© Courtesy of BioMed Central Ltc. From: Maddelena Arigoni et al. "A novel Dictyostelium RasGEF required for chemotaxis and development," BMC Cell Biology, 7 December 2005.

In a striking example of emergent behavior, cells of the slime-mold Dictyostelium form an aggregate in response to the signaling molecule cyclic AMP. The individual cells signal one another, then stream together to form a fruiting body in an emergent process called chemotaxis. Though the process seems simple on the surface, this collective behavior is extremely difficult to predict from first principles, and a thorough explanation of the physics leading to chemotaxis has eluded scientists for decades.