Tuesday, July 03, 2012

Was the Origin of Life a Lucky Accident?

Nick Lane has written a very nice article in the June 23, 2012 issue of New Scientist: Life: is it inevitable or just a fluke?.

Nick Lane is a biochemist who works on the origin of life at University College London (UK). He's also a science writer who recently won the Royal Society Prize for Life Ascending: The Ten Great Inventions of Evolution. His new book will be called, CHASM: The Outlandish Origins of Complex Life.

Lane is one of the main proponents of the "Metabolism First" view of the origin of life. The idea is that life began with the evolution of an energy source much like the main source of energy in today's cell. The energy is derived from a gradient of proton concentration across a membrane. This was first discovered by Peter Mitchell (chemiosmotic theory) and it was one of the most important discoveries in biochemistry—although most people, including most biochemists, haven't got a clue what it's all about [Ode to Peter Mitchell] [Why Are Cells Powered by Proton Gradients?] [Metabolism First and the Origin of Life ].

The ability to generate usable energy in this manner could have arisen near thermal vents in the ocean where the gradient exists between the alkaline interior of small cavities in the rock and the external seawater. The energy was then used to build up small organic molecules such as sugars and amino acids. Early metabolism arose when pathways for the simple molecules of life evolved using simple inorganic catalysts and/or simple peptide catalysts. "Metabolism First" is a much more reasonable scenario, in my opinion, than "Prebiotic Soup" [More Prebiotic Soup Nonsense].

If this view is correct then the origin of life depends only on the presence of simple chemical systems that create an energy producing gradient and this is likely to happen on many planets. However, the leap from simple cells to complex cells may have been the rate-limiting step in the evolution of sentient beings.

Lane suggests that large complex cells with large genomes could only have arisen if they discovered an abundant source of energy. That happened when some simple bacteria formed an endosymbiotic relationship with bigger cells to create a new life form with mitochondria [Energetics and genetics across the prokaryoteeukaryote
divide
]. This unique event was not inevitable. It happened only once in the history of life—about 2 billion years ago. This sort of lucky accident may never occur on other planets so it may just be a fluke that sentient beings evolved on Earth.

We may be alone in the universe.


14 comments :

  1. From what little I've read about this debate, it seems to me that astronomers tend to be more hopeful about the incidence of life throughout the universe than biologists are. I'm guessing that's telling.

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    1. I don't think people are skeptical of a bacteria-like life popping up. It's more the civilized, science-using, willing-to-communicate-with-us variety that leads to debate.

      -The Other Jim

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  2. That's because astronomers have a more accurate sense of perspective on how big the universe is.

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  3. and astronomers usually know nothing about biology. you don't see many biologists/biochemists prognosticating on the prevalence of extrasolar planets (for example). we leave that stuff to the astronomers.

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  4. We may be alone in the universe.

    I agree, it's very possible.

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  5. I must say I'm a bit surprised about the "bias" of the article - it assumes that what happened on earth is the only way life can evolve elsewhere. My problems are two-fold.

    Firstly, it is assumed here that the only way for a large complex cell to find a high-efficiency energy source is via endosymbiosis. I am unaware of any limitation that wouldn't prevent a "simple" high-efficiency cell to later evolve into a large complex one. At some point eukaryotes presumably evolved the complex parts that make them complex - i.e. the endomembrane system and "complex" genome; I am unaware of any limitations that would prevent some alien mitochondria-like organism from undergoing the same process.

    Secondly, it is also assumed that endosymbiosis itself is rare. We're not even sure this is true on earth - it happened at least twice with eukaryotes (mitochondria and chloroplasts), and other examples of endosymbiotic relationships are not all that uncommon in more discrete clades (i.e. nitrogen-fixing bacteria in the root cells of legumes, algae in corals, etc). Hell, most pathogens - be they of humans or bacteria - live inside of other cells. Not symbiotic, but definitely endo.

    And now that I think of it, I have a third problem with this. There are eukaryotes who have lost most of their mitochondria, and yet despite that deficiency (and a total dependency on anerobic metabolism*) they seem to have no issue being "big complex cells".

    *the residual mitochondria in these organisms tend to be involved in non-energy functions like lipid synthesis

    Dumb question for you Larry - does the idea that early amino acids were formed on earth, via catalysis by inorganic materials, count towards the "metabolism first" or "soup" models? I always associated that with the later (i.e. it was where some of the soup came from), but your comments seem to suggest otherwise.

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  6. Well, in my opinion Lanes views on the evolution of eukaryotic cells are the most obvious sort of non-sense (i.e. the kind that is demonstrably false). I probably shouldn't let a single paper full of non-sense color my view of someones work, but I REALLY dislike that paper. I've never found anyone who agrees with it.

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    1. I certainly understand why you prefer to remain anonymous!

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    2. Well! Thanks for a thoughtful comment. I was totally convinced by Lane's argument but you've turned me around completely with your highly sophisticated and well substantiated criticism.

      It's Lane's, nonsense and someone's, by the way. I hate to see a jewel like this spoiled by bad grammar.

      Are you a parson, by any chance?

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  7. It happened only once in the history of life—about 2 billion years ago. This sort of lucky accident may never occur on other planets so it may just be a fluke that sentient beings evolved on Earth.

    We may well be alone for other reasons, but the argument that something is a fluke because it only happened once has always bothered me. I think there is a possibility that something only happens once or rarely because the first lineage to get it right will exclude everybody who tries it next.

    Think of it as a severe distortion of the fitness landscape: there was a steep slope upwards before somebody could do it, but now that they are there everybody who "mutates in that direction" faces a deep valley of not being as good at it as the first to evolve that way.

    My favorite example is the colonization of land. Surely that is fairly easy when there is nobody on the land. But once that "niche" is filled, it is a completely different story. Only one group of plants every managed it, and if you try to imagine a contemporary red or brown alga trying to compete with mosses or vascular plants you immediately see why.

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  8. Generally the null hypothesis should be that the observed situation is not privileged. For example, we live on an earthlike environment planet. It is reasonable to suppose that other stars we see similar to the Sun will have similar planets. Indeed, such now appears to be the case. Similarly for the evolution of complex life on such planets.

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  9. It seems to me that biologists understand that life is far more complicated than people in the nonliving sciences who deal with things that are a lot simpler.

    You're never going to be able to say anything you can relate to the origin of life on Earth unless you find physical evidence directly related to that event, you're at even more of a disadvantage when you want to say something about the origin of life somewhere other than Earth. You might as well just skip the sciency math and chemistry and make up stories out of whole cloth if the goal is saying something you can be reasonably certain of about the actual universe. Your chances of guessing correctly don't seem to be very great as you don't even know what the target is or where it might be found.

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  10. Glad you enjoyed the article, but Nick is definitely not saying that "large complex cells with large genomes could only have arisen if they discovered an abundant source of energy".

    Rather, he is saying that when cells derive energy from a gradient across their membranes, they run into fundamental problems as the volume of the cells increases, and that these problems cannot be solved by incremental mutation and selection.

    Accuracy, accuracy, accuracy...

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  11. Strange it is that we hear in this summary of the leap from simple cells to complex ones, but we don't hear of the leap to simple cells. And even stranger, we now know cells are not (in the context of origin of life questions) simple--and Prof. Moran most certainly knows this. The first cell is really the main question.

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