Wednesday, December 2, 2009

more speculations on earth's early days

Canto: Going back to the origin of life on Earth, one of the most interesting pieces of early speculation came from Charles Darwin, who wrote to his friend Joseph Hooker in 1871:

It is often said that all the conditions for the first production of a living organism are present, which could ever have been present. But if (and Oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.
Jacinta: Yes, and much later, in the 1950s, the famous Miller-Urey experiment was carried out to try to reproduce life or something like it out of what were then thought to be the conditions of the primordial Earth's atmosphere, including lots of ammonia and methane.
Canto: In fact, the gases they used were those two plus hydrogen and water. Miller then ran a continuous electric current through this mix, to simulate lightning, again believed to be a common feature of the early earth system. After a week, the finding was that more than ten percent of the carbon in the system [provided by the methane] had been transformed into organic compounds, including amino acids, the building blocks of proteins. It seemed almost absurdly easy, then, to create, from these gases, material which was a giant step along the way towards life.
Jacinta: And was this experiment able to be replicated?
Canto: Oh yes and they have pushed further on. One of the interesting things is that, in the years following this experiment, the scientific consensus regarding the earth's early atmosphere moved away from NH3 and CH4 [ammonia and methane] to CO2, CO and N2 from volcanic activity, with UV radiation reducing the ammonia and methane to a short life span. 
Jacinta: But now things have changed again, and it's believed - by some of course - that Stanley Miller and Harold Urey, together with their brilliant predecessor Aleksandr Oparin, might've been right all along. Studies of the outgassing of chondrites [particular types of meteorite fragments representative of early planetary formation] show a preponderance of just the sort of gasses these early experimenters chose to work with.
Canto: So, just how significant was this work, and is it the only pathway to creating life? 
Jacinta: Well, there's the RNA pathway. Apparently, small segments of RNA can form quite easily, though it's less stable than DNA [they're actually very similar molecules], which makes more sense as info-storage molecule essential to life as we know it, but there has been talk for a long time now of a kind of pre-existing RNA world. Though maybe this thesis is already out-dated.
Canto: I don't know about that. Since the early days after Watson and Crick worked out the structure of DNA, when DNA and proteins were all the go, RNA, especially ribosomal RNA, has come to be recognised as surprisingly multifarious in its functions, both potential and actual. 
Jacinta: Good, we'll have to look more closely into all that next time.  

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