Hunting and Gathering
The earliest human societies relied on hunting and gathering for their food. Most were nomadic, since hunting and gathering cannot produce enough food to sustain a human hunting group within a day's walk. In Marxist terms, the earliest human societies practiced "primitive communism," in which goods and services were shared according to need. This was necessary for survival -- these groups could not produce a surplus over what they required, so everyone had to work in any case. Thus, the "primitive communist" hunting and gathering societies had no hereditary class distinctions, and little hierarchy of any kind. Of course, "big men" who were successful hunters and fighters, and "old women" who were wise in the finding and use of plants and in other things, could be especially influential.
The original economic system, then, is "share in order to survive." The exceptions prove to be consistent with the basic idea. Where hunting and gathering could produce more food than was necessary for the survival of the hunter-gatherer, usually because of very good fishing, complex societies with hereditary classes could grow up, as in the North American Pacific Northwest.