Travel Reference
In-Depth Information
the left and right hemispheres of the brain, the amount of thumb mobility,
the length of time spent nursing the young, and several other characters.
The great apes in the tree grouped fairly closely together, but humans stood
out dramatically. We occupied the end of a long branch that stuck out from
other, shorter branches like a fi shing pole. This tree-building exercise showed
clearly that we really are dramatically dif erent from even our close relatives.
Many of these dif erences between ourselves and the apes can be attrib-
uted to selective pressures that have increased our intelligence and toolmak-
ing ability, and that have encouraged the development of complex social
interactions within human groups. As our culture has become more com-
plex with time, we have evolved to take advantage of the opportunities that
have been provided by our culture. The result of this evolutionary feedback
loop, involving our brains, our bodies, and our culture, has been a uniquely
intelligent, uniquely dexterous, and uniquely social species.
But, remarkable as our evolution has been, there is no evidence that our
uniqueness has been caused by anything other than the basic evolutionary
processes we talked about in Chapter 2: mutations, natural selection, chance
events, and the genetic reshul ing of our gene pool that happens every gen-
eration as a result of sexual reproduction. Darwin always insisted that our
origins can be traced to the same evolutionary processes that have led to the
diversity of the rest of the natural world, and he was right.
Some of the evolutionary changes that have contributed to our emer-
gence as a separate species have nothing to do with our culture, but every-
thing to do with our resistance to disease. In 1998 Ajit Varki's group and
another group of scientists discovered the genetic basis for a molecular dif-
faces of the cells of our bodies, has driven us dramatically apart from our
closest ape relatives. To understand the nature of this dif erence, we must
delve into the submicroscopic world of molecules.
Each of the 100 trillion cells in the human body is surrounded by a deli-
cate cell membrane that shields it from harmful molecules while allowing
essential ones to pass through. The outer surface of this membrane, which
directly confronts the environment, is studded with a maze of treelike chains
of sugar molecules. These chains branch and join together to form a lattice-
