Ward's World Activity Guides

Paleontology (continued) (1) anagenesis, or gradual change within a lineage or evolving line of populations, such as the increase in size in successively younger populations of fossil oysters (Fig. 5c); or (2) cladogen- esis (speciation), or splitting of a species into two or more dis- tinct species, typically following geographic isolation (Fig. 5d). One of the most important observations about the fossil record is that, counter to the predictions of Darwin, many species arise geologically rapidly, probably during cladogenesis, but then show stasis (little or no change) for much of their history. This staccato pattern of evolution is referred to as punctuated equi- librium. This pattern has proven to be very common, especially in many marine invertebrate animals, and it suggests that most evolutionary change occurs during short bursts associated with cladogenesis. In addition, long-term trends commonly observed in the fossil record include pattern convergence— the independent evolution of similar adaptations in distantly related groups of organisms. Adaptation and exaptation The term adaptation refers to the way that organic form is fitted to function (Fig. 5). According to Darwinian evolution, this fit is shaped by natural selection, actually the elimination of suboptimal features and the accumulation, by default, of mutations or changes that promote survival or reproduction. Given the raw materials of life, an engineer could scarcely design a better flying machine than a bird, yet the bird evolved naturally from reptilian ancestors, which did not fly. As Darwin realized, however, it is the imperfections and oddities of nature that provide the best insights into evolution. Some features seen in fossil and living organisms are vestigial, such as the side splint bones in horses' hoofs, which as tracked through the fossil record are evidently remnants of once functional side toes. The sources of adaptive features are also varied, but constrained by evolutionary history. Many seemingly new features may appear abruptly simply by accelerating or slowing down devel- opment of existing features, a process referred to as heterochrony. For example, simple parasitic organisms may evolve by greatly ac- celerating their rates of reproductive matura- tion; alternatively, the proportions of the adult human cranium resemble those of juvenile great apes, suggesting that it may have evolved by retention of juvenilelike features into adulthood. Both of these scenarios repre- sent examples of paedomorphosis—adults of descendants are juvenilelike adults compared to their ancestors. Other adaptations arise from preexisting structures that were present as nonfunctional side products of evolution or were initially adapted to other functions. For example, the bamboo stripping "thumbs" of pandas are actually a modified element of their wrist bones. Such exaptations are undoubtedly very common in evolution (Fig. 5f). Paleontologists study the development of new adaptations by a variety of techniques. For example, consider the increase in brain size between apelike ancestors and modern humans. To study this, a paleontologist employs the following: (1) studying the rocks in which fossil bones are found to look for signs of any environmental change that might have made increased brain size advantageous; (2) determining the advantages of a larger brain, if possible; for example, it may have conferred, upon ancient humans, the ability to learn to control fire for warmth and cooking (charred bones of edible animals in caves inhab- ited by ancient humans could be used as evidence for such an assertion); and (3) applying mathematical techniques to obtain a precise measurement of the rate of evolution. Sketch of life history The range of life forms represented in the fossil record is extraordinarily broad, but can be subdivided into two realms— the marine and terrestrial ecosystems. Marine ecosystem Life on Earth probably originated in the early ocean, and possibly in mid-oceanic rifts, nearly 4 billion years ago (Fig. 7). The actual record of simple prokaryotic microbes ranges back + ward ' s science Fig. 7: Major evolutionary events in relation to physical events during the first 4 billion years (Precambrian interval) of Earth history. (Credit: Adapted from an original drawing by D. R. Prothero and R. H. Dott)

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