3 Mindblowing Scientific Paradoxes...

By Kevin Jennings

We often think of paradoxes as being thought experiments. These are logical conundrums, often involving unproven bits of science like time travel, that act as fun puzzles and ways to test our ability to reason through impossible situations. One thing they often tend to lack, however, is practical application.

Sure, it may be fun to contemplate what would happen if you traveled back in time and killed your father before you were born, or whether the statement “this is a lie” can ever be considered either true or false. But as fascinating as they may be, those sorts of paradoxes rarely lead to major innovations.

So today, instead of thought experiments, we’ll be looking at three paradoxical outcomes of empirical research. These are paradoxes rooted in modern science, some of which could lead to incredible advancements in modern medicine if we’re able to find a solution.

Peto’s Paradox

Cancer has plagued humanity for as long as we’ve existed, and scientists have spent centuries studying how it works and how we might combat it. The first environmental carcinogens were even identified 250 years ago, and there has been extensive research since then. However, there was one paradox related to cancer that nobody really seemed to notice until Richard Peto noticed it in 1977.

Over the course of any animal’s lifetime, cells are constantly replicating. We all begin as a single cell in utero, and that cell continues to divide to form the entire organism. An adult human has about 37 trillion cells, and those cells are constantly dividing to replace themselves. The rate at which they divide varies depending on what type of cell it is, but it all averages out to about once every 24 hours for human cells.

Each time a cell replicates, there’s the chance of genetic mutations. These happen all the time, and most of them are harmless. Some, however, will result in a cell that continues to replicate uncontrollably, causing cancer. But our bodies have a number of built in defenses for this.

There is a tumor suppressing gene called p53 that will trigger other mechanisms within the cell to repair damaged DNA. If the cell is too far damaged, p53 will force it to self destruct. The immune system can also recognize and kill cancer cells. In fact, people develop cancer cells on a pretty regular basis, but our body is usually able to eliminate them before it becomes a problem.

In order to for a tumor to form, the cancerous cell needs to have several very specific mutations simultaneously. It needs a mutation to cause uncontrolled reproduction, one to disable the p53 gene, and one to hide its presence from the immune system. The odds of all of the necessary mutations happening in a single cell are low, but over the course of a person’s life their body will have produced somewhere between 10 quadrillion and 100 quadrillion different cells. Even though the odds for any individual cell forming a dangerous tumor are low, the sheer volume of cells results in about 20% of people developing cancer at some point in their lifetime.

However, what Peto noticed in 1977 was that humans didn’t seem to be getting nearly as much cancer as we should. Across the entire animal kingdom, all cells are roughly the same size. Larger animals don’t have meaningfully larger cells, they just have more cells. What Peto observed was that even though humans have 1,000 times as much mass as mice and live 30 times as long, meaning we have 1,000 times more cells dividing for a much longer period of time, both species developed cancer at the same rate.

Logically, this didn’t make any sense. In 1998, a study even confirmed the idea that more cells should mean more cancer, as the data showed that humans who were either taller or heavier than average were more likely to get cancer. The rate of cancer has increased significantly as human life expectancy has increased as well. The same is also true for within mice populations, meaning that number of cells and longevity are both risk factors for cancer within the same species, even though humans weren’t getting cancer at thousands of times the rate mice do, as would have been expected.

Later research examined dozens of different mammalian species, and it found that there was no correlation between number of cells or lifespan of a species and the rates of cancer relative to other species. For example, elephants have about 70 times more cells than humans and blue whales have over 2,000 times as many cells as humans, but they aren’t developing cancer at higher rates than humans. In fact, cancer is extraordinarily rare in both species.

This observation is Peto’s Paradox: if all cells are equally likely to become cancerous, then mammals with more cells should have more cancer. So why don’t they? Why do giant mammals with many times more cells than humans or mice seem to get cancer at dramatically lower rates than smaller mammals?

For the most part, we don’t know. If we did, this probably wouldn’t still be called a paradox. But there are some theories why this might happen. To start, a lot of larger animals have slower metabolic rates, so as a result their cells don’t divide as often. While that does introduce a variable that wasn’t originally accounted for, human cells don’t divide anywhere remotely close to 2,000 times more often than blue whale cells, so there has to be something else going on.

Another theory is that these large animals do get cancer, we just don’t notice. The larger an animal is, the larger a tumor would need to be for it to become deadly. It’s possible that giant animals like blue whales are full of small tumors that would be deadly to a mouse or even human, but they aren’t large enough to affect the whale in any way.

