I Have Landed (38 page)

Interdisciplinary unification represents a grand and worthy goal of intellectual life, but greater understanding can often be better won by principled separation and mutual respect, based on clear definitions and distinctions among truly disparate processes, than by false unions forged with superficial similarities and papered over by a common terminology. In our understandable desire to unify the sciences of temporal change, we have too often followed the Procrustean strategy of enforcing a common set of causes and explanations upon the history of a species and the life of a star—partly, at least, for the very bad reason that both professions use the term “evolution” to designate change through time. In this case, the fundamental differences embody far more interest and insight than the superficial similarities—and true unity will only be achieved when we acknowledge the disparate substrates that, taken together, probe the range of possibilities for theories of historical order.

The Darwinian principle of natural selection yields temporal change—“evolution” in the biological definition—by a twofold process of generating copious and undirected variation within a population, and then passing only a
biased (selected) portion of this variation to the next generation. In this manner, the variation
within a population
at any moment can be converted into differences in mean values (such as average size or average braininess)
among successive populations
through time. For this fundamental reason, we call such theories of change “variational” as opposed to more conventional, and more direct, models of “transformational” change imposed by natural laws that mandate a particular trajectory based on inherent, and therefore predicable, properties of substances and environments. (A ball rolling down an inclined plane does not reach the bottom because selection has favored the differential propagation of moving versus stable elements of its totality, but because gravity dictates this temporal sequence and result when round balls roll down smooth planes.)

To illustrate the peculiar properties of variational theories like Darwin's in an obviously caricatured, but not inaccurate, description: Suppose that a population of elephants inhabits Siberia during a warm interval before the advance of an ice sheet. The elephants vary, at random and in all directions, in their amount of body hair. As the ice advances and local conditions become colder, elephants with more hair will tend to cope better, by the sheer good fortune of their superior adaptation to changing climates—and they will leave more surviving offspring on average. (This differential reproductive success must be conceived as broadly statistical, and not guaranteed in every case. In any generation, the hairiest elephant of all may, in the flower of youthful strength but before any reproductive action, fall into a crevasse and die.) Because offspring inherit their parental degree of hairiness, the next generation will include a higher proportion of more densely clad elephants (who will continue to be favored by natural selection as the climate becomes still colder). This process of increasing average hairiness may continue for many generations, leading to the evolution of woolly mammoths.

This little fable can help us to understand how peculiar, and how contrary to all traditions of Western thought and explanation, the Darwinian theory of evolution, and variational theories of historical change in general, must sound to the common ear. All the odd and fascinating properties of Darwinian evolution flow from the variational basis of natural selection—including the sensible and explainable, but quite unpredictable, nature of the outcome (dependent upon complex and contingent changes in local environments), and the nonprogressive character of the alteration (adaptive only to these unpredictable local circumstances and not building a “better” elephant in any cosmic or general sense).

Transformational theories work in a much simpler and more direct manner. If I want to go from
a
to
b
, I will have so much less conceptual (and actual) trouble if I can postulate a mechanism that will push me there directly, than if I must rely upon selection of “a few good men” from a random cloud of variation about point
a
, then constitute a new generation around an average point one step closer to
b
, then generate a new cloud of random variation about this new point, then select “a few good men” once again from this new array—and then repeat this process over and over until I finally reach
b
. When one adds the oddity of variational theories in general to our strong cultural and psychological resistance against their application to our own evolutionary origin—leading to the reconceptualization of
Homo sapiens
as an unpredictable and not necessarily progressive little twig on life's luxuriant tree—then we can better understand why Darwin's revolution surpassed all other scientific discoveries in reformatory power, and why so many people still fail to understand, or even actively to resist, its truly liberating content. (I must leave the issue of liberation for another time, but once we recognize that the specification of morals and the search for a meaning in our lives cannot be resolved by scientific data in any case, then Darwin's variational mechanism will no longer seem threatening, and may even become liberating as a rationale for abandoning a chimerical search for the purpose of our lives, and the source of our ethical values, in the external workings of nature.)

These difficulties in grasping Darwin's great insight became exacerbated when our Victorian forebears made their unfortunate choice of a defining word—“evolution,” with its vernacular meaning of directed unfolding. We would not face this additional problem today if “evolution” had undergone a complete transformation to become a strict and exclusive definition of biological change—with earlier, and etymologically more appropriate, usages then abandoned and forgotten. But important words rarely undergo such a clean switch of meaning, and “evolution” still maintains its original definition of predictable unfolding in several nonbiological disciplines—including astronomy.

