BIOLOGICAL EVOLUTIONIt is easy to see that some creatures are more like other creatures and construct a“tree” based on these similarities and differences (e.g. try humans, chimpanzees,wolves, cheetahs, worms, fruit flies, spiders, …). This is just the kind of categorizationthat can be constructed for any kinds of things—a car is more like a tank than a house,etc. The question that biology tries to answer is: How did these similarities anddifferences arise?How do we know that “evolution” occurred at all?All we really (think we) know is that all living species did not come into existence atthe same time, and the order of their appearance. Also, that organisms which appearlater seem more "complex" both physically and cognitively. Example: earliest rockscontaining fossil evidence for single-celled organisms, insects, vertebrates, … The onlyassumption made in this is that ages of artifacts can be reliably determined byradioactive dating (which we'll discuss shortly). So that life “developed” or “evolved”in some way during the history of the Earth is not part of any “theory,” it is more-or-less as simple as being able to tell by inspection that a book from 1922 is older than abook just published. The fact that the record of fossil evidence is incomplete and hasgaps has nothing to do with this conclusion "Evolution", as the term is usually used, means that these various appearances of"newer" species, and the categorization of the “tree of life” are linked by "inheritance."Darwin had no idea what this genealogical inheritance could be (Mendelian geneticscame later, and knowledge of DNA later still), he only postulated that there must besome such mechanism by which traits can be passed down through generations oforganisms. Darwin added that the steps are linked by gradual changes under the operation ofnatural selection. However it is a mistake to think that modern evolutionary biologyassumes either of these (although that natural selection plays some role is acceptedalmost universally, but with many who think that other things [which we will list: e.g.especially processes at the genome level, like “lateral transfer,” gene duplication, …]play just as important a role. It is this shift from the evolutionary views of a fewdecades ago (which persist in the popular literature and even in university courses) tothe current state of affairs, focused on evolution at the genome level and allowing formany important processes, some of which are surely yet to be discovered, that I wouldlike you to appreciate. This will allow us to think more clearly about how thedevelopment of life on other planets might have taken place. For a good briefpresentation of the older picture of evolution and its history, read pp. 56-58 in yourbook, and then read p. 59 on “Evolution at the molecular level.”The first thing to appreciate is the strong evidence that all life shares a number offundamental properties indicating that life developed from some original life form: Evidence for a common ancestor:1. Universality of genetic code.2. Same 20 amino acids in all species.3. Identical "handedness" of biological molecules.4. Energy carriers (e.g. ATP) and enzymes with identical functions in diverseorganisms.This is why evolutionists often write about a “last common ancestor” (LCA).But note that this “LCA” is supposed to already be complex, like a bacterium, not thefirst life on Earth, which would probably have been much more rudimentary.Getting the ages of rocks containing fossils or chemical evidence for life.How can we get the order and dates at which various crucial evolutionarydevelopments occurred? Relative ages of rocks containing fossil evidence comes fromsedimentary strata, as explained in detail on pp. 80-84 of your book. For absolute agesthere are two methods:1. Absolute dating using radioactivity—used for fossils or impressions in rocks.Although these methods give accurate ages for the rocks, the problem for the earliestlife forms (say 2 to 4 Gyr ago) is that it’s difficult to establish that the rocks have tracesof biological processes, i.e. evidence for life in them. But there are fewer and fewerproblems for times less than about 2 Gyr ago.Your textbook gives a detailed explanation of radioactive dating on pp. 85-90. Itis important to realize that this method is not especially “scientific” (you don’t have tounderstand “radioactivity” to use it!) and is no more mysterious than watching a clockor an hourglass.2. Relative dating by differences in DNA sequences—this is based on using the numberof accumulated mutations as a clock, and can be used in DNA, RNA, and amino acids.The results generally agree. However this method does not give accurate absolute agesbecause we don’t really know how the rate of mutation has varied. Instead it gives thelineage, or the relative point in the “tree of life” at which two organisms with differentDNA probably diverged. These relative ages can be put on an absolute scale if weknow the rate at which mutations accumulate, and that this rate has been constant—thatis a problem, but only introduces some uncertainty in the absolute ages, not in the order,and the ages inferred agree with ages from radioactive dating when comparison ispossible.This method is also described on pp. 116-117 of your textbook.Example: Assuming that the first sequence is the oldest, what is the order inwhich they probably arose?TTGGACCTGACGCTTGGGACATTGACCCTTGAACCRadioactive Dating: 14C as an exampleEach radioactive atom has its own half-life. If we know the amount present insome sample, and can estimate the original amount, then we can calculate how long it'sbeen decaying, which gives it's age.Example: Carbon-14 dating:Production:cosmic rays in -------> neutrons -------> combine with------>14Cearth's atmosphere14NDecay: 14C -----> 13C with half-life = 5730 yearsIn earth's atmosphere, the balance between production and decay gives anequilibrium 14C abundance, which is well-known, and constant.But 14C + 2 O ---> 14CO2 , which gets mixed with normal 12CO2.All plants take in CO2 , and animals feed on plants, so all living creatures havethe equilibrium ratio of 14CO2 to 12CO2. When an organism is alive, you get 15.3 decays per minute for each gramof carbon in it. After 5730 years, only 1/2 of 14C left, get 7.6 decays per minutefor each gram of carbon. After 2 X 5730 = 11,460 years, only get 3.8
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