Chicken, Egg, and Soup

Comments upon reading Iris Fry's The Emergence of Life on Earth Chapters 9-11

Which came first: the chicken or the egg? That is the fundamental question in the origin of life. Evolution manages to kick this can down the road a bit: the chicken and egg evolved together from earlier egg layers which in turn evolved from more primitive reproduction schemes. We even have samples of the more primitive reproduction schemes around to encourage us.

The discovery that life processes could be reduced to biochemistry, together with the Oparin-Haldane hypothesis positing a reducing atmosphere for the primeval Earth, encouraged origin of life researchers to believe that organic chemistry might naturally produce the molecules of life. However, developments from 1960 to the nineteen-eighties put a couple of severe dampers on such optimism.

First, and probably most grievously, explication of the mechanisms of heredity and metabolism revealed an intrinsic complexity that seemed guaranteed to prevent any easy synthesis of the substrates. Life as we know it depends critically on two very specific and chemically delicate types of polymers: proteins and nucleic acids. More crucially, proteins can't be made without nucleic acids, and nucleic acids can't reproduce without proteins. This presents us once again with the chicken and egg problem in a more fundamental form.

The second new problem was that the notion of an early Earth with a reducing atmosphere became dubious. It now looks likely that CO2 and H2O were the common molecules in the early atmosphere instead of NH3 and CH4. This makes early organic synthesis much more difficult and calls into question the notion of a rich prebiotic organic soup.

A third constraint also appeared. The early Earth was bombarded with a heavy rain of plantesimals that melted the surface and almost certainly made the place uninhabitable until about 3.85 billion years ago, but evidence of fairly sophisticated microbial life is clear by 3.5 billion years ago, and suggestive much earlier, perhaps 3.8 billion years ago. This appears to show that life arose very fast.

The idea of the "naked gene" arising from an "organic soup" was a clear casualty. There was no obvious way it could arise, and even if it did, no plausibility that it would survive and reproduce.

This crisis produced some new ideas. One of them is the idea of hetero cycles, collections of fairly small molecules catalyzing each other. Another biggie was RNA world. RNA plays at least three essential roles in mediating the conversion of DNA's information into proteins. Is it possible that RNA came first, before DNA and before proteins?

The components of RNA are somewhat simpler to synthesize than those of DNA. Unlike DNA, RNA tends to fold up in ways that suggest that it could have catalytic activity. The discovery of ribozymes - enzymes that are RNA instead of protein - confirmed that insight and quickly became a major focus of research. It too hits a very hard barrier though - initial formation of a set of ribozymes precise and complex enough to replicate seems too improbable to be credible.

Many became discouraged enough to adopt the counsels of despair - perhaps life did not originate on this planet, but somewhere else. Maybe primitive bacteria arrived on comets or on interstellar space craft. Clearly each possibility is very close to surrender to intelligent design.

To be continued...