Figure 2g from Tang et al. 2020. Proterocladus antiquus. Scale bar 200 μm.

Researchers from Virginia Tech and the Chinese Academy of Sciences have described some billion-year-old fossils that they interpret as green algae. What’s interesting about these fossils, aside from being older than previously known green algal fossils, is that they appear to be fully multicellular, with differentiated cells. This is a valuable find, because it shows that at least one of the many green algal lineages that have independently evolved multicellularity did so relatively early. Sadly, the article, in Nature Ecology & Evolution, is paywalled. The best I can do is link to the article’s page on ResearchGate, which has a read-only version. I requested a full-text through that page, and the lead author sent a pdf promptly.

How soon after the origin of green algae? We don’t know, exactly. We already knew, from the red algal fossil Bangiomorpha, that red algae and green algae diverged over a billion years ago. Molecular clock estimates place that divergence between 1 and 2 billion years ago, with most recent papers tending towards estimates close to the middle of that range, around 1.2-1.7 billion years ago (searching Ulva versus Porphyra on timetree.org will get you a summary of these studies):

If these fossils, and Bangiomorpha, are correctly identified, the youngest of those estimates is implausible. So this fossil is valuable in narrowing down the range of plausible dates for the origin of green algae.

At least one intelligent design creationist thinks its significance is far greater than that. Quoting a press release on ScienceDaily that “going back 2 billion years, Earth had no green plants at all in oceans,” Denyse O’Leary opines

It’s not “land” vs. “sea” that’s really significant here. It’s how much time was available for the development of photosynthesis. If the claim is that photosynthesis developed via natural selection acting on random mutations (Darwinism), then it must have somehow randomly happened in that billion years. Was there enough time? becomes an unavoidable question.

Aside from calling natural selection a random process, the problem with this is that the new fossils have nothing whatsoever to do with the origin of photosynthesis. Photosynthesis didn’t originate in green algae; it originated in cyanobacteria, and (all) algae acquired it through endosymbiosis. Cyanobacterial fossils go back around 3.5 billion years, so O’Leary is only off by about 1.5 billion years, or around a third the age of the Earth.

Stable links:

Tang, Q., K. Pang, X. Yuan, and S. Xiao. 2020. A one-billion-year-old multicellular chlorophyte. Nature Ecology & Evolution, doi: 10.1038/s41559-020-1122-9.