2.2.1 Ice, Ice, and More Ice

Scientists who study the changes that have occurred on Earth over billions of years have identified at least five significant periods of cooling on the planet. These are often called ice ages, and each has included multiple glaciation periods during which glaciers grew on the land.

A few factors can trigger an ice age, but generally such climate changes occur when insufficient sunlight is able to reach the planet’s surface. Then temperatures drop in northern latitudes, resulting in the accumulation of ice. As the glacial ice sheets grow and spread across the land, water is pulled from the oceans, causing sea levels to decline. Even areas closer to the equator, where ice is unlikely to develop, can experience dramatic climate change during these cooling periods. Otherwise-tropical areas can experience drying, causing rivers to disappear, lakes to turn into swamps, jungles into savannahs, and grasslands into deserts. These changes have a huge effect on plants and animals, leading to evolutionary adaptations in some and extinction in others. These are all natural processes, and each recorded ice age in our planet’s history has eventually come to an end when more sunlight reaches the Earth and causes the temperature to rise and ice to melt.

The most recent glaciation period began a little over 100,000 years ago and reached its peak about eighteen thousand years ago (Figure 2.14). The ice age of which this glaciation period was a part ended approximately twelve thousand years ago. At peak glaciation, ice sheets sometimes two miles thick covered the land around the North Pole and extended outward over much of present-day Russia, Scandinavia, the British Isles, Greenland, Canada, and the northern reaches of the United States.

The consequences of these climatic transformations for modern humans have been huge. It is probably not a coincidence that at approximately the same time Earth entered its last glaciation period, humans began their global expansion. Climate changes in Africa were likely a decisive factor in encouraging and enabling them to move into other parts of the world. Low sea levels allowed modern humans to expand into maritime Southeast Asia and Japan and reach Australia. And not long after Earth reached peak glaciation, the first human migrants entered North America from Siberia, by way of a strip of land exposed by low sea levels.

Modern humans who moved into colder conditions had to adjust to their harsh environments. For example, they created new forms of clothing, unnecessary in warmer climates but vital now, by removing the hides from hunted animals with various types of rock tools and scraping them clean. The earliest clothing must have been simple and likely functioned as blankets draped over the body to keep warm. However, by around thirty thousand years ago, modern humans had developed the earliest known sewing needles, making them of bone, wood, and ivory. Like their modern counterparts, these needles had sharp points at one end and a hole in the other. With thread made from animal remains or wild flax, humans could now piece together bits of soft animal hide from foxes, rabbits, and deer to produce far more sophisticated and tight-fitting clothing.

The five-thousand-year-old remains of a man discovered in the alpine region between Austria and Italy in 1991 and dubbed Ötzi provide us with some indication of the type of clothing that could be created (Figure 2.15). Ötzi was dressed in a heavy coat made of goat and sheep hides stitched together. He also wore tight-fitting leggings of similar materials, a bearskin cap with a chin strap, and shoes constructed from woven grass, tree fibers, and deer hide. This type of clothing was far more functional than earlier designs and would have allowed populations to survive in frigid areas.

The warming of Earth and retreat of the glaciers that began around seventeen thousand years ago submerged continental shelves around the world and created new lakes and rivers. These changes in turn created opportunities for exploiting fresh- and saltwater marine life in the new waterways and the warmer shallow waters along the coasts. Many human groups were now exposed to a greater variety of animals that they could use to supplement their diets. As other animals like reindeer adapted to life in cold environments and moved north, the human populations that hunted them moved north as well. The higher water levels also helped to isolate some groups, however. Those that had migrated into maritime Southeast Asia and Australia found themselves more secluded on islands in the south Pacific. Those that had crossed into the Americas from Asia were cut off from populations in the eastern hemisphere as sea waters rose in the Bering Strait. The civilizations they created in North and South America remained largely separated from the rest of the world until the fifteenth century CE.