Billion-year-old fossil seaweeds found in China could be the ancestors of all land plants. The tiny seaweeds have branching structures and disc-shaped features to fix them to rocks, making them the oldest complex plants yet discovered.
“The first time I saw these fossils I was very excited because I knew it could be something very important,” says Qing Tang of Virginia Tech in Blacksburg.
Tang and his colleagues went to Liaoning, China, to collect samples from the 1-billion-year-old Nanfen formation because they knew this ancient rock is well-preserved and might contain fossils of early complex cells. However, Tang only saw the fossil seaweeds for the first time when he examined the rocks closely back in the laboratory.
These ancient organisms – named Proterocladus antiquus – would have grown on rocks in shallow seas. They appear to have had specialised cells for reproduction and for forming spore-like cells that would have remained dormant when conditions got tough and regrown later.
The fossils are up to 3 millimetres in length, so the living plants would have been visible to the naked eye. They are very similar to some present-day green seaweeds.
Some biologists think green seaweeds first evolved in freshwater environments such as lakes and rivers, colonising the seas only relatively late in the history of life. But Tang’s discovery provides strong support for the rival idea that green seaweeds evolved in the oceans very early, and gave rise to the plants that evolved to live on land starting around 450 million years ago.
The evolution of plants began long before all this – around 2.3 billion years ago with the rise of simple cells capable of using the green pigment chlorophyll for photosynthesis. These cyanobacteria transformed the atmosphere of Earth by releasing oxygen.
Complex cells came later. Around two billion years ago, a complex single-celled organism swimming around in the oceans engulfed cyanobacteria and formed a symbiotic relationship with them. This event gave rise to all plants, with the cyanobacteria evolving into the photosynthetic organelles called chloroplasts.
Multicellular plants evolved more than once. Tang’s finding shows this happened more than 1 billion years ago in the lineage that gave rise to green plants – some previous estimates had put it as late as 620 million years ago.
Journal reference: Nature Ecology & Evolution, DOI: 10.1038/s41559-020-1122-9