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In-Depth Information
Thus, when the less-charged mutant sialic acid appeared in our ancestors
about two million years ago, it was a big deal indeed. It was strongly selected
for, because it immediately conferred resistance to this dangerous malaria.
Our lucky ancestors were not totally freed from malaria, because there are
three other less dangerous malaria parasites that also prey on us. Nonethe-
less, at one stroke, they had rid themselves of the most debilitating form of
this disease.
We do not know the full benefi ts that followed from the emergence
of this new immunity to
reichenowi
malaria, but it might have helped our
African ancestors survive dii cult times. They may have been given some
breathing room as they continued on their new evolutionary path towards
more sophisticated tool use and more complex social structures. It may not
be a coincidence that the fi rst stone tools, the fi rst human-like hands, and the
fi rst signs of organized hunting all appeared in Africa at about the same time
as the sialic acid mutation. Many other factors certainly contributed to these
essential events in our history as a species, but freedom from severe malaria
must have been a big help.
All good things come to an end, and at some point the inevitable hap-
pened. A mutant strain of
P. reichenowi
arose that could attach itself to the
less-charged sialic acid. This mutant strain eventually gave rise to the most
dangerous human malaria,
P. falciparum
. The
falciparum
DNA family tree is a
branch of the
reichenowi
tree, but so far it is dii cult to say when it branched
of . It may have been as recently as 10,000 years ago, or as long ago as a
million.
The mutation that allowed
falciparum
to attach itself to our ancestors' red
cells occurred in an invasion receptor gene of the parasite that is called EBA
175. When the mutant strain of malaria appeared it exploded in sub-Saharan
Africa and spread to the Middle East, but not to temperate regions because
the parasite cannot survive long winters. The result has been, as Jared Dia-
mond has pointed out in
Guns, Germs and Steel
, that societies in tropical areas
that suf er from malaria have been held back relative to those of less malaria-
ridden regions like Europe.
This story is far from complete. In humans, many genes are involved
in resistance to parasites and to bacterial and viral diseases. Ajit Varki and
