I supplement with vitamin c in gram amounts, and find the effects to be beneficial for me. One theory on this is that the loss of the ability to synthesize vitamin c was a less than beneficial mutation. Along those same lines, we have, apparently, also loss the ability to produce the enzyme uricase which converts uric acid to allantoin. While uric acid seems to have some benefits at low enough levels (even possibly augmenting intelligence to some degree), perhaps the lack of circulating allantoin slows bone and skin healing? Allantoin is considered a very safe topical ingredient with amazing healing properties, but I haven't found much discussion of its potential use as an refined oral supplement. Some herbs that contain it, like Comfrey, have serious downsides to their use over the raw ingredient.
My uric acid levels are normal so I'm not sure supplementing with uricase would be a good idea if that were even possible.
Here is an interesting paper, although it doesn't discuss allantoin supplementation.
http://www.ncbi.nlm....les/PMC2495042/