and extinction and also over the formation of valuable resources.

Dynamics of the Global Environment

The evolution of the environment is an important area of research in modern earth sciences. The study of ancient conditions, when climates were warmer than today, offers insight into potentials of modern greenhouse warming. In addition, studies of different global ecosystems of the past may reveal peculiarities of the present world that render it especially vulnerable to certain forcing factors. The most recent segment of the geological record provides the temporal resolution and geographic control needed to identify very sudden environmental changes.

During the first 4-billion-years of earth history, life arose and evolved through many intermediate stages to a point at which a variety of multicellular plants and animals existed. Evolution and extinction during this interval were tightly interwoven with profound changes in the physical nature of our planet, especially its atmospheric chemistry. This intimate relationship between life and environment serves to underscore an important point about future directions of research. Different intervals of earth history require distinct scientific strategies, because the intervals were characterized by different kinds and degrees of environmental change and are represented by different kinds of geological records.

The prospects are exciting. But they require a prodigious amount of research to chronicle global environmental change for a variety of past intervals with the resolution required. Once models can accurately simulate environmental conditions of key intervals during earth history, they can be used for predicting future conditions. Collaborative modeling projects that unite workers in the geological sciences with meteorologists and oceanographers are beginning to yield results.

Life Through Time

Numerous advances have breathed new life into paleontology. The contributions of the fossil record to the study of evolution and extinction uniquely document myriad forms unknown in the modern world. A cumulative chronology indicates the rates of evolution and extinction, and the timing of major events in the context of environmental change. Such a chronology establishes fluctuations and patterns that can point to new questions about the history of life. The development and application of quantitative techniques will continue to play a prominent role in future studies of life through time. Key methods will assess morphological change; patterns of evolution and extinction; and theoretical modeling of taxonomic, stratigraphic, and environmental data obtained from the fossil record.

The fossil record also provides evidence of the timing of evolutionary branching. The molecular clocks used to estimate the times when certain extant groups branched from others must be calibrated against fossil data, and conclusions must be tested against macroscopic evidence. Of special importance are fossil data that reveal the occurrence of adaptive breakthrough—evolutionary innovations that ushered in new modes of life that transformed the ecosystem. These can be recognized in the fossil record only by inferring function from morphology, an activity that merits increased support. Many adaptive breakthroughs have triggered adaptive radiations—diversification from a single life form that results in the invasion of a variety of ecological niches. Such radiations have special significance because they account for most of the major evolutionary changes in the history of life. Understanding the general diversification of life on Earth requires an understanding of adaptive radiations.

Sudden extinctions—the negative equivalent of adaptive radiations—have repeatedly reordered the global ecosystem and opened the way for new evolutionary directions. Mass extinctions can be understood only in the context of global environmental change. The taxonomic, temporal, and geographic patterns that have characterized these events are especially significant; compilation of new data demands the continued generation of high-quality biostratigraphic and taxonomic information.

Whether global biotic catastrophes have occurred in combination with background extinction or instead have been quantitatively or qualitatively distinct is a question that can be answered only by understanding the patterns and causes of noncrisis extinctions.

The Most Recent Past

Geologists know that the record of the most recent past is exceptional because its complex history can be better established than that of any earlier period. Powerful new techniques are determining the ages, isotopic compositions, and temperatures of materials deposited during the past 2.5-million-years. But the most significant recent development is the worldwide awareness of changes in the global environment caused by humanity. This new awareness has stimulated a need to better understand the



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