Gould, Tires to Sandals

vention that secured equal separation; prokaryotes, lacking chromosomes and other organelles, cannot produce an exact genetic halving.)

We now encounter the same conundrum faced in the last example: We can see why multicellular life required the evolution of organelles, but eukaryotic cells arose at least 800 million years before the origin of multicellular animals--so progress to multicellular complexity cannot be the reason organelles evolved.

A favored theory for the origin of some organelles (the mitochondrion and chloroplast but not, alas, the nucleus, for which no good theory now exists) invokes the process of symbiosis. Mitochondria and chloroplasts look uncannily like entire prokaryotic organisms (they have their own DNA and are the same size as many bacteria). Almost surely, they began as symbionts within cells of other species and later became more highly integrated to form the eukaryotic cell (so that each cell in our body has the evolutionary status of a former colony). Now, one can argue that the wedge drove the ancestors of mitochondria to a life of symbiosis. These bacteria, gaining protection or whatever, did not enter the primordial eukaryote in order to provide an opportunity for multicellular complexity a billion years down the road. This event happened for immediate Darwinian reasons of its own; then the wheel turned and the rubber made for symbiosis put the first footprints on the path of multicellular complexity.

3. The basic features of human consciousness. The wheel and the wedge then interact for more than half a billion years to the separation of our lineage from the ancestry of chimpanzees some six to eight million years ago. The wedge produces some forward motion (and more blind alleys of overspecialization), but the wheel inaugurates each domain of change--the limbs on an odd group of fishes, by an unusual arrangement of fin bones, can bear the body's weight on land; mammals get a chance after 100 million years in the backwaters because dinosaurs succumb in a mass extinction.

Australopithecus now begins the process that textbooks used to call, before we reformed our language to include all people, the "ascent of man." Doesn't the wedge finally prevail? Isn't the unreversed trend of increasing brain size, from Australopithecus to Homo habilis to Homo erectus to us, driven by ordinary natural selection working on the advantages of superior cognition? Let me take the most conservative argument of the wedge (not my actual view) and reply: Yes, fine; I agree that the human brain got large because natural selection directly favored some traits of cognition that gave bigger-brained people advantages in competition.

Does such an admission imply that the foundations of human cognition, the universal traits that we define as "humanity" or "human nature," were built directly by the wedge? Of course not--and no argument is more important for our understanding of human nature, yet less widely appreciated, than this. Yes, the brain got big by natural selection. But as a result of this size, and the neural density and connectivity thus imparted, human brains could perform an immense range of functions quite unrelated to the original reasons for increase in bulk. The brain did not get big so that we could read or write or do arithmetic or chart the seasons--yet human culture, as we know it, depends upon skills of this kind. If you label me as a hopelessly parochial academic for citing only the skills of an intellectual elite, I reply that the fortuitous side consequences of large brains include the defining activities of all people. What about language, the most widely cited common denominator and distinguishing factor of humanity? And I don't mean using sound or gesture for communication, as many complex animals do. I refer to the unique syntax and underlying universal grammar of all languages. I can't prove that language was not the selected basis of increasing brain size; but the universals of language are so different from anything else in nature, and so quirky in their structure, that origin as a side consequence of the brain's enhanced capacity, rather than as simple advance in continuity from ancestral grunts and gestures, seems indicated. (This argument about language is by no means original with me, though I ally myself fully with it; this line of reasoning follows directly as the evolutionary reading for Noam Chomskys theory of universal grammar.)

To cite another example, consider Freud's argument (one of his defendable propositions, I think) on the origin of religion--or at least of a belief in some form of persistence after death as a common feature of this institution. Freud held that all religions maintain some belief in personal persistence after death--whether in heaven, by reincarnation, in a universal soul, or merely by continuity of tradition. This belief marks the common basis of religion because our large brains "forced" us to learn and acknowledge the fact of personal mortality (a concept not clearly grasped by any other animal). Now you cannot argue that our brains became large so that we would appreciate the fact of our death; knowledge of mortality is an inevitable (and largely unfortunate) side consequence of mental power evolved for other reasons. Yet this unwanted knowledge forms the basis of an institution often regarded as the most fundamental consequence of human nature.

If such features as language and the basis of religion are side consequences of the wheel, not direct gifts of the wedge, then is human nature a predictable product of organic improvement, honed in the fires of competition, or a set of oddly cobbled side consequences rooted in an unparalleled neural complexity built for other reasons? We are a bit of both--though more, I suspect, the quirks of the wheel than the gifts of the wedge--and in this mixture lies our hope and our destiny.

If I have upset your equanimity by attributing the genuine complexity of human cognition to fortuity plied upon fortuity (with a little yardage for predictability after each spin of the wheel), then I must apologize for one further disturbance in conclusion. We talk about the "march from monad to man" (old-style language again) as though evolution followed continuous pathways of progress along unbroken lineages. Nothing could be further from reality. I do not deny that, through time, the most "advanced" organism has tended to increase in complexity. But the sequence from protozoan to jellyfish to trilobite to nautiloid to armored fish to dinosaur to monkey to human is no lineage at all but a chronological set of termini on unrelated evolutionary trunks. Moreover, life shows no trend to complexity in the usual sense--only an asymmetrical expansion of diversity around a starting point constrained to be simple. Let me explain that last cryptic remark: For reasons of organic chemistry and the physics of self-organizing system, life arose at or very near the lower limit of preservable size and complexity in the fossil record. Since diversity, measured as number of species, has increased through time, extreme values on the distribution of complexity can move in only one direction. No species can become simpler than the starting point, for life arose at the lower limit of preservable complexity. The only open direction is up, but very few species take this route. Increasing complexity is not a purposeful trend of an unbroken lineage but only the upper limit of an expanding distribution as overall diversity increases. We only focus on this upper tail and call its expansion a trend because we crave some evolutionary rationale for our perception of ourselves as predictable culminations.