The oldest green algae to have been preserved in three dimensions may provide evidence for an earlier origin of plants than previously thought.
The fossils date back more than 541 million years, placing them in the late Ediacaran epoch (635 million to 541 million years ago). This was on the eve of the Cambrian epoch (541 million to 485.4 million years ago), when life diversified in an instant known as the Cambrian Explosion. The fossils, which are only half a millimetre in diameter, are incredibly small but are preserved in wonderful detail, right down to the masses of fragile filaments that make up their centre and the bumpy tubular structures that line its outer layers. They originate from Shaanxi province in central China, which in the late Ediacaran had a shallow sea.
Green algae are members of the plant kingdom that first appeared at least a billion years ago, but the new discovery, which demonstrates that a variety of modern-looking algae species existed earlier than thought, the researchers wrote in the journal BMC Biology on September 21 that their findings “may push back the origin of the plant kingdom perhaps another 100 million years.” The ancient algae is surprisingly sophisticated, and it is nearly identical to the modern seaweed genus called Codium.
Cédric Aria, co-author of the study and a postdoctoral researcher in ecology and evolutionary biology at the University of Toronto and Royal Ontario Museum, said that the discovery of something so similar to Codium in the Ediacarian will probably push the origin of green algae and undoubtedly the origin of the entire plant kingdom further back in time.
A modern analogue
The specimens came from the Gaojiashan biota, a rich fossil deposit, where many of the fossils have been preserved in three dimensions as opposed to being squished flat. The five algae samples didn’t like anything else discovered from the late Ediacaran anywhere else in the globe, so the study’s co-authors from Northwest University in Xi’an, China, requested Aria’s assistance in examining them.
The structure of the fossils quickly revealed to Aria and his colleagues that they were made of green algae. However, the researchers couldn’t locate anything comparable in the known species of ancient algae, so they began started combing through more recent species. They then found that the ancient algae resembled the modern Codium almost perfectly. Green algae comes in a variety of shapes, from simple single-celled plants to complex multicellular seaweed. According to Aria, the only difference between the ancient algae and a modern unicellular Codium was that the ancient algae was around half the size.
That was undoubtedly unexpected, he said. Really, it was a Eureka moment.
According to Aria, organisms that existed before the Cambrian Explosion were usually believed to have been quite simple. However, the recent discovery of a very sophisticated green algae that dates back more than 541 million years implies that the Ediacaran has more diversity than was once thought. Aria said that it might be time to reexamine a few of the 2D fossils from this time period to determine whether these, too, might be Codium.
The new species was named Protocodium sinese, which roughly translates as “first Codium from China.” Aria believes the algae has survived. It is possible that this type of green algae found its evolutionary niche early and managed to survive through five mass extinctions and more than half a billion years of change since it has stayed basically un-changed since the late Ediacaran suggest. In many locations today, Codium species are invasive and outcompete local seaweeds & algae.
It started off with a winning hand and kept it, according to Aria.
The land plants of today do not directly descend from algae. The branch that would later give rise to land plants had already been split-off from branch by the late Ediacaran. The new research does indicate that this split and maybe the emergence of plant life in general have a longer evolutionary history than previously thought. Scientists try to estimate when organisms branch from each other by using inferences from the genomes of modern species and the mutation rate observed in paleontological records. If the early green algae were more diverse than previously thought, the whole evolutionary process likely began earlier than scientists thought, too, Aria said.