A billion years ago, complex green algae was evidently thriving in the tossing oceans, as deduced from fossils of this tiny but multicellular plant discovered – in abundance, no less – in northern China. The fossils are the oldest example of multicellular green algae ever discovered, though examples of complex red algae more than half a billion years older in India was reported three years ago.
Only 1 millimeter (0.04 inches) in size, the newfound multicellular green algae was found in the billion-year-old Nanfen formation, as reported in Nature Ecology and Evolution by Qing Tang of Virginia Tech, with Ke Pang, Xunlai Yuan and Shuhai Xiao of the Chinese Academy of Sciences.
The discovery sheds a glimmer of light on one question that has been tormenting paleobotanists: When the vast family of green plants arose (the answer now being “more than a billion years ago”).
For any tiny fragile organism to survive in fossil record is very rare, even if it has rigid cell walls with cellulose. So the discovery of “abundant” green algae fossils in Nanfen indicates they were positively flourishing in the seas covering that area at the time.
Red algae (Rhodophyta), of which some 7,000 species still exist, may have preceded green algae. Not one but two types of red algae fossils a cool 1.6 billion years old were found in Chitrakoot, India. Or not.
The team declared the newfound algae a new species, understandably, which they named Proterocladus antiquus. They add that they found 1,028 specimens in the Nanfen formation.
"Technically, red algae and green algae should have evolved simutaneously. If red algae presented in 1.6 billion years ago, then green algae should also present in the same [time] ago," Qing Tang tells Haaretz. "Genetic studies should eventually consistent with paleontological studies. However, the fossil record in the rocks is usually incomplete. If we believe that red algae existed in 1.6 billion years ago and Proterocladus is a green algae at 1 billion years ago, then there is still at least a 600-million-year fossil gap of green algae. But in any case, genetic studies indicate that red and green algae should come at the same time."
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So green algae even earlier than the new find could possibly be discovered in the future.
In any case, neither was likely the earliest form of life. Paleomicrobiologists suspect the first creatures were unicellular chemoautotrophs, who could use carbon dioxide as a carbon source and oxidize inorganic molecules to extract energy over 4 billion years ago.
Proto-bacteria seem to have arisen perhaps half a billion years later, still bereft of a cellular nucleus but capable of directly “eating” glucose for energy.
Then around 3 billion years ago (give or take a lot) came photosynthetic cyanobacteria, which are still around and poisoning polluted waters with toxic blooms big enough to be seen from outer space.
It seems algae only arose after that lot. We can be confident that the first, let’s call it plant, was single-celled. The evolution of complexity, i.e., multicellularity, is equally shrouded in fog.
As far as green algae are concerned, since the team reports that billion-year-old fossil was very complex and featured differentiated, branched cells and root-like structures (“holdfast structures”) – clearly, multicellularity had evolved long before, and going by the red algae, well over 1.6 billion years ago.
"I would agree," Tang says. "However, I would suggest being very cautious about the 1.6-billion-year-old red algae fossil in India. It turns out that not everyone agrees with the red algal interpretation as well as the 1.6-billion-year age assignment. The regional geology in that area where the red algal fossil comes from might be more complex than is understood." Its age needs to be further conformed by regional stratigraphic studies, he adds.
At the bottom line, it seems algae arose and speciated and became differentiated much earlier than had been thought, the authors explain. “Considering the abundant occurrence of Proterocladus in the Nanfen formation, chlorophytes [green algae] may have played notable ecological and geobiological roles, at least locally if not globally, before the Cryogenian [720 to 635 million years ago] when their biomarkers became abundant,” they write.
Remarkably for a plant that lived a billion years ago, the scientists managed to conclude from its “holdfast” and evidence of apical growth that it likely stood erect on the seabed. The dense, asymmetric branching they discerned in the fossils, and their tendency to aggregate, suggest that Proterocladus probably formed tufts, which may have facilitated colonization of the seabed, they write.
While on the topic, some believe there were complex animals crawling around at least 2.1 billion years ago, based on fossil micro-tunnels found in Franceville, Gabon, that seem unlikely to be of geological origin. Though no fossil animals were found, the Gabon theory would have kicked back the evolution of motility by at least 1.5 billion years.