Impacts: Historical Reflections

Working in a broadly interdisciplinary area is a fascinating experience, as I first noticed when I started my research on possible astrophysical triggers of mass extinctions on Earth. At first it was confusing, to say the least, to see how different the methodology and the underlying assumptions were in the different disciplines involved, from astrophysics and physics through nuclear chemistry all the way to geology and paleontology. Trying to all speak the same language sometimes seemed like an exercise in sociology more than anything else.

An obvious difference between physics and paleontology concerns the truth criterion for different types of evidence. In physics, a single convincing laboratory measurement may be enough to tip the scales in an argument. In paleontology, however, almost every type of evidence is indirect and may not be able to stand on its own. Instead, in most cases a theory is accepted only on the weight of a whole body of accumulated evidence from different sources.

Charles Lyell

Another reason for difficulties in communication stems from the fact that Earth scientists in general have been trained not to evoke unusual and sudden catastrophes. This goes back to the early nineteenth century, when geologists started to distance themselves from the biblical stories about the history of the Earth. Charles Lyell especially was an eloquent spokesman for the doctrine of what he call uniformitarianism: he only wanted to work with hypotheses that were uniformly valid in time, thereby excluding biblical miracles, in favor of more scientific explanations. Events such as earth quakes and vulcanic eruptions did not pose any problem, because they repeatedly occurred throughout human history. But mythological stories about a great flood, for example, were not allowed.

At first sight, stories about a comet hitting the Earth would seem to fit in the category of `miracles', and it is altogether understandable that Earth scientists would treat such hypotheses with a skeptical eye. In fact, while I was working with geologists and paleontologist on related research projects, we often found ourselves in after-dinner discussions on this topic. Our conclusion was that uniformitarianism is fine as a broad principle, as long as it is interpreted in a wide enough context. And certainly, impacts have occurred regularly throughout the Earth's history. This makes it likely that at least some of the majore mass extinctions (besides the one at the K/T boundary) have been triggered, directly or indirectly, by impacts of asteroids or comets. We have summarized our ideas in the paper

• Uniformitarianism and the response of earth scientists to the theory of impact crises, by Alvarez, W., Hansen, T., Hut, P., Kauffman, E.G. & Shoemaker, E.M., 1989, in Catastrophes and Evolution, ed. S.V.M. Clube (Cambridge Univ. Press), pp. 13-24.