Things don't always become bigger.

Paleontologists have believed for a century that organisms naturally evolved to larger and larger sizes. However, a study by University of Chicago paleontologist David Jablonski shows that this is not true. In fact, there is no more tendency for things to become bigger as they evolve than there is for them to become smaller.

Cope's Rule that evolutionary lineages have a tendency to evolve toward larger body size is one of the most long-standing evolutionary "laws." E.D. Cope first formulated his now-famous rule in a book published in 1896, and it is cited widely in textbooks from elementary to graduate school.

"Although Cope's Rule has been questioned for some time on a theoretical basis, this is the first time anyone has taken a quantitative look at a large enough data base to really draw a general conclusion," Jablonski indicates. "This is the empirical `nail in the coffin.'" Jablonski maintains that Cope, as have many others since him, tended to focus only on the largest animals in a given evolutionary lineage and to home in on only those lineages that did increase in size. This is a sampling bias that skews the data.

"Dinosaurs are a great example. People forget that there were plenty of tiny dinosaurs running around, even at the end of the group's history. And the evolution of the horse, from tiny Eohippus to the modern horse, is often cited as the classic example of Cope's Rule. But, in fact, horses show a broad range of sizes through most of their evolutionary history -- until the very end, when all became extinct except for one of the largest lineages. The last survivor just happened to be a large one. If you connect the small starting point with the big final survivor, you seem to get a straight line of size increase, but the real pattern is much more complicated."

Jablonski argues that paleontologists have to think about the entire range of body sizes when examining evolutionary trends, not just the extremes. He says that there are two reasons why Cope's Rule has permeated paleontological thinking so thoroughly, and both of them are psychological. "Size determines who you can eat and...