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The Evolutionary Truth About Living Fossils
Scholarly Journal
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Appearances to the contrary, no species is exempt from selection, even when changes are difficult to detect in the fossil record. To gaze upon a horseshoe crab is to glimpse a prehistoric Ordovician sea of nearly half a billion years ago. So moved was Charles Darwin by the appearance of these and similarly ancientlooking creatures-lungfish, lampreys, lampshells, and lycopods-that he coined the term living fossil to describe them. In his landmark 1859 treatise On the Origin of Species, he wrote that such apparently primitive species are "remnants of a once preponderant order... which, like fossils, connect to a certain extent orders now widely separated in the natural scale." These "anomalous forms," he wrote, nonetheless "have endured to the present day." Like modem biologists, Darwin was struck by the unusually archaic form of the three widely scattered lungfishes (Protopterus, Neoceratodus, and Lepidosiren of Africa, Australia, and South America, respectively), whose < large lungs; cartilaginous notochord; and fleshy, lobed fins resemble those of creatures known only from the fossil record. Similarly, the coelacanth (Latimeria), another lobe-finned fish, was well known from fossils but thought to have gone extinct in the Late Cretaceous until the discovery of living Latimeria in 1938. Likewise, the dawn redwood tree, Metasequoia, was known only from fossils (10 to 100 million years old) before it was found alive in a remote Chinese valley in 1943; today it is a common ornamental that grows readily in temperate regions. Living fossil is a perfectly appropriate and evocative term for such extant forms even today. It conveys the jarring surprise that such species conjure by retaining anatomical structures out of deep geologic time. This curious phenomenon raises evolutionary and ecological questions at once simple and profound, such as why these taxa appear to have persisted unchanged for so long, and...