Travel Reference
In-Depth Information
Humans
Old World
monkeys
Orangutans
Chimpanzees
Gorillas
Common
ancestor
Figure 83
The evolutionary tree that Ajit Varki and I built using behavioral and physical
characteristics of the humans and great apes. We humans are clearly different from our close
relatives. We stick out from the tree like a fi shing pole. (From C. Wills,
Children of Prometheus
(Cambridge, MA: Perseus Books, 1998), 200.) © Christopher Wills.
work, providing a fi rst line of defense against the invasion of bacteria and
viruses.
The sugar molecule chains are attached to proteins and fatty molecules
embedded in the cell's membrane. These embedded molecules are free to
slither around within the two-dimensional world of the membrane like skat-
ers on a skating rink. They drag the sugars with them, forming ever-shifting
cheveaux-de frise
of chemical defenses against invaders.
The outer tips of most of the chains of sugar molecules are occupied by
modifi ed sugars called sialic acids. The predominant form of sialic acid in
most mammals has a teeny negatively charged arm that stretches out to meet
the environment. These charged sialic acids interact readily with positively
charged molecules outside the cell.
The equivalent sialic acid molecules in humans, however, do not have
this negative charge. The tips of their chains of sugars are uncharged. And
this, as Ajit discovered, is because we have lost the enzyme that normally
