Humankind has made great conceptual and technological advances since we first walked the Earth. Thanks to our ability to build on others' ideas, we've progressed from the abacus to the computer, from the wheel to the modern-day car, and from simple observations about the world to our current knowledge of the laws of nature and the Universe. This ability to accumulate knowledge and improve on it, referred to as "cumulative culture," is unique to humans.

But what is it about humans that predisposes us to cumulative culture? Some claim that this "ratcheting" of ideas is a result of a particular cognitive ability, such as language or prosociality, that is unique to—or especially prevalent in—humans. Others believe that some social aspect of other species, such as the tendency to scrounge, may preclude them from building on each others' ideas as we can. A new study in Science this week suggests that a few different traits may actually be responsible for our success with cumulative culture.

The researchers used three species in their experiments: chimpanzees, capuchin monkeys, and humans (specifically human children, to minimize the impact of culture). The idea was to compare humans with two closely related species that have cultural traditions, but not cumulative culture itself.

The studies employed a puzzle box that required three consecutive actions to fully open. The idea was to measure how well groups of each species performed on a task with sequential steps, which mirrors the cumulative acquisition of knowledge. In the first step, the subjects had to slide a door open to reveal a somewhat desirable treat. Then, they needed to press a button that let the door slide further and allowed access to a more desirable treat. Finally, in the third stage, they had to turn a dial to slide the door the rest of the way, revealing a really desirable treat. For the chimps and the capuchins, the rewards were carrots, apples and grapes; for the human children, the treats were various types of stickers.

While each group was working with the box, the researchers watched for instances from a set of specific behaviors. They then correlated the frequency of each behavior with how far that species got with the box. Behaviors that were common in groups that progressed through the most stages are likely the same ones that promote cumulative culture.

After 53 hours with the puzzle box, just one capuchin monkey had reached the second stage, and none figured out the third stage. Chimpanzees fared only slightly better: just one figured out stage three, and only four more had reached stage two. As expected, there wasn't any evidence that these species are capable of cumulative culture. Meanwhile, after just 2.5 hours of exposure to the puzzle box, more than sixty percent of the 3- and 4-year old children had figured out stage three.

So what distinguished the humans’ performance from that of the other primates? The researchers found that several behaviors seemed to make the difference, both in terms of which species progressed the furthest, and which individuals within each species did the best.

Only in the human groups did the researchers witness teaching behavior, either through language ("push that button") or gesturing. Furthermore, the children who received the most instruction were more likely to make it to the third stage. The researchers also found that "matching"—when an individual who picks up the box imitates what the last individual using it did—had a great impact on progress. One of the significant measures of prosociality was how often more than one individual manipulated the box simultaneously; these instances can be interpreted as cooperation or, at the very least, tolerance. Humans worked on the box together much more often than either the chimps or the capuchins.

There was no evidence that nonsocial cognition or any aspect of social structure, such as scrounging or social hierarchy, affected performance.

Although there wasn't a quantitative measure, the researchers noted that the humans tended to approach the box in a very different way than either the capuchins or the chimpanzees. The children used the box together, helping each other and sharing the rewards; meanwhile, the other primate species seemed to interact only with the box simply to procure resources.

There are obviously some shortcomings in this research; the experiments took place under artificial conditions, and the "ratcheting" of each idea had to occur over a very short time scale. However, it does give us an idea of the traits that are likely to contribute to cumulative culture on a much larger—and more difficult to study—scale.

Science, 2012. DOI: 10.1126/science.1213969 (About DOIs).