Fossil algae hold clues to origin of modern photosynthesis

The debate over the origin of the lineage that led to multicellular life — and thus plants and animals — has raged for decades. To identify when these ‘eukaryotes’ emerged, researchers need well-preserved fossils, complete with characteristics such as complex internal structures surrounded by membranes. Now, a newly discovered set of specimens that are about 1.6 billion years old may help to reveal the truth.

Stefan Bengtson, a paleobiologist at the Swedish Museum of Natural History in Stockholm, Sweden who led the team that made the discovery, thinks these fossils could represent the oldest red algae, and therefore the oldest eukaryotic specimens, found so far. If they are indeed red algae, they could also push back the date for the origin of photosynthetic algae and plants by several hundred million years.

The researchers found three sets of these fossils, which are described in a study published on March 14 in PLoS Biology, in a region of central India. The first set is arranged like a stack of coins and is probably a colonial bacterium that the authors name Denaricion mendax. The other two, which the team calls Rafatazmia chitrakootensis and Ramathallus lobatus, look like long filaments separated into smaller chambers.

Seeing the light
“People have found older fossils that might be eukaryotes,” says Stefan. But so far, no one has been able to see their internal structures to confirm that. Based on X-ray images of the fossils, researchers found what look like complex, well-preserved structures inside Rafatazmia. These include what could be a plantlike cell wall and internal dividers called sept. According to Stefan, the septum’s structure shows that these fossils are definitely red algae, and are therefore eukaryotic and capable of photosynthesis.

If that’s true, these fossils will help researchers to narrow down the age of a major evolutionary event, says Debashish Bhattacharya, an evolutionary biologist at Rutgers University, New Jersey. That would be the point in time at which an organism engulfed photosynthetic cyanobacteria. But instead of being destroyed, those cyanobacteria eventually evolved into the cellular machinery responsible for photosynthesis in eukaryotes.

Current estimates for when this first happened range from 600 million to 1.5 billion years ago. However, Debashish isn’t sure that these fossils truly represent the ancestor at the base of the red algal evolutionary tree, as Stefan and his colleagues suggest. Debashish thinks it more likely that the fossils represent a very ancient side branch. But they are certainly red algae of some kind, and definitely eukaryotic, Debashish says.

Nicholas Butterfield, a paleobiologist at the University of Cambridge, on the other hand, is not persuaded. The specimens may share characteristics with red algae, he says, but it would take more than a few septums to convince him that they are true eukaryotes. Perhaps additional similar finds, or the discovery of structures that are definitively eukaryotic, such as an irregularly shaped cell wall, he says. Still, the paleobiologist thinks that these new fossils are better than some specimens other researchers have put forth as examples of the oldest eukaryotes.

It can be difficult to pinpoint exactly where ancient fossils lie on the tree of life because, billions of years ago, many organisms were superficially similar. “That’s the problem with this field,” says Nicholas. “You stand back and squint, and say, ‘Well, the fossil kind of looks like X.’” Stefan acknowledges that it is difficult to peg his fossils’ exact place. “We can never prove their affinity with 100% certainty,” he says. “But we are very confident we have made the best guess.”