Why Apples Want to Be Eaten
There’s a mutually beneficial relationship between you, your gut bacteria, and fruit. So, what’s in it for the apple?
Most living things spend a lot of time and effort trying to avoid being eaten. The cactus has its spines, the turtle has its shell, and organisms which lack such physical defenses are often full of chemical deterrents—like the bitter and astringent tannins in an acorn. But in this regard, fruits stand apart from other living things, in that they seem to want to be eaten: Everything about them, from their bright colors to the easy nutrition they provide, is an invitation. Why?
The simple answer is that, for the seeds of fruiting plants, being eaten is not the end. It’s an essential part of the life cycle. It’s how new trees get planted. It’s a case study in mutualism, because the easiest way for a fruiting plant to improve its own fitness is by improving the fitness of the animals that eat it—and in this, there’s a window not only into our evolutionary relationship with plants, but also into symbioses more generally, including the mutually beneficial relationship between humans and their gut bacteria.
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What Humans Have That Apples Need
One sentiment that’s common to nearly all life forms is that living in your parents’ basement can be kind of a bummer. This is a particularly pressing problem for plants, because they’re effectively immobile: When a plant drops its seeds straight down to the ground, it creates a situation that’s bad for both the parent (which now has to compete with the budding plant for water and nutrients) and the offspring (which is shaded from vital sunlight by its parent’s leaves). Evolution has yielded countless solutions to this problem, each more interesting than the last. Think about the seed of the maple tree—with its delicate helicopter wing, engineered to provide horizontal distance as it falls—or the dandelion, which can travel great distances on a gentle breeze.
But nowhere is the creativity of nature better on display than in the design of endozoochorous plants—those which rely on animals’ digestive tracts for seed dispersal. These are the plants which, rather than trying to solve the problem of mobility for themselves via clever engineering, have opted instead to strike a deal. They have something we need—carbohydrates—in abundance. They’re able to spin sugar out of nothing but thin air and sunlight. Meanwhile, we have something they need: legs.
It’s a match made in heaven.
By wrapping their seeds in a coat that protects them against digestion, and then in an extra layer of something sweet and nourishing, fruiting plants have found the simplest and most effective way to disperse their offspring far and wide: getting eaten. And not only does this help ensure that the seeds end up far from the mother plant, it also guarantees that when they’re deposited back into the soil after their journey through an animal’s GI tract, they’ll find themselves conveniently located in a pile of fertilizer—giving them the best head start a plant can hope for.
But this means that a fruit only wants to be eaten by certain creatures, at certain times. An apple that is eaten by worms or rodents doesn’t see any benefit, because these creatures are too small to swallow the seeds whole. Likewise, a fruit doesn’t want to be eaten before the seeds have developed. As a result, fruits are highly specialized to discourage the “wrong” animals from eating them, or eating them in the wrong way. Think of the banana, its peel an astringent admonition, its flesh starchy and unappetizing until it’s ripe. Apple seeds notoriously contain a chemical that can turn into the poison cyanide, but this only occurs if you chew them—swallowed whole, they’re harmless.1
Symbiosis: An Evolutionary Argument For Cooperation
In its simplest form, the relationship between fruiting plants and the animals that are supposed to eat them (their “dispersal vectors”) is almost like a trick, or a transaction: sugar in exchange for seed dispersal. But if this were the only incentive at play, fruits might not be the miracle that they are. In a purely transactional relationship, you’d expect plants to make the minimum investment: to produce fruit which contains the smallest possible amount of sugar that could still entice an animal to eat it.
But, over millennia, this kind of transactional dynamic paves the way for a genuine symbiosis to emerge. When a plant comes to rely on an animal for seed dispersal, their fates are tied together, and the plant starts to “want” the animal to be healthy. A healthy animal ranges farther and faster, meaning the plant’s seeds are more likely to end up far away from home, increasing the plant’s geographic spread. Not only that, a healthy animal is more likely to have offspring of its own, which means more dispersal vectors for the plant.
This kind of intimate symbiosis, where a plant improves its own fitness by improving that of its animal friends, is everywhere. The polyphenols in the peel of an apple simultaneously dissuade insects from chowing down on it while acting as health-promoting antioxidants for larger animals like ourselves.2 Bananas, which are highly specialized for dispersal by apes, contain specialized proteins that help fight off some of our species’ oldest enemies, like the virus that causes herpes.3
From a classical, purely “selfish” view of evolution, this sounds almost too good to be true. But seen through the lens of our eons-long relationship with these plants, it makes perfect sense: If humans and all the rest of Earth’s apes went extinct tomorrow, the banana would likely soon follow, because fruit-bearing plants have become almost entirely dependent on animals like us for survival and success. The fact that a diet rich in a variety of fruits is one of the easiest and best ways to stay healthy is no coincidence—it’s the result of millions of years of evolutionary pressure. Competition is still the driving force behind evolution by natural selection, but the strength of symbiosis is a reminder that in any competition, cooperation is often the most powerful strategy.
The Key Role of Our Gut Microbiome
Fruiting plants aren’t the only organisms we have this kind of dynamic with, though. The relationship between humans and our gut bacteria has a largely similar structure. Many of our gut microbes are passed down reliably from generation to generation,4 and are found nowhere on Earth besides the human gut. When we eat, they thrive. If we die, most of them do too, because the majority are strict anaerobes, meaning they can’t grow in the presence of oxygen. For our gut bacteria, the best shot at continued survival is to ensure that their animal hosts (you, for example) live long lives and have lots of offspring. In that sense, our relationship with our gut bacteria is almost the same as our relationship with our own genes.
