abiogenesis: a goer?
Abiogenesis was a term coined by T H Huxley in the nineteenth century. He wrote an essay, ‘Biogenesis and Abiogenesis’, first published 1870. In the essay he imagined himself on the planet eons ago, ‘a witness of the evolution of living protoplasm from non-living matter’. Of course we have no such witnesses, and so we can only speculate, and experiment, and search the universe for other life forms, or conditions supportive of life (as we know it, or maybe not as we know it). Creationists are generally convinced that the principle of abiogenesis is undiscoverable, that there is no other life, that it cannot be created by humans etc, but hey let’s leave those guys on the sidelines for a while (they really are scary). There’s a fair literature on the subject, e.g. The Spark of Life: Darwin and the Primeval Soup, by Christopher Wills and Jeffrey Bada (excellent, apparently, as a general overview), and The Molecular Origins of Life: assembling pieces of the puzzle, by Andri Black (for the more technical, scientifically educated reader). Other writers on this subject include Christian De Duve, Thomas Gold, John Maynard Smith, Robert Shapiro and Lynn Margulis.
The abiogenesis idea (and that of exobiology – life elsewhere) received something of a boost from the experiments of Stanley Miller back in 1953. Sending an electric current through a mixture of methane, ammonia, hydrogen and water, he was able, apparently, to ‘create’ organic compounds such as amino acids. There’s a very interesting 1996 interview with Miller here. I’ll summarise some of the data from that interview. The Earth is quite reliably dated as 4.55 billion years old. The earliest known life forms on Earth date back some 3.8 billion years. We don’t know what the pre-biotic Earth’s atmosphere was like precisely, but it is generally assumed, but that might be putting it too strongly, that the Earth had a reducing atmosphere, one that contained ammonia, methane, hydrogen and water. Such an atmosphere is found in all the outer planets of our solar system. Some speculate that some enabling molecular compound was introduced from outer space, on comets or meteors, though nobody has yet suggested what precisely this compound might be. There’s also a panspermia theory which we won’t dwell on here – it’s about life being everywhere, and having no beginning except in the sense that the universe had a beginning. Problems there, obviously. Whether life arrived here from elsewhere, or originated here, the origin problem is basically the same. It seems more likely though that life began on Earth, and independently elsewhere.
As to the spontaneous generation of life, Pasteur’s experiments in the mid-nineteenth century refuted the popular notion that life could sprout and teem from a bundle of lifeless rags. This did not necessarily refute the idea of abiogenesis, as some wish to argue, it merely shows that organisms do not spring from non-living material as a matter of course, all the time. The origin of life on Earth is a one-off, which is one of the reasons scientific investigators have such difficulties with it. Science is more often about discovering general principles to explain more or less common events.
The idea of the reducing atmosphere was first posited by the Russian scientist Oparin, who kicked off modern explorations of the origin of life in 1924. His first important idea was the heterotrophic hypothesis, the idea that the first organisms were heterotrophic, obtaining organic material from outside themselves. He also suggested that it is easier to form organisms where there is less biosynthesis (I don’t really understand what this means). His idea of the reducing atmosphere was independently arrived at by Harold Urey, under whom Stanley Miller conducted his famous experiments. Interestingly and importantly, a meteorite that landed near Murcheson in Australia some years later was found to contain many of the same amino acids in roughly the same proportions as developed in Miller’s work.
Reflections on the nature of the first replicated molecule led Miller, and others, to consider RNA and pre-RNA. We start getting into complicated chemistry here, in dealing with racemic mixtures, D and L amino acids, asymmetric carbon and peptide nucleic acid (PNA). Anyway, Miller speculates on the role of amino acids, the prebiotic conditions for life (drying lagoons and ocean borders are likely candidates, and temperature is v important) and the evidence from Mars. It’d be interesting to get an update on some of this stuff, as the field of abiogenesis has burgeoned in the ten years since this interview. More on that later.
The abiogenesis idea (and that of exobiology – life elsewhere) received something of a boost from the experiments of Stanley Miller back in 1953. Sending an electric current through a mixture of methane, ammonia, hydrogen and water, he was able, apparently, to ‘create’ organic compounds such as amino acids. There’s a very interesting 1996 interview with Miller here. I’ll summarise some of the data from that interview. The Earth is quite reliably dated as 4.55 billion years old. The earliest known life forms on Earth date back some 3.8 billion years. We don’t know what the pre-biotic Earth’s atmosphere was like precisely, but it is generally assumed, but that might be putting it too strongly, that the Earth had a reducing atmosphere, one that contained ammonia, methane, hydrogen and water. Such an atmosphere is found in all the outer planets of our solar system. Some speculate that some enabling molecular compound was introduced from outer space, on comets or meteors, though nobody has yet suggested what precisely this compound might be. There’s also a panspermia theory which we won’t dwell on here – it’s about life being everywhere, and having no beginning except in the sense that the universe had a beginning. Problems there, obviously. Whether life arrived here from elsewhere, or originated here, the origin problem is basically the same. It seems more likely though that life began on Earth, and independently elsewhere.
As to the spontaneous generation of life, Pasteur’s experiments in the mid-nineteenth century refuted the popular notion that life could sprout and teem from a bundle of lifeless rags. This did not necessarily refute the idea of abiogenesis, as some wish to argue, it merely shows that organisms do not spring from non-living material as a matter of course, all the time. The origin of life on Earth is a one-off, which is one of the reasons scientific investigators have such difficulties with it. Science is more often about discovering general principles to explain more or less common events.
The idea of the reducing atmosphere was first posited by the Russian scientist Oparin, who kicked off modern explorations of the origin of life in 1924. His first important idea was the heterotrophic hypothesis, the idea that the first organisms were heterotrophic, obtaining organic material from outside themselves. He also suggested that it is easier to form organisms where there is less biosynthesis (I don’t really understand what this means). His idea of the reducing atmosphere was independently arrived at by Harold Urey, under whom Stanley Miller conducted his famous experiments. Interestingly and importantly, a meteorite that landed near Murcheson in Australia some years later was found to contain many of the same amino acids in roughly the same proportions as developed in Miller’s work.
Reflections on the nature of the first replicated molecule led Miller, and others, to consider RNA and pre-RNA. We start getting into complicated chemistry here, in dealing with racemic mixtures, D and L amino acids, asymmetric carbon and peptide nucleic acid (PNA). Anyway, Miller speculates on the role of amino acids, the prebiotic conditions for life (drying lagoons and ocean borders are likely candidates, and temperature is v important) and the evidence from Mars. It’d be interesting to get an update on some of this stuff, as the field of abiogenesis has burgeoned in the ten years since this interview. More on that later.
posted by Stewart at 10:37 am
0 Comments:
Post a Comment
<< Home