The world we live in is not what it was when our anatomy and physiology took their present form. If we compare our environments to those of modem hunter-gatherers (which may be more similar to our current environment than that of our Pleistocene ancestors), we find that population densities are greater; social, economic, and political structures are more complex; group sizes are larger; and the range of altitudes and latitudes inhabited is greater. Multinational corporations now dominate the production of goods and services. Industrial pollution is a serious problem in almost every part of the world. International conflicts seem to be a normal part of our lives. These, and a plethora of other differences, interact to produce a world that our Pleistocene ancestors would likely find strange and unnerving.