Structural Constraints, Spandrels, and Exaptation 1241
death messenger" (p. 64) follows from its primary role and allows these molecules to
"serve as harbingers of impending cell death when released from their normal cellular
compartments": "Release of proteins from their normal locations in the cell may have
originally been a symptom of cell death, rather than a cause of it. Evolution may then
have exploited the accidental [their italics] release of these proteins (and possibly
others) to build, amplify, and eventually fine-tune the death circuitry."
Finally, in a third phase, the term enters the literature as a standard item of
professional lingo, requiring no further citation of original sources (probably
unknown to the authors in any case) than any other word of professional jargon.
Thus, Jablonski and Chaplin (1999, p. 836) view manual dexterity and eventual tool
use as human exaptations of a bipedal posture that originally arose as part of a
common threat display in ancestral apes, and Roy (1996) analyzes exaptations for
defense in the fossil history of stromboid gastropod shells.
I take greatest pleasure, however, in the spread of exaptation as a term of art in
the most pervasively expressed domain of molecular evolution. For example, in a
review article on "interspersed repeats and other mementos of transposable elements
in mammalian genomes," Smit (1999) consistently uses exaptation to describe
coopted utilities of multiply repeated and dispersed transposable elements (the classic
molecular items that inspired the concept of "selfish DNA," see pp. 693-695 for
further discussion).
In a section of a paper on "domestication of individual transposable elements"
(p. 661, and I do appreciate his witty and apposite metaphors from the vernacular),
Smit writes: "Throughout time, host genomes have rummaged through the novel
sequences accumulated by transposition and have recruited numerous elements...
Far from merely expanding genomes with interspersed repeats, their legacy ranges
from spliceosomal introns and antigen-specific immunity to many recent recruits in
highly specialized functions" (1999, pp. 661-662). Although Smit notes an apparently
reduced exaptive role for such transposons in humans vs. mice, "leading to
speculations on host defense mechanisms" (p. 657) in our species, he also lists an
impressive array of potential human examples, "some with household names" (p.
661). For example, Smit cites a fascinating exaptation of all higher primates,
probably essential to the existence of this book and any reader's kind and current
attention thereto: "BC200, the only known fully recruited SINE in humans, is a brain
specific RNA that is part of a ribonucleoprotein complex preferentially located in the
dendrites of all higher primates. It is presumably derived about 50 million years ago
from a monomeric Alu and has since been selectively conserved in all studied
descendants."
Note Smit's proper conceptual separation of initial cooptation from later
retention by natural selection. Following both Darwin and Nietzsche, the secondary
adaptive enhancement by natural selection (in this case, perhaps, merely a selective
retention without further structural change)—that is, the promotion of the current
utility—does not permit a conclusion about the different cause of historical origin (in
this case, presumably as an exaptation