Map of life expectancy at birth from Global Education Project.

Friday, May 23, 2008

Cancer's War on Us

Cancer is frightening, but it's also damn interesting. It's a very stark reminder of what we are, as metazoans, and that's something quite astonishing.

There are two major inventions that make us metazoa fundamentally different from all the other life forms except the plants and fungi (which are actually closely related to the metazoa). They can't really be separated, but I'm going to say that the most fundamental is the transfer of the reproductive capacity from the level of the cell, to the level of the organism. The second is the organism itself -- a colony of cells with identical genomes but which have many different patters of gene expression and hence constitute various tissues with differing functions, organized (and note the etymological link) into a complex structure.

In the other kingdoms of life, individual cells reproduce by division. Bacteria may form structures called biofilms. (Think of the slime that may adhere to the bottom of a sink strainer that hasn't been cleaned for a while.) These may even have some structure, and cells in different locations may even behave differently, but biofilms cannot reproduce themselves. Only the individual cells within them reproduce, and whether they end up forming a biofilm depends on whether they happen to encounter the right conditions.

Our cells, in contrast, have given up their individual rights. They reproduce only under rigidly constrained circumstances. If they suffer DNA damage, or happen to be in the wrong place at the wrong time, they even kill themselves. Evolution has favored these behaviors because they produce the best chance of the cell's DNA being passed on to another organism, or more precisely, that an identical copy of half of its DNA will be passed on, although this may occur multiple times. This may seem a sacrifice not worth making, but it is well worth it because it is only through sexual reproduction, relegated to specialized germ cells, that metazoan evolution can occur.

Cancer means that this system has broken down, that a cell and its progeny no longer obey the rules. What is remarkable is not that it happens, but that it doesn't happen more often. It only takes a single cell out of the trillion or so that make up you to become sociopathic, and you have cancer. On the other hand, with intelligent design, it would never happen.

The origin and evolution of the sexes is at the core of the evolution of complex organisms. I've been thinking about this a bit lately just because it's so damn fascinating. I'll be away from your Intertubes for a couple of days now but I'll have more to say about it on my return.

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