An article published in the journal “PLOS Biology” describes the study of two types of fossil plants discovered in India dating back 1.6 billion years. A team of the Swedish Museum of Natural History led by Stefan Bengtson studied these two different species that look like red algae calling them Rafatazmia chitrakootensis and Ramathallus lobatus. The oldest red algae known so far date back to 1.2 billion years ago and the new discovery indicates that complex life evolved earlier than expected.
The reconstruction of the evolution of eukaryotic organisms consisting of complex cells, meaning animals, plants and fungi, is complex due to the shortage of fossils and the difficulty in analyzing them and interpret the characteristics with reasonable certainty. For this reason, the estimates on the age in which the last common ancestor of eukaryotes lived are extremely vague, going from 800 million to 2.3 billion years ago.
That’s why the discovery of eukaryotic organisms such as red algae dating back to 1.6 billion years ago constitute an extraordinary event but at the same time raise doubts about the interpretation of the fossils discovered. The two species of red algae identified were found in layers of fossil stromatolites, which instead are common because they’re unicellular organisms with a much older origin and already widespread 1.6 billion years ago.
The new fossils are well preserved and show the traces of a shallow-water marine environment and there were many organisms capable of photosynthesis. Modern technologies are increasingly used by paleontologists to examine these types of extremely ancient but also very small fossils. In this case, the researchers used an X-ray tomographic microscopy technique to examine the inside of the fossil algae.
Thanks to this technique, the researchers found three-dimensional cellular and subcellular structures that convinced them that they were red algae. They have also been able to identify tubular shapes that were classified as Rafatazmia chitrakootensis in the case of the ones consisting of concatenated cell-like compartments and Ramathallus lobatus in the case of the ones with a structure that looks like a stack of coins.
The researchers think that the dating is robust because it’s the result of a number of geological studies carried out over the years. This is an important point given that these fossils might be very important to understand the timeline of eukaryotes’ evolution. In particular, they could help figure out when plants as we know them today emerged, the ones capable of photosynthesis.
As always happens when such old fossils are discovered, many doubts are also raised about their interpretation. To begin with, are those ancestors of the subsequent red algae or rather belonged to a side branch of this group? Even modern technologies allow to identify only part of these fossils’ features that were preserved so at least today it’s impossible to give final answers.