Is cancer the price for our big brains?

[The Spartan]

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Is cancer the price for our big brains?
Chimps have a surprisingly low cancer rate compared to humans
By Rachael Rettner
LiveScience
updated 11:02 a.m. CT, Wed., June 10, 2009
Our large brains may make us cognitively superior to chimps, but, according to a new hypothesis, we could be paying a price for our sizable cerebrum: a higher rate of cancer.

Chimpanzees are thought to be the closest evolutionary relative to humans, and we share around 98 percent of our genes with these primates. But for years, scientists have observed that chimps have a surprisingly low cancer rate compared to humans.

To find out why this might be, John McDonald, a researcher at Georgia Tech, turned to the human and chimp genome.

Many past studies have looked at differences in how genes are expressed among humans and chimps. McDonald and his colleagues re-examined the data of a previous gene expression study and added some previously excluded information.

They looked for differences in gene expression in several tissues, including the brain, liver, testes and kidneys.

McDonald wanted to test a hypothesis that the difference in cancer rates between the species could be due to differences in the way their cells self-destroy themselves — an important biological process known as programmed cell death or apoptosis.

The researchers saw that some of the genes for apoptosis were expressed differently in humans than in chimps, and their data suggests that human cells are not as efficient at carrying out programmed cell death as chimp cells, at least in the brain and other studied tissues.

What does apoptosis have to do with cancer?

Reduced amounts of apoptosis have been associated with an increased risk of cancer. Also, several genes involved in apoptosis are thought to "malfunction" in cancer cells. This makes sense: cancer cells divide uncontrollably and somehow seem to override the signal to self-destruct.

Paying the price
And what does that have to do with a large brain? During human evolution, it is thought that people were naturally selected for larger brains and increased cognition. There is also another hypothesis that to get these larger brains, we needed to have a high rate of neuron synthesis.

The researchers are tying these two hypotheses together. They think that reduced apoptosis may have helped people acquire their large brains. But it may also have made us more prone to cancer.

"It's kind of hard to explain why we could have evolved to have a less efficient apoptotic system," says McDonald. "So the hypothesis we came up with was that maybe selection to increase brain size was what put the selective pressure on the system to reduce apoptosis." And even though less apoptosis may have meant more cancer, there would not have been selective evolutionary pressure against it since most cancers don't appear until after reproductive age, McDonald adds.

The hypothesis is slightly different from many previous ideas about how we evolved large brains. "We have larger brains than chimps, [and] most people have focused on the fact that we may be producing neurons at a higher rate," McDonald told LiveScience. "But I think the other side of the story is that we could also not be destroying them."

The research was also supported by recent studies that observed that people with certain cognitive diseases — diseases that are associated with an increase in neuron apoptosis — have lower cancer rates.

So far, the published evidence is only from gene expression patterns. Additional studies are being carried out to look at the actual rate of cell death in chimps and humans. And even with a lot more evidence, no one can ever know for sure why we evolved to be a certain way.


"It's a hypothesis, so the question is whether it's true or not," says Stephen Hubbell, a professor of evolutionary biology at UCLA, who was not involved in this study. "It would be really interesting if ... selection on humans relative to chimps might have changed the expression of these genes and affected the likelihood of getting sick."

"I thought this really could galvanize some really interesting research in directions where evolutionary biology is brought to bear on medical questions," he added.

The results were published online in May in the journal Medical Hypotheses.

Honestly, I don't really have the biology knowledge to comment intelligently on this particular article, but I thought it was interesting enough that I should share it with the class.

[Tolya]

Looking at the genome is one thing, but there seems to be a link between our surroundings and lifestyle. I've read about a study that compared cities to countryside - and not surprisingly, city dwellers are at much higher risk of cancer.

So while we may be genetically predisposed, it's not the end of the story. Stress, anxiety, unhealthy food, polluted air - everything is important. Chimps don't live in big cities, don't have stressing jobs and eat healthy.

[Nephtys]

I was under the impression that we have higher cancer rates than animals, by the virtue of humans living longer, when excluding obvious environmental factors (carcinogenic whatevers, smoking).

How long do chimps live? At best, I can see with my google-fu, some 70-odd years, with 50 being average compared to 70+ for humans at least in industrialized nations.

[Shroom Man 777]

Maybe it's because chimps live in environments not-so-contaminated with artificial pollutants? Or that they've got natural selection going for them, in that sickly chimps get eaten by wild elephants and don't survive long in the wild? Whereas we've got society and civilization and witch doctors keeping everyone alive, including those genetically predisposed to getting sick - who then get to pass their tainted seed to the next generation?

[Anguirus]

Looks like a plausible hypothesis to me, but you have to keep in mind that it's a very early one. They found some confirming evidence, but I'm actually more interested in the study they mentioned at the end, looking at the actual apoptotic rates.

[Simon_Jester]

Maybe it's because chimps live in environments not-so-contaminated with artificial pollutants? Or that they've got natural selection going for them, in that sickly chimps get eaten by wild elephants and don't survive long in the wild? Whereas we've got society and civilization and witch doctors keeping everyone alive, including those genetically predisposed to getting sick - who then get to pass their tainted seed to the next generation?

Wouldn't most of the people with a genetic predisposition for sickness already be dead by that logic? Humanity hasn't lived in civilized societies with competent doctors for long enough to have natural selection shake things up much.

[Knife]

Looking at the genome is one thing, but there seems to be a link between our surroundings and lifestyle. I've read about a study that compared cities to countryside - and not surprisingly, city dwellers are at much higher risk of cancer.

