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Scientists at Georgia Tech last week published research in the journal Medical Hypotheses (abstract only) that suggests a link between intelligence and cancer risk. Specifically, the researchers believe that the increased brain capacity in humans, compared to chimpanzees, may explain why we get cancer more often than our primate relatives. Cancer results from errors in apoptosis, or programmed cell death, which is when a cell commits suicide. If a cell acquires a mutation it can't fix, or encounters other serious damage, apoptosis is critical to prevent a damaged cell from replicating and potentially affecting the rest of the body. Apoptosis is also a normal, healthy part of the life cycle - a classic example is during frog development; tadpole cells undergo apoptosis to get rid of the tail and make a mature frog. (See the super-short, but helpful apoptosis video below).
When apoptosis this process goes awry, damaged cells can divide uncontrollably, leading to cancerous tumors. The scientists behind this new study argue that since human brains are larger (and therefore require more cells), the increased brain size may have required the cell-death process to be less efficient. As a result, the authors of the study suggest that cancer could be an unfortunate side effect of our increased intelligence.
I'm not so convinced.
Apoptosis
It's really tricky with scientific studies to know what is causative (when something actually causes an effect) versus correlative (when an effect happens to correlate with a potential cause). While it's possible that the increased brain size may have lead to this greater cancer risk, it's possible that the findings are simply correlative - humans get cancer more than chimps for lots of reasons, and that it just so happens that our brains are bigger, too. Seems reasonable, and perhaps more easily tested experimentally than the causative model (especially since we can't re-create human evolution in a lab).
More interesting, though, might be an argument of an indirect relationship between brain size and cancer. We do know that cancer is caused by multiple errors in the cell, due to exposure to chemicals, UV light and other damaging agents. So what if the authors are sort-of correct? Could it be that our increased intelligence - which has led to toxic industrial emissions, preservatives, pesticides, designer drugs, and even tanning - be indirectly causing our higher cancer rates?
I should note that this paper is in a journal with the word "hypotheses" in the title, so it's clearly not being presented as proven fact. And after reading the entire article (unfortunately, it's not free-access, so you'll have to pay to read it), the scientists do present an interesting argument on how natural selection for smarter primates (us) required lower apoptosis activity. But ruling out the alternative explanations is a difficult, but important, next step to understanding this potential link.
-Rabiah
Author's Note: Sources:
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WN2-4W75RWV-8&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=e713ef2414b1563b280fda1f6442df18
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Apoptosis.html
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowSection&rid=dbio.figgrp.180
http://www.sciencedaily.com/releases/2009/06/090610091222.htm
http://evolution.berkeley.edu/evolibrary/article/evo_25
Editor's Note: For more blogs from Dr. Rabiah, visit Science Chicago's website at: http://www.sciencechicagoblog.com
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