Common Descent Part 1

Pretty much everyone accepts some degree of common descent; creationists generally believe that all canines (the dog "kind") evolved from a single species. However, I am talking here about universal common descent, the claim that all living things are related, and are descended from a common ancestor. When I say common descent on this blog, I mean universal common descent, rather than limited common descent.

Let me say up front that there are issues in science revolving around that common ancestor. It may be that there was actually a whole bunch of ancestors that appeared independently, and subsequently exchanged genes. It was a very long time ago, and the evidence available today is scant. So the argument can be summarised as the claim by mainstream science that humans and (say) bananas share a common ancestor, and the claim by creations that they do not (and indeed, usually that even humans and chimps do not share a common ancestor).

Common descent is consistent with theistic evolution, and, of course, with modern evolutionary theory. It is not consistent with the creationist claim that God produced the various "kinds" of organisms separately.

Common descent is a fascinating issue in the evolution debate for a number of reasons as we shall see...

Michael Denton and Common Descent

Michael Denton is an "intelligent design" advocate who wrote a book, Evolution: A Theory in Crisis, published in 1985. In the book he spent a lot of time arguing against common descent. However, he subsequently abandoned his claims. In his later book, Nature's Destiny, he embraces common descent. The fact that the author of these arguments now accepts they are flawed does not concern the creationist one jot. Their faith informs then common descent is wrong, and so they accept any argument against it, without bothering to actually think.

Here is what Denton has actually said (from here):

It is important to emphasize at the outset that the argument presented here is entirely consistent with the basic naturalistic assumption of modern science - that the cosmos is a seamless unity which can be comprehended ultimately in its entirety by human reason and in which all phenomena, including life and evolution and the origin of man, are ultimately explicable in terms of natural processes. This is an assumption which is entirely opposed to that of the so-called "special creationist school". According to special creationism, living organisms are not natural forms, whose origin and design were built into the laws of nature from the beginning, but rather contingent forms analogous in essence to human artifacts, the result of a series of supernatural acts, involving the suspension of natural law. Contrary to the creationist position, the whole argument presented here is critically dependent on the presumption of the unbroken continuity of the organic world - that is, on the reality of organic evolution and on the presumption that all living organisms on earth are natural forms in the profoundest sense of the word, no less natural than salt crystals, atoms, waterfalls, or galaxies. (Nature's Destiny, pages xvii-xviii).

Dave Scot and Common Descent

Dave Scot is an IDist who at one time ran the Uncommon Descent blog site for William Dembski. Here is what he says of objections to common descent (ironic, given the name of the blog):

You certainly don’t have to agree here with descent with modification from a common ancestor but I’m going to start clamping down on anyone positively arguing against it. It’s simply counter-productive to our goals and reinforces the idea that ID is religion because nothing but religion argues against descent with modification from a common ancestor.

This was the thread he said it on:
http://www.uncommondescent.com/index.php/archives/744

Uncommon Descent has a revisionist attitude to history, and Scot's thread has since been deleted (perhaps by Dembski, who rejects common descent, which does explain the name of the web site). The thread is preserved here:
http://immunoblogging.blogspot.com/2006/01/intelligent-design-has-new-party-line.html

Predictions from Common Descent

Common descent is a scientific claim. That means that certain prediction follow inevitably if it is true, and those predictions can be tested. If common descent is true, then certain features of the biological world have to be true. If they are not, then common descent is refuted.

A Nested Hierarchy

Common descent involves a "tree of life" in which, over billions of years, species branch off, evolve and die out. If you can reconstruct that tree, and point to any node in it, it necessarily follows that all the organisms that are descended from the organism at that node are closer related than those not (for example all mammals are descended from a common ancestor, and so are much more alike than those animals that are not). Follow the branch down (i.e., back in time), and pick another node somewhere along it. Now you have a larger set of organisms descended from an older ancestor, which includes the first node, and must necessarily include all the organisms that branched from the first node - just as your grandson's children must necessarily be related to you (so the mammals are all nested inside the vertebrates, because the common ancestor of the mammals was itself a descendant of te common ancestor of all vertebrates). The organisms from the first node are necessarily nested inside the organisms from the second node (your grandson's descendants are nested inside the grouping of your descendants). Thus a nested hierarchy has to be the case if common descent is true.

Curiously, the nested hierarchy was first shown by a creationist, Linnaeus. In fact, it should be noted that the discovery of the nested hierarchy actually predates Darwin's theory of evolution. Predictions in science do not have to be made before the event, they just have to be a necessary consequence of the theory (the orbit of Mercury was well known before Einstein used relativity to predict it, for instance).

A Common Fundamental Chemistry

If all life is descended from a single ancestor, you would expect some of the basic biochemistry of that ancestory to survive in every organism today, at a bare minimum, the mechanism of inheritance. All life on this planet uses DNA/RNA to handle genetic code, and it all codes for amino acids in the same way. The relationship between the three nucleotide codon mapping to a certain amino acid is pretty much universal across all living things.

Codons are discussed here:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Codons.html

Molecular Relatedness Closely Fits Phenotype Relatedness

The prediction is that if two species are similar, and are near each other in the tree of life, then this is because they are closely related, and a necessary consequence of common descent is that their biochemistry is also closely related.

It turns out that proteins can vary quite a lot. The famous one here is cytochrome-c, which is about 100 amino acids long, 30 of which are specific, the other 70 can vary somewhat, without having any impact on functionality. The prediction from common descent, then, is that the variation in cytochrome-c is large for organisms a long way apart in the tree of life, and small for closely related organisms. And this is what we find.

For more, go here:
http://www.rtis.com/nat/user/elsberry/evobio/evc/argresp/sequence.html

In fact, we find this with all proteins. In some cases the protein is only present in one branch of the tree, so can only be tested for in a small group of organisms. Also you can reach a saturation point with some proteins; the variation is too great to get meaningful results for more distantly related organism. Neither of these issues refute common descent; they are what you would expect.

Compare to creation; why would the supposed design bother with different sequences anyway? They all do exactly the same thing. If he craves diversity, why swap the sequences around in such a specific manner?