Without this context, it’s easy to see why some are skeptical of the ever-growing body of research showing that the microbiome is one of the most powerful modulators of human health: It seems implausibly ideal.5 But seen through the lens of cooperative evolution and the powerful incentives at work, it’s clear that the benefits of maintaining a healthy gut microbiome are likely to be just as great as—or even greater than—those of a fruit-rich diet, which few would dispute is one of the best ways to improve our health.
Of course, it’s hard to have one without the other. Fruit isn’t just the embodiment of our symbiotic bargain with plants, it’s also critical to maintaining a healthy microbiome. In most people’s diets, fruit is a key source of fiber, which gut bacteria ferment into health-promoting short-chain fatty acids like butyrate.6 The polyphenols in fruit provide selective food for certain beneficial gut bacteria, and in consuming these chemicals, our microbes help transform them into more bioavailable versions that the body can absorb.7
In this, there’s a reflection of the interconnectedness of biological systems, and the importance of microbes in all of them. If a plant wants to encourage its dispersal vectors to be healthy, oftentimes the easiest way to do so is by encouraging a healthy microbiome in the animal—with the added bonus (for the plant) that its seeds will end up in a more diverse pile of “compost” when it’s excreted back into the soil. Microbes may also be key to the health benefits of fruit in another way: Recent research has revealed that fruits contain a diverse array of bacteria.8,9 Although more research is needed to understand how much of the health benefits of fruit are attributable to the fruit’s microbes, there’s reason to believe it may be a substantial portion. Bacteria are like modular packets of genes, which allows organisms to “adopt” these genes, rather than relying on random mutations to the genome in order to develop fitness-enhancing new traits.
The three-way relationship among animals, plants, and microbes has evolved over tens of millions of years, but it’s undergone some drastic changes in the past few millennia. With the invention of agriculture and indoor plumbing, we’ve broken our end of the bargain in some respects. Seedless varieties of grapes now populate the shelves of grocery stores, and clonal bananas are picked green before being chemically ripened once they reach their destination. Of course, the plants haven’t caught on to this fact. Nevertheless, it’s worth behaving as if they might. The point of a fruit’s existence is still to get you to swallow its seeds, and this is reflected in its structure and biology: An apple’s core contains denser fiber, and more of those potentially probiotic bacteria, than any other part of the fruit.8
So, if you find yourself wandering the rows of an orchard this autumn, enjoying the fresh air and the crisp flesh of an apple, take a moment to appreciate this everyday miracle, and to honor the ancient relationship that gave rise to it. Eat the core. Swallow the seeds. Just don’t chew them—the tree probably wouldn’t like that.
- Bolarinwa, I. F., Orfila, C., & Morgan, M. R. A. (2015). Determination of amygdalin in apple seeds, fresh apples and processed apple juices. Food Chemistry, 170, 437–442. https://doi.org/10.1016/j.foodchem.2014.08.083
- Singh, S., Kaur, I., & Kariyat, R. (2021). The Multifunctional Roles of Polyphenols in Plant-Herbivore Interactions. International journal of molecular sciences, 22(3), 1442. https://doi.org/10.3390/ijms22031442
- Batcha, A. T. M., Wadhwani, A., & Subramaniam, G. (2020). In vitro antiviral activity of BanLec against herpes simplex viruses type 1 and 2. Bangladesh Journal of Pharmacology, 15(1), 11–18. https://doi.org/10.3329/bjp.v15i1.42320
- Ley, R. E., Hamady, M., Lozupone, C., Turnbaugh, P. J., Ramey, R. R., Bircher, J. S., Schlegel, M. L., Tucker, T. A., Schrenzel, M. D., Knight, R., & Gordon, J. I. (2008). Evolution of mammals and their gut microbes. Science (New York, N.Y.), 320(5883), 1647–1651. https://doi.org/10.1126/science.1155725
- Walter, J., Armet, A. M., Finlay, B. B., & Shanahan, F. (2020). Establishing or Exaggerating Causality for the Gut Microbiome: Lessons from Human Microbiota-Associated Rodents. Cell, 180(2), 221–232. https://doi.org/10.1016/j.cell.2019.12.025
- den Besten, G., van Eunen, K., Groen, A. K., Venema, K., Reijngoud, D. J., & Bakker, B. M. (2013). The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism. Journal of lipid research, 54(9), 2325–2340. https://doi.org/10.1194/jlr.R036012
- Cardona, F., Andrés‐Lacueva, C., Tulipani, S., Tinahones, F. J., & Queipo-Ortuño, M. I. (2013). Benefits of polyphenols on gut microbiota and implications in human health. Journal of Nutritional Biochemistry, 24(8), 1415–1422. https://doi.org/10.1016/j.jnutbio.2013.05.001
- Wassermann, B., Müller, H., & Berg, G. (2019). An Apple a Day: Which Bacteria Do We Eat With Organic and Conventional Apples?. Frontiers in microbiology, 10, 1629. https://doi.org/10.3389/fmicb.2019.01629
- Zhang, H., Serwah Boateng, N. A., Ngolong Ngea, G. L., Shi, Y., Lin, H., Yang, Q., Wang, K., Zhang, X., Zhao, L., & Droby, S. (2021). Unravelling the fruit microbiome: The key for developing effective biological control strategies for postharvest diseases. Comprehensive reviews in food science and food safety, 20(5), 4906–4930. https://doi.org/10.1111/1541-4337.12783