12 The History of Life

12.6Primate EvolutionKEY CONCEPT Humans appeared late in Earth’s history.MAIN IDEAS• Humans share a common ancestor withother primates.• There are many fossils of extinct hominids.• Modern humans arose about 200,000years ago.VOCABULARYprimate, p. 379prosimian, p. 379anthropoid, p. 380hominid, p. 380bipedal, p. 381Connect In terms of the geologic time scale, the evolution of humans hasoccurred only very recently. Many fossils of our early ancestors consist of partialskeletons from which details must be inferred through careful study. Though farfrom complete, this fossil record offers a fascinating glimpse of our past.MAIN IDEAHumans share a common ancestor withother primates.The common ancestor of all primates probably arose before the mass extinctionthat closed the Cretaceous period 65 million years ago. Primates make upa category of mammals with flexible hands and feet, forward-looking eyes—which allows for excellent three-dimensional vision—and enlarged brainsrelative to body size. Primates also have arms that can rotate in a circle aroundtheir shoulder joint, and many primates have thumbs that can move againsttheir fingers. Primates include lemurs, monkeys, apes, and humans. In additionto sharing similar physical traits, primatesshare strong molecular similarities.TAKING NOTESMake a concept map of primateclassification. Add more shapesas needed.primatesincludeprosimiansanthropoidssuch as such as such asPrimate EvolutionSimilar to other groups of related organisms,the relationship among the primate groupsforms a many-branched tree. At the tree’s baseis the common ancestor of all primates. Justabove this base, the tree splits into two mainsubgroups: the prosimians and the anthropoids.Prosimians (proh-SIHM-ee-uhnz) arethe oldest living primate group, and most aresmall and active at night. This group ofnocturnal animals includes the lemurs, thelorises, and the tarsiers, like the ones shownin FIGURE 12.17. Tarsiers have been called livingfossils, as their physical traits have changedlittle since their appearance in the fossilrecord more than 40 million years ago.FIGURE 12.17 Prosimians, such asthese tarsiers, are the oldest livingprimate group. They are active atnight and have large eyes and ears.Chapter 12: The History of Life 379