When someone gets under your skin, you don’t want them there, because they make you forget what it’s like to be you. But what does it mean to be you? If whoever got under your skin went on to lodge themselves there, would they eventually become part of who you are? What if you grew to arrange yourself around them, your actions aligning with their interests? This is the stuff of slow, creeping nightmares, and also the stuff of parasites.

The essence of a parasite is its inability to live on its own. Parasites take up residence in other creatures, siphoning off their hosts’ nutrients to feed themselves. Many will grow, mate, and have babies within their hosts, the hosts suffering from or even succumbing to this imposition. And this already lopsided dynamic often reaches fascinating heights when the parasite needs to make its way between different hosts, or into a new environment. This jump is a gamble, because it depends on bringing a creature into an alien environment, or on tying together the biologies of two creatures that may otherwise never interact. But without risking that jump, the parasite would perish.

Consider, for example, the hair worm. Juvenile hair worms are parasitic, but adults aren’t. Adult hair worms swim freely in streams, but juveniles infect and grow in insects that live on land, insects that tend to avoid water. How then, in the leap from juvenile to adult, does the hair worm get from land to water? By somehow making its host behave in ways that are entirely uncharacteristic of its kind. Terrestrial insects infected with hair worms will wander towards waterbodies and jump right in. Within minutes, an adult hair worm burrows its way out from inside the insect and swims towards the rest of its life. The insect soon dies.

The hair worm has evolved to manipulate its insect hosts into going where the worm needs them to go, at a severe cost to the hosts themselves. Other parasites influence their hosts in even more complex and dramatic ways. Fungal parasites of canopy-dwelling ants induce their hosts to position themselves just off the ground, an area that these ants rarely go to but that happens to be the perfect place for the fungus to reproduce. Once there, an infected ant will bite precisely into a leaf vein — its jaw muscles have atrophied, and it remains stuck. Soon, the ant dies, and the fungus springs into action. A stalk erupts from the dead ant’s head so that the fungus can spread its spores far and wide, reaching new colonies of ants.

A parasitic worm in the tropical rainforests of Central and South America induces the abdomen of its ant host to turn from jet black to bright red. Passing birds mistake this abdomen for fruit, and try to peck at it. Unlike a healthy ant, this red-bellied parasite-vehicle has dulled aggressive responses, and will not attack a pecking bird. Damaged nerves and a weakened exoskeleton mean that the reddened abdomen falls off easily, like ripened fruit. And because these same ants feed bird faeces to their young, the worm’s life cycle continues.

When faced with tales of parasites’ relationships with their hosts, we confront a choice of how to view the world — we can see clockwork precision tinted with destiny, which is how narratives of evolution are often framed, or we can see the power of chance in shaping how life evolved. Peering closer at some of these behaviours that parasites induce in their hosts reveals the extent to which their evolution depends on a series of lucky accidents. Take the infected cricket that jumps into water to let its hair worm escape — if its behaviour were perfectly shaped by the parasite’s needs, the cricket would seek out water. But harbouring a hair worm does not give the cricket an uncanny ability to find water. Instead, infected crickets move more erratically than their healthy counterparts, and just happen upon suitable streams. And fungus-ridden canopy ants don’t resolutely make their way to lower heights — they simply fall, and cannot get back up.

So what do we make of how these happenstances align with parasites’ needs? We consider how many parasites must have gone extinct because their effects on hosts’ behaviour led them too far away from where they needed to be. We emerge from being scared at a host’s loss of agency to marvel at how, over millions of years and across millions of species, natural selection whittles away at those series of accidents. We embrace that the evolution of life on Earth is messy, complicated by which organisms interact and who they were before they met, even as it is fuelled by natural selection — a process that is breathtakingly simple.

I’ve been imagining myself as an infected ant as we leave behind 2017, a year dominated by narratives devoid of concern for our fellow human beings and for the world we live in. If I were that ant, would I see myself as a maverick, sensing some compelling, unappreciated reason to descend from the canopy, or would I recognise my fall as a symptom of something bigger? As we move forward into 2018, it calms me to remember that we can approach the future through messy narratives braided together with simple ones, that while such a confluence can look terrifying in certain lights, it can also bring us somewhere beautiful.

Ambika Kamath is a behavioural ecologist, currently based at the University of California, Santa Barbara; ambikamath@gmail.com

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