preserving their shape.
According to the scientists, the seaweeds’ multiple branches, upright growth and specialised cells suggest the fossil is a green seaweed that is about one billion years old.