When astronomers talk about the evolution of a star, they clearly do not invoke a variational theory like Darwin's. Stars do not change through time because mama and papa stars generate broods of varying daughter stars, followed by the differential survival of daughters best adapted to their particular region of the cosmos. Rather, theories of stellar “evolution” could not be more relentlessly transformational in positing a definite and predictable sequence of changes unfolding as simple consequences of physical laws. (No biological process operates in exactly the same manner, but the life cycle of an organism certainly works better than the evolution of a species as a source of analogy.)

Ironically, astronomy undeniably trumps biology in faithfulness to the etymology and vernacular definition of “evolution”—even though the term now holds far wider scientific currency in the radically altered definition of the biological sciences. In fact, astronomers have been so true to the original definition that they confine “evolution” to historical sequences of predictable unfolding, and resolutely shun the word when describing historical cosmic changes that
do
exhibit the key features of biological “evolution”—unpredictability and lack of inherent directionality.

As an illustration of this astronomical usage, consider the most standard and conventional of all sources—the
Encyclopaedia Britannica
article (fifteenth edition, 1990) on “stars and star clusters.” The section on “star formation and evolution” begins by analogizing stellar “evolution” to a preprogrammed life cycle, with degree of “evolution” defined as position along the predictable trajectory:

The fully predictable and linear sequence of stages in a stellar “lifetime” (“evolution” to astronomers) records the consequences of a defining physical process in the construction and history of stars: the conversion of mass to energy, with a depletion of hydrogen and transformation to helium:

The same basic sequence unfolds through stellar lives, but the rate of change (“evolution” to astronomers) varies as a predictable consequence of differences in mass:

More-complex factors may determine variation in some stages of the life cycle, but the basic directionality (“evolution” to astronomers) does not alter, and predictability from natural law remains precise and complete:

In the most revealing verbal clue of all, the discourse of this particular scientific “culture” seems to shun the word “evolution” when historical sequences become too meandering, too nondirectional, or too complex to explain as simple consequences of controlling laws—even though the end result may be markedly different from the beginning state, thus illustrating significant change through time. For example, the same
Britannica
article on stellar evolution notes that one can often reach conclusions about the origin of a star or a planet from the relative abundance of chemical elements in its present composition. But the earth's geological history has so altered its original state that we cannot make such inferences for our own planet.

In other words, the earth has undergone a set of profound and broadly directional changes—alterations so extensive that we can no longer utilize the present state to make inferences about our planet's original composition. However, since this current configuration developed through complex contingencies, and could not have been predicted from simple laws, this style of change apparently does not rank as “evolution”—but only as being “affected”—in astronomical parlance:

I don't mention these differences to lament, to complain, or to criticize astronomical usage. After all, their concept of “evolution” remains more faithful to etymology and the original English definition; whereas our Darwinian reconstruction has virtually reversed the original meaning. In this case, since
neither side will or should give up its understanding of “evolution”—astronomers because they have retained an original and etymologically correct meaning, evolutionists because their redefinition expresses the very heart of their central and revolutionary concept of life's history—our best solution lies simply in exposing and understanding the legitimate differences, and in explicating the good reasons behind the disparity of use.

In this way, at least, we may avoid confusion and the special frustration generated when prolonged wrangles arise from misunderstandings about words, rather than genuine disputes about things and causes in nature. Evolutionary biologists must remain especially sensitive to this issue, because we still face considerable opposition, based on conventional hopes and fears, to our emphasis on an unpredictable history of life evolving in no inherently determined direction. Since astronomical “evolution” upholds both contrary positions—predictability and directionality—evolutionary biologists need to emphasize their own distinctive meaning, especially since the general public feels much more comfortable with the astronomical sense—and will therefore impose this more congenial definition upon the history of life if we do not clearly explain the logic, the evidence, and the sheer fascination of our challenging conclusion.

Two recent studies led me to this topic because each discovery confirms the biological, variational, and Darwinian “take” on evolution, while also, and quite explicitly, refuting a previous transformational interpretation—rooted in our culturally established prejudices for the more comforting astronomical view—that had blocked our understanding and skewed our thought about an important episode in life's history.

1.
Vertebrates all the way down
. In one of the most crucial and enigmatic episodes in the history of life—and a challenge to the old and congenial idea that life has progressed in a basically stately and linear manner through the ages—nearly all animal phyla make their first appearance in the fossil record at essentially the same time, an interval of some five million years (about 525 to 530 million years ago) called the Cambrian Explosion. (Geological firecrackers have long fuses when measured by the inappropriate scale of human time.) Only one major phylum with prominent and fossilizable hard parts does not appear either in this incident, or during the Cambrian period at all—the Bryozoa, a group of colonial marine organisms unknown to most nonspecialists today (although still relatively common), but prominent in the early fossil record of animal life.