Thursday, March 15, 2007

Why Do We Develop Cancer?

No doubt this is a question we have all asked at some stage in our lives as cancer is a ever constant and present threat to the life prospects of everyone. And the usual answer is one that provides a causal explanation of why a particular person developed a particular kind of cancer. Given that cancer is a multifactorial disease this causal explanation will be one that emphasizes the important role played by our genes, age and environmental influences (like smoking, diet, etc.). Check out this useful site for an informed answer to this question.

But why do we, as a species, develop cancer? That question is a more profound and complex question. And Mel Greaves has a fascinating opinion piece in the latest issue of Nature Reviews Cancer where he offers some answers to this question. The answer, he contends, can be offered by Darwinian science. Here are a few snippets from this insightful piece:

From Mel Greave's "Darwinian medicine: a case for cancer" (Nature Reviews Cancer, March 2007)

A pragmatic focus on immediate, or proximate, causal mechanisms in cancer has been very productive. Epidemiologists identify cause as, for example, chronic exposure to cigarette tar, and molecular biologists indict gene mutations as mechanistic drivers. In turn, these insights provide genuine, practical advances in prevention, screening, differential diagnosis, prognosis and innovative treatments. We should all be happy. But there are two difficulties here, both related to our expectations of what the word 'cause' actually means and what level of understanding we aspire to. Environmental exposures and mutations are self-evidently not autonomous entities, but crucial components of a causal chain of events or components of a causal network. Second, even with a more realistic compound view of risk factors and molecular, biochemical mechanisms that produce cancer, we might still lack a coherent framework that can help us understand vulnerability. Why should potentially lethal cancer be such a common biological phenomenon, and why do we, as a species, seem to be especially vulnerable? Why is the lifetime risk of breast cancer as high as one in ten? And, superimposed on what seems to be a species vulnerability, why are some individuals more at risk of particular cancers than others?

…Overall, these data suggest that cancer risk is underpinned by intrinsic fallibility, and that risk increases with increasing age and is greatly exacerbated by some aspects of human activity.

…Any engineer confronted with a recurring fault in a complex machine or plant would look not only at the immediate source and cause of the fault, but at system design, its compromises and limitations. The engineer will resort to a blueprint; we have evolutionary biology.

The essential tenet of the new discipline of evolutionary or Darwinian medicine is that susceptibility to malfunction and disease must in part reflect historical or evolutionary legacies. The corollary is that we might then benefit from stepping back to take a broader look at human history and our protracted evolutionary trajectory. Even a cursory consideration of human anatomy reveals structural imperfections that are pregnant with potential for malfunction. For example, no intelligent designer would place the optic nerve and retina or prostate and urethra in the anatomical relationships in which we find them. The reality is of course that we have not been 'designed' or 'engineered' at all. The evolutionary processes involved in the diversification of molecules, cells, tissues and physiological processes rely on options generated randomly from previous templates. This is coupled with the selection of beneficial traits, by contingency or chance, or neutral drift. Evolutionary biologists continue to debate the relative importance of the mechanisms of selection, particularly as claims that traits were positively selected (the adaptionist argument) cannot always be substantiated. Irrespective of these uncertainties, the processes involved will inevitably result in 'designs' that have constraints or limitations on board, and trade-offs, collateral damage or negative impacts. Ultimately, inherent flaws are tolerated, at some level, as long as they do not impact deleteriously on reproductive fitness.

…Intrinsic vulnerability to cancer (or other chronic diseases) must be counterintuitive to anyone who views our bodies as the product of purposeful design or engineering. Evolutionary or Darwinian medicine provides the opposite view: the blind process through which we and other species have emerged carries with it inevitable limitations, compromises and trade-offs. The reality is that for accidental or biologically sound, adaptive reasons, we have historically programmed fallibility. Covert tumours arise constantly, reflecting our intrinsic vulnerability, and each and every one of us harbours mutant clones with malignant potential.

The complete article is well worth the read. The link is here (though you will need a subscription to the journal).