The primate in question is the common marmoset (right), NOT the pygmy marmoset (left) which fits in the palm of one's hand.
Below is the brief story from the Smithsonian, entitled the "Evolution of Sharing".
Common marmosets display what researchers in Zurich call "unsolicited prosociality"—that is, they will slide a tray of food to a marmoset in a nearby cage even if there's no chance of getting something in return, and even if the other animal is unrelated. So far, they're the only animals other than human beings known to be so altruistic. Whether they say "Please" and "Thank you" is unknown.