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How inevitable was the origin of life on Earth?

MAJIKTHISE: Seven-and-a-half million years? What are you talking about?

DEEP THOUGHT:
I said I'd have to think about it didn't I? And it occurs to me, that running a program like this is bound to cause sensational public interest.

Evolutionary biologists mainly study how life has diversified and changed or how it is diversifying and changing, rather than how it originated. But I thought this article was really interesting. "Chance or necessity? Bioenergetics and the probability of life", published in the Journal of Cosmology by Nick Lane (University College, London), doesn't have any original data, but cites lots of other papers that do.

This journal publishes some strange stuff, but journal review processes are never more than an somewhat-useful filter. If you can't get your hypothesis or data published in any peer-reviewed journal, you're probably a crackpot. On the other hand, there's no guarantee that a paper published in a top journal will stand up to subsequent criticism. Anyway...


First, Lane argues that "the emergence of life is probable on any wet, rocky planet." Serpentization, a chemical reaction between water and rock, leads to complex mineral structures on the sea floor, with moderate temperatures, cell-size pores, and gradients in hydrogen-ion concentration similar to those which, when they occur across cell membranes, power life (ATP synthesis, rotation of bacterial flagella, transport across membranes, etc.). Given these conditions, he suggests, life is virtually certain to arise eventually.

On the other hand, he argues that complex life is far from inevitable. If cellular processes depend on energy stored as hydrogen-ion gradients across membranes, then cells can't get very big. As a cell doubles in diameter, its volume goes up by a factor of 8, while its surface membrane area only goes up by a factor of 4. To get more energy per unit volume, cells need more membranes-with-gradients per unit volume. We humans have solved this problem. We just pack our cells with mitochondria, each a little power plant with its own membranes. It's now generally accepted that mitochondria, which have their own DNA, are descended from symbiotic bacteria. Lane provides various lines of evidence that plants, animals, and fungi are all descended from the same mitochondrion-containing ancestorm i.e., that this key step only happened once. Various single-celled organisms that look like intermediate steps, he argues, are instead examples of secondary losses of complexity. For example, cells with real nuclei, like ours, but without mitochondria, are all apparently descended from ancestors that had mitochondria and lost them.

For more detail, read the paper. But I do have a couple of comments.

First, if cell-size pores with hydrogen-ion gradients and a few other characteristics (simple organic molecules that can arise from nonbiological processes, minerals that act as simple catalysts, etc.) have a high probability of producing life, can we repeat the process in the laboratory? I'm thinking lots of ceramic sheets, each with billions of cell-sized pores, with different inorganic chemical solutions on the two sides to generate the right gradients. Of course, it might take a few million years to get results. Then again, it might not. Seems like it might be worth trying.

Second, is there some good reason why bacteria couldn't get bigger and more complex, without increasing their volume:membrane ratio, by growing as branching filaments or nets?

Comments

Look up Epulopiscium, which grows up to a million times the size of a normal e. coli by multiplying its entire genome to 100000 copies or more in order to be able to manufacture key enzymes close to every spot in the membrane. It works but with only a tiny fraction of the efficiency of mitochondria which only have to retain key genes for local control and leave the rest to the nucleus. Epulopiscium can only afford it because it is an intestinal parasite. Branching filaments would have similar problems.

Mark,

Thanks for the interesting comment, but I don't get your last sentence. Branching filaments would have the high surface:volume ratio of small cells, but the overall structure could be large and complex.

Without a nucleus to hand off all but the genes needed to handle respiration requirements on the spot, like cytochrome c oxidase, Epulopiscium and those filiments just aren't going to be able to compete in terms of energy efficient replication with either mitochondria or bacteria/archaea with much fewer copies of the genome. Those filiments are going to need all those multiple copies just as much as Epulopiscium does or risk losing control of the respiration process.

Perhaps this recent paper by Lane and Bill Martin will help.

http://www.nature.com/nature/journal/v467/n7318/full/nature09486.html

If you don't have money to burn, these might help:

http://www.newscientist.com/article/dn18734-why-complex-life-probably-evolved-only-once.html

http://esciencenews.com/articles/2010/10/20/energy.revolution.key.complex.life

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