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Bacterial fossil doubts resolved

I raised a few questions about those 3.4 billion year old bacterial fossils, primarily that I was bugged by the large size and that they cited a discredited source to say that they were in the appropriate range of diameters for bacteria. Now my questions have been answered by Chris Nedin, and I’m satisfied. In particular, he shows data from 0.8 and 1.9 billion year old fossils in which the bacterial sizes are in the same range. It’s also a good review of the other evidence used to infer that they actually are bacterial microfossils.

(Also on Sb)

Comments

  1. Nom de Plume says

    No, don’t link to Chris Nedin! His blog is my little secret. Well, was.

    What can we infer from these fossils in terms of how long life must have been around at that point (3.4bya)? Would it be reasonable to assume that it must have arisen at the dawn of the archean, or is that too much speculation?

  2. says

    Related only under the broad category of evolution…

    Did anyone else see “Life after Dinosaurs” on NatGeo? I had never heard of some of the early mammals, but the CGI was weird and kind of funny.

  3. mck9 says

    I have a pet hypothesis about why early prokaryotes might have been larger than what we’re used to. I’m no microbiologist, so I may be completely off base, but I’m willing to be corrected by my betters.

    For cell division to be successful, each daughter cell needs to get a complete set of genes from the parent cell. A mistake might be occasionally be tolerated, or even beneficial, like any other kind of mutation, but most of the time it would be harmful if not fatal.

    Modern cells have more or less elaborate mechanisms to ensure an even division of the genes between the daughter cells. Eukaryotic cells form a mitotic spindle to separate the two sets of chromosomes. Then they divide along a plane transverse to the long axis of the spindle, so that the ends of the spindle wind up in separate cells, along with the chromosomes attached to them.

    Prokaryotes presumably do something comparable when they divide. Maybe each copy of the chromosome (typically a single circular chromosome) attaches the cell wall at one end or the other. Probably someone here can explain how it works.

    In very early cells, this kind of partitioning may not have evolved yet. Cell division might even have happened, not through any kind of machinery devoted to the purpose, but by more or less accidental mechanical breakage of the bag of goo that we call a cell, followed by a resealing of the lipid bilayer around each of the fragments.

    In such a regime, the best bet for survival would be for each cell to maintain multiple copies of its genome, spread throughout the interior in a sort of prokaryotic syncytium. When the cell breaks, for whatever reason, each major fragment (with any luck) will contain at least one complete copy of the genome.

    This strategy would favor large cells and small genomes, to improve the odds that a broken-off fragment would contain at least one copy of the genome.

    The presence of multiple copies would also help overcome one drawback of large size: the difficulty of getting things to diffuse fast enough over longer distances, in the absence of the intracellular transport machinery typical of eukaryotes. If there are multiple copies of the genome, then no part of the cytoplasm needs to be very far away from one of the copies.

    Diffusion might still be a problem for other things. However long stringy things (such as RNA transcripts) and big bulky things (such as ribosomes) don’t diffuse as well as smaller things (such as sugars and electrolytes). If you can solve the diffusion problem for things that diffuse slowly, then you can grow big enough that diffusion becomes a problem even for things that diffuse quickly.

    Since those early cells are long gone, I don’t see an obvious way to test my hypothesis. However it wouldn’t surprise me to see echos of the early regime even in modern cells.

    Are there some kinds of large prokaryotes where it’s normal to have multiple copies of the genome in the same cell?

  4. says

    Oh good. I wasn’t sure whether or not to accept this since you (who are a clear expert in the area and know a lot more about it than I do) had issues with it, and the issues as you explained them seemed worrisome. Thanks for keeping us updated. This is really neat.

  5. says

    mck9 – my simpler hypothesis would be that earlier cells were lucky to exist at all and likely less tuned for efficiency. Smaller cell-size gave greater efficiency (less waste) and greater survivability (until mutations that gave access to greater resources gave those strains other advantages).

    And if Szostak (et al.) are correct, in part, the first “cells” may have been just natural lipid vesicles which would be much larger and more porous.

    A move from larger, more porous, less well-organized structures towards smaller, more survivable, controlled structures would seem to be supportive of this.

  6. Nix says

    Smaller cells give greater efficiency to this day, because bacteria generate energy the same way we do, by stripping electrons from food and using them to pump protons across a membrane (for bacteria, their outer cell membrane, so they end up trapped between that and the cell wall), then using the resulting membrane potential to power an ATPase. So the bigger the surface area relative to the volume, the more ATP they can generate and the faster they can replicate. (Eukaryotes are freed from this trap because we have internal mitochondrial membranes to do the job and don’t need to use our external cell membrane.)

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