Around 450 million years ago one of the most important developments in the history of Earth occurred: plants colonized the land for the first time. Now a fossil has been found that could resemble the first species that developed this capacity, from which all modern land plants descend. Most remarkably, the fossil comes not from around the time of this epic event, but from more than twice as long ago.
Professor Shuhai Xiao of Virginia Tech announced the discovery of fossilized green seaweed in billion-year-old ocean sedimentary rocks from Liaoning Province, north-eastern China, in Nature Ecology and Evolution under the name Proterocladus antiquus. P. antiquus exceeds the previous record for green seaweed by some 200 million years.
Like many paleobotanists, Xiao thinks all land plants evolved from green seaweeds, which continue to compete in the oceans with brown and red seaweeds, both of which failed to make the same transition to land. However, he acknowledged in a statement, "Not everyone agrees with us; some scientists think that green plants started in rivers and lakes, and then conquered the ocean and land later.”
Dr Qing Tang, who brought the fossils to Xioa's attention noted, “These seaweeds display multiple branches, upright growths, and specialized cells known as akinetes that are very common in this type of fossil.”
As a forerunner of the tallest redwood or the extraordinary expanse of quaking aspen, Xiao's fossils are underwhelming, being just 2 millimeters (0.08 inches) long. Proverbs about mighty oaks and tiny acorns are underselling the truth.
Some of P. antiquus' descendants, however, appear to go by the principle that when you are on to a good thing, stick to it.
“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.” Intriguingly, P. antiquus bears considerable similarity to the next oldest fossil of this type, P. major, which has been found in 800 million-year-old rocks at Svalbard. The P. major fossils were less well preserved, however, making confirmation they are the same species difficult, and boosting hopes we will learn a lot more from the new find.
Red seaweeds fossils date back 1.05 billion years ago, but previously no green seaweeds had been found of similar ages. Molecular clocks suggest the first green photosynthesizing plants appeared somewhere between 1,600 and 720 million years ago, but scientists find such a wide range unsatisfying and hope fossil discoveries will narrow it down.
The discovery raises the question of why green seaweeds took so long to move onto dry land, given the speed with which they adapted to most environments once they did.