“If you cut them too early, they will ripen very slowly, if too late, some fruits will fall off during cutting and rot before you know it,” Eli explained. Being the owner of such a beautiful orchard, Eli had to know how and when to cut bananas.

One day, when I returned to the base after exploring the Costa Rican jungle, a huge bunch of bananas hung over the camp table. My Spanish was just warming up so the fruit were from “someone” in gratitude for “something”. People are undisguised racists about bananas. My friend Luc tolerates only the light green ones, my roommate always awaits the transition from green to yellow, and my parents’ friend is a fan of the black ones. I look for the evenly yellow ones. As you can see, the ones in the photo were not ready yet. Except for the one and only one at the bottom… This August Landmesser of bananas, this allegory of nonconformity just right for a banal internet meme, had haunted me.

I was hoping that the ethylene emitted by the ripe banana would cause the neighboring fruit to change their flag. I knew that the physics were against me, as the air, infinite reservoir, would dilute the ethylene to practically nothing. Days went by and the other bananas were not ripening. The bunch had been cut too early. But I was still perplexed why this one and only one banana had deserted. Why had it broken symmetry?

Symmetry breaking is not only a figure of speech. It is also a defined set of processes of fundamental importance in physics and biology. In a broader context, in nature, disruption of the regular pattern often conceals a puzzle, the solution of which opens up new perspectives. This was also the case here. When I finally decided to consume the renegade, it turned out that some insect’s larvae had nested inside. What a beautiful twist! August Landmesser turned out to be a wormy element, rotten from the inside. A history for a politburo!

One could end the story here and say: the mystery has been solved and my curiosity is satisfied! 

Really?

Similar stories end at this point for most non-biologists and, unfortunately, also for some biologists: it turned out that the banana was yellow because there were worms inside.

Indeed, it would be quite a coincidence if the wormy banana had just changed color by chance. However, if one banana in the bunch had parasites, there was a good chance that the others were infected too. If all the bananas were wormy, the color change would probably not be worm-related. I did not do a control experiment to see if the green bananas were healthy. Would one healthy green banana prove my assumptions? How many bananas would I need to check for the power of the test (a technical term) to be high? Hundreds of statistical methods have been developed to address these non-trivial questions.

Suppose I screened all the green bananas and only the yellow one turned out to be wormy. Does this support my conclusion: the banana was yellow because there were worms inside? The opposite is also possible: there were worms inside, because the banana was yellow. I did not know what the sequence of events was. The banana could change color before it was attacked. If green bananas can defend themselves more effectively against attacks or yellow bananas are a more attractive morsel for insects, then such a scenario would have a strong foundation. Of course, the question then remains why the banana changed its color. The mystery remains unsolved. This shows that science, and biology in particular, often faces the serious problem of establishing causality. The concept itself is methodologically difficult to define.

There is another problem with causality, but in a more subtle, biological sense. Namely was it the banana that changed its color, or was it the parasite that caused the banana to change its color. Parasite-host relationships are one of the most fascinating topics in ecology and evolutionary biology. Parasites have evolved incredible methods of manipulating the physiology and behavior of the host. Hosts, especially primary hosts, have also evolved methods of fighting the parasite. It is often not clear what we are dealing with. For example, is coughing the result of a struggle between immune cells and the virus, or is it a behavior caused by the virus to spread throughout the population? A similar question can be asked about our bananas. Perhaps the larvae, due to their appetite for simple sugars, secreted compounds that activated the genetic program of fruit ripening. Or maybe it was the fruit that turned on this program to fall away from the bunch as soon as possible and prevent further invasion?

The answers to these questions may already be known, but that’s not the point. There are many such questions and possible scenarios. The point is, the next time you see or hear the memically motivating “Be special,” remember that it is a non-trivial thing.