the saber-toothed cat were among the many species that evolved. Then, about 3 million years ago, North and South America were reconnected as a consequence of the movement of the Earth’s tectonic plates. Mammals of South American origin, such as armadillos, porcupines, and opossums, migrated north. Meanwhile, many kinds of North American mammals, including deer, raccoons, mountain lions, bears, and dogs, eventually made their way across the isthmus to the south.
Charles Darwin and other 19th-century biologists arrived at their conclusions despite knowing almost nothing about the molecular basis of life. Since then, the ability to examine biological molecules in detail has provided an entirely new form of evidence about the mechanisms and historical pathways of evolution. This new evidence has fully confirmed the general conclusions drawn from the fossil record, the geographic distribution of species, and other types of observations. In addition, it has provided a wealth of new information about the evolutionary relationships among species and about how evolution occurs. DNA is passed from one generation to the next directly from a parent to its offspring (in asexually reproducing organisms) or through the union of
DNA-containing sperm and egg cells (in sexually reproducing organisms). As discussed earlier, the sequence of nucleotides in DNA can change from one generation to the next because of mutations; if these changes give rise to beneficial traits, the new DNA sequences are likely to spread within a population