Prolouge
It was a blazing hot day at Olduavia George¸ East Africa¸ in 1959. Back in camp¸ Louis Leakey lay in his tent¸ suffering from a bout of influenza. Meanwhile¸ Mary Leakey¸ sheltered by a beach umbrella¸ was excavating the small scatter of broken bones and crude artifacts deep in the gorge. For hours she brushed and pried away dry soil. Suddenly¸ she jumped into her Land Rover and sped up the track to camp. “ Louis¸ Louis¸” she cried¸ as she burst into their tent¸ “ I´ve found Dear Boy at last.” Louis leapt out of bed¸ his flu forgotten. Together¸ they excavated the fragmentary remains of a magnificent¸ robust human skull. The Leakeys named it Zinjanthropus boisei (“African human of Boise”)¸ a Mr. Boise being one of their benefactors. With this dramatic discovery¸ they changed the study of human evolution from a part–time science into a international detective Story.
Biologist Thomas Huxley called it “the question of questions¸” the nature of the exact relationship between humans and their closest living relatives such as the chimpanzee and the gorilla–the question of human origins. Ever since his day¸ scientists have been locked in controversy as they trace the complex evolutionary history of humanity back to its very beginnings. At first, they thought in terms of simple¸ ladder–like evolutionary schemes. These theories have now given way to highly tentative studies of early human evolution process fraught with difficulty when bone fragments are the raw materials. It is a matter of fine and careful judgment¸ the comparing of anatomical details¸ the weighting of different characteristics¸ and the assessment of chronology and stratigraphy. The problems is complicated by an extremely thin fossil record between 4 million and 1 million years ago¸ representing less than 1500 individuals. Most of these are single teeth found in fossil–rich South African caves. Very few are skull fragments or jaws¸ the most valuable of all fossil finds. During this 3 million– year r=time period¸ our ancestors went through dramatic transformations¸ visible only through an incomplete paleontological lens. We know that many hominid forms flourished in tropical Africa during this period. Which of them¸ however¸ were direct human ancestors? We can only achieve an understanding of human evolution by getting to know as many species as we can¸ and this task has hardly begun. We will see in this section how humans examine some of the controversies that surround the biological and culture evolution of humankind and describe what we know about the behavior and life–ways of our earliest ancestors.
The Great Ice Age (1.8 million to 15,000 years ago)
T he story of humanity begins deep in geological time, during the later part of the Cenozoic Era, the age of mammals. For most of geological time, the world's climate was warmer than it is today. During the Oligocene epoch, some 35 million years ago, the first signs of cooling appeared with the formation of a belt of pack ice around Antarctica 11 million years ago. As temperatures fell, large ice sheets formed on high ground in high latitudes. About 3.2 million years ago, large ice sheets formed on the north continents. Then some 2.5 million years ago, just as humans first appeared in tropical Africa, glaciation intensified even more, and the earth entered its present period of constantly fluctuating climate. These changes culminated during the Quaternary period or Pleistocene epoch, the most recent interval of earth history, which began at least 1.8 million years ago. This period is sometimes called the Age of Humanity, for it was during this epoch, the Great Ice Age, that humans first peopled most of the globe. The major climatic and environmental changes of the Ice Age from the backdrop for some of the most important stages of evolution.
The words “Ice Age” conjured up a vision of ice–bound landscapes and frigid, subzero temperatures that gripped the earth in a prolonged deep freeze. In fact, the Pleistocene witness constant fluctuations between warm and intensely cold global climates. Deep–sea cores lifted from the depths of the world's ocean produces a complex picture of Ice Age climate. These cores show that climate fluctuations between warm and cold were relatively minor about 800,000 years ago. Since then, periods of intense cold recurred about every 90,000 years, with minor oscillations about 20,000 and 40,000 years apart. Many scientists believe that these changes are triggered by long–term astronomical cycles, especially in the earth's orbit around the sun, which affect the seasonal and north–south variations of solar radiation received by the earth.
There were at least nine glacial periods that mantled northern Europe and North America with great ice sheets, the last one retreating only 15,000 years ago. Interglacial periods, with climates as warm or warmer than that of today, occurred infrequently, and the constant changes displaced plants and animals, including humans, from their original habits. During colder cycles, plants and animals general fared better at lower altitudes and in warmer latitudes. Population of animals spread slowly toward more hospitable areas, mixing with population that already lived there, and creating new comminutes with new combinations of organisms. For example, paleontologist Björn Kurten estimated that no fewer than 113 of the mammalian species living in Europe and adjacent Asia evolved during the past 3 million years. This repeated mixing affected human evolution in many ways.
The earliest chapter of human evolution unfolded during a period of relatively minor climatic change, indeed before the Pleistocene truly began. Between 4 million and 2 million years ago, the world climate was somewhat warmer and more stable than it was in later times. The African savanna, the probable cradle of humankind, contained many species of mammals large and small, including a great variety of the older primates, of which humans are a part.
