Koppl argues that contractual exchange likely emerged from gift exchange through an evolutionary process in which Darwinian natural selection played an important role. While economists and other social scientists have studied the role of Darwinian evolution in shaping human social practices, economists have generally neglected the distinction between gift exchange and contractual exchange. In gift exchange the timing and nature of repayment is indeterminate when the initial gift is given. In contractual exchange the reciprocation is agreed upon before any exchange of values occurs. When economists have noted the distinction between gift and contractual exchange the two have been viewed as distinct and unrelated. The evolutionary origins of gift exchange have been studied in anthropology and other social sciences. The same cannot be said of contractual exchange. The argument that contractual exchange emerged from gift exchange helps to close a gap in the literature on Darwin and the institutions.