When you bite into sushi, you might not realize that the seaweed keeping the rice and fish bundled has a very, very long history. Nori, the seaweed used in sushi, is a variety of red algae – and a team of paleontologists has 1.6-billion-year-old rocks that they say contain red algae fossils.
If they are right, this could support the idea that eukaryotes, the branch of life that ultimately led to humans, first arose on Earth quite early. And this could also hold clues into the origins of photosynthesis. The new fossils are described in a paper published Tuesday in the journal PLOS Biology.
"The general picture of the Proterozoic at this time is that we had an almost exclusively microbial biosphere," study lead author Stefan Bengtson, a paleozoologist at the Swedish Museum of Natural History, says in a phone interview with The Christian Science Monitor.
Those microbes, also known as prokaryotes, are single-celled organisms without a nucleus or the other organelles that characterize eukrayotic cells.
With these new fossils, Dr. Bengtson says, "I think we have to rethink the way we calibrate the tree of life, and perhaps push back the origin of not only the red algae but also other eukaryotic groups farther back in time."
Scientists who study the history of life still debate when the first eukaryotes emerged on Earth. Some say these organisms have been around for as long as 2.3 billion years or more, explains Roger Summons, a geobiologist at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass., who was not involved in the new study, in an email to the Monitor. But, he adds, "others say as young as 800 million years."
Eukaryotic life is thought not to have begun to really flourish until the so-called Cambrian explosion, around 600 million to 500 million years ago.
"This work falls on the side of the older ages because, if the authors are right, and they belong to crown group red algae (that is similar to the red algae that exist today), then their ancestors must be older and, perhaps as old as 2.3 billion years or more," Dr. Summons says.
Bengtson points to cellular structures in the fossils that he says are reminiscent of red algae found today. But the most intriguingly plant-like feature hints that these 1.6-billion-year-old organisms may have been photosynthesizing.
The ancient organisms appeared to have little platelets in their cells that the team interprets as belonging to chloroplasts, the organelles within plant and algal cells associated with photosynthesis.
But not everyone agrees with Bengtson and his colleagues' assessment of these fossils.
"The fossils are interesting, but, in my books, far from convincing, either as red algae or as eukaryotes. Yes there are some red algae and eukaryotes that exhibit features comparable to those seen in the fossils, but none of these are fundamentally exclusive to red algae or eukaryotes," writes Nicholas Butterfield, a paleobiologist at the University of Cambridge who was not involved in the research, in an email to the Monitor.
Perhaps a few more distinctive eukaryotic features would help, he says.
"That's not to say that there aren't unambiguous eukaryotes at this time, but we're not looking at them here," he adds.
Bengtson admits, "We can't be 100 percent sure that these things are actually red algae because these cell arrangements and cell structures may sometimes occur in different kinds of groups because they independently evolved them or because they inherited them from the same common ancestor."
Dominic Papineau of the University College of London has faced similar skepticism. Earlier this month he and colleagues reported what he says are the "oldest fossils on the planet" at somewhere between 3.77 billion to 4.22 billion years old. Dr. Papineau’s discovery significantly challenges previous ideas of when the first life on Earth appeared, so naturally some paleobiologists have been skeptical of the team's interpretation of the fossils.
"The reality is," Papineau tells the Monitor, fossils this old don't have "written on them what they were and what they were doing." As such, he says, identifying ancient organisms can be challenging and takes multiple lines of evidence.
But he thinks Bengtson and his colleagues make a good case. "I would say that they have really documented in very excellent details, unprecedented to my knowledge, these novel structures which do indeed look like algae."
"They used new synchrotron-based (that's a particle accelerator) micro-analytical techniques to visualize inside the microfossils and revealed their intracellular structures in unprecedented details," he writes in a follow-up email. But, he adds, stable isotope analyses would have helped confirm the identify of these organisms as red algae.
To Papineau, these new fossils add to the mounting evidence that life was beginning and diversifying earlier than most scientists had previously thought.
Shuhai Xiao, a paleobiologist at the Virginia Polytechnic Institute and State University who was not involved in the new research, says that these 1.6-billion-year-old fossils look remarkably like some 600-million-year-old fossils that he has been studying.
This could mean "there's a much deeper origin of the red algae," as Bengtson and his colleagues suggest, Dr. Xiao tells the Monitor. Or, he says, "maybe they have nothing to do with the younger fossils that I have been studying and these forms evolved multiple times."
Xiao agrees with Bengtson's team that these fossils could have significant implications for the origin of photosynthesis in eukaryotes.
"If it is indeed red algae, that would push the origin of photosynthetic eukaryotes at least back to 1.6 billion years," he says. "They're certainly much older than that, because these things are not the most primitive photosynthetic eukaryotes," Xiao adds.
[Editor's note: This article has been updated.]