A British astronomer and mathematician who proposed in 1691 that if the original oceans were fresh water, one could calculate the minimum age of the Earth by dividing the total amount of salt now present in the oceans by the amount added each year from the world's rivers and streams. While he actually did not have the full necessary data available, he used this approach to conclude that the Earth is minimally 100,000,000 years old.

An ancient Egyptian historian who counted the durations of all dynasties of pharaohs and gods that reigned down to his time (3rd century B.C.). This allowed him to conclude that the Earth is about 38,000 years old (from our time).

An early 17th century Archbishop of Armagh, Ireland who accepted the Judeo-Christian Old Testament as being literally true and subsequently determined the age of the Earth by counting biblical generations. With this method, he calculated that the Earth began in 4004 B.C. on October 23 (about 6,000 years ago).

An English canal and mine engineer who in 1799 expanded on James Hutton’s idea of geologic time by dividing sedimentary rock layers into 6 main divisions based, in part, on the fossil evidence of life that they contained. These main divisions, or eras, that he proposed became the basic framework for the geologic time scale of the Earth that we still use today.

An English geologist who in 1867 estimated the age of life on Earth to be about 240,000,000 years based on his assumption of the amount of time for the successive changes in animal species found as fossils in sedimentary rock layers.

An American chemist who in 1907 estimated the age of the Earth to be at least 2 billion years based on the decay of uranium-238 to lead-206 in rocks.

The most likely age of the Earth, based on radiometric dating of meteorites.

The date for the earliest signs of life on Earth. These are organic chemicals that were presumably produced by single-celled microscopic organisms. This evidence is not convincing to some researchers because it consists of the chemical markers of life in rock rather than fossils of actual cells.

The date for the earliest clear evidence of early life on Earth. This is fossil colonies of single-celled microscopic organisms.

The geologic era during which there was the appearance and evolution of early multicellular life forms including invertebrates, vertebrates, and plants. In addition, life moved onto the land for the first time. This era existed about 542-251 million years ago.

The geologic era during which mammals first appeared and large reptiles dominated the planet. This was the age of dinosaurs. It occurred about 251-65.5 million years ago.

The geologic era following the extinction of the last dinosaurs. During this era, more advanced mammals (placental mammals) rapidly evolved and became the dominant large animals. It was only in this last geologic era that primates evolved.

The number of known major global extinction events that have occurred on Earth.

at least six (they occurred about 488, 444, 360, 251, 200, and 65.5 million years ago)

The Earth's outer rock shell consisting of about a dozen enormous rigid tectonic plates and many smaller ones that are more or less constantly moving relative to each other at a rate of a few centimeters a year.

The super-continent that was created when all of the continents came together and fused early in the time of the dinosaurs. It began forming about 285 million years ago. It was complete by about 210 million years ago and began drifting apart again 10 million years later. The name of this super-continent literally means "all of the Earth" in Greek.