The geological record represents the only source of data available for documenting long-term historical patterns of extinction intensity and extinction susceptibility. Such data are critical for testing hypotheses of extinction causality in the modern world as well as in deep time. The study of extinction is relatively new. Prior to 1800, extinctions were not accepted as a feature of the natural environment. Even after extinctions were recognized to have occurred in Earth's geological past, they were deemed to have played a minor role in mediating evolutionary processes until the 1950s. Global extinction events are now recognized as having been a recurring feature of the history of life and to have played an important role in promoting biotic diversification. Interpretation of the geological extinction record is rendered complex as a result of several biasing factors that have to do with the spatial and temporal resolutions at which the data used to study extinctions have been recorded: fluctuations in sediment accumulation rates, the presence of hiatuses in the stratigraphic sections/cores from which fossils are collected, and variation in the volumes of sediments that can be searched for fossils of different ages. The action of these factors conspires to render the temporal and geographic records of fossil occurrences incomplete in many local stratigraphic sections and cores. In some cases, these stratigraphic and sampling uncertainties can be quantified and taken into account in interpretations of that record. However, their effects can never be eliminated entirely. Testing hypotheses of global extinction causality requires acknowledgment of the uncertainties inherent in extinction data, the search for unique predictions of historical patterns of variation or associations that can, in principle, be preserved in the fossil record and tied logically to the operation of specific causal processes, and to adoption of an explicitly comparative approach that establishes the presence of multiple instances of the predicted cause-effect couplets within a well-documented chronostratigraphic context.

The discovery of many substantial objects in the outer solar system demands a reassessment of extraterrestrial factors putatively implicated in mass extinction events. These bodies, despite their formal classification as minor (or dwarf) planets, actually are physically similar to comets observed passing through the inner solar system. By dint of their sizes (typically 50–100 km and upward), these objects should be considered to be giant comets. Here, I complement an accompanying paper by Napier, who describes how giant comets should be expected to cause major perturbations of the interplanetary environment as they disintegrate, leading to fireball storms, atmospheric dustings, and bursts of impacts by Tunguska- and Chelyabinsk-class bodies into the atmosphere, along with less-frequent arrivals of large (>10 km) objects. I calculate the terrestrial impact probability for all known asteroids and discuss why the old concept of single, random asteroid impacts causing mass extinctions is deficient, in view of what we now know of the inventory of small bodies in the solar system. Also investigated is how often giant comets might be thrown directly into Earth-crossing orbits, with implications for models of terrestrial catastrophism. A theme of this paper is an emphasis on the wide disparity of ideas amongst planetary and space scientists regarding how such objects might affect the terrestrial environment, from a purely astronomical perspective. That is, geoscientists and paleontologists should be aware that there is no uniformity of thought in this regard amongst the astronomical community.