Cancer as Evolution, part 2

When I posted part 1, I didn’t realize that Scientific American would be coming out with an entire special issue devoted to cancer in the same month, including an article by Carl Zimmer entitled “Evolved for Cancer?“.

I had hoped that the article would be about the somatic evolution of cancer, and while it touches on this aspect briefly and tangentially, it mostly talks about the evolution of defenses against cancer within the human population as a whole.  There is a critical distinction here: somatic evolution occurs on the cells within a single body in the course of a single human lifetime,* while human evolution happens in a population of many humans over millions of years.**

The reason it is important to clearly understand this distinction is that it has critical importance in understanding cancer as well as dire implications on how to prevent, detect and treat it.  For instance, those who understand the implications of somatic evolution are very skeptical of the ability of chemotherapy or radiation as reasonable approach.  Why?  Because just as virus populations evolve in response to selective pressures (i.e. things trying to kill them), so do cells in your body.  In the case of chemo, it’s called drug resistance.  The problem with non-targeted approaches like chemo and radiation is that the are also very toxic to normal, healthy cells and hence may kill you before eradicating the last cancer cell.  The question becomes, can we get beyond drug resistance and make the chemo approach work?  If you understand somatic evolution, the answer is probably not.***

The larger point is not to single out chemotherapy — though there certainly are some perverse incentives in the drug industry to push ineffective and harmful treatments — but rather that understanding somatic evolution, how it relates to human evolution, and the distinction thereof are all critical if we are going to lead longer, healthier lives that are not destroyed prematurely by cancer.

hat tip: Ali Tabibian for sending me the SciAm issue.

* With the caveat that in some cases (Tasmanian devils, dogs, human transplant patients), somatic evolution can span generations — in theory indefinitely — as the cancerous cells are transmitted from organism to organism.

** Kirschener and Gerhart in The Plausibility of Life shed some fascinating light on the intimate and complex relationship between somatic and organismic evolution, however they use terms like adaptation instead of somatic evolution, presumably to avoid this very confusion

*** Ironically, SciAm published an article in 1985 by John Cairns, professor of microbiology at Harvard University which stated, “Aside from certain rare cancers, it is not possible to detect any sudden changes in the death rates for any of the major cancers that could be credited to chemotherapy. Whether any of the common cancers can be cured by chemotherapy has yet to be established.”  Note that according to the National Cancer Institute itself, the percentage of Americans dying from cancer is the same now as it was in 1950 (and has remained flat the entire time).  Thus it is not reasonable to dismiss Cairns’ proclamation as a single or outdated voice.

  • kevindick

    I think your analysis of chemotherapy and radiation suffers from applying a single figure of merit. In networks, we distinguish between two quality of service measures: throughput and latency. It seems like we can make an analogous distinction for cancer treatment effectiveness.

    It seems to me like the cancer death rate is equivalent to throughput. We may very well not be able to affect that much with chemotherapy and radiation. However, they may improve (in this case, increase) the latency. What’s the evidence say about mean survival times for cancer at various stages?

    Note that (as you well know) it’s important to have a common longitudinal baseline for the cancer stage because detection technology advancements make cancers detectable earlier.

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