You probably believe that merpeople – sirens, tritons – are imaginary creatures, but how can you be absolutely sure? Benjamin Franklin boasted about his rationality, yet in 1736 he informed readers of his Pennsylvania Gazette that a ‘Sea Monster’ had recently been spotted in Bermuda, ‘the upper part of whose Body was in the Shape and about the Bigness of a Boy of 12 Years old, with long black Hair; the lower Part resembled a Fish’. Of course, as a discerning reader, you know that not everything printed in a newspaper is true, but during the first half of the 19th century, the British press reported twenty-four different sightings of merpeople. Would you have been justified in dismissing all of them as false? As their champions argued, perhaps you had never been lucky enough to be in the right place at the right time.

In any case, over-scepticism can bring its own problems. In 1797, when the first stuffed platypus arrived from Australia, London’s top zoologists condemned it as a Chinese fake; the specimen preserved in the British Museum still shows marks from their attempts to prise off the bill, which they thought was artificially attached. Like merpeople, pterodactyls and centaurs, platypuses were borderline creatures that challenged accepted classification systems for organising the world. Deciding whether a mermaid was a fish, a woman or a fantasy required grappling with questions of what it meant to be a human being.

Whatever their ontological status, these elusive, slippery phenomena have been swimming around human cultures for millennia. Vaughn Scribner’s Merpeople is a comprehensive catalogue of our marine counterparts. Yet – to paraphrase Arnold Toynbee – this is no mere history of one damn merperson after another. Scribner has structured this study chronologically, emphasising the importance of medieval Christianity and its global imperialising influence.

Scribner launches his quest by tackling an obvious question: why are merpeople overwhelmingly female? While Babylonian and Greek mythology featured piscine gods of both genders, he writes, early church leaders emphasised woman’s depravity by reworking ancient myths of monstrous harpies. As Scribner puts it, the ‘Christian Church was on a mission to dethrone femininity’. In religious buildings all over Europe (the fan-vaulted roof of Sherborne Abbey, for example), carvings survive of mermaids with flowing hair to represent fertility and with mirrors and combs in their hands to symbolise worldly vanity. Some of them even have their tails split apart to exhibit their genitalia. Like pictures of Eve tempting Adam with an apple, these visible messages were designed to reinforce the notion that women are intent on luring men away from the straight and narrow.

By the end of the 15th century, mermaids had been so successfully incorporated into Christian culture that their existence seemed no less real than that of elephants, rhinoceroses and other unfamiliar animals reputed to live in strange lands far away across the oceans. Cartographers drew them frolicking around foreign coastlines; opportunistic merchants brought back dried specimens, which were snapped up by wealthy noblemen for their cabinets of curiosities; detailed illustrations appeared in learned scientific journals.

Scribner’s analysis falters somewhat when he explores the question of how different cultures and traditions influenced each other. A snake’s forked tongue might or might not be the source of a mermaid’s split tail. Are Hindu depictions of a male god emerging from a fish’s mouth necessarily related to Christian images of a fish penetrating a woman sexually? Juxtapositions are fascinating but do not prove relationships. However, Scribner does convincingly suggest that as Europeans spread their influence around the world, other civilisations gradually absorbed Christianised mermaids into their own rosters of marine hybrids.

Debates about the origins of fossils added further layers of complication. Had they been produced by God at the creation of the world, or were they proof that strange creatures used to roam the earth and had now become extinct? As Thomas Jefferson pointed out, denying the existence of mermaids and dragons requires the same level of proof as showing that a species is extinct. ‘Have all parts of the earth been sufficiently explored to authorize a confident assertion?’ he wondered.

Preserved bones and shells bolstered early theories of evolution long before Charles Darwin was born, but so too did supposedly confirmed sightings of merpeople. Speculation circulated that life originated beneath the ocean, in the form of minute organisms that gradually developed into fish, which in turn evolved into land-based creatures. In this scenario, merpeople were the intermediaries between fish and humans. However improbable such theories might now sound, it’s worth remembering that Darwin himself put forward the equally bizarre conjecture that whales had developed from black bears swimming for hours with their mouths wide open to catch insects.

Scientists continued to weigh up the arguments for and against the existence of merpeople until the middle of the 19th century. The question was definitively resolved in 1843, when a group of scientists demonstrated that one of the most famous examples, the showman P T Barnum’s ‘Feejee Mermaid’, was a clever combination of a monkey’s head grafted on to a fish’s body. Mermaids then migrated into the realm of fantasy, but that did not make them any less popular. On the contrary, they underwent a positive revival, proliferating on film posters, as well as in sexually suggestive advertisements for men’s shirts, tonic water and tuna fish.

Scribner reminds us that the world’s most aggressively expansionist coffee chain took its name from a character in Moby-Dick and that its logo was based on the mermaids in medieval churches. Over the last five decades, Starbucks has progressively sanitised the image: originally, the mermaid’s breasts were exposed and her tail was split suggestively. Whether you fancy a quick dip or a marathon swim, this is a delightful book to splash around in, a gloriously illustrated and meticulously researched study of our closest aquatic relatives.