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A very Darwin-like god ponders what the nature of life will be like. From Monty Python’s The Meaning of Life.
The origin of life presents a number of fundamental difficulties to science. One of these is the seemingly irreducible complexity of life itself. For instance, DNA codes for the molecules that are essential to life. Some of these molecules, however, are the very enzymes that help DNA code for molecules. It is difficult to imagine DNA works without these enzymes, but the enzymes exist in a cell because, in part, of the activities of DNA. Even the basic process of moving molecules into cells to use in the construction of DNA or other molecules requires molecules that are thusly constructed.

What’s an evolutionary biologist to do????

Well, we can start with research, such as modeling ancient systems, looking for clues about the nature of the ancient environments in which life may have originated.

ResearchBlogging.orgWith respect to early life, most of this research assumes that some kind of “proto cell” or cell-like environment existed allowing a subset of the known biological processes to happen. In this view, we start with existing life and take away parts, working backwards to simpler forms, occasionally substituting one kind of system with a different kind of system. For instance, some models of early life have geology providing cell membranes; Bubbles in certain kind of rock, or the space between layers of mica serve as cell-like spaces.

I think there are problems with this approach, but this is a start.

A new paper in Nature addresses one of the problems with early life, specifically, the way important molecules can be transported across a membrane-like protective boundary. In addition, these researchers have made a contribution to the question of replication of DNA-like molecules in early life or protolife.

Living cell membranes are made up of phospholipids. Phospholipid “heads” are hydrophilic (attracted to water), but the tails are hydrophobic (they are “afraid of” water). So, a bunch of phospholipid cells floating around in water tend to arrange themselves into a two-layered configuration with the heads pointing outwards. This mass of molecules forms a membrane impermeable to nutrients or nucleotides. So while the naturally arranged phosophlids form a nice barrier, they do not serve the role of selectively transporting molecules into or out of a cell.

However, some fatty molecules have more complex structures, so when they are included in a membrane, the result is a surface that has room for the passive movement of certain biologically important molecules. The researchers managed to construct such a membrane, and thus created a sort of “protocell” made out of fatty acids likely to have been present on Earth during the early evolution of life.

When single stranded DNA is encapsulated inside of such a membrane and the appropriate molecules are in the environment, these molecules manage to end up inside the protocell and, using the template, replicate the original DNA.

This is not quite enough to fully explain how cells got started, but it is a significant advance in this area of research.

Mansy, S.S., Schrum, J.P., Krishnamurthy, M., Tobé, S., Treco, D.A., Szostak, J.W. (2008). Template-directed synthesis of a genetic polymer in a model protocell. Nature DOI: 10.1038/nature07018

Comments

  1. #1 J-Dog
    June 13, 2008

    Excellent! Thanks for updating me on the latest in this area.

  2. #2 Mike Haubrich, FCD
    June 13, 2008

    This is what’s bugging me about abiogenesis. I think that “eveolutionists” are too quick to say that abiogeneis is not evolution. I don’t see how they can be separated, considering that the definition of life is not fixed.

    I heard on “Science Friday” one day a few weeks ago that someone said definitively and without question that there was a single abiogenesis event. Even as we read through the article on the viability of e. coli which live on citrates at The Loom, we can’t define what a species is when it comes to a bacterium, how can we possibly state that there was a “first cell?”

    The beginning of life had to be nebulous, as are species. Sure, DNA and RNA are common to all life, but as the precursors to cellular life evolved from crystalline or nanobacterial form (or whatever) to live, as phospholipids survived to form membranes they would have been spread through a planet covered by 140 million square miles of water I find it hard to conceive that life only started once and evolution began at that point.

    Life began in fits and starts, and our problem with finding abiogenesis is one of the philosophical definition of life, not a fault of science.

  3. #3 Papilio
    June 14, 2008

    Life began in fits and starts, and our problem with finding abiogenesis is one of the philosophical definition of life, not a fault of science.
    The point is that the first replicators, whatever they were, must have spread rapidly over the entirety of the available habitat (even if it wasn’t the primordial soup of some of the older theories). I imagine it a bit like a wildfire sweeping across the globe, then merely smouldering as most of the available resource is used up – although this may not be an appropriate picture. In fact, one of the problems to be addressed is the question of whether new life originating in an isolated habitat (underground, for example, as some theories suggest) would have exhausted available chemicals quite quickly, leading to extinction of these first replicators.

  4. #4 Papilio
    June 14, 2008

    When I said underground, I meant in a puddle (the slow seep of nutrients underground vs the static nutrients in a puddle, is what I was trying to refer to)

  5. #5 Tom Buckner
    June 14, 2008

    Papilio wrote: “In fact, one of the problems to be addressed is the question of whether new life originating in an isolated habitat (underground, for example, as some theories suggest) would have exhausted available chemicals quite quickly, leading to extinction of these first replicators.”

    Well, one way to deal with the problem of replicators exhausting nutrients is to suppose that this problem does not appear wherever there is what you might call a “sterilizing barrier.” Take for example the deep-sea hydrothermal vents (“smokers”) where superheated water emerges, filled with nutrients. Organisms depend on the smoker, but because the water is so hot as it comes out, no organism can follow the nutrients to the source and exhaust them. The sterilizing barrier in a smoker is the line between livable temperatures (generally below 100C) and temperatures in which no replicator is known to be able to live (water leaves the vents at as much as 400C). So perhaps the first replicators spread like wildfire and then all starved, except around vents.

  6. #6 Owlmirror
    June 15, 2008

    Random layperson thought:

    I’ve seen the hypothesis that replicators came first. I’ve also seen the hypothesis that metabolism came first (in Robert Shapiro’s June 2007 SciAm article).

    It occurred to me to wonder, though, if it couldn’t have been both simultaneously: Replicators forming in a particular set (or sets) of chemical and energy environments; metabolisms forming in different sets of chemical and energy environments…

    And then the replicators infect the metabolisms.

    Wacky hijinks ensue, which eventually result in cellular replicators.

    Anyway, I thought it was an idea worth throwing out there to be chewed upon.

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