In the novel they imply that it's a punishment among the cannibal
clans to cut peoples' fingers off (a relatively minor one,
apparently), though it only features once in the book near the end
when an outcast from one of the clans steals all their stuff while
they are foraging and they have to take it back. However, the guy is
missing all the fingers off of one hand, not just a thumb. Hard to say
what liberties the movie took. I have seen it yet because it isn't
playing here.
One might also suppose they meant to imply that people gnawed their
own thumbs off out of starvation but if the movie doesn't explain it
and it ain't from the novel who the hell really knows? One would think
if they were going to make a point of showing something like that they
would feel compelled to explain it rather than make one guess at the
signifcance, even if it's from the book. that are apt to be folks who
didn't read the book but will see the film and so you would think you
could expect a bit of exposition. Would be easy to have the kid ask
his dad what was up with the thumbs and the dad explain it to him and
to th audience at the same time, done deal. And maybe they did but the
missing thumbs "ended up on the cutting room floor" as they say in
hipster moviespeak...
Okay, thanks.