The skeptic and common descent
The common descent of all life is one of the theses of evolutionary theory. The skeptic is right to call common descent and even evolution into question. It is not an easy task. The difficulty for the skeptic is not with the evidence for evolutionary dogma but rather that there are few credible alternatives. As for evolution itself, the evidences are far weaker than many in this newsgroup admit.
Consider the supposed historical fact. The sundry departments of science have discovered perhaps 10% of the extant species of life, perhaps less. Of these most are known in the form of mounted exhibits and a page or so of notes detailing a few superficial features and some probably inaccurate guesses about habitat. This is for multi-cellular life forms, of course. Our knowledge of prokaryotes and viruses is abysmal.
Though our knowledge of current life forms is seriously deficient, it is encyclopediac compared to our knowledge of past life forms. It is doubtful that we are aware of even .001% of all past species, let alone their life styles, the ecologies in which they existed, or their reproductive habits. In short, our ignorance is profound.
It is on the basis of this profound ignorance that we have based that wildest of inductions, the doctrine of common descent. What is the evidence? Evidently it is not knowledge of the actual lineages of life; of these, we know almost nothing. No, the argument rests on assumptions. Thus we observe that life forms reproduce and do not observe spontaneous biogenesis. The assumption is that our observations are complete and sufficient. And yet we know (or believe we know) that abiogenesis must have happened. We do not know how it happened or in what ways it could have happened. At best, our knowledge consist of some pretty experiments with suggestive results.
For all we know, abiogenesis could be going on right now in some form or way that we simply don't recognize. The argument that it couldn't happen now in the presence of existing life is simply a bit of specious handwaving.
Another assumption underlying the doctrine of common descent is that the commonality of basic biochemistry of life is universal and is evidence for common descent. There are problems here. One is (yet again) our profound ignorance. We don't the details of the biochemistry of all lifeforms, particularly those of our more eccentric prokaryote cousins. More importantly, commonality of biochemistry does not guarantee common descent. Horizontal transfer, the transfer of biochemicals between organisms, is continually occurring and, as far as we can tell, always has been throughout the history of life.
A third assumption is that life forms are unitary individuals with a single heredity. That is, we are composed of cells that all have the same DNA as the original fertilized egg. This assumption is at best a simplification and can be seriously misleading. As physical organisms we are, all of us multi-cellular life forms, chimeras, communities of lifeforms with different heredities and biological interests. Some of us, lichens for example, are integral collaborations. Even the eurkaryote cells themselves are collections of coordinated lines of descent.
In short, the doctrine of common descent is at best a superficial simplification and perhaps even seriously wrong. Why, then, is it treated as a simple, obvious truth. The major reason, perhaps, is that it is hard to think of plausible alternatives and amendments. Some adduce this as evidence for common descent. It is not. Failure to think of alternatives to a thesis is not evidence for a thesis.
This page was last updated June 1, 2008.