I learned many interesting things at this weekend's MIT Center For Art, Science & Technology Symposium, but there is one fact that totally blew my mind. In her fascinating and wide-ranging talk on multi-dimensional space and human consciousness, Tauba Auerbach briefly mentioned the fact that after an organism dies its amino acids will gradually change "handedness" — from an entropy defying left-handed favoritism back to 50-50 over many thousands of years.
Non-biological amino acids produced in organic chemistry labs come in equal mixtures of their mirror-image flavors. In living cells, however, amino acids are almost entirely left handed, with only a few exceptions. All new protein chains made by a living cell will be have 100% left-handed amino acids, but over time a tiny percentage will randomly flip to being right handed. Proteins that aren't frequently recycled in the body will accumulate these flipped amino acids over the course of a lifetime. Using an estimate of around one flip per thousand amino acids per year, researchers have been able to guess the age of humans by looking at the handedness of the amino acids in their tooth enamel, or the age of whales by the amino acids in the lenses of their eyes.
Dating organisms that have been dead for a long time using the handedness of amino acids is trickier because the rate of random flipping depends on the temperature (in a warm blooded animal like a human or whale, the temperature is stable enough over time to give a pretty good estimate while it's still alive). In places with a similar "thermal history," amino acid dating can be used to determine the relative ages of organisms over much longer time-scales.
Astrobiologists think about the handedness of amino acids a lot too. The fact that all living things on Earth use the left handed version leads to many intriguing questions: Is there a "shadow biosphere" of right-handed organisms that we haven't been able to detect? Does the slightly higher proportion of left-handed amino acids found in meteorites indicate the existence of extraterrestrial life? How did life "choose" the left-handed version?