As described in Chapter 5, context and dating are vital for the interpretation of fossils. Because primate evolution extends so far back in time, paleoanthropologists reconstruct this evolution in conjunction with information about the geologic history of the earth.
The geologic time scale is unfamiliar because few people deal with hundreds of millions of anything, let alone years, on a regular basis. To understand this type of scale, astronomer Carl Sagan correlated the geologic time scale for the history of the earth to a single calendar year. In this “cosmic calendar,” the earth itself originates on January 1, the first organisms appear approximately 9 months later around September 25, followed by the earliest vertebrates around December 20, mammals on December 25, primates on December 29, hominoids at 10:15 am on New Year’s Eve, bipeds at 9:30 pm, with our species appearing in the last minutes before midnight. In this chapter, we will consider human evolutionary history beginning with the December 25 appearance of the mammals in the Mesozoic era, roughly 245 million years ago.
Over such vast amounts of time, the earth itself has changed considerably. During the past 200 million years, the position of the continents has shifted through a process called continental drift, which accounts for the rearrangement of adjacent landmasses through the theory of plate tectonics. According to this theory, the continents, embedded in platelike segments of the earth, move their positions as the edges of the underlying plates are created or destroyed (Figure 6.3). Plate movements are also responsible for geologic phenomena such as earthquakes, volcanic activity, and mountain formation. Continental drift is important for understanding the distribution of fossil primate groups whose history we will now explore.
World History
