Re: Can I have a brief overview of the validness of the Hetertroph Hypothesis

As you probably know, this is a particularly difficult question to answer because anything happening so long ago wasn't observed by anyone (or anything, for that matter), and its really difficult to reconstruct what oceans, land and air would have been like so long ago.

For starters,

One important dimension is what we mean by life there's no point in trying to explain the origin of life if we don't really know what we're asking about, is there? There is as much debate here as in the harder science stuff. Many biologists, especially evolutionary biologists, like me, think that we'd call anything that evolved by natural selection "alive" in an important sense. At the moment, that only includes living organisms like animals, plants, bacteria, viruses and probably prions (the tiny molecules that cause diseases like BSE (mad cow disease) and nvCJD (the human form of BSE). It is when we get to tiny things like viruses and prions that the definition becomes important. Viruses may consist of only a couple of different molecules, and don't do very much in the way of biochemistry or metabolism, but they DEFINITELY evolve. I call them alive, but some people only want to call organisms alive if they do that kind of complex biochemistry like respiration, protein synthesis, DNA synthesis etc etc etc that we think of as typical of living things. Viruses can't do these things they hijack a cell to do these things for them, but that's another and even longer story.

Back to the beginning, anyway. If you think that life originated when the first replicating, mutating molecules appeared (something like the replicators that Richard Dawkins talks about in his "The Selfish Gene", although the idea doesn't originate with him - it's an excellent book). Then you're looking somewhere else for the origin of life than people who think that metabolism is all-important. I think Dawkins also has a very sensible and readable discussion about the origin of life in one of his other books probably "The blind watchmaker."

As an extreme example, Graham Cairns-Smith, in his excellent book "Seven clues to the origin of life," suggests that replicating clays could have been the first replicators, and so the origin of these clays is the "origin of life". The idea sounds mad, but it does make sense (although I don't know anybody else who would suggest it as the MOST likely explanation, we can't rule it our). The book is excellent in very clearly explaining the problems associated with thinking about the origin of life, and I definitely recommend you reading it, too. It was reprinted recently (1991, I think).

Most people would suggest that a molecule called RNA was the first replicator, a close relative of the DNA that serves as the molecule of heredity in almost all living things (some viruses still use RNA).

I should also point out that many people think that some kind of primitive metabolic process generating energy originated before any replication, and that this was the most important event in the origin of life. They can suggest various combinations of reactions that could appear on certain sorts of environments - funnily enough, the surface of the same clays that Cairns-Smith talks about are popular suggestions - where molecules could reach sufficiently high concentrations that metabolism in some sense could appear, whether using sunlight as energy (photoautotrophy) or using inorganic molecules (chemoautotrophy) such as sulphates, nitrates etc. etc. There’s not much evidence for any of these that I’m aware of, but several are vaguely plausible.

Whatever route people prefer to thing of the first living systems originated by, there are still many problems to solve. Some of the hypotheses answer some problems better than others, but none of them are known to be correct or more correct than the others. I think that the heterotroph hypothesis is among the most sensible, but would want to point out that there are serious doubts as to whether a suitably rich "organic soup" of molecules could be generated by the sort of processes modelled in the experiments of Stanley Miller and others. Most people think that the processes would have to be confined to some particular location for a sufficient build up of molecules to allow any reasonable chance of, for example, two ribonucleotides to collide to form a simple RNA chain. Suggestions for these locations include ocean foam, around deep-sea hydrothermal vents, in clouds and even in what basically boils down to a common or garden rock pool! Take your pick.

Briefly, I'm not sure what you mean about the "evidence from space." There is no particularly convincing evidence that life has ever existed anywhere else in the universe, and the composition of the clouds of organic molecules that exist in space aren't really exactly what's needed for life to originate. And if we argue that life arrived on earth from elsewhere, whether on a spaceship or meteorite, that still leaves us with the problem of explaining how life originated elsewhere!

All hypotheses about the origin of life have to answer a number of important questions, and all of them have to propose a certain amount of blind luck or chance for the critical steps to occur. There is a great deal of research on the origin of life, including looking at conditions on the young earth, theoretical work on what the earliest living things would have been like and experiments on how simple molecules interact to form more complex molecules and simple metabolisms. Even so, the question is still "which hypothesis is least unlikely", and its to some degree a matter of personal judgement. The heterotroph hypothesis is still probably a front-runner, but many of the steps that must have occurred for life as we know it to appear still seem frighteningly unlikely to many biologists, but the fact that we're here today tells us that something like these ideas suggest must have happened, and I'd argue with anybody that the heterotroph hypothesis is infinitely more likely than any divine intervention!

Thanks, Kera, for a fascinating question and I hope I've gone someway to explaining the very real problems in thinking about this. I'd very much encourage you to read Cairns-Smith's book, and there are plenty of text books available that probably discuss the more "classical" ideas such as Oparin and Miller's work in detail - Im afraid I'm not really in a position to recommend one in particular.

Come back and ask lots more challenging questions in future,

Cheers,

James