In our struggle to understand the history of life, we must learn where to place the boundary between contingent and unpredictable events that occur but once and the more repeatable, lawlike phenomenon that may pervade life's history as generalities.

Contingency is rich and fascinating; it embodies an exquisite tension between the power of individuals to modify history and the intelligible limits set by laws of nature. The details of individual and species's lives are not mere frills, without power to shape the large-scale course of events, but particulars that can alter entire futures, profoundly and forever.

Environments without oxygen are excellent for the preservation of soft parts: no oxidation, no decay by aerobic bacteria. Such conditions are common on earth, particularly in stagnant basins. But the very conditions that promote preservation also decree that few organisms, if any, make their natural home in such places.

It has become, in my view, a bit too trendy to regard the acceptance of death as something tantamount to intrinsic dignity. Of course I agree with the preacher of Ecclesiastes that there is a time to love and a time to die - and when my skein runs out I hope to face the end calmly and in my own way. For most situations, however, I prefer the more martial view that death is the ultimate enemy - and I find nothing reproachable in those who rage mightily against the dying of the light.

Evolving life must experience a vast range of possibilities, based on environmental histories so unpredictable that no realized route - the pathway to consciousness in the form of Homo sapiens or Little Green Men, for example - can be construed as a highway to heaven, but must be viewed as a tortuous track rutted with uncountable obstacles and festooned with innumerable alternative branches. Any reasonably precise repetition of our earthly route on another planet therefore becomes wildly improbable even in a trillion cases.

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Many similarities of basic design among animal phyla, once so confidently attributed to convergence, and viewed as testimony to the power of natural selection to craft exquisite adaptation, demand the opposite interpretation that Mayr labeled as inconceivable: the similar features are homologies, or products of the same genes, inherited from a common ancestor and never altered enough by subsequent evolution to erase their comparable structure and function. The similarities record the constraining power of conserved history, not the architectural skills of natural selection independently pursuing an optimal design in separate lineages. Vertebrates are, in a certain sense, true brothers (or homologs) - not mere analogs - of worms and insects.

Thus, when we tackle the greatest of all evolutionary questions about human existence—how, when, and why did we emerge on the tree of life; and were we meant to arise, or are we only lucky to be here—our prejudices often overwhelm our limited information. Some of these biased descriptions are so venerable, so reflexive, so much a part of our second nature, that we never stop to recognize their status as social decisions with radical alternatives—and we view them instead as given and obvious truths.

Leftist scientists are more likely to combat biological determinism just as rightists tend to favor this quintessential justification of the status quo as intractable biology; the correlations are not accidental. But let us not be so disrespectful of thought that we dismiss the logic of arguments as nothing but an inevitable reflection of biases—confusion of context of discovery with context of justification.

Creative thought in science is exactly this - not a mechanical collection on of facts and intuition, bias, and insight from other fields. Science, at its best, interposes human judgement and ingenuity upon all proceedings. It is, after all (although we sometimes forget it), practiced by humans.

No one supposed that dinoflagellates might actively kill fish as an evolved response for their own specific advantage, including a potential nutritional benefit for the algal cells. And yet the dinoflagellates do seem to be killing and eating fishes in a manner suggesting active evolution for this most peculiar reversal.

We can now determine, easily and relatively cheaply, the detailed chemical architecture of genes; and we can trace the products of these genes (enzymes and proteins) as they influence the course of embryology. In so doing we have made the astounding discovery that all complex animal phyla - arthropods and vertebrates in particular - have retained, despite their half-billion years of evolutionary independence, an extensive set of common genetic blueprints for building bodies.