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Chapter 26 The Tree of Life: - page 7 / 18





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Because their properties were heritable, they could be acted on by natural selection.

The most successful of these protobionts would have increased in numbers, because they could exploit available resources and produce a number of similar daughter protobionts.

Once RNA sequences that carried genetic information appeared in protobionts, many further changes were possible.

One refinement was the replacement of RNA as the repository of genetic information by DNA.

Double-stranded DNA is a more stable molecule, and it can be replicated more accurately.

Once DNA appeared, RNA molecules would have begun to take on their modern roles as intermediates in translation of genetic programs.

The “RNA world” gave way to a “DNA world.”

Concept 26.2 The fossil record chronicles life on Earth

Radiometric dating gives absolute dates for some rock strata.

The relative sequence of fossils in rock strata tells us the order in which the fossils were formed, but it does not tell us their ages.

Geologists have developed methods for obtaining absolute dates for fossils.

One of the most common techniques is radiometric dating, which is based on the decay of radioactive isotopes.

An isotope’s half-life, the number of years it takes for 50% of the original sample to decay, is unaffected by temperature, pressure, or other environmental variables.

Fossils contain isotopes of elements that accumulated while the organisms were alive.

For example, the carbon in a living organism contains the most common carbon isotope, carbon-12, as well as a radioactive isotope, carbon-14.

When an organism dies, it stops accumulating carbon, and the carbon-14 that it contained at the time of death slowly decays to nitrogen-14.

By measuring the ratio of carbon-14 to total carbon or to nitrogen-14 in a fossil, we can determine the fossil’s age.

With a half-life of 5,730 years, carbon-13 is useful for dating fossils up to about 75,000 years old.

Fossils older than that contain too little carbon-14 to be detected by current techniques.

Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-7

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