Gould, The Ediacaran Experiment

more complex design through time--amidst all the fluctuations and backings and forthings that must characterize a process primarily devoted to constructing a better fit between organisms and changing local environments. Darwin certainly thought so when he wrote:

The inhabitants of each successive period in the world's history have beaten their predecessors in the race for life, and are, insofar, higher in the scale of nature; and this may account for that vague yet ill-defined sentiment, felt by many paleontologists, that organization on the whole has progressed.

I regard the failure to find a clear "vector of progress" in life's history as the most puzzling fact of the fossil record. But I also believe that we are now on the verge of a solution, thanks to a better understanding of evolution in both normal and catastrophic times. We need a two-tiered explanation for patterns (or nonpatterns) in the history of life.

I have devoted the last ten years of my professional life in paleontology to constructing an unorthodox theory for lack of expected patterns during normal times--the theory of punctuated equilibrium. Niles Eldredge and I, who must admit responsibility for this particularly uneuphonious name, argue that the pattern of normal times is not one of continuous adaptive improvement within lineages. Rather, species form rapidly in geological perspective (thousands of years) and tend to be highly stable for millions of years thereafter. Evolutionary success must be assessed among species themselves, not at the traditional Darwinian level of struggling organisms within populations. The reasons that species succeed are many and varied--high rates of speciation and strong resistance to extinction, for example--and often involve no reference to traditional expectations for improvement in morphological design. If punctuated equilibrium dominates the pattern of normal times, then we have come a long way toward understanding the curiously fluctuating directions of life's history. Until recently, I suspected that punctuated equilibrium might resolve the dilemma all by itself.

I now realize that the fluctuating pattern must be constructed by a complex and fascinating interaction of two distinct tiers of explanation--punctuated equilibrium for normal times, and the different effects produced by separate processes of mass extinction. Whatever accumulates by punctuated equilibrium in normal times can be broken up, dismantled, reset, and dispersed by mass extinction. If punctuated equilibrium upset traditional expectations, mass extinction is even worse. Organisms cannot track or anticipate the environmental triggers of mass extinction. No matter how well they adapt themselves to environmental ranges of normal times, they must take their chances in catastrophic moments. And if extinctions can demolish more than 90 percent of all species, then we must be losing groups forever as a result of pure bad luck among a few clinging survivors designed for another world.

Heretofore, we have thrown up our hands in frustration at the lack or expected pattern in life's history--or we have sought to impose a pattern that we hoped to find on a world that does not really display it. Perhaps now we can navigate between a Scylla of despair and a Charybdis of comforting unreality. If we can develop a general theory of mass extinction, we may finally understand why life has thwarted our expectations--and we may even extract an unexpected kind of pattern from apparent chaos. The fast track of an extraordinary meeting in Indianapolis may be pointing the way.

Stephen Jay Gould teaches biology, geology, and the history of science at Harvard University.