They way that bearded capuchins crack nuts open is a window into human evolution-- specifically, the evolution of tool-making.
These monkeys don't just bash indiscriminately. First of all, they take years of experimenting with different types of nuts and hammers to find a winning combination (just like humans, they must learn effective tool use). Second, the monkeys use "anvils," such as log and boulders, to hold the stone and help with the cracking.
Thirdly, and mot interestingly, the monkeys understand how much force is required to open a particular nut. They make a strike, then take a look at the result and modify their force for the second strike. This explains why the monkeys are able to get at the tasty insides of the nut without smashing it apart!
This evidence of manual dexterity in monkeys suggests that nut-cracking may have been a precursor to stone tool-making in our human ancestors. The motion is relatively similar, and dexterity-- and force modulation-- are both necessary aspects of stone tool-making. Thus, this behaviour provides one more valuable clue about the evolution of tool use in hominids; the observation of intermediate states is necessary to understanding the evolution of a trait.
New observations in the southeastern state of Piauí, Brazil (map) reveal that the animals carefully regulate the force they use in nut-cracking. After each strike, the monkeys evaluate the condition of the nut and then tailor the force of the next blow accordingly. (See National Geographic's monkey pictures.) That's called dexterity, "a very surprising skill we never expected to find in a non-human animal," says Madhur Mangalam, also of the University of Georgia at Athens. (Read about how clever crows use one tool to acquire another.)