Life between the appearance of the first organisms, 3.8 billion years ago (bya), and the first written record of history, several thousand years ago, is called prehistoric. It included sudden increases in life and mass extinctions, for example during an ICE AGE.
Bacteria and other single-celled organisms lived in the sea and remained the only life forms for billions of years. Some of these released oxygen into the air, allowing the evolution of organisms that could use oxygen. The first animals probably appeared 600 million years ago (mya), those with hard shells and body cases about 550 mya, and vertebrates (animals with backbones), such as fish, 500 mya.
The first land plants evolved from green algae found at the edge of the sea and rivers 440 mya. In time they gave rise to horsetails and club mosses. Forests then evolved and were the home to the first land animals. Scorpions and centipedes, as well as earthworms and leeches, first appeared about 400 mya. They were followed by the first land vertebrates, which evolved from fish and were the four-legged ancestors of amphibians.
An ice age is a period in Earth’s history when the climate is far colder than usual and large areas of Earth’s surface are covered by ice sheets. There have been 20 ice ages in the past two-and-a-half million years, each lasting about 100,000 years.
Changes in Earth’s orbit around the Sun produced cooler summers, so winter snows did not melt. As ice sheets formed, they reflected sunlight back into space so it did not warm the planet. These changes also affected the oceans, increasing the numbers of plantlike plankton, which took in carbon dioxide from the atmosphere. Since this gas helps to retain heat around Earth, a drop in its levels accelerated cooling.