[cross posted at TSZ A Question for those that doubt Common Descent]
Recent posts by Sal remind me that there are some intelligent educated people who doubt Common Descent. What I don’t understand what they think the alternative is. Put simply I take Common Descent as the position that :
* At one time there was only very simple unicellular life on earth (this is not a debate about how that unicellular life originated)
* Complex life forms (eukaryotes) are created by slight modifications from other life forms (which are their parents). We have never observed them being created any other way!
* All complex life forms are the descended from a very small number of simple life forms – quite possibly just one.
The alternatives I can imagine are:
* Complex life descended many different times from simple life forms – so e.g. mammals descended from a different simple life form from fish. This flies in the face of the fossil record and the hierarchical nature of complex life but I can sort of understand it.
* Complex life from time to time gives birth to wholly different species – massively implausible.
* Complex life is created anew by some process never imagined or observed – even more implausible but presumably what Young Earth creationists believe.
But maybe there is another option?
If Sal or someone could explain I would be interested.
[My response (and readers can add theirs).]
Take a look at this graph. I personally fall into the CRS (Creation Research Society) members group. Monophyly means those that accept common descent, and polyphyly means those that accept independent lines of descent.
(image by Paul Nelson: http://www.evolutionnews.org/2007/05/_most_people_including003582.html)
From the polyphyletic column we see two varieties of mechanisms for the independent lineages:
1. special creation by some Creator or intelligent designer
2. independent origin of separate lineages by some naturalistic means
Each has its challenges.
One can always invoke a Creator (like God). God is always a sufficient explanation, but is it a necessary one? Further, God as a mechanism isn’t a repeatable mechanism and hence is outside operational science. Some don’t believe God exists or view His existence irrelevant to the evolution of life. Others would believe if they saw God, but will not invoke a creator until they see Him in operation, so they provisionally invoke naturalistic mechanisms. I respect those objections, but those are views I don’t hold…
Surprisingly there are some naturalistic theories that reject common descent that don’t involve God or some Designer. I don’t necessarily endorse these, but I can appreciate that they exist. Notice Woese and Doolittle are put in the category of non-Design and non common-ancestry (polyphyly). In addition to those two we can add Periannan Senapathy. They accept naturalistic mechanisms of independent (rather than common) descent. They believe in some sort of pool and Horizontal Gene Transfer. I don’t know much about Lamark and Haeckel. These naturalistic theories don’t solve the problem of ORFans.
Why do I accept design and polyphyly (essentially special creation) even though I’ve not witnessed God? Life looks designed, therefore a designer seems reasonable. The designs evidence more capability than humans, so much so the designer seems God-like, and God is postulated as a real entity by some physicists: Quantum Enigma of Consciousness and the Identity of the Intelligent Designer.
Thus if the designer is God, is the evidence more in favor of God using an evolutionary mechanism that evolves all life from one simple cell, or an evolutionary mechanism that evolves life from many complex created kinds? If one believes in a Creator, the latter seems more reasonable. Paraphrasing the way Darwin posed the question: “which way did the Creator choose to diversify life, via common descent or special creations?” Darwin left out the third possibility which Blyth and other creationists allowed for, namely the Creator created many created kinds which diversified over time. I hinted at this here: Deorigination of species by means of reunion..
IMHO, the reason some design theorists sometimes criticize common descent is that evidence of special creation is a sufficient but not necessary condition for demonstrating design. That was the line of reasoning I laid out here: Cocktail: Relevance of YEC to ID, and Cocktail: Astrophysics vs Darwinist paleontology, and Cocktail: C14, DNA, Collagen in dinosaurs, and Cocktail: Falsifying Darwinism by Falsifying the geological column.
Intelligent beings do not behave in ways that are always amenable to repeatability, so I can also accept that hypothesis of creation without repeatability of the creation events in real time. There are many uncertainties in all viewpoints. As nice as it would be to actually have all humanity see God in action as the children of Israel supposedly did in the time of Moses, I follow the old saying, “play the cards you are dealt”. To me, the best play on the data we’ve been dealt is to accept design and polyphyly (special creation), but others will obviously disagree.
1. the above graph conflicts with the way I described Denton in my TSZ response, so I defer to Paul Nelson’s categorization of Denton and provisionally retract my description of Denton. Holye (who believed in Extra Terrestrials and is an ID proponent) is sort of a class of his own.
2. Creationists disagree on how much polyphyly there is. That is the topic of Discontinuity systematics and Baraminology. ORFans and hybridization experiments and taxonomy are of keen interest in these disciplines which are intended to resolve how much polyphyly there is.
3. John Davison would probably argue Berg is in the Design category. Speaking of which Davison would put him self in the design and polyphyly category, but invoke far fewer lineages than most creationists (like Berg did).