Perhaps the greatest challenge for the study of complex adaptive systems lies in the historical fields of natural science, whether astronomy, geology, evolutionary biology, paleontology, or archaeology. In each of these fields, experimental approaches are limited, studying modern systems may not provide much insight into processes in the distant past, and chance (contingency) often plays an important role. This tension between the role of chance and the search for regular patterns that underlie historical processes is also found in a number of social and behavioral sciences, where the SFI community has been increasingly active.