[quote=Kaga]
Found it!
[/quote]

Glad I watched this first, cuz that's a large part of what I was gonna say (the cancerous-growth and anti-cancer gene controls). The other operative theory I've heard about is, there's a pretty strong correlation in most animals between heart rate and lifespan; basically, the mechanical structures in your organs wear down with use, and while you can regrow most natural organic damage, it does eventually take a toll. Even a well-oiled engine eventually fails. [url=http://gizmodo.com/5982977/how-many-heartbeats-does-each-species-get-in-a-lifetime]We get more beats-per-life than most of the other animals[/url], but still, in principle, there's an average limit out there and most living things adhere pretty closely to it. Obviously, there's more than just a heartbeat going on in your squishy-parts at any given time -- your liver is in constant use, your kidneys are in constant use, hell, even your BRAIN turns on every once in a while. So your life is a constant battle between mechanical breakdown and organic regrowth -- and that's what ultimately drives cell death, cell mutation, and eventually cell dysfunction, organ failure, and flatlines.