Cyanobacteria of today look much like their ancestors of 2 billion years ago

[lpetrich]

The Fossil Record of Cyanobacteria | SpringerLink with The Fossil Record of Cyanobacteria - Semantic Scholar by J. William Schopf

Several hundred fossils of cyanobacteria are know known from the Precambrian, not only microfossils but also macroscopic ones: stromatolites. These are multiple layers of sediment trapped by microbes like cyanobacteria. Stromatolites became much rarer during the Cambrian and afterward, because of the evolution of multicellular animals that could efficiently eat them. In the present, stromatolites only form in very salty lakes and lagoons, like Shark Bay in Australia, the saltiness keeping out potential stromatolite-eaters.

Most Precambrian cyanobacterium fossils are found in the Proterozoic, after the Great Oxygenation Event of 2.5 billion years ago. Fossil stromatolites before then are much rarer, though some go back as far as 3.5 billion years ago. Although cyanobacteria are likely older than the GOE, it is not very clear how much older. Oxygen-releasing photosynthesis likely had predecessors that did not release oxygen, and it is not very clear how much older those are. A complication is methanogen metabolism, which produces a similar carbon-isotope signature, and sometimes a strong one. This evidence goes back 3.5 billion years. Also going back to around then is evidence of Gram-positive bacteria (Firmicutes, "strong skins"), and sulfate-reduction metabolism.


A remarkable feature of fossil cyanobacteria is the remarkable resemblance of many of them to present-day ones, even over 2+ billion years. Many of them fall into five taxonomic families constructed for present-day ones (Oscillatoriaceae, Nostocaceae, Chroococcaceae, Entophysalidaceae and Pleurocapsaceae, two filamentous and three that form small or irregular clusters), thus extending their range for that 2+ billion years. Thus, cyanobacteria are extreme "living fossils", even more than such Phanerozoic ones as horseshoe crabs (not much change over the last 450 million years). This is especially remarkable when one considers the short generation times that prokaryotes can have. Doubling time - Cyanobacteria Synechocystis PC - BNID 111252 = 12 hours and Doubling time - Cyanobacteria Synechococcus el - BNID 111253 = 6 - 7 hours. Heterotrophic ones can reproduce even faster. The favorite laboratory bacterium Escherichia coli can have a doubling time of 20 minutes or less (The distribution of bacterial doubling times in the wild, Growth of Bacterial Populations).

 

[Speakpigeon]

Remarkable but not surprising.

And, chloroplasts for example are descendant of cyanobacteria and they don't look the same.
EB

 

[Gun Nut]

I guess they really scratch that niche!

 

[lpetrich]

Considering the question of rates of evolution, a traditional expectation of evolutionary biology is gradual evolution. But a problem is that species-to-species gradualism has been hard to find. This has traditionally been attributed to the patchy nature of the fossil record, but in 1972, Niles Eldredge and Stephen Jay Gould proposed that the fossil record is too good to plausibly claim that, and that transitions are too fast to show up very often in the fossil record. This is "punctuated equilibrium", or "punc-eq" for short. It states that evolution in macroscopic features occurs in bursts in offshoot populations that give rise to new species. Then each species does not have much macroscopic-feature evolution over the rest of its existence, thus being in a sort of stasis.

But even if species-to-species transitions are hard to find, larger-scale transitions are sometimes very evident.

A related issue is of  Living fossil, organisms not much different from their ancestors of a few hundred million years ago, even if several species over that time. For instance,  Horseshoe crabs have four present-day species and several past ones over the last 445 million years, for instance. Another one is  Lingula (brachiopod), whose ancestors have not changed much in 500 million years, since the early Cambrian. What I posted about in my OP goes even farther, with cyanobacteria not changing much over the last 2.5 billion years or more.

This offers an interesting conundrum in rates of evolution.  Rate of evolution quotes

The question of evolutionary change in relation to available geological time is indeed a serious theoretical challenge, but the reasons are exactly the opposite of that inspired by most people’s intuition. Organisms in general have not done nearly as much evolving as we should reasonably expect. Long term rates of change, even in lineages of unusual rapid evolution, are almost always far slower than they theoretically could be. The basis for such expectation is to be found most clearly in observed rates of evolution under artificial selection, along with the often high rates of change in environmental conditions that must imply rapid change in intensity and direction of selection in nature.

(Williams, G.C. (1992). Stasis. In Natural Selection: Domains, Levels and Challenges. p. 128. New York: Oxford University Press.)

 

[Speakpigeon]

Considering the question of rates of evolution, a traditional expectation of evolutionary biology is gradual evolution.

I don't see why the same species couldn't survived unchanged as long as the environment in which it is optimally adapted to begin with doesn't itself change. Is there any notion that entropy or something leads inevitably species to change?
EB

 

[lpetrich]

Considering the question of rates of evolution, a traditional expectation of evolutionary biology is gradual evolution.

I don't see why the same species couldn't survived unchanged as long as the environment in which it is optimally adapted to begin with doesn't itself change.

Yes, that's completely reasonable, and it likely explains a lot of species stasis.

Is there any notion that entropy or something leads inevitably species to change?
EB

It's more like not much of it has happened over humanity's history. Thus, for it to happen, it must be too slow for it to be noticeable over humanity's history. Geological time was eventually discovered to be much greater than humanity's recorded time, allowing plenty of temporal room for slow evolution.

Punc-eq changes this picture a little bit. We see most species during times of stasis, though we do seem to have caught some species in the act of speciation: Observed Instances of Speciation, Some More Observed Speciation Events


Here is some more fossil evidence. Fossils of the red alga Bangiomorpha pubescens were discovered in rock that is about 1,047 million years old, rock that is now in the Canadian Arctic. That species was named from its close resemblance to present-day Bangia red algae. This gets close to the amount of stasis that many cyanobacteria show in their evolution.


The stasis that I have been discussing here is stasis in fossilizable phenotypes. There is, however, much less stasis in gene-sequence evolution. But most gene-sequence evolution is selectively neutral ( Neutral theory of molecular evolution), a form of genetic drift. Part of it is from how genes code for proteins. Each amino acid (protein building block) is coded for by a triplet of nucleotides (gene building blocks). There are 4^3 = 64 possible triplets or codons, and they code for 20 protein-forming amino acids, thus giving some redundancy. This means that selection is neutral between different triplets that code for the same amino acid. Proteins are composed of strings of amino acids, and some parts of these strings are under much stronger selective constraint than others -- they are much more "conserved". So much protein-sequence evolution is also selectively neutral, as is the corresponding gene-sequence evolution.