MACROEvOLuTiON: EvOLuTiON AbOvE THE SPECiES LEvEL 541
(say E′, not E) if any of many antecedent events had been different (C′ rather than
C, or B′ rather than B). Perhaps, in principle, we could know the series of events
that caused B′ and C′, rather than B and C, and therefore E′ rather than E, to occur,
but realistically, we can never know all of the incredibly vast number of possible
causal chains. In this view, which Gould championed, the history of life would be
very different if it were to start again from any point in the past. The concept of
contingency has been familiar in human history at least since the seventeenth-
century philosopher Blaise Pascal mused about the effect of Cleopatra’s nose: if
Mark Antony, one of the triumvirate that ruled Rome from 43 to 33 bc, had not
been smitten by Cleopatra’s beauty and become her lover, Octavian would not
have battled and vanquished him, and there never would have been a Roman
Empire. On a slightly less grand scale, most of us can think of “chance” events that
changed the course of our own lives.
There is certainly some predictability in evolution, and it is the basis of a great
deal of the evolutionary theory presented in this book. Some of the selection equa-
tions in population genetics, for example, deterministically predict allele frequency
changes in large populations (see Chapter 5). Organisms conform to physical prin-
ciples, so massive terrestrial vertebrates such as elephants have, predictably, dispro-
portionately thick leg bones. Many features of organisms are more or less success-
fully predicted by “optimality” theories of life history evolution, behavioral ecology,
and functional morphology (see Chapters 10 –12). These theories are successful
largely because of the high incidence of convergent evolution: the many instances in
which similar adaptations to similar environmental selection pressures have evolved
independently. Conway Morris depends on convergent evolution to support his
argument that humans, or humanoids, were an inevitable outcome of evolution [12,
13]. Along similar lines, many people are convinced that there must exist intelligent
humanoids elsewhere in the universe; this conviction is the basis of SETI (Search for
Extraterrestrial Intelligence) and similar projects.
Some—probably most—evolutionary biologists reject this position and argue
for a strong role of contingency in the history of life. The course of evolution can
depend on rare mutations or combinations of interacting mutations [6] and on the
sequence of environmental changes. Consequently, convergence between closely
related lineages may be close, but convergence between remotely related lineages
is usually more superficial [62].
Although examples of convergence abound, so do unique events in the history
of life. As far as we know, life originated only once, as did flowers, vertebrates,
terrestrial vertebrates, the amnion, the feather, the mammalian diaphragm, and
countless other examples. Moreover, extinction, including mass extinction events,
has cut short the possible evolutionary future of the vast majority of lineages that
have ever lived, and no equivalents of trilobites, ammonoids, dinosaurs, and many
other extinct groups have ever replaced them. As Gould emphasized [35], if any
species in our long line of ancestors, back to the first vertebrate or even beyond,
had become extinct, intelligent hominids would probably never have evolved.
Among the billion or more species of organisms in Earth’s history, only one
evolved human intelligence, and this happened only after at least 3 billion years of
cellular life. We have no reason to suppose that any human equivalent would have
evolved in our stead. For these reasons, George Gaylord Simpson [101] and Ernst
Mayr [71], two of the most influential biologists of the twentieth century, argued
that the probability that there exists another intelligent life form in the universe
that we have the faintest hope of detecting, much less communicating with, is, for
all intents and purposes, zero, and that our own evolutionary history was far from
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