Humans may have evolved altruistic traits as a result of a cultural “tax” we paid to each other early in our evolution, a new study suggests.The research also changes what we knew about the genetic makeup of our hunter-gatherer ancestors. The origin of human altruism has puzzled evolutionary biologists for many years (see Survival of the nicest). In every society, humans make personal sacrifices for others with no expectation that it will be reciprocated. For example, we donate to charity, or care for the sick and disabled. This trait is extremely rare in the natural world, unless there is a family relationship or later reciprocation.