animals and from a paleontological study of the past over about the same two-century interval. Both of these endeavors have consumed the lives of men and women who have traveled to the ends of Earth to discover, classify, and catalog the extant and extinct biodiversity.
The when of this history has proven as difficult to discover as the what but for different reasons. Early geologists had no conception of the true age of Earth and had absolutely no way of discovering that great age. The one or two efforts to estimate the age of the planet, such as Lord Kelvin’s innovative (and ultimately hugely wrong) effort to use the heat flow from Earth to estimate its age, yielded woefully low estimates. Discovering the age of individual fossils was even more impractical, and thus the chronology of life’s history obtained from the fossil record was only a relative one. The succession of strata, one layer piled upon another, was slowly and painfully discerned by a century of stratigraphic study, eventually producing the geological timescale of eras, periods, and ages in use today. The first and last appearances of the major vertebrate groups that make up the what of life’s history could, with the construction of the geological timescale, be put into relative succession. But the actual age of each event, such as the first appearance of mammals, or the last of dinosaurs, remained unknown until the inception of rock dating using radioactive age determination. Yet even without knowing the actual period in which mammals arose or dinosaurs died, paleontologists had, by the middle part of the 1950s, arrived at an accurate picture of animal evolution. While in the 50 or so years since then science has filled in innumerable details and developed a better understanding of the length of this history, in the larger view little has changed.
The third question of life’s history—the how—has also remained little changed for a long time—since the nineteenth century, in fact. Charles Darwin, with his great theory on the evolution of organisms, gave an explanation as to why there has been a history of life. The diversification of the great vertebrate groupings came about through evolutionary processes. We obviously now know a great deal more about how evolution works than did the life historians in Darwin’s day, but the overall explanation of how this works remains the same: evolution is responsible for providing the mechanism in this history of life.