One of the most challenging concepts in thinking about evolution is the idea of "deep time," the profoundly different timescale involved in geological and evolutionary processes than those we deal with in our daily lives. It is rare for those of us in North America to see buildings that are more than 100 years old. If the geological history of the world were represented by the height of the Empire State Building, the time since mastodons walked across North America 14,000 years ago would be represented by the thickness of a single dime.

Fig. 5.2, p. 86 from Ancient Earth, Ancient Skies, by Brent G. Dalyrmple, shows the locations of some of Earth’s oldest rocks.

By Joel R. Primack and Nancy Ellen Abrams

Understanding why living organism resemble each other has fascinated human beings for millenia, long before evolution provided a unifying concept for biology.

The term "homology" was originally coined in the 19th century by Robert Owen, a British comparative anatomist. Owen noticed the similarities between certain structures in different organisms, such as the similarities of forelimbs in vertebrates. He thought that some kind of abstract ideal plan accounted for the similarity of structure among groups of animals.

by Steven Newton, Public Information Project Director

The age of the Earth is 4.54 billion years (Ga or Gyr), a number which geologists have determined by several independent methods. Because of the recycling of crustal rocks by plate tectonics, no direct material from the earliest Earth still exists. However, geologists are able to analyze meteorites and lunar rocks returned by the Apollo missions.

Methods