Two biologists, Jim Brown and Brian Enquist, and a physicist, Geoffrey West, had been considering this problem independently. Brought together at the Santa Fe Institute, which encourages interdisciplinary problem-solving, they began to collaborate in the mid-1990s. Their approach centered on the geometry of an organism’s circulatory system, with these assumptions:

• An organism’s circulatory system extends throughout the organism.
• The circulatory system operates as efficiently as possible, requiring the minimum possible energy to transport nutrients.
• The smallest vessels, capillaries in animals and xylem in plants, all have the same size.

From these simple assumptions, the three scientists built a model of circulatory systems that, among other things, explained the relationship between organism mass and metabolic rate, creating a sensation among biologists and physicists alike. Other physicists weighed in with a simpler theory, and then Brown, Enquist, and West responded with a substantial revision of their own ideas. Much controversy remains, and this area of research promises to be exciting for some time to come.

Branching veins in a leaf (courtesy of John W. Kimball)

Capillaries in a bird's lung, with red blood cells clearly visible. The white spaces are passageways for air that loop among the capillaries (photo courtesy of Thomas Caceci Virginia-Maryland Regional College of Veterinary Medicine)