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As the host species are driven further and further apart by these selective
pressures, the parasites and herbivores that prey on them must also become
more diverse. The result is an ecosystem with such a diverse set of host species
that no single pathogen or herbivore is versatile enough to attack every host.
The preservation of diversity, at the level of both the visible and the
invisible worlds, is essential if ecosystems are to remain healthy and bal-
anced. Trees and bushes, pollinating insects and birds, seed-eating pigs and
browsing tapirs, frugivorous bats, birds and primates, the fi erce ants and the
stingless bees that defend their host trees, beetles and bugs in unimaginable
profusion, fungi that can benefi t or damage their hosts, and the myriad bac-
teria and viruses that form the invisible base of this complex edifi ce—all are
linked in a fragile web of life and death. It is these dynamic interactions that
make every encounter with a complex ecosystem such an adventure for an
alert Darwinian tourist.
Ecology, malaria, and the origin of humans
Up to this point we have been talking about trees in the rainforest, but of
course the ecological interactions that encourage the maintenance and evo-
lution of rainforest diversity must be operating in the rest of the living world
as well. And that includes the evolution of our own species. Like the trees of
the tropical rainforests, we have evolved to fi ght of our pathogens. And, like
the tree species, we have been driven apart genetically from our nearest rela-
tives as we have succeeded in keeping our shared pathogens at bay.
My colleague Ajit Varki at the University of California San Diego has been
studying the unique evolution of our species for much of his scientifi c career.
Twelve years ago he and I set out to quantify a set of anecdotal observations.
When we compare ourselves with our nearest living relatives, the great
apes, we seem to be extremely dif erent in our appearance and behavior. We
decided to build an evolutionary tree in order to visualize this relationship.
To build the tree we used a wide variety of physical and behavioral char-
acters that we share with chimpanzees, bonobos, gorillas, and orangutans.
These included the ability to walk bipedally, the dif erence in size between
 
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