Structural Constraints, Spandrels, and Exaptation 1265
an almost caricatured version of adaptationism as the only ground of evolutionary
explanation for the origin of traits.
- At a sufficient depth and distance, original adaptations now act primarily as
historical constraints, and must be so characterized and analyzed (the central theme of
Chapter 10). When we recognize a cognitive universal of human mentality as ill-fit to
the complexities of modern social life, we do not then achieve an explanation of its
human origin in adaptationist terms simply because we can state a good case for its
initial phyletic appearance as an adaptation. We need to specify the evolutionary
distance and the environmental context of initial appearance before we can render
any judgment. In general, I would accept the statement that if we can locate the
feature's adaptational origin in the last common ancestor of Homo sapiens, or even as
far back as the common ancestor of the hominid line (after splitting from the lineage
of great apes), then we may legitimately argue that this initial adaptive context
establishes the "evolutionary meaning" of the feature in our quest to understand its
appearance in human phyletic history.
But suppose that the feature had a far more ancient, but still fully adaptational,
origin in a distant ancestor of very different form and neurological function, and also
living in a very different environment—say, in the basal gnathostome fish of early
Paleozoic times. Suppose also that this mental attribute has persisted ever since as a
plesiomorphic aspect of the basic operation of the vertebrate brain. When we then try
to explain the evolutionary significance of this mental mode in contemporary human
life—especially when we try to identify its role in quirky and clearly suboptimal
characteristics of human reasoning in the modern world—would we wish to claim
that an adaptational analysis (in recognition of the feature's Darwinian origin in such
a distant ancestor) will provide our best understanding? Clearly, we do not so proceed
in most evolutionary analyses—and for good reasons discussed at length in Chapter
10 on the evolution of development. Rather, we treat such features predominantly as
historical constraints because, as invariant and plesiomorphic traits of our entire clade
(not only of all hominids and primates, but also of all mammals and tetrapods), they
operate as unchanging constraints upon any subsequent evolution of mental modes,
despite their adaptational origin in such a distant ancestor of such different form and
environment.
I suspect that many puzzling features of human mentality would be better
resolved if we conceptualized them as historical constraints derived from distant
adaptational origins. To cite a hypothetical example (that would attract my substantial
and favorable wager were I a betting man): I agree with a major theme of structuralist
philosophy and research, as developed most cogently in our times by Claude Levi-
Strauss and his followers, that identifies our tendencies to parse natural variety into
pairs of opposed and dichotomous categories as an inherent property of human
mental functioning—with male and female, night and day, and culture vs. nature as
primary examples. I think that most people would also identify this strong preference
as a constraint with highly unfortunate consequences for human life—not only
because