128 CHAPTER 6 | Macroevolution and the Early PrimatesBut with the demise of the dinosaurs, all sorts of op-
portunities became available, allowing mammals to begin
their great expansion into a variety of species including
our own ancestors, the earliest primates. Therefore, an
essentially random event—the collision with a comet or
asteroid—made our own existence possible. Had it not
happened, or had it happened at some other time (before
the existence of mammals), we would not be here.^4
The history of any species is an outcome of many such
occurrences. At any point in the chain of events, had any
one element been different, the final result would be mark-
edly different. As Gould puts it, “All evolutionary sequences
include... a fortuitous series of accidents with respect to
future evolutionary success. Human brains and bodies did
not evolve along a direct and inevitable ladder, but by a cir-
cuitous and tortuous route carved by adaptations evolved for
different reasons, and fortunately suited to later needs.”^5
Given that humans arrived here by chance relatively late in
the history of life on earth, the drastic changes humans have
imposed on the environment and on the other species with
which we share this earth are particularly shocking. As de-
scribed in this chapter’s Original Study by Australian zoologist
Sir Robert May, it is now time for humans to create solutions.humans, who occupy the top rung of a ladder of progress.
However, even though one-celled organisms appeared long
before multicellular forms, single-celled organisms were not
replaced by multicellular descendants. Single-celled organ-
isms exist in greater numbers and diversity than all forms of
multicellular life and live in a greater variety of habitats.^3
As for humans, we are indeed recent arrivals in the
world (though not as recent as some new strains of bacte-
ria). Our appearance—like that of any kind of organism—
was made possible only as a consequence of a whole string
of accidental happenings in the past. To cite but one exam-
ple, about 65 million years ago the earth’s climate changed
drastically. Evidence suggests that a meteor or some other
sort of extraterrestrial body slammed into earth where the
Yucatan Peninsula of Mexico now exists, cooling global
temperatures to such an extent as to cause the extinction
of the dinosaurs (and numerous other species as well).
For 100 million years, dinosaurs had dominated most
terrestrial environments available for vertebrate animals
and would probably have continued to do so were it not
for this event. Although mammals appeared at about the
same time as reptiles, they existed as small, inconspicuous
creatures that an observer who came to earth from outer
space would probably have dismissed as insignificant.The characteristic long legs of prosimians and humans are not the result of a close evolu-
tionary relationship. This is instead the result of convergence of homologous structures. The
long legs of prosimians allow them to follow their characteristic pattern of locomotion, called
vertical clinging and leaping. On rare occasions for the briefest periods of time, they are also
capable of taking a bipedal step or two.Visual Counterpoint© Wolfgang Kaehler/Corbis © Pete Saloutos/Corbis
(^4) Gould, S. J. (1985). The flamingo’s smile: Reflections in natural history
(p. 409). New York: Norton.
(^5) Ibid., p. 410.
(^3) Gould, S. J. (1996). Full house: The spread of excellence from Plato to
Darwin (pp. 176–195). New York: Harmony.