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July 7, 2009

Once in Galapagos a Lady ...

This happened in the late 1970s. I was returning from a European business trip on a wide-body jet. As was my habit, I had a book with me and once we were airborne, started to read it. Its title was Evolution, its author, John Maynard Smith. A Penguin trade paperback.

In due course a meal was served and I placed the book in the pocket in front of me.

“I see you’re reading that book on evolution.”

It was the woman to my left. A fellow American, a dignified lady of a certain age who had the appearance and manner of someone who had the time (widow?) and money (life insurance proceeds?) to travel the world at her leisure and who spent her time and money doing just that.

I glanced at her and saw from the unsmiling tightness of her mouth that this was not a pleasant query. Oh oh, I thought. Here it comes.

“So, do you believe in it? In evolution?”

To have given a complete – and completely honest – answer I would have replied that yes, I’m absolutely convinced that evolution did happen but the accepted theory fails to explain the existence of complex animals. It has a fatal flaw, but – not to worry – I’ve figured out how to fix it. But I took the easy way out and, nodding my head and mumbling, let her know that I found it quite convincing but (the primary function of mumbling?) didn’t care to talk about it.

My fellow passenger then delivered what I am sure she considered the final word on the matter, a triumphant conclusion to our extremely brief conversation.

“Well, young man ... ”

Did I say this was a few years ago?

“ ... I’ve actually been to the Galapagos ...”

I turned to face her.

“ ... and I didn’t ... ”

Somewhere on the planet someone executed a perfect drum roll.

“ ... see any evolution there!”

That was it. We said nothing further.

Perhaps I should have replied with something clever like “Oh, I can beat that. I’ve been to ocean beaches all over the world and I never saw any of that continental drift. Or, I’ve been listening for decades and never heard any Big Bang or even an echo.”

Over the years I’ve occasionally dined out this little tale and it never fails to get a chuckle. But when I reflect on it hadn’t that silly woman actually reached her conclusion using her own version of a tool employed every day in laboratories run by scientists? She had a hypothesis (that animals change over time) went to the place where a lot of change had supposedly happened (Galapagos) made her observation and, seeing no changes, reached her conclusion: evolution did not happen.

Although he never wrote about coping with the occasional anti-evolutionist one might encounter on an airplane, one biologist who thought that laboratory science was not the best way to understand evolution was the late Ernst Mayr. In The Growth of Biological Thought Mayr gets so many things right it’s a pity all biologists are not required to read it, especially those who write on evolution. (He was, however, wildly wrong to claim that the evolutionary synthesis required no “major modification”.)

Mayr addresses at some length the inappropriateness of relying exclusively on observational science:
No one questions that the appropriate technique for the study of functional phenomena is the experiment: but it must be emphasized that the causal explanation of historical (evolutionary) phenomena ordinarily must rely on inferences from observations.

There are many places in Cancer Selection where I rely on “inferences from observations” but it was particularly appropriate in considering the question of cancer selection’s possible role in the evolutionary history of insects. In their report of what apparently is the first unequivocal finding of cancer in Drosophila Elizabeth Gateff and Howard A. Schneiderman list a number of characters found in all insects that seem to explain cancer’s relative rarity. In the book I summarized their list of reasons:

(1) Unlike humans and other vertebrates, adult insects have few cells that divide. Cells that do not divide cannot possibly have cancerous offspring.
(2) Most insects undergo metamorphosis during development and during metamorphosis the adult animal is created from "imaginal disc" cells, not from the division of somatic cells that comprise larval tissue. Any tumors that might exist in the larval tissue could be discarded.

(3) Much insect growth occurs without cell division; the cells simply get bigger and, in some cases, DNA replicates inside the cell. Again, unless they divide, no cells, not even those in which a major mutational event has taken place, will have cancerous offspring.
(4) Insect DNA is spectacularly good at replication. When normal (noncancerous) vertebrate cells are kept alive in laboratory vessels, abnormalities appear in DNA after a few cell divisions. Eventually all the cells die. In vitro, some human cells cease to divide after a mere 50 generations. In contrast, observed insect cells divide more than 1,500 times without abnormalities. Because mutagens are carcinogens DNA that successfully resists somatic mutations will experience little cancer.

To Gateff and Schneiderman’s list of characters that might explain extremely low levels of cancer in insects I added some of my own.

(5) Insects shield larvae from sunlight and other naturally-occurring environmental radiation.

(6) Insects are small. Smaller animals are smaller targets for mutagenic radiation.

(7) Most insects are short-lived. Drosophila go from egg-to-egg in ten days. The possibility of lethal cancer interrupting this link in the chain of life is minimized by the shortness of the link.
So what do these facts about insects and cancer tell us?

If we take the approach favored by my seat mate – the lady who saw no evolution in Galapagos – we would conclude that the present low level of cancer in insects settles the matter: cancer selection played no significant role in the life history of insects.

The alternative that I favor is (big surprise!) that because surviving gene pools always work on here-and-now problems the abundance of insect features with apparent cancer-defensive characteristics in present-day specimens is sufficient evidence for concluding that, contrary to present-day laboratory observations, cancer selection played a very significant role in the insects’ past.

The alternative explanation: these characters were selected for reasons other than cancer-defense. The fact that they minimize the likelihood of cancer is merely coincidental, cancer selection had nothing to do with it.

(Please note that I do not deny that other factors played a significant role in the emergence of these characters. The fact that birds cannot eat larvae growing under rocks was significant. I do not claim an exclusive role for cancer selection.)
What, then, am I claiming? These characters are so fundamental to insects am I saying that all insects owe their existence to cancer selection? Yes, that is my claim. Throughout the book I argue that cancer played an essential role in the origin of all the Bilaterians including insects.

Perhaps I ought to say especially insects for, unlike the marine arthropods from which they descended the terrestrial insects lived in an environment which, unlike the sea, provided no universal protection against sunlight and other carcinogenic radiation. Moreover, unlike the land vertebrates insects did not possess an adaptive immune system functioning as a second line of defense, a defense that would kill off cells already in the cancerous state. Insects had to avoid at all costs the initiation of cancer. Its present-day extremely low incidence seems to suggest that they succeeded.

Copyright 2009 by James Graham

Gateff, E. & Schneiderman, H.A. Neoplasms in Mutant and Cultured Wild-Type Tissures of Drosophila in National Cancer Institute Monograph 31 Neoplasms and Related Disorders in Invertebrates and Lower Vertebrate Animals (1969)

Mayr, E. The Growth of Biological Thought, Harvard University Press (1982)