Related to how large the tumors need to become to be dangerous is another theory called hypertumors. The idea is that, as cancerous cells reproduce, they continue to mutate. Some of these cells mutate to form a separate tumor that competes for resources with the original tumor so aggressively that it winds up cutting the main tumor off from the vascular system, killing both of them in the process. Essentially, the tumor gets a tumor, and the name hypertumor comes from hyperparasites, which are parasites of parasites.

It’s possible that humans die of cancer before hypertumors have the ability to form, whereas larger mammals like whales need more cancer cells for a tumor to become deadly and thus they have more time for hypertumors to form and kill the original tumor. Note that while there is some indirect evidence that such a thing might be possible, there is no direct evidence of hypertumors anywhere in the animal kingdom.

While there have been observations of one tumor outcompeting another nearby tumor in the same person, causing one of the tumors to die, there aren’t any documented cases of a hypertumor existing, let of one alone resulting in the mutual destruction or the tumor and hypertumor. This theory is largely based off of a single paper from 2007 that used computer simulations to prove the theoretical possibility of hypertumors.

While this is considered a paradox, the answer is an important area of research, as discovering how giant mammals avoid cancer could help humans in our own fight against it. And when it comes to elephants, we’ve even discovered an answer.

We mentioned the p53 tumor suppression gene earlier, and how it helps prevent cancerous cells from multiplying, requiring that the cell not only mutate to become cancerous but also to disable that gene. Nearly all animals have a single p53 gene, but researchers have discovered that elephants have 20 copies of the gene. Having so many copies gives their cells incredible redundancy in the ability to prevent cancer from spreading, but this particular adaptation seems to be limited exclusively to elephants.

Other large mammals like whales, hippos, rhinoceroses, and so on each only have a single p53 gene, yet they seem to be equally resistant to cancer. It’s possible that there may be other genetic adaptations present in these animals that we just haven’t discovered yet.

Alternatively, it may not be the other large animals that are the outliers. It’s possible that humans might just have an abnormally high rate of cancer stemming from environmental factors and our diets.

The Exercise Paradox

Exercise is good for you. There is a mountain of research showing all of the ways in which regular exercise is beneficial for humans. We want to make that as clear as possible at the start, as this point often gets lost or confused when it comes to discussions about the exercise paradox and its implications.

That said, exercise may not be as useful as previously thought when it comes to losing weight. People eat food every day, and we use the calories from that food to power our bodies. If you consume more calories in a day than you use, the excess gets stored as fat for later use. Since physical activity burns calories, conventional wisdom has always been that the two keys to losing weight were to exercise more and to eat less.

However, new research has shown that this may not actually be the case. The study that really kicked off discussions about the Exercise Paradox was a 2012 study involving the Hazda tribe of Tanzania. But the results weren’t an outlier, and a number of previous studies had already supported what the researchers found, even if the previous research didn’t fully look into the paradox.

The 2012 study followed a number of members of the Hazda tribe, a group of hunter-gatherers who routinely walk up to 9 km each day while hunting and foraging. Members of the tribe were given special water containing rare isotopes of hydrogen and oxygen. By examining the amount of these isotopes in their urine, researchers could calculate the amount of carbon dioxide their bodies were producing in a day, and thus the number of calories they were burning each day. This is called the doubly labeled water method, and it has been the standard method for measuring energy expenditure ever since it was discovered.

The numbers from the Hazda tribe were then compared against sedentary office workers in industrialized nations, and the results were rather shocking. Hazda men burned around 2,600 calories each day on average, and the women burned around 1,900 calories each day. This energy expenditure was nearly identical to people who were mostly sedentary, despite the Hazda tribe being far more physically active.

The assumption was always that energy expenditure would be additive. It takes a certain amount of energy to keep your body functioning, with the human brain using up about 20% of your body’s total daily energy. So if your body needed 2,000 calories just to function, then doing 300 calories worth of exercise should logically burn a total of 2,300 calories for the day. Or so we always thought.

However, this research suggests that energy is actually constrained, and that there is essentially a predetermined amount of energy that the body will use each day. If the body isn’t using that energy for physical movement, it may instead use it on other processes within the body.

For example, inflammation is an immune system response to a variety of different stimuli. In people who exercise regularly and are using up their daily calorie allotment, the immune system has to be as efficient as possible with things like inflammation, as it requires energy. But in a sedentary person whose body needs to use all that excess energy somehow, they may experience greater levels of inflammation or even chronic inflammation. Chronic inflammation can have some severe negative consequences, so the paradox ironically revealed yet another reason why exercise is good for you, as its best not to have our bodies use excess energy on things like inflammation.

