Once in Galapagos a Lady ...

What follows is a re-posting of my initial (July 2009) entry to this blog. In it I suggest that perhaps scientists who fail to "see" cancer's role in Bilaterian evolution are emulating a woman who didn't approve of my airplane reading.  
  

This happened in the late 1970s or maybe in the early 1980s. I was returning to New York 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, I started to read it, a Penguin trade paperback entitled Evolution, its author, John Maynard Smith. 

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 her money doing exactly 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, 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. Instead, 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 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. And I’ve been listening for decades and never heard that Big Bang, not 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 my book 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 was 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 abandoned.

(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 the 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 that 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 conclusion I favor is that, because surviving gene pools always worked 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 is that 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 could not eat larvae growing under rocks was significant. I do not claim an exclusive role for cancer selection.

What, then, am I claiming? Am I saying that because these characters are so fundamental to insects 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 evolution of all the Bilaterians, including insects.

Perhaps I ought to say especially in insects for, unlike the marine arthropods from which they descended, the insects lived on land in an environment which, unlike the sea, provided no universal shield against sunlight and other carcinogenic radiation. Moreover, unlike the land vertebrates, insects did not possess an immune system capable of functioning as a second line of defense against cancer, one that could exterminate cells already transformed to 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. 

REFERENCES 

Gateff, E. and 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)

Comments and questions to the author ... are welcomed here.

At this site you will find links to additional material including my original Letters to the Journal of Theoretical Biology and  the 1992 Nature review of my book.

Copyright © 2009, 2013 by James Graham

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