I was looking through the list of human universals in the back of Steven Pinker's The Blank Slate the other day when I saw an item I hadn't previously noticed: sucking wounds. Then I remembered reading something about a compound in dog's saliva having been identified that speeds up healing. I couldn't find the dog saliva story again, but did find a study (press release) published back in July that concluded human saliva promotes healing. Specifically, saliva contains a peptide called histatin that not only kills bacteria, but seemingly accelerates wound closure. This finding, together with the observation that wound licking is a human universal, is exceptionally interesting, but before going on to speculate rather wantonly about its significance, a couple of caveats are in order. Firstly, the study was conducted in vitro, not in vivo, and thus its net clinical significance is unknown. Secondly, the researchers used epithelial cells - those that line the cheeks - in their experiments and so one has to extrapolate, if fairly reasonably, to the conclusion that saliva accelerates wound closure in other types of cells. (Though, there is some evidence that saliva promotes healing in skin cells). Finally, the research I cited is quite preliminary, and thus will have to be replicated before we can be confident about its findings.
Doubts aside, this is fascinating stuff. Why? Because if wound licking really promotes healing (but not obviously so), and all human beings lick their wounds, then it's plausible to suggest this behavior is adaptive. Or, in other words, if it increases fitness and if nearly everybody does it, it's possible that licking one's wounds is a behavior that evolved by natural selection. (Do note that I say 'plausible' and then 'possible'. The evidence I've presented here is merely suggestive and far, far from definitive). Moreover, if wound licking behavior occurs widely among mammals - and a couple of searches in Google Scholar suggests it is, including among several species of non-human primate - then it might be a very ancient behavior indeed. That is, because specific characteristics that different but related organisms share is likely to be inherited from a common ancestor, wound licking could have arisen tens of millions of years ago.
So next time you instinctively pop your injured finger into your mouth, remember that you're engaging in a behavior that may be significantly older than even the hominid lineage. You animal you.