The Earth is about 4.5 billion years old. Fossil evidence suggests that the first living things were simple cells that appeared about 3.5 billion years ago. The time line in Table above shows that the period of time from the beginning of Earth to 700 million years ago, the largest part of the Earth’s past, is known as the Precambrian era.
The Paleozoic era began about 570 million years ago and lasted until 280 million years ago. Fish, insects, amphibians, and reptiles were some of the major groups of animals that developed in this period. Both terrestrial and aquatic plants also formed in this time span. The Mesozoic era extended from 250 million years ago until 135 million years ago. A period dominated by reptiles, the Mesozoic is known as the age of the dinosaur. Late in the era, mammals and birds developed. The most recent period, the Cenozoic era, began 65 million years ago and extends to the present. During this time, birds and mammals flourished. Humans made their appearance late in the era, about 3 million years ago.
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| The geologic time scale shows significant events in the development of life on Earth. |
To visualize the amount of time that has passed since the first coral reef appeared on Earth 2 billion years ago, one can compare time to a human’s walking stride. For example, a person’s stride, a distance of about 3 feet (0.9 m), could represent a period of 50 years. In such an analogy, walking two steps back would take one back a century in time. The distance of 40 steps would represent the time that has passed since the birth of Jesus (the beginning of the Christian era [C.E.]), and 200 strides would bring one to human’s prehistoric period. Yet, to reach the time when reefs were first formed on Earth, one must walk a distance equal to the Earth’s circumference at the equator (24,902 miles [40,076 km], or 43,827,520 strides)!