Talmudic Question #25
by guest author Bill Martin
This is our first multiple-part question.
If one believes (as some out there do) that there was a free-living LUCA (Last Universal Common Ancestor), then what kind of lipids did it have?
I. If eubacterial, then explain why it would evolve the archaebacterial type in the presence of a good lipid synthesis solution. (Bonus question: And why was that only done once?)
II. If archaebacterial, then return to I) for the converse solution (with bonus).
III. If both, explain why we do not invoke differential loss for everything that distinguishes archaebacteria from eubacteria, and explain what is wrong with Ford Doolittle's strong Genome of Eden argument, while recalling that LGT (lateral gene transfer) only explains similarities, not differences (an interesting observation).
IV. If none, suggest what might have served to contain organics from their environment en route to self replication, recalling the so-called concentration problem (molecules must meet to react).
V. If dissatisfied with I‑IV, suggest where the LUCA might have got going as a non-free-living entity.
VI. If disgusted with all of the foregoing, suggest a solution to compartmentation from the environment and the universality of chemiosmosis (except in a few derived fermenters that live off autrotrophs) that circumvents I‑V, recalling that thermodynamics is not just a good idea, it's the law. Put another way, thermodynamics is as close as it gets to a genuine logical constraint in biology.
VII. If this leads you into origin of life literature, explain why it is written mostly by chemists and physicists who, by trade, do not understand much about microbes (their explanandum).
VIII. Alternatively, formulate a better question.
Bill is at the Botanical Institute III, University of Düsseldorf
I can only offer feeble speculations. It's a slippery slope, but here I go: Imagine an early phase of pre-Darwinian evolution (Doolittle and Brown 1994, PNAS), where phenotype was not yet tightly coupled to genotype, especially for lipids. Imagine early biotic reactions encapsulated in random, spontaneously assembling lipid envelopes. (Lipid bilayers form spontaneously, but ribosomes and polymerases take a little more planning). After ribosomes and translation developed, true lineages become possible. An early Archaean forebearer stumbles onto genes that allow consistent production of tetraether lipids, which allows them to survive in stupidly hot environments, leading to rapid radiation. the Ur Bacterium stumbles upon its own unique solution, and takes off in a different direction (in the remaining niches including everything below about 100 C). Both approaches were clearly successful, so it would be hard for further innovations to have competed once these lineages became established.
So the answer to what type of lipids did the LUCA have is: Soup of the Day!
Remember during this fascinating conversation that geology did not wait for life (au contraire of course). That is, one has to expect that gobs of soup were often buried in sediment and exhumed; that burial would mix your organic molecules up with clays, sulfides, diagenetic stuff (diffusion reactions and making concretions and all sorts of radial and dendritic mineral patterns). Maybe there were lots of trials, two survivors at opposite poles and no one LUCA, as suggested by Lipson.
I wish I had something really smart to say about this! A hugely interesting question to me, but one I'm utterly unqualified to even start answering.
Still, that never stops me...I think one issue to confront is the difficulty of the LUCA idea, and how it intersects with the world that was. When I think about this, it helps me to think about "mitochondrial eve", and the notion that everyone on earth has a common ancestor x kya (not sure what x is, maybe 40–50?). This obviously doesn't mean that there was just one person — a banal statement that bears on our more interesting discussion! The Universal Common Ancestor doesn't need to have been a monoculture! There may be many things that lived that do not have living progeny in the current world (that we know of...). So, this leaves us with the interesting question of just how weird they might have been.
Returning to the question at hand. I have always been baffled by the seeming paradox of different lipids between Eubacteria/Eukarya on one side and Archaea on the other, combined with closer relationships at the gene expression level between eukarya and archaea. It seems to me easier to believe that there might have been a period of time where lipid bilayers were produced containing both types of linkages? I'm not a biochemist, so maybe these would be unstable for some reason I don't know, but it doesn't seem like a big problem. So, if the LUCA had both types of linkage, it could then have lost one or the other.
So I guess I come down in favor of III. We can't use differential loss to explain all the differences, because it is much harder to envision having two extensive, complex, and separate systems for controlling transcription, but much easier to envision lipid synthesis genes with redundancy.
It also seems to me that the archaeal membrane might have added stability compared to the eubact/eukaryote membrane because of it's chemistry (again, not a biochemist!). So organisms with either machinery might have been at a disadvantage to an organism with both phospholipid types in certain environments. Also, we assume based on current single celled organisms that carrying extra DNA is a fitness burden, but maybe it wasn't back then? If we assume that LUCA had Archaeal lipids and Eubacterial lipids, and Archaeal txn machinery, we still have to figure out where Eubacterial txn came from. I've got no idea! I'll have to read that paper before I comment further, though!