So while we may be genetically predisposed, it's not the end of the story. Stress, anxiety, unhealthy food, polluted air - everything is important. Chimps don't live in big cities, don't have stressing jobs and eat healthy.

True, threshold theory suggests that all those environmental factors could mean jack and shit if a person is genetically predisposed NOT to get cancer while someone who is likely to get cancer would or could need little to no environmental help with it. Seems like a good hypothesis though, if a cell doesn't auto kill itself then it will probably continue on with plasia in some respect. Displasia or malplasia will obviously be a greater risk if the cells don't die.

[Oskuro]

Layman opinion here, I agree with the notion that our extended lifespan is to blame here, since not only does it take us beyond our theoretical expiration date, it also means we are exposed to enviromental factors for a longer time, increasing the likelihood of developing cancer.

[Ziggy Stardust]

I couldn't find the article from the OP, but I did manage to scrounge up another study about cancer in humans and chimps. It claims:

Nutritional differences and other ecological causes are responsible for disparity in cancer incidence, but the tenfold increase, as observed for carcinomas of breast, ovary, lung, stomach, colon and, rectum cancer, between chimpanzee and human cannot be explained coherently by such arguments. This directs the focus on intrinsic factors like susceptibility, and tolerance, which depend on genetic factors.

I understand the argument they are trying to make (which is also, likely, the one the study in the OP is based off of), but it seems like an utter oversimplification, with no evidence provided as to why they made that simplification. It seems like they just said that they think the disparity is too big to be explained by environment and lifestyle, alone, so they are focusing on genetics. I'd really like to know why they made that decision, because it seems utterly arbitrary to me. (Note, in the article I linked to, they provide a citation for the claim. However, the article that they cite, to the best of my knowledge, does not support it at all. You can find it here.)

[Akhlut]

Too lazy to look it up at the moment, but I would at least be wary of anything saying that our environments (human ones) aren't sufficient to cause a super increased incidence of cancers in humans compared to chimps. Consumption of red meat on the scale that Americans do is known to increase cancer risk substantially, and everyone uses potential dangers constantly with little protection (cleaning products and petroleum products for instance).

While the hypothesis is intriguing and I'm sure it has some amount of validity, I'd still think environmental considerations are stronger, though this might make one more likely to develop cancer when exposed to potential carcinogens as opposed to just developing it randomly (which does occur from time to time).

[Mayabird]

I can certainly see this genetics thing being a factor, one more of many, and maybe even one that's a bit more important than some other factors, but also on the genetics pile there's the little issue that our species as a whole is incredibly inbred - one single chimp troop has more genetic diversity in it than all of H. sapiens. And then there's the whole environmental pile and the "we live long enough to let all this crap in our genome and accumulated damage over our lifetimes express themselves" pile. Nobody should think that this is the end all, be all of why humans get cancer, and nobody seems to be doing that. Anyone who tries to use this article to justify why they shouldn't stop smoking should be slapped. And I don't see anybody doing that either so we're cool.

But it is an interesting hypothesis. That is published in a journal called Medical Hypotheses, I note.

[The Spartan]

"we live long enough to let all this crap in our genome and accumulated damage over our lifetimes express themselves"

Just out of curiosity, does anyone happen to know the cancer rates of, say, tortoises? I know they're quite a bit further removed from us than chimps, so you can't exactly compare them to us, but I wonder what the results would be if we compared them to other reptiles with lower life expectancies.

[wolveraptor]

Just out of curiosity, does anyone happen to know the cancer rates of, say, tortoises? I know they're quite a bit further removed from us than chimps, so you can't exactly compare them to us, but I wonder what the results would be if we compared them to other reptiles with lower life expectancies.

Some reptiles (dunno if tortoises are among them) are able to regenerate lost tissue without risking cancer. I'm not sure, but I think it has something to do with the fact that they can suppress their immune systems.

Point being, they seem to be less susceptible to cancer than more "advanced" mammals.

[The Spartan]

Just out of curiosity, does anyone happen to know the cancer rates of, say, tortoises? I know they're quite a bit further removed from us than chimps, so you can't exactly compare them to us, but I wonder what the results would be if we compared them to other reptiles with lower life expectancies.

Some reptiles (dunno if tortoises are among them) are able to regenerate lost tissue without risking cancer. I'm not sure, but I think it has something to do with the fact that they can suppress their immune systems.

Point being, they seem to be less susceptible to cancer than more "advanced" mammals.

I was actually thinking of reptiles that don't regenerate. Presumably, if you compare, say, short lived tortoise species to a long lived tortoise (or maybe different crocodillians) you'd be able to see if there's a difference. It wouldn't necessarily give you an answer as to the mechanism in mammals but it might give at least some credence to there being a correlation.

Of course, if reptiles don't have enough of a cancer rate to be significant enough to study, or if they do and the numbers just aren't reliable enough because of, well, any number of reasons, then it's a moot point, though perhaps there's another set of species that are closer to a human/chimp like relationship from a genetic standpoint that could be compared.

Unfortunately, I can't answer any of those questions myself.

[Starglider]

Cancer risk is highly correlated with metabolic rate (e.g. free radicals causing DNA damage, although there are multiple mechanisms), so it's really not surprising that reptiles have a low incidence of cancer. What's more impressive is how little cancer birds have, despite their very fast metabolisms.

[The Spartan]

I see. Well, that's a dead end then.