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adds the extra charge. Although all our ape relatives have an intact charge-
adding gene, the form of the gene that all humans carry has been devastated
by a DNA deletion that ef ectively destroys its function.
This mutational change spread through our remote ancestors, replacing
the allele that coded for the functional enzyme. The change made the sur-
faces of the cells of those ancestors dif erent from those of all their close rela-
tives. When did the mutation happen? DNA evidence suggests that it took
place about two million years ago, which puts it back at around the time
when ancestors of ours called
Homo erectus
fi rst left Africa for Asia.
Biochemical evidence now reinforces this genetically derived date. Less
than a million years ago, our lineage diverged from the ancestors of the
Neanderthals, who left Africa to populate Europe and the Near East. Nean-
derthals are now extinct, but through a chemical tour de force Ajit and his
colleagues were able to isolate sialic acid from Neanderthal bones and show
that it was chemically identical to that carried by modern humans. This
independent evidence shows that the mutational change must have hap-
pened before the human-Neanderthal split, which puts it at a minimum of
about a million years ago.
What drove this change? Ajit and his colleagues note that the most dan-
gerous human malaria parasite,
Plasmodium falciparum
, is able to attach itself
fi rmly to the less-charged form of sialic acid that is found on the surface of
best-studied chimpanzee malaria parasite,
Plasmodium reichenowi
, attaches
itself to the more charged form of sialic acid that is found on chimpanzee red
blood cells. The human parasite cannot attach to and invade chimpanzee red
blood cells, and vice versa.
Other pathogens that cause human and ape diseases also bind to sialic
acids, but malaria is by far the deadliest. Malaria parasites account for more
than three million deaths annually, and many more millions of people are
left severely debilitated. The relative impact of this disease must have been
much greater on the smaller human populations of the past.
Our ancestors began to diverge from those of chimpanzees six million
years ago, in Africa. For four of those six million years they continued to be
susceptible to the chimpanzee malaria parasite,
P. reichenowi
.
