Tuesday, March 8, 2011

How Does It All Stack Up

The ultimate appeal for those seeking to demonstrate the theory of evolution is the fossil record. But the reliability of that record depends on the chronological ordering of the fossils. How is this ordering accomplished?

One can start by looking at the order of the rock layers the fossils are found in.  But the rock layers are not normally all neatly exposed in one place, but are found in various outcroppings. As William Smith, who made the first geological map of England, noticed, there are similar types of rock in different places in the geological sequence. How do you know which layer a particular rock outcropping belongs to if there are identical types? Smith's solution was to distinguish them by the fossils in them, an approach that was ultimately generally adopted. Now this makes sense when applied over a limited area. But it creates questions when applied in general, especially when fossils are used to correlate different types of rock. How do we know that the same plants and animals did not flourish at different times in different localities or happen to be preserved by some accident that did not have any connection to the time they lived in?

But Smith, who appears to have been mainly a pragmatist who did not particularly hold any set theory, was followed by those with a philosophical axe to grind. Charles Lyell and Rodrick Murchison, who followed James Hutton in believing in the development of the earth by gradual processes, rejected the existence of coal deposits in rock layers they regarded as too early for coal before they even examined the evidence. Geologists have since then continued to develop methods to  get around the fact that the fossils are not necessarily found in the order that fits their theory. If there are rock layers found with the expected layers between them missing, it is claimed that during this time period the area was above water. But after supposedly large numbers of years of erosion, what is left looks deceptively like a bedding plane. Or if the layers are found out of order, it is assumed there was a thrust fault (a major earth movement that would normally be expected to leave things like slickened surfaces and metamorphized rock) even if there is no or only marginal evidence for its existence.

In this system, the fossils are dated by the rocks and the rocks are dated by the fossils. Which makes sense if the system as a whole has been established, but is a problem if the system itself is in question. But what about radioactive dating? Radioactive samples associated with rocks laid down by water are frequently not a closed system, and whether they are a closed system is commonly determined by whether they match the geologic ages already concluded for the rock layers. One is left with the feeling the fossils are being dated to fit the theory rather than the theory being based on the existing order of the fossils.

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