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Page history last edited by gerryc 11 years, 2 months ago

M13. The universal presence of DNA is strong evidence for the common ancestry of all living things.


Student Outcome: M13.1

Know that DNA is universal to most living things.

Living organisms do indeed share a common mechanism that copies and translates heritable genetic information. All living organisms translate the genetic code using ribosomes, tiny protein-building factories, they all translate it with the aid of small molecules called transfer RNA, they all read it in the same direction, and they all read it in the same way, translating the code 3 letters at a time into sequences of amino acids, the building blocks of proteins.




The genetic code and common descent

The fact that there is a standard genetic code, with very few variations, that is used by all species, suggests an argument that life had only one common ancestor.

The genetic code, it is argued, is to some extent arbitrary. It is true that the redundancy of the code has convenient features, as we have noted above, but there would be many other way of selecting a code that also had these useful properties.

So, it is argued, if creatures now living are not descended from a common ancestor, then the same genetic code must have originated twice. Since the code is arbitrary, this could not be a result of convergent evolution, but would also have to involve a massive coincidence. The more probable explanation, then, is common descent from a single common ancestor.

The minor variations in the genetic code from species to species do not particularly invalidate this argument so long as it is possible for such variations to evolve (which they can, as we have seen in the previous section). For the massive similarities that remain after we have acknowledged the existence of these minor variations are still too great to be reasonably accounted for by coincidence alone.

We should note that this argument for a common ancestor is only relevant to people who are already convinced, on other grounds, that life evolved from (in Darwin's words) "a few forms or one", and wish to know whether it was one form or a few: it is an argument from and not for evolution.


Source: http://skepticwiki.org/index.php/Genetic_Code

Student Outcome: M13.2

Know that DNA has diversified over billions of years.

Finally a worthwhile introduction to this idea - however, it contains other information about the origins of life on earth 


This revision of Protein synthesis is the best I can do.


This video is long and you need to allow about 20 minutes for full viewing.



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