124 CHAPTER 6 | Macroevolution and the Early Primates
Today, humans are the only primate existing globally. We
inhabit every continent, including areas as inhospitable as
the icy Antarctic or the scorching Sahara Desert. This ex-
tended geographic range reflects the adaptability of Homo
sapiens. By comparison, our relatives in the hominoid
superfamily live in very circumscribed areas of the Old
World tropical rainforest. Chimpanzees, bonobos, and go-
rillas can be found only in portions of Central, East, and
West Africa. Orangutans are limited to the trees on the
Southeast Asian islands of Sumatra and Borneo. Gibbons
and siamangs swing through the branches of a variety of
Southeast Asian forests.
Such comparisons between humans and the other
primates appear natural to biologists and anthropolo-
gists today, because they accept that modern humans,
apes, and monkeys are descended from the same pre-
historic ancestors. However, a century and a half ago,
when Charles Darwin published On the Origin of Spe-
cies (1859), this notion was so controversial that Dar-
win limited himself to a single sentence on the subject.
Today, anthropologists, as well as the global scientific
community in general, accept that human origins are re-
vealed in the evolutionary history of the primates. We
now know that much of who we are, as culture-bearing
biological organisms, derives from our mammalian pri-
mate heritage.
Although many of the primates discussed in this chap-
ter no longer exist, their descendants (as we discussed in
Chapters 3 and 4) now live in South and Central America,
Africa, Asia, and Gibraltar at the southern tip of Spain,
and in zoos and laboratories all over the world. The suc-
cessful adaptation of the primates largely reflects their
intelligence, a characteristic that provides for behavioral
flexibility. Other physical traits, such as stereoscopic vi-
sion and a grasping hand, have also been instrumental in
the success of the primates.
Why do paleoanthropologists attempt to recreate pri-
mate evolutionary history from ancient evidence? The
study of these ancestral primates gives us a better un-
derstanding of the physical forces that caused these early
creatures to evolve into today’s primates. It gives us a fuller
knowledge of the processes through which an insect-
eating, small-brained mammal evolved into a toolmaker,
a thinker, a human being. In addition, the continued
survival of our species and of our world now depends
on understanding evolutionary processes and the way all
organisms interact with their environment.Macroevolution and the
Process of Speciation
While microevolution refers to changes in the allele fre-
quencies of populations, macroevolution focuses on the
formation of new species (speciation) and on the evolu-
tionary relationships among groups of species. To under-
stand how the primates evolved, we must first look at how
the evolutionary forces discussed in Chapter 2 can lead to
macroevolutionary change. As noted in that chapter, the
term species is usually defined as a population or group of
populations that is capable of interbreeding and producing
fertile, viable offspring. In other words, species are repro-
ductively isolated. This definition, however, is not alto-
gether satisfactory, because in nature isolated populations
may be in the process of evolving into different species,
and it is hard to tell exactly when they become biologically
distinct without conducting breeding experiments. Fur-
thermore, this definition can only be tested among living
groups. Breeding experiments cannot be conducted with
sets of fossilized bones.
Certain factors, known as isolating mechanisms, can
separate breeding populations and lead to the appear-
ance of new species. Because isolation prevents gene flow,
changes that affect the gene pool of one population can-
not be introduced into the gene pool of the other. Random
mutation may introduce new alleles in one of the isolated
populations but not in the other. Genetic drift and natural
selection may affect the two populations in different ways.
Over time, as the two populations come to differ from each
other, speciation occurs in a branching fashion known as
cladogenesis (Figure 6.1) (from the Greek klados, mean-
ing “branch” or “shoot”).
Some isolating mechanisms are geographic— preventing
contact, hence gene flow, between members of separated
populations. Biologic aspects of organisms can also serve
as isolating mechanisms. For example, early miscarriage
of the hybrid offspring or sterility of the hybrid offspring,
as in the case of closely related species such as horses and
donkeys (producing sterile mules), serve as mechanisms
to keep populations reproductively isolated from one
another.
Isolating mechanisms may also be social rather than
physical. Speciation due to this mechanism is particu-
larly common among birds. For example, cuckoos (birds
that do not build nests of their own but lay their eggs in
other birds’ nests) attract mates by mimicking the song
of the bird species in whose nests they place their eggs.macroevolution Evolution above the species level.
speciation The process of forming new species.
isolating mechanism A factor that separates breeding
populations, thereby preventing gene flow, creating divergent
subspecies and ultimately (if maintained) divergent species.
cladogenesis Speciation through a branching mechanism
whereby an ancestral population gives rise to two or more
descendant populations.