"Nemesis has not been found, although Richard A. Muller of the University of California at Berkeley continues to track dim red stars to discover if one of them might be a companion to the sun. Also, evidence of impact (such as iridium-rich clay layers or widespread shocked minerals) has not been found in association with all the major extinctions, as would be predicted by the hypothesis. There is excess iridium at the end-Triassic (208 million years ago) and Late Eocene events, as would be expected if impact occurred then, but no such evidence is seen at other events in the periodic series that have been studied in detail.

"Hence, we have an empirical observation of periodicity but no new hypotheses that suggest interesting tests. This situation is not conducive to scientific effort or to intellectual curiosity, so interest in the question of periodic extinctions has died down. But not completely: there is an interesting scientific paper on the problem published by M. R. Rampino and B. M. Haggerty in Earth, Moon, and Planets (Vol. 72, Nos. 1-3, pages 441-460; 1996), entitled "The 'Shiva Hypothesis': Impacts, Mass Extinctions, and the Galaxy."

Finally, Carlton E. Brett in the department of earth and environmental sciences at the University of Rochester adds his point of view on this controversy:

"Raup and Sepkoski were onto something quite significant, although perhaps not in the specific formulation they presented. I am strongly convinced of one very important implication of Raup and Sepkoski's 'worldview,' that is, that episodic crises have been crucial to evolution. Indeed, one might argue that relatively little evolutionary change would have taken place in life's history without such events. Raup and Sepkoski did make that argument, and so do I, albeit from a different and more detailed perspective.

"My main slant on this issue stems from more than two decades of intensive field study of marine animal fossil assemblages from the Silurian and Devonian (about 438 to 375 million years ago) of eastern North America. The pattern that emerges here--and in at least some other intervals--is that there are more numerous but less regular extinction events spaced a few million years (in the range of one to about 10 million years) apart. These events have strongly shaped the course of life.

"I recently proposed that one important pattern in the history of life is what I term 'coordinated stasis.' This hypothesis, related to but broader than the concept of punctuated equilibrium, suggests that very little evolution is going on during most of geologic time and that most lineages do not effectively speciate (split into two or more species) or undergo much within-lineage change for periods of several million years. Moreover, the structure of biofacies or communities remains fairly similar for vast spans of time, represented by these one- to 10-million-year blocks of stability. But at the ends of these intervals, fairly radical changes occur (for instance, local extinction of up to 80 percent of lineages, speciation, immigration or ecological restructuring). These changes appear to have been relatively rapid: periods of instability and restructuring last a few thousand to at most a few hundred thousand years, as compared with millions of years of near stability

"The events that triggered such major biological turnovers remain for the most part obscure, but we are going back to the geologic record to try and sleuth out correlations that may indicate causal factors. Rapid climatic change and major sea-level fluctuation seem the most plausible triggers. Such events occurred fairly regularly in geologic time, a pattern reflected in the rather similar durations of the geologic stages long recognized by scientists; the divisions of geologic time are usually defined by notable change in the fossil record. Without these crises there would be little evolutionary change; one might argue that extinctions are necessary to open opportunities for new evolution.

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