My (extremely rudimentary) understanding of Biology is that many of the building blocks of life are chiral and that life on earth is entirely made up molecules with the same chirality.

Does this have any interesting repercussions for our understanding of abiogenesis?

EDIT: Just to be clear I am interested in understanding how life developed from non-life (on Earth in particular and, more speculatively, in the universe broadly). For instance, the article linked in Matt's answer that it may be due to the polarization of light in our area of the galaxy is very interesting.

I see several (not necessarily exclusive) hypotheses (and probably am certainly missing some) about the asymmetry of the chirality of existing lifeforms of and was wondering if there was any sort of consensus (or at least informed speculation) on which was more likely or less.

  1. Abiogenesis is an incredibly rare phenomena and so the opposite chiral life never developed.
  2. Which every chiral form of life appeared first and this prevented the formation of the other chiral form of life.
  3. There is some sort of selection principle that selects for the chirality we appear.
  4. If simple versions of life and mirror life occur simultaneously it precludes or retards the development of complex lifeforms (A sort of anthropic principle).
  5. ``Mirror life" is not actually possible--there is some subtle phenomena that requires the chirality we have on Earth.
  6. Something else...
  • $\begingroup$ Is there no documented evidence of any life using the "other" chirality at all? Across all taxonomic rankings? We are always finding new species such as extremophiles that live in ways we never anticipated. If it could be more strongly stated that it's never been documented in any species that would be an interesting point to add to the question. $\endgroup$ – Karl Jun 10 at 16:44
  • $\begingroup$ Don’t look for consensus, that road leads to phlogiston, look for evidence. So you want to know how life formed from non-life and somehow you can’t find any idea that is supported by convincing evidence on searching the internet. If there was any, wouldn’t you think it would be widely known? Oh, and this is a question and answer site, not a site for speculative discussion. $\endgroup$ – David Jun 11 at 13:28
  • $\begingroup$ @Karl — No. Genome sequencing would have indicated a kingdom completely unrelated to those known. Metabolomic analysis would have detected abnormal amounts of D-amino acids or L-sugars. $\endgroup$ – David Jun 11 at 13:34

As far as I know, there isn't too much we know about why life is "left-handed". So far the prevailing hypothesis is that in space there may be a tendency for amino-acids to favor one chiral-state over another, and there's some very smart people who are testing this, as opposed to the 50:50 shot here on Earth.

To answer your question, however, the general consensus is that life came from non-living material. Pan-spermia, intelligent design, and the rest are more fringe ideas, so the question is how life formed from non-life. One of the ideas is that the basic building blocks came from space (likely a comet or meteor), and if the idea that amino acids favor one chiral direction over another in space can help support that hypothesis.

  • 1
    $\begingroup$ Abiogenesis refers to both the actual way life emerged and fringe ideas, so no need to "call the OP out". I have always imagined that the single chirality was the result of enzymes being usually able to bond to only one chiral izomer substrate. If there was a single ancestor to all current life, it might very well be the reason. It would have been a 50/50 chance for either chirality to take over $\endgroup$ – Francis L. May 10 at 0:56
  • $\begingroup$ @FrancisL. I realized I should probably have been clearer (and less open ended) in my question. One thing I was wondering was whether the fact that there is only one chirality means that the odds of generating life given the right preconditions was actually very rare (as otherwise we would expect things to be symmetric) or if there was some sort of symmetry breaking phenomena. $\endgroup$ – Rbega May 11 at 14:58

There is no reason to expect to find opposing chiral life today even if it did exist during the early stages of life's evolution. Everything not incorporated into more recent organisms has been lost, selective process will destroy early life as better adapted life evolves. Remember the earliest forms of life will be very flimsy compared to what comes later as it has no competition.

Which ever form evolves first will quickly deplete the oceans of available material for other forms to emerge. In addition even if both emerge whichever one emerges sooner has a massive advantage as opposing nucleotides has a tendency to mutually inhibit each other. Once nucleotides are synthesized by early organsms opposing chirality will be completely eliminated until only whichever evolved first remains. Building RNA out of both chirality does work but it is severely limited so it will not persist in the face of much more flexible single chirality.

It is the same reason we don't see codon diversity in life, each may work equally well, but once one evolves it quickly outcompetes and prevents others from evolving.


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