Travel Reference
In-Depth Information
others have found mutant forms of some of these genes in our species. These
mutant forms separate us even further genetically from the apes. Perhaps the
spread of these mutant genes has also been driven by their ability to protect
us from the diseases that we used to share with our close relatives.
This story of humans and malaria resembles the one I proposed earlier
for our hypothetical pair of plant species in the forest. I pointed out that the
two plant species would still continue to be susceptible to a shared set of
parasites as they begin to speciate and move apart genetically. Just as has
happened with humans and malaria, they would then be selected for resis-
tance to the parasites and pathogens of the other species. And the parasites
and pathogens would evolve in turn, driving the plant species even further
apart.
As we examine the underlying processes of evolution, the same themes
emerge again and again. Earlier in this chapter I pointed out that the pattern
of recruitment and survival in rainforests around the world shows how the
diversity of the rainforest itself may have evolved. I suggested that the diverse
tree species of the rainforest may have been driven apart genetically by two
factors. The tree species were able to avoid the pathogens and predators that
they shared with closely related species, and they were able to invade regions
of the forest that had previously been closed to them.
Now, evidence is emerging that humans have been subjected to selective
pressures similar to those of our hypothetical plant example. We escaped
some of our most dangerous parasites in Africa by acquiring a sialic acid that
was dif erent from the one belonging to our chimpanzee relatives. Perhaps
our ability to escape from our shared parasite gave us the chance to acquire
the technology and cultural cohesion necessary for us to escape from Africa
itself.
At fi rst blush there might seem to be a huge gap between the origins
of the ecological and genetic diversity of rainforests and the origins of the
dif erences between our own species and our ape relatives. But as we learn
more about the evolutionary forces involved, a common theme emerges. In
the topic's second part, we will look further at how our own history has been
shaped by such evolutionary pressures. In the process we will discover addi-
tional connections between ourselves and the rest of the living world.
 
Search WWH ::




Custom Search