Let's start with a fundamental assumption; Life began once and only once on our planet, created from (insert correct answer here, we don't have it yet) to form self-replicating strands of nucleic acids and the proteins for which they code, somewhat like viruses or prions. This isn't that poor of an assumption, given the low probablity of spontaneous generation of life in the first place. It had to happen somewhere, given the numbers of stars and planets, and obviously it happened here or we couldn't write and read blogs written by graduate students of no current academic importance, but proposing it occured multiple times is a bit of a mathematical stretch. If you'd like, we can concede that life was unlikely to start more than once at a time, allowing the possibility that early life began several times sequentially, and that will not affect this dicussion at all. So, given that life occured once, or once to start, then the proteins incorporated in that first organism were likely of one chirality, given the simplicity of early life-like molecules and the coding for proteins written into whichever nucleic acid (likely RNA, from current evidence) was employed. That life had to replicate, using the available amino acids, proteins, and other molecules, and would replicate using the same structure without fundamental changes in the RNA (mutations). Molecules of the wrong chirality couldn't even interact chemically with those of early life, it'd be life trying to use the mirror image of a key. (Gleiser uses the mirror analogy to describe chiral molecules first, and it's apt, so I'm stealing it. Thanks Marcelo, you'll never likely see this, but I'm giving credit anyway)
Briefly, then, what we're seeing in the exclusivity of "left-handed" proteins is a what Dawkins calls a frozen accident, the vestige of the structure of our most distant ancestors and the necessary structures for their replication and survival, all the way down to us. There's no reason to suppose that life on other planets would necessarily follow this blueprint, and actually finding samples would be a requirement for making any statements of that kind.
Gleiser states it thusly:
If one traces life’s origins from its earliest stages, it’s hard to see how life began without molecular building blocks that were “chirally pure,” consisting solely of left- or right-handed molecules. Indeed, many models show how chirally pure amino acids may link to form precursors of the first protein-like chains. But what could have selected left-handed over right-handed amino acids? My group’s research suggests that early Earth’s violent environmental upheavals caused many episodes of chiral flip-flopping. The observed left-handedness of terrestrial amino acids is probably a local fluke.
Obviously we're not far apart on most of this view, it's the "it's hard to see how" statement that vexes me... his local fluke may be much more local than he believes, as in local to the particular location on the planet where life began.
