An exceptionally long time scale is necessary for the mechanisms described in this chapter to drive evolutionary change.

For thousands of years, people relied on religion to determine the age of the Earth. Unfortunately, religion offered arbitrary and erroneous dates. None of the estimations came close to the mark! Darwin was concerned with the planet’s age because he understood that evolution required exceedingly long periods of time to occur.

If the Earth was as young as the religious people of his time believed, his theory would simply crumble to the ground. In the mid-20th century, scientists estimated that the Earth was 4.567 billion years old.

The Earth’s age of 4.567 billion years is usually divided into four great eons:

• Hadean: From the formation of the Earth to about 4 billion years ago. The principal characteristic of this eon is the absence of life on the planet.

• Archaic: From 4 billion years ago to about 2,500 million years ago. The first evidence of primitive life, prokaryotes, unicellular organisms without a nucleus, appears during this eon. These prokaryotes include cyanobacteria and photosynthetic bacteria.

• Proterozoic: From 2,500 million years ago to about 543 million years ago. Eukaryotes appeared, unicellular organisms containing a nucleus, along with the first multicellular organisms.

• Phanerozoic: From 453 million years ago to the present. This eon is distinguished by the appearance of hard-shelled animals and enormous species diversification. The Phanerozoic itself is divided into three eras:

  • Paleozoic: Beginning 453 million years ago, when the explosion of life forms took place, known as the Cambrian era, up to 251 million years ago, when the great Permian extinction took place.

  • Mesozoic: From the Permian extinction up to 65 million years ago, when the dinosaurs disappeared.

  • Cenozoic: From the extinction of dinosaurs to our present day. This era is also known as the era of mammals.