HOW DO COMPLEX ORGANISMS FORM? A Darwinian mechanism of natural selection plus random mutation is not quite enough to explain the complex features of life on earth. For example, it does not predict or anticipate the fact that an ecosystem or a global community has a hierarchical structure, with interactions that take place at several size scales. For example, people communicate with each other in an organization; and organizations communicate with each other in a larger community. Barbara Drossel of the University of Manchester in England (011-44-161-275-4201, barbara.drossel@man.ac.uk) has introduced a simple mathematical model for describing how originally independent units may develop into a complex organism with a hierarchical structure. In her model hierarchy comes about because of the increase of a quantity she calls "productivity" (similar to "fitness" in biology and "utility" in economics). Individual units communicate with each other to increase productivity which leads, at the very least, to larger groups. Drossel's model incorporates the additional idea that the size of a group is restricted by the limited capacity of individuals to communicate and to travel. Therefore, she introduces a "communication cost" per partner and per unit distance to the partner. This encourages the formation of groups and ultimately the formation of supergroups and groups of supergroups which interact with each other. (Drossel, Physical Review Letters, 21 June 1999.)