It’s important to note that while there is a lot of evidence supporting the exercise paradox on sample populations as a whole, each person is different. While total energy expenditure for humans seems to be constrained on average rather than additive, there are clear examples where we know this isn’t the case.

High performance athletes like endurance athletes or those competing in strongman competitions tend to need several thousand calories of food each day just to maintain their weight, especially during particularly strenuous training cycles. Even for people who are just exercising for health rather than attempting to push their body to its limits, there is a lot of individual variation with the extent to which this theory holds true, even if the aggregate data supports the idea.

When a person who was previously sedentary begins daily moderate exercise, it also seems to provide an initial shock to the system. Even if they don’t change what they eat, individuals who take up exercising may see weight loss early on until their body adapts to the new levels of activity, reducing energy spent on other internal functions to compensate for the extra energy expenditure from the exercise. Even before the exercise paradox was identified, this idea of early weight loss followed by a frustrating plateau was something that millions of people had already witnessed for themselves.

Because the exercise paradox is contradicting centuries of conventional wisdom, it’s still a new area of research with questions yet to be answered. It’s quite possible that energy is neither fully additive nor constrained, but that the reality lies somewhere in the middle. Regardless, based on the best data that is currently available, it seems that regulating calorie intake is far more important than exercise when it comes to weight loss. But again, exercise is still good for you and is very important for human health, even if it doesn’t appear to directly cause weight loss in the way that it was always believed to.

The Paradox of Enrichment

Wouldn’t it be great if there were more resources? It certainly sounds like that would be great, but it turns out it could actually be disastrous. The term “Paradox of Enrichment” was coined in 1971 by ecologist Michael Rozenzweig, as it relates to population ecology.

We generally see two types of predator-prey relationships. Some are cyclical in nature, where increases in the prey population result in increases in the predator population. The predators then begin to run out of food and their population decreases, allowing for the prey population to expand again and the cycle repeats.

While these cyclical relationships do exist in nature, they tend to be pretty rare. It’s what we might expect from a theoretical predator-prey model, but in the real world where things like disease and competition over territory exist, we usually see predator and prey populations reach an equilibrium.

So what if the environment was enriched such that the prey population was able to grow? More importantly, what if resources became so abundant that the prey population was able to grow completely unbounded? That may sound like heaven for the predators, and yet the results of laboratory experiments showed very different results.

One of the most common examples of the Paradox of Enrichment involves two single celled organisms: Didinium nasutum as the predator and Paramecium aurelia as the prey. When the two were put into an environment together, the Didinium would eat all the Paramecium and both would go extinct, as expected.

When a source of food was added for the Paramecium, there was typically some amount of oscillation between the populations, but they would eventually reach equilibrium. However, when an effectively infinite supply of food was added to the environment for the Paramecium, the species were unable to coexist and it was the predator who went extinct, rather than the prey.

Essentially what was happening was that the increased number of prey would cause the predator population to balloon to unsustainable levels. This eventually resulted in the predators all dying off, as prey were able to hide from and evade the competing predators long enough. Rozenzweig described this relationship as a paradox because of the irony that attempting to enrich an ecosystem could instead lead to its instability and even to species extinction.

Fortunately, infinite resources aren’t a thing that exist in the real world. While the Paradox of Enrichment has been demonstrated in labs many times, we don’t tend to see this actually occur in nature. There are also a number of factors in nature that can prevent predator populations from growing to unstable levels as well, such as how difficult the prey in question is to catch and eat, or potential toxicity of the prey that makes eating them less desirable.

Real world factors like these help prevent throwing an ecosystem into upheaval even in the presence of an explosion in the prey’s population. Of course, it’s also quite possible that we could see this paradox destroy a natural ecosystem rather than being limited to laboratory settings, we just haven’t seen it happen with the right predator-prey combination yet.

Key Takeaways

• Peto’s Paradox questions why larger animals with more cells don’t have higher cancer rates.
• The Exercise Paradox suggests that physical activity may not burn as many calories as previously thought.
• The Paradox of Enrichment shows that abundant resources can lead to predator extinction in ecosystems.
• Elephants have multiple copies of the p53 gene, which helps prevent cancer.
• Exercise reduces inflammation, highlighting another health benefit beyond calorie burning.

Sources

• Original Side Projects video: 3 Mindblowing Scientific Paradoxes…

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