Late in August, the planet Mars made its closest approach to the Earth in 60,000 years. Though the practical scientific importance of the event was marginal, it provided a good occasion to reflect on what we know about our celestial neighbor.

One thing we know for sure is that there are no canals. Not a trace. Not even any streaks that could be taken for canals. Just a crater-scarred, dust-blown red globe. Yet I clearly recall that in my grade-school science books, the canals were presented as a plain observational fact. They were mysterious, to be sure: natural phenomena or the work of intelligent beings? But these questions were of the "what" variety, not "whether." That such a well-established idea could be so quickly overturned makes me wonder what other beliefs are on the verge of being obsolete.

As far as we know, the first astronomer to refer to "canals" on Mars was one Rev. Angelo Secchi, who in his 1858 map of Mars called one prominent surface feature the "Atlantic Canal." Two decades later, Giovanni Schiaparelli referred casually to seeing "channels" (canali) on the planet, a term whose mistranslation as "canals" fired the imagination of later observers.

Foremost among these was Percival Lowell, a gifted and energetic astronomer, who began observing the planet in 1894. In his book Mars, he left no doubt as to there being a network of green-gray canals covering the planet. In later, even more speculative work, he elaborated his vision of a Martian civilization not only intelligent, but extraordinarily so. Not surprisingly, the public and the press climbed onto the bandwagon. But the professionals were no less enthusiastic in embracing the reality of canals and offering theories to explain them.

Not everyone was convinced. As early as 1894, several prominent astronomers reported their inability to detect canals. And as late as 1940, George Hale and Earl Slipher published directly contradictory observations. For the next two decades, nothing happened to resolve the issue.

Then, in 1965, Mariner 4 flew by Mars and abruptly ended the 70-year debate. There are no canals. It was all an optical illusion, reinforced by wishful thinking. The Red Planet remains mysterious, but not in the way we had thought.

Dentists, like astronomers, are human. Once we learn a thing, especially a complex and difficult thing, we tend to hang on for dear life until evidence or circumstance finally forces us to accept that yet another of our cherished beliefs needs revision. And though it’s reasonable, even wise, to take a cautious view of a new idea, there’s no merit in an obstinate insistence on the rightness of the old.

Thinking back on what I have been taught over the years, it’s easy to come up with examples of "knowledge" that has become, or in some cases always was, just plain wrong.

Examples:

– "Never use tooth-colored fillings in posterior teeth." This was the conventional wisdom when I was in dental school, but now we know that composites, properly applied, can provide a serviceable and esthetic restoration for the long term.

– "Dental implants are experimental." Right then, wrong now. Today’s implants are effective substitutes for missing teeth in a great many situations, with success rates above 95 percent in the anterior mandibles of healthy patients. Again, it takes intelligent treatment planning and good technique to get the most from implants, but experimental they’re not.

– "Tooth loss is a normal part of the aging process." While prevalence of tooth loss increases with age (how can it decrease?), this is the cumulative effect of a lifetime of oral disease. Humans aren’t programmed to lose their teeth. With good preventive and corrective care, and absent special circumstances, there’s no reason why our older patients should not expect to keep their natural dentition.

– "Don’t spend too much time on elderly patients—they’re too old to benefit." This is wrong on so many levels I hardly know where to start. Yes, our bodies become more fragile and prone to systemic problems as we get older, making dental treatment perhaps more complex. But just as edentulism is not a normal condition, pain, infection and poor Aesthetics do not become more tolerable with age.

– "Oral health doesn’t affect general health." Apart from a few spectacular cases in which an infected extraction site caused fatal septicemia, the old "focal infection" theory was considered thoroughly debunked decades ago. While nobody is proposing a return to prophylactic full-mouth extractions, there’s a growing body of evidence that the health of the mouth affects the body in profound and unobvious ways. The possibility is intriguing, but I’d have to say that the jury is still out on this one.

No doubt you can cite other examples of theories and techniques that were treated with unquestioning respect not so long ago, only to be refuted and abandoned in the face of new knowledge. My point is not that we shouldn’t respect what we’ve been taught, but rather that this respect must not degenerate into blind submission.

We all carry in our heads a simplified model of how the world works, the cumulative result of our individual experience and reflection. To keep that model orderly, we need to balance our instincts for skepticism and open-mindedness when assimilating new information. Personally, I apply an intuitive 80/20 rule to the dental literature: what I "know" gets an 80 percent weighting, and the new material gets about 20 percent (or less—junk science gets zeroed out at the start). In this way, new, unfamiliar and even unwelcome ideas gradually make their way into my world view, while "noise" is filtered out. Rarely do I change my mind overnight.

But back to Mars. The Martian surface is actually quite difficult to see clearly from even the largest earth-based telescopes, so it isn’t surprising that astronomers differed as to what lay behind the purple haze they saw through their eyepieces. It is more difficult to explain why what is essentially a figment of the imagination was so universally accepted for 70 years. Any of the dozens of research observatories, not to mention thousands of amateurs around the world, were perfectly free to make their own observations. There was nothing to see, so why did so many people see it?

Or put differently, why do we believe what we’ve been told in the face of evidence to the contrary? There are two factors, I think, that help explain this peculiarity of human nature.

For a start, unambiguous data often are hard to come by. It was true of Mars, and it’s true of just about any biomedical research. As dentists, we rarely encounter a truly definitive study; more usually, the arguments are statistical, the effects small and the applicability limited. It is the cumulative weight of the evidence that convinces us, gradually, rather than a single breathtaking photograph, and we are rightly reluctant to abandon a workable theory without very good reason. In the case of the canals, though, we see extremes of conservatism and credulity: Lowell appears to have applied a 5/95 rule when he proclaimed his canals, while his followers used 95/5 to ignore all evidence to the contrary.

There’s another instructive aspect to the canal story. Lowell was an eminent scientist, probably the most influential astronomer of his day, and something of a media star. His pronouncements on canals were forceful and unequivocal. It is not surprising that others in his profession, whether out of fear or respect for the great man, chose to follow his lead. How sincere they were, we have no way of knowing. But it is likely that the progress of Mars research (such as it was) was retarded by decades because of this reluctance to challenge so formidable a personality.

A certain amount of hero worship is natural, but, as we have seen, it can get in the way of progress. At the word of a famous athlete, thousands rush out to buy sneakers or snack foods. Are we really so different when we stick dogmatically to the teachings of the Greats of Dentistry, or, alternatively, parrot the theories of a popular lecturer?

Ours is a science-based profession, where discoveries can and should change the way we work. In the next decade, a sizable fraction of what we "know" will be proven false, so we may as well get used to the idea. The theory of canals on Mars was a bandwagon to nowhere, one that scientists were too eager to board and too slow to abandon. Let’s try to be a little more discerning.