Regarding the point raised by Deirdre M., this is one of the hoariest chestnuts in around, certainly dating back to Adam Smith, and indeed much earlier, at least to Aristotle.  Its modern manifestation has been as the central debate in economic anthropology, with spillovers into economics involving the issue of institutionalism in particular.  In econ anthro, it could be characterized as Malinowski and Karl Polanyi (the "substantivists") versus especially Chicago-based "formalists" who in effect argued for the universal applicability of the standard, neoclassical law of supply and demand to all societies and institutional frameworks.  The substantivists argued that ritual reciprocity within religious frameworks was something different in an important way from regular market trade.
     Regarding the deeper issue of humans versus animals, we so observe certain kinds of reciprocity in nature.  Now, I am willing to grant that symbiotic coevolution of species is not the same thing as human trade, whether of the ritualistic reciprocal sort or the more regular market for gain sort (assuming there is even a difference between these, which the formalists fundamentally deny).  However, we do observe certain kinds of reciprocal behaviors among chimpanzees, both the bonobos and the not so nice more regular kind, and we may have observed some similar kinds of conduct among certain other "higher mammals," such as dolphins and elephants, although, indeed, some of these may have some "primitive" forms of aural communication.

Barkley Rosser