Question

62. Which are the three common hypotheses we discussed in class for the evolution of primates, i e., why become a primate, after the great K-Textinction 65 mya. Include in your answer a description of primate traits that can be related to each of these hypotheses.

Answer 1

Carbon Dating Carbon dating is a variety of radioactive dating which is applicable only to matter which was once living and presumed to be in equilibrium with the atmosphere. taking in carbon dioxide from tlhe air for photosynthesis cosmic ray proton He rn ucleus of atom Cosmic ray protons blast nuclei in the upper atmosphere. producing neutrons whiclh in turn bombard nitrogen. the major constituent of tlhe rn не This neutron bombardıment produces the radioactive isotops carbon-14. The radioactive carbon-14 commbines with oxygen to form carbon dioxide and is incorporated into the cycle of living things Radioactive carbon-14 is present in al living organisms. dioxide cO2 appea The carbon-14 forms at a rate which appears to be constant. so that by measuring the radioactive emissions from once-living matter and con]paring its activity with the sauilibrium level of living things. a measurement of the time elapsed can be made Co2 takes it into the food cycle O2 Cosmic rays from the sun collides with atoms in the upper atmosphere producing energetic neutrons neutronNitrogen-14 Carbon-14 proton The energetic neutron collided with a Nitrogen-14 atom to produce a Carbon-14 atom and a proton The Carbon-14 oxidises to Carbon-14 dioxide and is transported to the lower Plants absorb Carbon dioxide photosynthesis and take in Carbon-14 in a ratio similar to that of Carbon-14 in the atmosphere Carbon-14 is taken in by animals and humans through the food chain The wood from felled trees used for construction or paper manufacture also undergo Carbon-14 decay When plants and animals die, they stop taking in Carbon-14 Following death or burial, the unstable Carbon-14 in bones undergo beta decay to form Nitrogen-14 Carbon-14 undergoes beta decay to form Nitrogen-14. Over time the ratio of Carbon-14 to other Carbon atom decreases. Carbon-14 has a half life of 5,730 years. By measuring the amount of Carbon-14 in a sample and comparing it to a fresh sample, scientists can determine its age. Beta Decay LB Neutron Proton + electron (electron leaves the atom with high energy as a beta particle) Carbon-14 Nitrogen-14 O Pass My Exams

The Limitations of Carbon 14 Dating Using this techmique, almost any sample of organic material can be directly dated. There are a number of limitations, however. First, the size of the archaeological sample is important. Larger samples are better, because purification and distillation remove some matter. Although new techniques for working with very small samples have been developed, like accelerator dating, these are very expensive and still somewhat experimental Second, great care must be taken in collecting and packing samples to avoid contamination by more recent carbon. For each sample, clean trowels should be used, to avoid cross contamination between samples. The samples should be packaged in chemically neutral materials to avoid picking up new C-14 from the packaging. The packaging should also be airtight to avoid contact with atmospheric C-14. Also, the stratigraphy should be carefully examined to determine that a carbon sample location was not contaminated by carbon from a later or an earlier period. Third, because the decay rate is logarithmic, radiocarbon dating has significant upper and lower limits. It is not very accurate for fairly recent deposits. In recent deposits so little decay has occurred that the error factor (the standard deviation) may be larger than the date obtained. The practical upper limit is about 50.000 years, because so little C-14 remains after almost 9 half-lives that it may be hard to detect and obtain an accurate reading. regardless of the size of the sample Fourth, the ratio of С 14 о С 12 in the atmosphere is not constant. Although t was originally thought that here has always b en about the an era o ra ücca ons ni taken and co ot eci u ite dendrochronology have shown that the ratio of C-14 to C-12 has varied significantly during the history of the Earth This variation is due to changes in the intensity of the cosmic radation bombardment of the Earth, and changes in the effectiveness of the Van Allen belts and the upper atmosphere to deflect that bombardment. For example, because of the recent depletion of the ozone laver in the stratosphere, we can espect there to be more C-14 in the atmosphere today than there was 20-30 years ago. To compensate for this variation, dates obtained from radiocarbon laboratories are now corrected using standard calibration tables developed in the past 15-20 years. When reading ar chaeological reports, be sure to check if the carbon-14 dates reported have been calibrated or not Finally, although radiocarbon dating is the most common and widely used chronometric technique in archacology today, it is not infallable. In general, single dates should not be trusted Whenever possible multiple samples should be collected and dated from associated strata. The trend of the samples will provide a ball park estimate of the actual date of deposition. The trade-off between radiocarbon dating and other techniques, like dendrochronology, is that we exchange precision for a wider geographical and temporal range. That is the true benefit of radicarbon dating, that it can be employed anywhere in the world, and does have a 50.000 year range. Using radicarbon dating. archaeologists during the past 30 years have been able to obtain a much needed global perspective on the timing of major prehistoric events such as the development of agriculture in varous parts of the world