BIOLOGY Volume 3 - A Guide to General Biology - 2004

APPENDICES

Appendix 4. Geological Time Scale

The rocks that make up the Earth's crust are stratified, meaning they consist of layers or strata superimposed on one another. Barring disturbances related to crustal mobility, rock strata are younger the closer they lie to the Earth's surface. In the 18th century, William Smith noted a correlation between groups of organisms represented by fossils and specific strata. The sequence of fossils he discovered reveals a gradual increase in Organism complexity from the lowest layers to the uppermost, indicating that certain life forms grew more complex over geological time.

Radiometric dating has made it possible to establish the approximate age of the oldest rocks associated with each period. The geological time scale, along with the distinct biological events linked to each period as determined by the fossil record, is presented in Table App.4.1.

Table App.4.1. Geological time scale and The history of life (Age = years · 10⁶)

App.4.1. Age of the Earth

According to current estimates, the planet Earth was formed approximately 4.6–4.9 · 10⁹ years ago. These estimates are based primarily on determining the age of rocks (geochronology) using radiometric dating Methods.

In Section App.1.3, we discussed the fact that certain chemical elements exist in A number of varieties called isotopes, some of which are radioactive. Radioactive elements decay at a constant rate independent of Temperature, gravity, magnetism, or any other factors. This rate of decay is characterized by a value known as the half-life.

Currently, three Main methods of radiometric dating are used (Fig. App.4.1). The First and Second methods are employed to determine the age of rocks forming the Earth's crust, whereas the third method — radiocarbon dating — is used to determine the age of fossils and is directly relevant to the history of life on Earth.

Fig. App.4.1. Radiometric dating methods.

Radiocarbon Dating

Carbon-12 (¹²C) is a stable, non-radioactive isotope of carbon. The radioactive isotope ¹⁴C is present in very small quantities (< 0.1%) in the air, surface waters, and living organisms. It is continuously produced in the atmosphere As a result of cosmic rays interacting with nitrogen and oxygen atomic nuclei, and strong evidence suggests that The rate of ¹⁴C production has remained constant for several millennia. A dynamic equilibrium has been established whereby The formation of ¹⁴C balances its loss through radioactive decay. ¹⁴C naturally occurs as part of ¹⁴CO₂, and The ratio of ¹⁴C to ¹²C theoretically remains constant. Throughout their lives, living organisms absorb ¹⁴C either as carbon dioxide or as Organic compounds. After an organism dies, carbon uptake ceases, but ¹⁴C decay continues at a rate determined by its half-life. By measuring the ¹⁴C content in a dead organism and comparing it with that of a living organism, the age of the dead specimen can be determined. For instance, if the ¹⁴C content in a fossil mammal bone is one-fourth that of the same bone in a recently killed mammal, then, since the half-life of ¹⁴C is 5.6 · 10³ years, the theoretical age of the fossil bone is estimated to be 11.2 · 10³ years. Using this method, it is possible to date remains of organisms that lived up to 10.0 · 10⁴ years ago.

Radiometric dating involves numerous potential sources of error; therefore, age estimates obtained via the methods discussed above should be considered approximate. Nevertheless, they have proven invaluable in significantly expanding our knowledge of the Earth.