Resource: Deep Time

Since taking hold on Earth more than 3.8 billion years ago, life has never let go. Evolutionary transformations along the way -- changes in the forms and functions of living things -- have yielded tremendous diversity. As much as evolution is about life and its many forms, however, Darwin's theory involves more than can be explained by biology alone. To fully understand the conditions in which life has evolved, we must integrate other sciences, including geology, chemistry, and physics, into our study of life on Earth.

The physical environment determines how much of the fossil record is preserved for scientific analysis. Based on the work of taphonomists, scientists who study how organic remains are preserved, we know that a rare set of circumstances must combine to produce fossils. For instance, the body of a Paleozoic reptile had to have fallen on the muddy soil of a streambed when it died. That streambed would, in turn, have to have been quickly covered by other sediment layers that protected the remains from oxidation and microbes, and then it had to have been left undisturbed as long as it took for minerals in the ground to replace those in the bones.

Because Earth is such a dynamic planet, hundreds of millions of species may already have become extinct. Knowledge of these species is likely to elude scientific discovery even if the organisms did happen to leave fossils behind. With Earth's crust in constant motion, as new material surfaces through volcanoes and undersea ridges, older material is consumed through earthquakes, weathering, and other natural processes. When Earth's rocks and features are destroyed, evidence of past life that might otherwise help scientists in their construction of a "tree of life" is also destroyed. The fossil record, like a puzzle with missing pieces, will always contain gaps.

To learn about how paleontologists use fossils to reconstruct an organism's appearance, check out Fossils: An Ancient Sea in Indiana.

To view images of fossils from different geologic time periods, check out Fossils.

To learn more about how to find the ages of rock, check out Radiometric Dating.

To learn more about radiocarbon dating, check out The Dating Game: Radioactive Carbon.

To learn more about geologic time, check out The Wall of Time.

• Use the timeline to find out when fish, reptiles, birds, mammals, and humans appeared. What geological changes were occurring at the time of their appearance?
• How has the complexity of life changed over time?
• When did oxygen become part of Earth's atmosphere? What other changes occurred at the same time?
• List the major extinctions and the hypothesized causes of each.
• If you could trace your family back to early humans, how many generations would there be given a 50-year life span?
• What helps you understand deep time?