hospitable habitats sustain more varieties of life and harbor more biological groups, which means that they can support greater total rates of speciation.

Origins of higher groups frequently reflect the evolution of adaptive innovations. Studies of ancestors and descendants in the fossil record can reveal the morphogenetic mechanisms that gave rise to innovations. Among these mechanisms are changes in the relative timing for development of different morphological features. This area of research offers great potential for fruitful interaction between paleobiologists, developmental biologists, and geneticists.

Phylogenetic Reconstruction

During the past two decades, cladistic analysis—the study of similarities resulting from common origins—has emerged as a powerful quantitative method for reconstructing the genealogical relationships called phylogenies. It is based on assessment of how traits that have evolved only once are distributed among taxonomic groups. Some proponents of cladistic analysis disregard the stratigraphic distribution of groups, grounding reconstruction of phylogenies on judgment as to which characteristics are primitive and which are derived. An alternative approach reconstructs phylogenies by evaluating stratigraphic and morphological distances between groups. Comparisons have shown that the two approaches sometimes yield identical results. Such comparisons are of great value, and methods that combine the two approaches warrant further examination. Molecular data also can reconstruct phylogenies; however, some of the techniques, including DNA hybridization, remain controversial, and their results must be compared to those achieved with the other approaches.

Catastrophes in Earth History

Geologists call sudden violent changes catastrophes, and they contrast catastrophes with the changes in the rock record attributed to constant gradual processes. There is obviously a continuum between frequent events, moderate events, and the occasional violent happening, and this simple relationship is readily expressed in empirical laws. Such relationships can, for example, link earthquake frequency with earthquake magnitude.

Nearly two centuries ago polarized positions were assigned to geologists—they were either catastrophists or uniformitarians. Since then geological interpretation has accommodated the occurrence of occasional violent events, of the kinds experienced within our own lifetimes—hurricanes, earthquakes, and volcanic eruptions. A uniformitarian approach has dominated because it is very effective in analyzing the rock record and in making correlations.

The possibility that the four or five great biological extinctions of the past few hundred-million-years marked catastrophic events never received serious attention because no evidence of a promising catastrophic mechanism had been recognized. That changed a decade ago when researchers, studying a few sites scattered over the globe, reported that strata that marked the mass extinction of about 66-million-years ago contained anomalously high concentrations of iridium and related platinum-group metals. Similarily, high concentrations have now been reported from dozens of localities worldwide of the same age. They are considered strong evidence of a catastrophic event close to the time of extinction of many groups of animals, of which dinosaurs are the most notable. This latter extinction event is one of the five largest extinction events to have occurred since fossils became abundant—the largest, 250-million-years ago, is represented by a less detailed record. But many things were happening 66-million-years ago; evidence indicates that the atmosphere and ocean were cooling and that the sea level was rising again after having fallen. Because the last vestiges of some life forms ceased to appear in strata that date to before the time of the iridium anomaly, it is argued that a catastrophic event may have been the final blow at a time of general environmental deterioration.

Two kinds of nonbiological catastrophic perturbations that have been suggested are a global volcanic episode and the impact of a meteorite. A high platinum-metal concentration could indicate origins from within the mantle, though such a volcanic episode would have to be extremely intense, and one of the largest lava eruption events of the past few hundred-million-years formed the Deccan basalts in India at just the right time. Catastrophic perturbation by extraterrestrial impact seems more probable, not only because meteorites exhibit high concentrations of platinum-group metals in the right proportion but also because of an association of the iridium-rich horizon with quartz grains showing the effects of intense shock. Shock features are commonly found with meteorite impacts but are unknown in volcanos. And perhaps even more significant is the existence of small beads of glassy material of the type produced by the heat released by meteorite impacts—microtectites. Several mechanisms have been proposed that show how meteorite impact could

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