… what possible criterion can validate this claim … ?
This is from Full House: The Spread of Excellence from Plato to Darwin by Stephen Jay Gould (1996):
… I believe that the most knowledgeable students of life’s history have always sensed the failure of the fossil record to supply the most desired ingredient of Western comfort: a clear signal of progress measured as some form of steadily increasing complexity for life as a whole through time. The basic evidence cannot support such a view, for simple forms still predominate in most environments, as they always have.
… I do not challenge the statement that the most complex creature has tended to increase in elaboration through time, but I fervently deny that this limited little fact can provide an argument for general progress as a defining thrust of life’s history. Such a grandiose claim represents a ludicrous case of the tail wagging the dog, or the invalid elevation of a small and epiphenomenal consequence into a major and controlling cause.
… [You] might say, “Okay, you win. I understand your point that the evidence of supposed progress, the increasing right skew of life’s bell curve, is only an epiphenomenal tail that cannot wag the entire dog — and that life’s full house has never moved from its modal position. But I am allowed to be parochial.
[line break added] “The right tail may be small and epiphenomenal, but I love the right tail because I dwell at its end — and I want to focus on the right tail alone because this little epiphenomenon is all that matters to me. Even you admit that the right tail had to arise, so long as life expanded. So the right tail had to develop and grow — and had to produce, at its apogee, something like me. I therefore remain the modern equivalent of the apple of god’s eye: the predictably most complex creature that ever lived.”
Wrong again, even for this pitifully restricted claim (after advancing an initial argument for intrinsic directionality in the basic causal thrust of all evolution). The right tail had to exist, but the actual composition of creatures on the tail is utterly unpredictable, partly random, and entirely contingent — not at all foreordained by the mechanism of evolution.
[line break added] If we could replay the game of life again and again, always starting at the left wall and expanding thereafter in diversity, we would get a right tail almost every time, but the inhabitants of this region of greatest complexity would be wildly and unpredictably different in each rendition — and the vast majority of replays would never produce (on the finite scale of a planet’s lifetime) a creature with self-consciousness. Humans are here by the luck of the draw, not the inevitability of life’s direction or evolution’s mechanism.
In any case, little tails, no tails, or whoever occupies the tails, the outstanding feature of life’s history has been the stability of its bacterial mode over billions of years!
… [R.D.K. Thomas] holds, however, that progress still compels our attention as the “main” effect among all of evolution’s incidental consequences. But what possible criterion can validate this claim beyond the parochial and subjective desire to designate as primary an effect that both led to human life and placed us atop a heap of our own definition? I think that any truly dominant bacterium would laugh with scorn at this apotheosis for such a small tail so far from the modal center of life’s main weight and continuity.
[line break added] I do realize that bacteria can’t laugh (or cogitate) — and that philosophical claims for our greater importance can be based on the consequences of this difference between them and us. But do remember that we can’t live on basalt and water six miles under the earth’s surface, form the core of novel ecosystems based on the earth’s interior heat rather than solar energy, or serve as a possible model for cosmic life in most solar system [all of which bacteria can do].