Geochronology and Evolution

Version:1.0 StartHTML:0000000172 EndHTML:0000005842 StartFragment:0000002833 EndFragment:0000005806 SourceURL:file://localhost/Users/sbowring/Desktop/aaas%202011.doc

Time is an essential part of any historical science, whether it is the study of the formation of our universe or the evolution of life on the planet we inhabit. An appreciation of the enormity of geological time and how our atmosphere, oceans, and climate have changed over hundreds of millions of years is essential for understanding and ameliorating the impact of human dominance of the planet. The science of determining the age of rocks and minerals, or geochronology, has seen remarkable development since the discovery of radioactivity a little more than a century ago. Even centuries before this discovery, however, those who studied the geologic record proposed it to be millions to billions of years old, not thousands as many at the time believed.  The science of geochronology is based on using the decay of radioactive isotopes and has been employed to date the oldest objects in our solar system at approximately 4.567 billion years.  There is no credible scientific debate about the age of the earth, moon, and meteorites, and multiple radioactive isotope systems with very different rates of decay agree remarkably well.  Measurement techniques continue to improve and it is now possible to date events that range from < 1 million years to greater than 4 billion years with a precision of ± 0.05%, or ± 5000 years for a 1 million year old rock.  Those who insist that Earth is thousands of years old must either ignore the science of geochronology or attempt to prove it wrong.  There is no shortage of attempts and most hinge on untestable hypotheses such as variable radioactive decay rates through time or emphasis on samples that do not meet the basic closed system requirement, which prescribes that the ratio of parent to daughter isotopes in a mineral or rock change only as a result of radioactive decay.  Long before Darwin’s “On the Origin of the Species” was published, 17^th and 18^th century natural scientists and paleontologists worked to construct a time scale using relative ages of fossil-bearing rocks based on first and last appearances of distinctive fossils. Most attention was focused on the part of Earth history after the first appearance of abundant animal fossils. The relative order and subdivisions of the geologic timescale were established before the discovery of radioactivity and remain today, now calibrated with absolute numerical dates using high-precision geochronology.  This allows the calculation of evolutionary, tectonic, and sediment accumulation rates in past environments preserved as rocks that are hundreds of millions of years old.