"But the history of life, as I read it, is a series of stable states, punctuated at rare intervals by major events that occur with great rapidity and help to establish the next stable era. Prokaryotes ruled the earth of three billion years until the Cambrian explosion, when most major designs of multicellular life appeared within ten million years. Some 375 million years later, about half the families of invertegrates became extinct within a few million years. The earth's history may be modelled as a series of occasional pulses, driving recalcitrant systems from one stable state of the next.

The Panda's Thumb p 226. . . . "Even I will admit that at some point in the story of human evolution, circumstances conspired to encourage mentality at our modern level. The usual scenario holds that attainment of upright posture freed the hands for using tools and waepon, and feedback from the behavioural possibilities thus provided spurred the evolution of a larger brain.

But I believe that most of us labor under a false impression about the pattern of human evloution. We veiw our rise as a kind of global process encompassing all memeber of the human lineage, wherever they may have lived. We recognize that Homo erectus, our immediate ancestor, was the first species to emigrate from Africa and to settle in Europe and Asia as well ("Java Man" and "Peking Man" of the old texts). But we then revert to the hypothesis of global impetus and imagine that all Homo erectus populations on all three continents moved together up the ladder of mentality on a wave of predictable and necessary advance, given the adaptive value of intelligence. I call this scenario the "tendency theory" of human evolution. Homo sapiens becomes the anticipated result of an evolutionary tendency pervading all human populations.

In an alternative view, recently given powerful support by reconstructions of our evolutionary tree based on genetic differences among modern groups (Cann, Stoneking and Wilson, 1987;Gould, l987b), Homo sapiens arose an an evolutionary item, a definite entity, a small and coherent population that split off from a lineage of ancestors in Africa. I call this view the "entity theory" of human evolution. It carries a cascade of arresting implications: Asian Homo erectus died without issue and does not enter our immediate ancestry (for we evolved from African populations); Neanderthal people were collateral cousins, perhaps already living in Europe while we emerged in Africa, and also contributing nothing to our immediate genetic heritage. In other words, we are an improbable and fragile entity, fortunately successful after precarious beginnings as a small population in Africa, not the predictable end result of a global tendency. We are a thing, an item of history, not an embodiment of general principles.

This claim would not carry startling implications if we were a repeatable thing--if, had Homo spaiens failed and succumbed to early extinction as most species do, another population with higher intelligence in the same form was bound to originate. Wouldn't the Neanderthals have taken up the torch if we had failed, or wouldn't some other embodiment of mentality at our level have originated without much delay? I don't see why. Our closest ancestors and cousins, Homo erectus, the Neanderthals, and others, possessed mental abilities of a high order, as indicated by their range of tools and other artifacts. But only Homo sapiens shows direct evidence for the kind of abstract reasoning, including numerical and aesthetic modes, that we identify as distinctively human. All indications of ice-age reckoning--the calendar sticks and counting blades--belong to Homo sapiens. And all the ice-age art--the cave paintings, the Venus figures, the horse-head carvings, the reindeer bas-reliefs--was done by our species. By evidence now available, Neanderthal knew nothing of representational art.

Run the tape again, and let the tiny twig of Homo sapiens expire in Africa. Other hominids may have stood on the threshold of what we know as human possibilitis, but many sensible scenarios would never generate our level of mentality. Run the tape again, and this time Neanderthal perishes in Europe and Homo erectus in Asia (as they did in our world). The sole surviving human stock, Homo erectus in Africa, stumbles along for a while, even prospers, but does not speciate and therefore remains stable. A mutated virus then wipes Homo erectus out, or a change in climate reconverts Africa into inhospitable forest. One little twig on the mammalian branch, a lineage with intresting possibilities that were never realized, joins the vast majority of species in extinction. So what? Most possibilities are never realized, and who will ever know the difference?

Arguments of this form lead me to the conclusion that biology's most profound insight into human nature, status, and potential lies in the simple phrase, the embodiment of contingency: Homo Homo sapiens is an entity, not a tendency.

Wonderful Life, p 319-320.

"new species usually arise, not by the slow and steady transformation of entire ancestral populations, but by the splitting off of small isolates from an unaltered parental stock. The frequency and speed of such speciation is among the hottest topics in evolutionary theory today, but I think that most of my colleagues would advocate ranges of hundreds of thousands of years for the origin of most species by splitting. This may seem like a long time in the framework of our lives, but it is a geological instant, usually represented in the fossil record by a single bedding plane, not a long stratigraphic sequence. If species arise in hundreds or thousands of years and then persist, largely unchanged, for several million, the period of their origin is a tiny fraction of one percent of their total duration. Therefore, they may be treated as discrete entities even through time. Evolution at higher levels is fundamentally a story of the differential success of species not the slow transformation of lineages.

Of course, if we happen to encounter a species during the geological microsecond of its origin, we will not be able to make clear distinctions. But our chances of finding a species in this state are small indeed. Species are stable entitles with very brief periods of fuzziness at their origin (although not at their demise because most species disappear cleanly without changing into anything else). As Edmund Burke said in another context: "Though no man can draw a stroke between the confines of day and night, yet light and darkness are upon the whole tolerably distinguishable."

Evolution is a theory of organic change, but it does not imply, as many people assume, that ceaseless flux is the irreducible state of nature and that structure is but a temporary incarnation of the moment. Change is more often a rapid transition between stable states than a continuous transformation at slow and steady rates. We live in a world of structure and legitimate distinction. Species are the units of nature's morphology.

The Panda's Thumb pp 212-213