Greg EganSamples from asteroid Bennu contain A, G, C, T & U nucleotide bases, and 14 of 20 amino acids used by life — but while we use only left-handed versions of these molecules, Bennu has a roughly equal L/R mixture, puncturing theories that the bias on Earth came from an initial cosmic seeding.
Martin Vermeer FCD@gregeganSF It also confirms that these can be produced by non-life processes.
dagelf@dagelf @gregeganSF Because life processes are known to propagate the chirality of molecules, once established. It all derives from the original chirality of DNA, or whatever molecule served as genetic carrier on the hypothesized parent body of Benno.
It is true though that we already knew this for some organic molecules thanks to lab experiments going back to Miller-Urey. The interesting thing is finding one natural environment in which this happened.
It is true though that we already knew this for some organic molecules thanks to lab experiments going back to Miller-Urey. The interesting thing is finding one natural environment in which this happened.
@martinvermeer @dagelf @gregeganSF I wouldn't use the term 'confirm' though, I mean, life could use, and produce both. Isn't an abundance of such nucleotides most likely attributable to living processes? As is an imbalance?
@dagelf @gregeganSF No, that's not how this works. Yes, in the original pre-biotic 'soup' there will be a 50/50 mix, racemic. But the moment anything self-replicating appears, it will be either/or. And as it spreads, it imposes its chirality on the whole pool of organic materials. And note that also natural selection kicks in: if multiple self-replicators of differing chirality appear simultaneously, one of them will eventually win out and become the 'latest common ancestor' of all life on that planet.
So, a racemic mix is not a problem: abiotic origin. Also a pure one-chirality mix is easy: biotic origin. But, a slightly non-racemic mix poses a problem...
@martinvermeer @dagelf @gregeganSF My thinking would be that yes a prebiotic mix would be 50/50, but would also likely not produce much, where as soon as its biotic there would be an explosion of production...
I'm curious, how can we be sure that life would pick only one? Surely they should be able to compete effectively, much like different species do... even not at all, if they need different resources to propogate? I'm probably lacking some fundamental knowledge about this though...
@dagelf @gregeganSF One possibility is that the reservoir consists of geometrically separated sub-domains, and different replicators of different chirality would take over only the separate parts. But as soon as you have life spreading though one world ocean, this is no longer the case, and there will at some point be one latest common ancestor for the whole ocean. Of course even then, small isolated sub-domains may survive, though on a tectonically active planet typically not long, cf. Lake Vostok...
BTW one way in which a slightly non-racemic mix could originate is if the parent body disintegrates at a point when still primitive life has only taken over a small part of the reservoir, and gets mixed with the prebiotic rest.