Green plants (e.g., trees on the land and green seaweeds in the ocean) are everywhere on modern Earth. They add colors to our world, but more importantly they maintain the ecological balance of our planet by producing organic carbon and oxygen through photosynthesis. But when did the first green plants evolve? Did green plants start in the ocean or on the land?
These questions have been debated by paleontologists and biologists for many years. Paleontologists have been looking for fossils of green plants preserved in rocks. The oldest green land plants are about 450 million years old, and fossils of green seaweeds go back to at least 800 million years ago. However, the fossil record is never complete, and only tells us the minimum divergence age of the respective fossil groups. Molecular biologists approach these questions from a different perspective; instead of examining the fossil record, they use molecular clocks to date the divergence of different green plant groups. However, the molecular clock estimates vary widely, partly because of the scarcity of reliable green plant fossils, particularly early green seaweed fossils, to calibrate the molecular clocks. In fact, the molecular estimates of green plant divergence time range from 1200 to 600 million years, with large uncertainties on the order of several hundred million years. Such large uncertainties highlight the importance of reliable fossils of green seaweeds from older rocks.
Our recent study, published in Nature Ecology & Evolution https://www.nature.com/articles/s41559-020-1122-9, recovered an exceptionally preserved green seaweed taxon from ca. one-billion-year-old rocks in northern China. This taxon, described as Proterocladus antiquus, is a multicellular thallus with branching filamentous structures. The multiple orders of branches and the presence of a root-like anchoring structure indicate that Proterocladus antiquus was an erect epibenthic seaweed. More importantly, the filamentous thallus of Proterocladus antiquus is characterized by large siphonous cells and a unique branching style that align them with a group of green seaweed called siphonodaleans. Proterocladus antiquus represents the earliest green seaweed thus far, providing a key fossil calibration point for molecular clock analysis. Prior to this study, the oldest green seaweed was about 800 million years old; thus, Proterocladus antiquus pushes back the divergence of green plants by 200 million years. In addition, Proterocladus antiquus occurs abundantly in our rocks, indicating that green seaweeds may have played important ecological and geobiological roles at least locally a billion years ago (Fig. 1).
Fig. 1. Ecological reconstruction of the green seaweed Proterocladus antiquus.
In short, Proterocladus antiquus tells us that the ubiquitous green plants we see today can be traced back to at least one billion years ago, and they started in the ocean before they expanded their territory to the land.