Billion-year-old green algae

Proterocladus antiquus

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.

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Fungi are old

Earlier this week, I wrote that fungi are weird. Fungi are also old. Probably.

Recently discovered fossils from Arctic Canada have been interpreted as fungi, an important finding since the sediments in which they were found date to 0.89-1.01 billion years ago, around half a billion years older than the next oldest unambiguously fungal fossils. Corentin Loron and colleagues have described microfossils from the Grassy Bay Formation and presented several lines of evidence that they represent ancient fungi.

Loron et al. 2019 Fig. 1

Figure 1 a-g from Loron et al. 2019. Microphotographs of Ourasphaira giraldae specimens. a, Sketch of O. giraldae, displaying the main features of the microfossil. b–g, Unornamented terminal sphere (spore). Transmitted light microscopy images show specimens with secondary branching at a right angle (b, d–g), with terminal spheres connected together (c), with a bulbous connection (e) and with tertiary branching (d, f, g). Arrows show septate connections.

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Fungi are weird

I think about the evolution of multicellularity a lot, and I talk about it with colleagues. One of the things we talk about is what general principles we can infer from the many independent origins of multicellularity, for example in land plants, animals, red algae, brown algae, green algae, and fungi. Those are the groups that have evolved what we might call complex multicellularity, and one of the things we notice is that they all develop clonally; that is, they start out as a single cell, and when that cell divides, the daughter cells stick together. We notice that complex multicellularity has never evolved in species with aggregative development, when free-living cells come together to form a multicellular body, as they do in cellular slime molds and myxobacteria. Some aggregative developers have evolved a couple of different cell types, but all of the groups that have reached higher degrees of complexity develop by cell division and the products of cell division staying together. All, that is, except for fungi. Fungi are weird.

Fungi don’t really develop clonally in the way I’ve described, but they don’t really not develop clonally either. That’s because their cells don’t divide in the way we’re used to thinking about, through repeated rounds of mitosis. In mitosis, duplication of the genome is coupled to cell division: the chromosomes duplicate, they move to either end of the cell, then the cell divides. The chromosomes double, then they halve, so the daughter cells end up with the same number as the mother cell. That’s not how it works in fungi. Instead, they form filaments called hyphae (singular hypha) that grow at the tip. In some cases, partitions called septa (singular septum) form behind the growing tip, dividing the hyphae into individual cells. In some cases, no septa form, and each hypha is effectively one long, skinny cell with lots of nuclei (this is called a coenocyte).

So fungi don’t really develop by repeated rounds of cell division in the same sense that animals, plants, etc. do. Hyphae just grow, and they are divided into cells as sort of an afterthought, if they are divided into cells at all. Fungi with coenocytic (or aseptate) hyphae aren’t really even multicellular in the same sense as plants and animals are. Different people have different qualifications for what counts as multicellular, but it’s a stretch to call something multicellular that doesn’t have multiple cells. Fungi are weird.

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Dissent on Bangiomorpha

Bangiomorpha pubescens

Figure 5 from Butterfield 1990. Bangiomorpha pubescens.

In my post on Bangiomorpha, I said

…Bangiomorpha was probably a red alga. This conclusion seems to be accepted by most everyone in the field. In fact, I don’t know of any dissenters, and that kind of consensus is rare for fossils this old.

I guess I didn’t look hard enough, because reader not the FTB Stewart commented

Cavalier-Smith dissents (dissented?) from the consensus
https://books.google.co.uk/books?id=lE6r5q5op94C&pg=PA63&lpg=PA63#v=onepage&q&f=false

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New fossil proves plants are younger than previously thought

That’s not a headline you’re likely to see again. Hopefully it made you think something along the lines of “how does that work, exactly?” Because it doesn’t. If your estimate of the age of a taxon is based on its oldest known fossil, finding a newer fossil isn’t likely to change that estimate. If it’s an extinct group, a newer fossil might show that it stuck around later than you thought, but not that it originated later. Paleontologists recognize that fossil-based estimates of ages are almost always underestimates, since the fossil record is spotty (and generally spottier the further back you go).

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One of the problems with a big tent

…is that the people in your tent only share some of your views. And one of the problems with having a blog is that it’s searchable, so that when you say ‘no one in my tent ever said x,’ it’s easy to show that it’s a lie.

Within the intelligent design tent, there are people like Michael Behe, who believe that species change over time and that they evolved from a common ancestor, differing from evolutionary biologists only in their insistence that some aspects of biology must have been designed:

I am not a creationist and have no reason to doubt common descent.

There are also people in the tent like Casey Luskin, Stephen C. Meyer, and Jonathon Wells who doubt, and spend a lot of their time attacking, common descent (see “Intelligent design’s relationship with common descent? It’s complicated.“).

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(Probably not) Precambrian Volvox

A new(ish) paper in National Science Review evaluates the evidence for various interpretations of Ediacaran microfossils from the Weng’an biota in South China (Xiao et al. 2014. The Weng’an biota and the Ediacaran radiation of multicellular eukaryotes. Natl. Sci. Rev., 1:498–520.). I recommend checking it out; it’s open access, and there’s a lot of interesting stuff in there that I’m not going to address.

These fossils are undoubtedly multicellular, probably eukaryotic, and extremely enigmatic. Their age (582-600 million years) means they could have important implications for the evolution of multicellularity, and their exceptional preservation in great numbers creates the potential for reconstructing their life cycles in great detail. Some of the Weng’an fossils have been interpreted as volvocine algae, an interpretation that I find highly unlikely.

Some of the Weng’an fossils are thought to represent red algae, and this would not be terribly surprising, since red algae have been around for at least 1.2 billion years. Others, for example the tubular fossils, are more problematic, with interpretations as diverse as cyanobacteria, eukaryotic algae, crinoids, and cnidarians.

Fig. 8 from Xiao et al. 2014

Figure 8 from Xiao et al. 2014: Schematic diagram showing diagnostic features of the five recognized species of tubular microfossils in the Weng’an biota.

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