Hearts on the Run (or How Parasites Gave Us Love)

We often think of love as a spiritual, emotional or relationship good. Something deeply human, irreducible and perhaps even transcendent. Yet there is a credible evolutionary account that suggests love may not be a cosmic gift or moral ideal at all. Instead, it may be a kind of collateral benefit: a downstream byproduct from one of nature’s most persistent kinds of threats—namely, parasites. Creatures that make their living free-riding on a host without providing any benefit in return.

It is a strange thesis, to be sure. But there is more to the story than mere provocation. Consider that in the Red Queen view of evolution, many of life’s most complex traits—especially those related to sex, reproduction and sociality—are not signs of forward progress in the moral sense, but signs of constant adaptation under pressure. From this perspective, the reason we fall in love, build families and feel heartbreak may be traceable—not to divine design or moral wisdom—but to one of the most enduring features of the natural world: the microscopic arms race between host and parasite.

Running to Stay in Place

The Red Queen hypothesis, introduced by Leigh Van Valen and developed by W.D. Hamilton and others, proposes that species must constantly evolve just to maintain their current level of fitness in a dynamic, co-evolving biosphere. Why? Because the species they interact with, among other threats, are parasites that are also evolving. As fast as organisms improve their defenses, their adversaries improve their attacks.

Parasites, in particular, have a knack for targeting the most common genotypes in a population. Asexual reproduction, which produces genetic clones, is thus a parasite’s dream: predictable, uniform, vulnerable. Sexual reproduction, by contrast, scrambles the genetic deck every single generation (for the most part). That shuffling makes it harder for parasites to adapt to any one host type. It introduces novelty and unpredictability. This is the enemy of exploitation.

This, perhaps, is why so many species reproduce sexually even though it’s wildly inefficient in other respects. Asexual reproduction can be faster, less risky and more energy-efficient. But sexual reproduction has one unbeatable advantage: it throws off parasite tracking.

It is a bit like Alice running next to the Red Queen in Lewis Carroll’s “Through the Looking-Glass:”

“Well, in our country,” said Alice, still panting a little, “you’d generally get to somewhere else—if you ran very fast for a long time, as we’ve been doing.”

“A slow sort of country!” said the Queen. “Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!”

In evolutionary terms, the “running” is genetic innovation, and the “place” you’re trying to stay in is relative fitness. Recombination becomes a survival strategy, a way to stay one step ahead of the enemy that is always adapting to who you were yesterday. But this is only the first link in a much longer causal chain.

Love as a Byproduct of Constraint

Sexual reproduction, for all its parasite-thwarting brilliance, is not cheap. Big-brained primates face a paradox: the larger the brain, the more intelligence and flexibility you get, but the more fragile and dependent your offspring become. Human infants, in particular, are born premature by mammalian standards. We are bipedal, and bipedalism constrains pelvic width requiring that babies be born before their brains are fully cooked. As a result, human children require years and decades of parental investment and protection.

That dependency creates new adaptive challenges. The kind of serial short-term mating that works for many animals is no longer sufficient. To protect offspring, acquire resources and transmit culture, humans needed more stability. Something stickier, like love. Emotional bonding, attachment, pair formation, and kin investment. We are drawn to partners. We cling to our children and form enduring bonds, often at great cost to ourselves. These are not sentimental accidents or arbitrary social conventions. They are rather evolved responses that ensure humans survive long enough for the next generation to do the same.

Why were human offspring so helpless in the first place? Because their brains were large. Why were their brains large? Because intelligence conferred fitness in a highly variable environment. But big brains also require long developmental periods. And those periods were only feasible if parents stuck around. Which only happened if there was selective pressure for pair bonding and emotional commitment. Which, in turn, only became necessary due to sexual reproduction. Here again the Red Queen story emerges as part of the explanation.

And so, love—the fierce, tender, sometimes irrational felt commitment to others –may be at least a partial echo of a long-forgotten evolutionary skirmish between hosts and freeloaders. Not the beginning of the story, but the tail end.

Parasites as Unwitting Architects

We’re not used to thinking of parasites as creative forces. They are the villains of the biological story that are silent, invasive, remorseless. And yet, in evolutionary terms, parasites are as much sculptors as they are saboteurs. They help shape the immune system. They drive the evolution of sexual dimorphism. They influence sociality, jealousy, disgust, even aesthetic preferences—since what we find attractive often tracks markers of health, symmetry and immune fitness.

Evolution lacks intent and design. It is a blind, iterative process: selection pressures, mutations and reproductive outcomes. But the results can be intricate. Parasites, by constantly probing the defenses of their hosts, force those hosts to innovate. Over time, the traits that resist exploitation survive—and so do the social systems that support those traits. The machinery of love may have been one of those systems.

This lens does not strip love of its emotional or moral significance. If love emerged under pressure—if it is nature’s response to nature’s own threats—then it is all the more remarkable. Love is neither a luxury nor an accident. It is a hard-won achievement. A stabilizing force in a world built on contingency and risk. Not only that, but to make such a pronouncement would be to make several highly controversial philosophical assumptions that may or may not be right.

The Contingency of What We Cherish

This story is not meant to be reductive. It is not an attempt to disprove the reality of love, or to explain it away with a clever evolutionary just-so story. It is, rather, a genealogical meditation. Would love have evolved without parasites? Possibly. But the odds and the shape would be different. Without the Red Queen treadmill, we might still reproduce clonally, like many plants and simple animals do. In that world, there would be no sex, no pair bonding, no dependent offspring—at least not in the form we know them. There would be no adaptive need for long-term emotional commitment. And therefore, no reason for natural selection to favor the psychological and behavioral architecture we now call love.

In that sense, love is not universal. It is contingent. A historical solution to a historical problem. One that arose because nature is dangerous, because bodies are vulnerable, because parasites adapt and because intelligence is expensive. That is a sobering, and perhaps inspiring, thought. Love, then, is not the pristine ideal it is often portrayed to be. Nor is it illusory. It is instead a practical strategy with beautiful side effects. Love is perhaps one of nature’s best answers to one of nature’s worst questions.