Shakespeare, hip hop, airplanes and millions of other innovations are all products of one of mankind's most distinguishing characteristics: human culture.
While it's clear that our brains hold a remarkable capacity to think and create, other animals demonstrate what some consider cultural behaviors. How the astounding complexity and diversity of human cultures sprang from the much simpler traditions found in animal communities has remained a puzzle.
"We really know very, very little about the kind of roots of culture, and the biological origins of culture, and how the forms of culture we see in our species are similar to or different from those seen in animals," said zoologist Alex Thornton of Cambridge University.
Much research has focused on the ingredients of human cultural evolution and other studies have sought to sort out the presence of simple animal traditions. "What's really lacking is an understanding of how the two relate to each other," Thornton added.
What is culture?
One of the problems inherent in answering this question is how to define culture.
Anthropologists use a fairly specific definition that requires the use of symbols to transmit cultural knowledge.
"If you define culture according to that, then culture is necessarily something that you find only in humans," Thornton said.
But biologists and animal behaviorists tend to define culture and tradition as any behavior that is learned by observing or interacting with others, Thornton said.
Taking this broad definition, some argue that simple traditions can be seen in animals like guppies, which will follow each other to a food source, so that a particular path to that source becomes a "tradition" in that guppy community.
So instead of looking at culture as something that humans came up with in the last million years or so, as some anthropologists do, biologists, particularly primatologists "think it's probably much older than that," said Frans de Waal of the Yerkes National Primate Research Center in Atlanta. "We basically perfected a system that already existed."
So if the foundation for the capacity for culture is rooted in our biology, Thornton argues, one of the ways to find these roots is to study the simpler traditions seen in other animals.
Clues in our cousins
One of the clearest places to look for clues of our cultural capacity is in one of our closest relatives: chimpanzees. Researchers have observed chimps performing certain behaviors that differ between populations and that seem to be passed by social learning, just as they are in human cultures.
For instance, some chimp populations have invented a means of cracking open a nutritious but hard-shelled nut, while other communities haven't.
The problem with this analogy is that researchers aren't certain that these traditions are really learned by observing others. They could be learned individually or could vary with environmental influences.
"So it's not totally clear that these are actually traditions, and we need to do experiments to really tease that out," Thornton told LiveScience.
These are exactly the kinds of experiments de Waal is conducting at Yerkes, where researchers teach one chimp a skill and watch as it is disseminated to the other members of the group, showing that chimps can learn by observing others and spread a behavior through a population.
Thornton cautions though, that "what an animal can do in the lab does not necessarily reflect what it does do in the wild."
The missing link is how intelligence and language — exclusively human characteristics — played a role in moving us from the simpler traditions seen in animals to the incredibly complex cultures seen in humans.
Accounting for complexity
One aspect of human culture that makes it so complex is that it is cumulative, as people build on the inventions of past generations.
"We adapt now culturally to an extent that's unparalleled in any other creature," said anthropolgist Jon Marks of the University of North Carolina at Charlotte. As a human product, technology evolves separately from human biology. (For example, you don't need to talk about the biology of the makers to discuss the evolution of the airplane.)
De Waal says that chimps might actually have the capacity for cumulative traditions. Nut-cracking, for example, is a complex skill that involves placing a nut between an anvil stone and a hammer stone and coordinating the movements to hit the nut just right.
"It's unlikely that some chimp all of a sudden did all these things at the same time, and probably they must have started with something simpler," he said.
But one of the biggest differences between human and animal culture is "the fact that we have language and writing, and we can record our cultures and transmit them in that way," Thornton said.
Language allows us to talk about abstract ideas such as happiness or love, about the past and the future, and to combine words to express an infinite variety of ideas. The forms of communication that animals use are much more limited — they can express a desire to mate, or warn of the approach of a predator, but those calls cannot be combined to mean something new.
To trace the exact effects of language and intelligence on the development of human culture will require a multi-disciplinary effort examining ancient human cultures, animals in the wild, human psychology and many other areas of science, Thornton said.
Only then, he said, will "the breakthroughs start to emerge."