Unlike most of the other species on Earth, humans have the ability to manipulate symbolic information. This level of computation allows them to create mazes of abstraction and shape their behavior in unusual and sometimes absurd ways.
As you may recall, only a fraction of the human population does any useful work; to wit, produce food. I should probably expand my definition of useful to include building dens as well- shelter is important. This small population of humans provides the basics of life for all of the others.
Obviously, for the species to survive, they must want to do this; if they chose to do otherwise, the population would crash. But natural selection operates on the level of the organism (or the gene), so there is no selection pressure that encourages them to behave thus; humans are not an ant hive wherein a small genetic legacy controls and rules sterile workers. How, then, do the colonial humans engaged in superfluous tasks convince the worker castes to provide for their needs?
This is where that advanced symbol manipulation capacity comes into play. Most colonies of humans are entirely structured around small tokens that represent value. It's very difficult to explain what is meant by "value" in this case. Small tokens, often fashioned from processed vegetable matter or metal, are thought by humans to have worth. A food producer will happily exchange his food for these tokens, because he can exchange these tokens with other humans for most anything he likes.
These tokens are extremely abstract; they do not represent a fixed quantity of goods. One day, five tokens may purchase a meal, on another day the same meal may cost ten tokens! This is example is a little extreme- the necessities of life like food tend to keep a fairly stable relationship with these tokens.
Because of this highly abstract notion of value, there is no objective explanation of value either. One human may be willing to exchange ten tokens for food, whereas another human is only willing to exchange five tokens for the same exact meal. This creates a complicated economy of token exchanges.
This economy is so complex, in fact, that some humans exchange these tokens for absolutely nothing at all. Some humans are tasked with the job of merely moving these tokens around; they give tokens to other humans or human colonies (or other social structures- humans break themselves up into many, many groups) in exchange for goods or promises of goods or more tokens. These humans who move tokens around are rewarded with yet more tokens.
Everything is valued in terms of these tokens. Food, information, entertainment, even sex (which among humans serves as entertainment). Even the tokens themselves are defined relative to tokens produced by other human groups.
Here's a real brainbuster that will give you a feeling for what it is like to be a human being. These tokens are usually produced by a small group of humans; in exchange for providing their services in the design and manufacture of these tokens, they are rewarded with the very tokens they produce.
The next question one must ask is: "Why?" Could such a strange behavior possibly have any evolutionary advantage? Or, perhaps better worded: what possible evolutionary advantage could this behavior have?
Simply put- humans could not have such large and varied colonies without this abstract value system. I keep repeating the fact that most humans are not employed in any useful work. This is somewhat true. Certainly, the real useful work is the production of food, shelter and other necessities. But humans are not hunter-gatherers. They use tools to grow food and manage domesticated livestock. Due to their specialization, the humans that produce the food are not the ones that make the tools- they exchange these economic tokens with the tool makers.
Well, certainly, the food-producing humans could negotiate with the tool-making humans; "I will give you food if you give me tools." And this works, to a point. But the tool-makers cannot make tools in vacuum. They in turn use tools made by other tool-makers. They in turn look to other tool-makers, and there are other individuals tasked with creating new tools and methods used by the other tool-makers and food-producers. As they add layer upon layer of abstraction, the simple task of growing food becomes increasingly complex; the number of "hops" between the food-producer and the root tool-makers that need the food are too great.
Food-producer A depends on tool-maker B, who in turn depends on tool-maker C, D and E, each of which depend on tool-makers F, G, H or I. And so on- the tree of society spreads out until we reach researcher Z. This researcher produces nothing of material value, but uncovers new tools that can be used by tool-maker Y. Y must give something of value to Z in exchange for this information- but what? If food or sex was passed along the chain, by the time it reached Y, there would be none left, or it would have spoiled.
This complex network requires a common medium of exchange. An abstraction of value allows all tool-makers and researchers from B-Z to interact directly with each other and with food-producer A. Instead of using food, which spoils and is consumed, as the basis of an economy, humans have taken the novel approach of using the abstract concept of value as embodied in symbols.