Revolutionary Worker #1157, June 30, 2002, posted at http://rwor.org
There are many reliable dating techniques, such as various radiometric dating techniques based on measures of radioactive decay, which were not yet available to scientists in Darwin's time. Radioactivity was only discovered at the end of the 19th century, and it was in the 1950s that scientists figured out that various radioactive substances, which occur naturally in various materials, will actually "decay" at constant and predictable rates, turning into non-radioactive stable forms (isotopes) of the same elements.
By concretely measuring the relative amounts of radioactive isotopes vs. stable forms of the same elements present in a sample, scientists can figure out just how long the radioactive decay process has been going on, and in this way determine how old the object is. And since different kinds of radioactive isotopes (of carbon, potassium, rubidium, and so on) decay into their particular non-radioactive forms at different rates, scientists can often doublecheck the age of a sample by using more than one dating technique. This kind of method has been used to very precisely figure out the age of different kinds of rock layers (and even moon rocks!): the rate of decay of certain isotopes found in rocks is so constant and predictable that scientists sometimes refer to them as "clocks in the rocks." Measures of the decay of such things as potassium isotopes to argon, rubidium-87 to strontium-87, thorium-232 to lead-208, uranium-238 to lead-206 are all commonly used and mutually corroborating techniques for dating many different kinds of rocks.
Organic plant and animal remains contain carbon, so measuring the decay of radioactive carbon-14 to the more stable isotope carbon-12 can provide the age of these remains. This is because all living plants and animals take in radioactive carbon-14 from the environment but as soon as they die they stop taking in carbon-14 and the amount that they had in them starts to decay and turn into the stable carbon-12 isotope. And this happens at a known, constant and predictable rate. So by measuring how much carbon-14 is still left in the remains of a dead organism, and comparing this to how much has already turned into carbon-12, it is possible to directly calculate the time when something died, going back as far as 50,000 years or so. Different methods of radiometric dating measuring the decay of other kinds of isotopes can be used to date materials older than 50,000 years. And even though such radiometric dating techniques cannot be used to directly date fossils which are found in sedimentary rocks, these fossils can also generally be consistently and reliably dated indirectly, simply by directly measuring the age of igneous (volcanic) rock layers found right above and below those fossils.
In recent decades advances in molecular biology have added yet another important kind of dating technique to the scientific repertoire--that of "molecular clocks," which involve measuring the amount of "neutral" mutations which have accumulated over time in different lineages of related species. These types of mutations are considered to occur at relatively constant rates, so that calculating the amount of certain kinds of molecular differentiation between related species can provide a pretty good estimate of the amount of time which has passed since their lines diverged (split) from a common ancestor. Additional methods, such as DNA hybridization techniques, can also assess the degree of similarity or difference in the DNA of different species, and this has been very helpful in providing more specific calculations of how closely different species are related, and how far back in time they must have shared a common ancestor. [Return to "The Science of Evolution, Part 1"]
This article is posted in English and Spanish on Revolutionary Worker Online
Write: Box 3486, Merchandise Mart, Chicago, IL 60654
Phone: 773-227-4066 Fax: 773-227-4497
(The RW Online does not currently communicate via email.)