Evolution, 4th Edition

THE HiSToRy oF LiFE 463

arbitrary whether they are called primates or not. The first monkeys are known

from the Oligocene, the first apes from the Miocene, the earliest hominins from

the late Miocene (about 7 Mya), and the first Homo sapiens from about 200,000

years ago. In Chapter 21 we shall return to the story of human evolution.

Pleistocene events

Because of its recency and drama, the last Cenozoic epoch before ours, the Pleisto-

cene, is critically important for understanding today’s organisms. Most living species

evolved in the Pleistocene, or shortly before.

By the beginning of the Pleistocene, the continents were situated as they are

now. North America was connected in the northwest to eastern Asia by the Bering

Land Bridge, in the region where Alaska and Siberia almost meet today (FIGURE

17. 33A). North and South America were connected by the Isthmus of Panama.
    Global temperatures began to drop during the Pliocene, about 3 Mya, and then,
    in the Pleistocene, underwent violent fluctuations at intervals of about 100,000
    years. When temperatures cooled, continental glaciers as thick as 2 km formed at
    high latitudes, and then receded during the warmer intervals. At least four major
    glacial advances, and many minor ones, occurred. The most recent glacial epi-
    sode, termed the Wisconsin in North America and the Würm in Europe, reached
    its maximum about 20,000 years ago (see Figure 17.33A), and the ice melted back
    between 15,000 and 8000 years ago. During glacial episodes, sea level dropped as
    much as 100 m below its present level. This drop exposed parts of the continental
    shelves, extending many continental margins beyond their present boundaries and
    connecting many islands to nearby land masses (FIGURE 17.33B). Temperatures in
    equatorial regions were apparently about as high as they are today, so the latitudi-
    nal temperature gradient was much steeper than at present. The global climate dur-
    ing glacial episodes was generally drier. Thus mesic and wet forests became restricted
    to relatively small favorable areas. Grasslands expanded, contributing to the diver-
    sification of grazing mammals in Africa. During interglacial episodes, the climate
    became warmer and generally wetter.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_17.32.ai Date 01-06-2017

Eocene Oligocene Miocene Pliocene Pleistocene

(A)

(B)

(D)

(C)

(E)

FIGURE 17.32 Proboscidea, the order of elephants, has only two living genera but

was once very diverse. A few of the extinct forms are (A) an early, generalized probos-

cidean, Moeritherium (late Eocene–early Oligocene); (B) Phiomia (early Oligocene);

(C) Gomphotherium (Miocene); (D) Deinotherium (Miocene); and (E) Mammuthus, the

woolly mammoth (Pleistocene). (After [77].)

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