If one person is making almost all of the commits, that a sign there could be trouble. One way to find out (a method I've heard used by consultants to demonstrate the problem to clueless bosses) was to have the Evan-type person stay home for a week -- paid vacation, in a sense -- just to see what would happen in his absence. If the team (or department) grinds to a halt or descends into chaos it's pretty clear there's an unhealthy dependency on one person that needs to be addressed before that person is hit by a bus, wins the lottery, falls in love, or runs away to join the circus.