Billion-Year-Old Green Seaweed Fossils Discovered by Scientists

Researchers have uncovered tiny fossils of ancient green seaweed that date back around one billion years ago.

According to a study published in the journal Nature Ecology & Evolution, these remains—which measure just two millimeters in length—represent the earliest green seaweed ever found, being around 200 million years older than the previous record holder.

The team identified the microfossils while using an electronic microscope to investigate rock samples collected near the city of Dalian, northern China—an area that was home to a shallow ocean when these organisms were alive.

"We used rock maps to identify the locality where the right types of rocks with the right age are present, and then explored the area with rock hammers and hand lenses," Shuhai Xiao, an author of the study from Virginia Tech, told Newsweek.

"We took the rock samples back and looked at them under the microscope to get nicer pictures. We also threw them in acid to dissolve the rocks, but the fossils weren't dissolved, so we fished out the fossils from the acid and examined them using high-power light and electron microscopes to look at the find details," he said.

The seaweeds—which belong to the species Proterocladus antiquus—became preserved after being buried beneath piles of sediment. This sediment eventually emerged from the water as the ocean was replaced by land, according to the researchers.

"These seaweeds display multiple branches, upright growths, and specialized cells known as akinetes that are very common in this type of fossil," Shuhai Xiao, an author of the study from Virginia Tech, said in a statement.

"Taken together, these features strongly suggest that the fossil is a green seaweed with complex multicellularity that is circa one billion years old. These likely represent the earliest fossil of green seaweeds. In short, our study tells us that the ubiquitous green plants we see today can be traced back at least one billion years," he said.

Seaweeds are technically forms of algae—known as "macroalgae"—which inhabit the seas, coming in three main varieties: green, red and brown. The oldest known red seaweed has been dated to around 1.047 billion years ago.

The green seaweed described in the study, Proterocladus antiquus, is a previously unidentified species. However, it shares several characteristics with other known organisms, despite its age.

ancient seaweed
Image of the ancient fossilized green seaweed. Virginia Tech

"There are some modern green seaweeds that look very similar to the fossils that we found," Xiao said. "A group of modern green seaweeds, known as siphonocladaleans, are particularly similar in shape and size to the fossils we found."

The researchers argue that green seaweeds, such as Proterocladus antiquus, are the forefathers of all land plants that exist today.

"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," Xiao said. "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."

However, not all experts support the hypothesis that green seaweeds in the ocean are the ancestors of all land plants.

"Not everyone agrees with us; some scientists think that green plants started in rivers and lakes, and then conquered the ocean and land later," Xiao said.

This article was updated to include additional comments from Shuhai Xiao.