Well, the wikipedia article on human hair growth says that all hair goes through the shedding cycle, but that it can grow longer by being in the Anagen (growth) phase for a longer period of time. We certainly do shed hairs, at least to a certain degree, so I imagine that the length of time spent in that phase probably varies from person to person..and thus some able to grow hair indefinately, and others have a fixed length that the hair will converge to over time.
So as to how we ended up with having a long Anagen cycle for some hair types (scalp..maybe beard?) is something we'll need to research to understand. Maybe some others know why this is the case, but I'll see what I can dig up.
On the question of evolving to a longer cycle *because* of shaving (and its benefit to avoiding issues with long hair), I hesitate. 30,000 years is not a super long time in the evolutionary sense. That said, there has been quite a bit of controversy in this area recently about how fast evolution can actually occur, with some evidence that significant changes can occur with relatively few generations. I don't recall all of the factors that can accelerate the selection process (I'm an engineer as well)..another thing I'd need to go look at more deeply.
Ah, another quote from that article:
"In most people, scalp hair growth will halt due to follicle devitalization after reaching a length of generally two or three feet. Exceptions to this rule can be observed in individuals with hair development abnormalities, which may cause an unusual length of hair growth." 
Last edited by jamesspo; 04-10-2012 at 09:01 AM.
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