(Third post in a series about the four reasons Dinesh D'Souza gives for his acceptance of evolution. Reading the previous posts might enhance the reader's understanding of this one.)
So, what about the "single invariant trajectory" of the fossil record to which D'Souza appeals? Let us assume (contrary to fact) that the record does indeed show the invariant trajectory from simple to complex life that D'Souza supposes. What would this observation prove with regard to distinguishing between evolution and, say, intelligent design or some form of creationism?
First, it would seem that the observation itself is somewhat meaningless without any attempt to define complexity. That is, it seems to be circular reasoning to say that more complex life is that which has arisen later, by way of evolution, and then to use this increasing complexity as proof that evolution has occurred. I don't believe that this is what D'Souza means to claim. Given his acceptance of evolution, however, I suspect that his thinking here hasn't really risen above uncritically seeing increasing complexity from an evolutionary perspective.
There are, in fact, single-celled organisms alive today that--in their ecologies and life histories--are quite a bit more complex than many multicellular organisms. Likewise, there are invertebrates whose life histories are significantly more complex than those of many vertebrates. Take, for instance, the caterpillar whose entire body goes through a complete dissolution in midlife and starts anew with a totally different morphology (indeed, not only the morphology but the diet, physiology, mode of mobility, reproductive capability and a number of other things about the butterfly couldn't be any different than it is). If we define complexity according to life history and ecology, then D'Souza's claim is refuted.
If nothing else, these considerations demonstrate that D'Souza's single-cells to invertebrates to fish to amphibians to reptiles to mammals trajectory is simplistic. To be sure, it also seems intuitively true that the butterfly is more complex (in some ways) than the earliest invertebrates, or even the earliest members of the same phylum (the earliest arthropods would probably be the seemingly ubiquitous extinct marine creatures known as trilobites). But again, it seems to beg the question to discuss increasing complexity without defining it independent of alleged evolution.
Comparative anatomists and physiologists would tend to define increasing complexity according to a movement from single cells to conglomerations of unspecialized cells to collections of tissues to organisms whose specialized tissues form specialized organs to those with organ systems. On this view, the organism with the most different organ systems (circulatory, nervous, muscular, digestive, etc.) is more advanced than the one with fewer organ systems, which in turn is more advanced than the one with organs but no organ systems...
The main problem (for evolution) of this view is that life on earth leapt from the lower levels of this scale to the highest levels of this scale overnight (as it were, in geological time) during the Cambrian explosion (530 million years ago). Prior to that time, there existed single-celled organisms, multicellular creatures with little or no cell specialization, and some living things that were multicellular that had specialized cells (differentiated cells, or those with different roles). Suddenly, there arose--fully formed and fully adapted--organisms on the extreme advanced end of this anatomical and physiological spectrum, animals with not only specialized tissues and organs but with a number of organ systems. Members of all of the known animal phyla existed by the end of the Cambrian explosion, though members of only a few existed prior to it. And so D'Souza's claim finds no support here, if complexity is viewed in this traditional (and reasonable) way.
In short, one cannot use the history of life on Earth as evidence for evolution unless one has first been able to get outside of an evolutionary perspective to ask more penetrating questions about the differences between "advanced" and "primitive" living things. I believe that doing so will enable one to arrive at a more correct explanation for the history of life, one that--far from supporting Darwinian evolution--shows at every stage the desire of an intelligent Creator to fill the planet with the most (and the most 'advanced') life possible.