Basics of Evolution - O.P. Korzh - 2006

Part III. PATHS OF THE EVOLUTION OF LIFE

Chapter 17. Geochronology

17.1. Methods for Determining Geological Age

According to current estimates, the entire period of the planet Earth's existence spans 4,600 million years and is divided into several geological periods (epochs). These characterize The Development of the planet's geographic, climatic, and other features, which have shaped the specific course of biological evolution. To understand Evolutionary Processes, it is essential both to comprehend and to determine the duration of the periods within which these processes unfolded. At the same time, one must bear in mind that in nature, long-term processes coexist with fleeting ones, and the temporal scale of even recent geological epochs extends far beyond our ordinary perceptions.

The complexity of life lies in the integration of multiple time scales: phylogenesis and ontogenesis occur simultaneously, remaining interconnected and interdependent. The identification of such scales depends directly on the technical capabilities of the dating Methods employed.

All dating methods are divided into two major groups: relative dating, where the age of events or objects is determined in relation to one another, and absolute dating, where age is measured in astronomical time. The primary method of relative dating is stratigraphy, or superposition. When several geological strata lie one upon another, their relative positions provide Evidence of the chronological sequence of geological events.

One of the most important relative dating techniques is biostratigraphic correlation, in which fossil remains of extinct animals or plants are used to sequence geological deposits. Layers containing the same species or assemblages of organisms can be regarded as evolutionarily

contemporaneous; that is, it is assumed that creatures preserved as morphologically similar fossils existed on Earth at approximately the same time. Biostratigraphy is used primarily to establish a general chronology based on the evolutionary lineages of fossil organisms.

One particularly interesting biostratigraphic object of study is trilobites, which existed from the Cambrian (about 600 million years ago) to the end of the Permian period (about 130 million years ago) and have been thoroughly investigated by paleontologists. Throughout this span, these animals underwent significant morphological changes. Taking these changes into account, scientists can determine the relative age of an entire studied sedimentary layer simply by discovering a particular trilobite species.

To achieve greater dating precision, supplementary techniques are employed, the most well-known being fluorine dating. This method relies on the capacity of fossil remains to absorb fluorine or other elements from surrounding sediments. Consequently, the concentration of such elements in the samples under investigation can indicate their relative age, or at least the synchronicity of events. The application of this method is limited by variations in The chemical composition of rocks across different regions.

Absolute dating relies on methods derived from the exact sciences, grounded in THE PRINCIPLE OF radioactive decay. These techniques are based on the natural occurrence of radioactive isotopes that decay into stable isotopes at a predictable rate. The half-life is constant; during this interval, half of the existing quantity of an unstable isotope transforms into its stable form, thereby determining The rate of decay. Thus, The ratio of stable to unstable isotopes makes it possible to date past events.

Among the earliest methods of this group to be applied were uranium-lead dating (based on the half-life of radioactive uranium converting into lead) and thorium-lead dating (based on the half-life of thorium converting into lead). The margin of error for these methods is a few million years, and they are typically used in conjunction with one another.

A more precise and modern technique is the potassium-argon method, which is based on the radioactive decay of potassium into argon. However, due to the long half-life of potassium, dating via the ⁴⁰K/⁴⁰Ar ratio is used exclusively for relatively ancient events dating back less than 500 thousand years.

To study younger strata, the radiocarbon method is employed, which is based on the capacity of living matter to absorb the unstable carbon isotope ¹⁴C from the atmosphere. Upon death, this process ceases, and the time of death can be calculated from the ¹⁴C/¹²C ratio. This method yields satisfactory results for a relatively brief time span—no more than 40,000 years. Several other Methods for determining rock age also exist, based on the half-lives of different radioactive isotopes.

In addition to those mentioned above, various lesser-known techniques are also utilized (such as rubidium-strontium dating, thermoluminescence dating, Amino Acid Racemization—which measures the loss of chiral purity after death—electron paramagnetic Resonance, paleomagnetism, and the analysis of stable oxygen isotopes in fossils, among others).