The Great Ice Age
1.8 million to 15,000 years ago
The 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 theQuaternary 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.
