Is there something fishy about mermaids? Yes, but maybe not their tails. Look again: their tails are held perpendicular to the plane of their bodies.
Evolution is played by rules
On the Origin of Species notwithstanding, nature may indeed jump. Darwin cited the old saw natura non faltet saltum, insisting–gratuitously, as even his “bulldog” Thomas Henry Huxley noted–that evolutionary changes had to be infinitesimally small and gradual. But even if nature does sometimes make radical adaptive changes on a relatively brief time-scale (a geological not a human one, of course), it doesn’t jump tracks. Having once proceeded down a particular branching of paths, it can’t skip back to before the point of divergence.
The evolution of species and the development of individual organisms are directional: you can’t go home again. Hox genes tell a developing fruit fly, first make a head end, then a middle, then a tail end. And not just fruit flies. Anciently conserved Hox genes gave the same message to your developing embryo and mine. Even yeast have Hox genes. So, while biology can build many wondrously complex organisms by the iteration of a limited number of simple processes, half-humans/half-fish are not among them. The trains that carried away fish and mammals left the station heading in different directions too long ago.
But, despite descriptions of mermaids as half-human/half-fish, depictions of mermaids, reveal their tails, though often scaly, are not so piscine after all. While they may be no less impossible for it, mermaids’ tails are marine mammal – rather than fish-oriented.
“There’s a porpoise close behind me… “
Have you noticed that, while the tails of fish are held vertically, in the same plane with their bodies, those of whales and dolphins, seals and sea lions, manatees and dugongs are held horizontally, perpendicular to their bodies? Have you wondered why?
The lineages that evolved into fish, ancient and modern, and those that evolved into tetrapods, all the four-limbed creatures from dogs to dinosaurs and pachyderms to pelicans, began diverging sometime after 530 mya (million years ago), when vertebrates, animals with spinal cords enclosed in a bony column, first appeared in the world’s oceans. This divergence occurred during the rapid diversification of animal life known as the Cambrian Explosion: those first vertebrates would have been the last common fish/human ancestors.
Tetrapods are also remarkable for having made the marine-terrestrial transition, adapting to ‘permanent’ land-based living. Some fish, of course, can breathe oxygen from air and live on land for extended periods, while among the tetrapods, amphibians (toads and frogs) still hatch their eggs and spend at least parts of their life cycles in (fresh or brackish) water. A number of modern reptiles–axolotl lizards, crocodilians (crocodiles, gaurs and alligators), sea snakes–have reverted entirely or nearly so to an aquatic way of life, some even in salt water.
Return to before Eden
Re-adaptation to ocean life by terrestrial species actually seems fairly common. Three extinct orders of reptiles contemporary with the dinosaurs returned to live in marine environments. Ichthyosaurs developed forms similar to modern whales and dolphins. Plesiosaurs resembled giant sea turtles… if you imagine a 15-foot long, viciously carnivorous, shell-less turtle weighing up to half a ton. Mosasaurs, the last to evolve, may have come from an ancient amphibian.
Four extant kinds of mammals have also adapted completely or partially to marine life. The orders Cetacea–whales, dolphins, porpoises–and Sirenia–manatees, dugongs–are completely aquatic: obligate ocean dwellers. Their ability to regulate oxygen, water and heat are compromised out of water and their locomotion is severely restricted; their vestigial hind limbs are buried deep within the blubber of their flukes (tails).
Pinnepedia–seals, sea lions, walrus–sometimes considered to form their own order and sometimes classed as a sub-order of the Carnivora, are semi-marine. While they feed exclusively at sea, they return to land (or ice flows) to reproduce and nurse their young, to rest, to bask in sunshine.
Seals are the most fully sea-going among them, like the cetaceans having combined hind limbs and tail. While the limbs of sea lions and walruses, like the forelimbs of seals, have evolved into flippers–paddles for swimming–they remain distinct external structures and can be rotated from a swimming position to a walking one.
Two other living carnivores also developed a semi-marine way of life–the polar bear (Ursus maritimus) and the sea otter (Enhydra lutris). (Fresh water otters and various rodents–muskrat, nutria, beaver–are more or less aquatic, but not equipped to live in salt water.) Polar bears and sea otters have retained fully articulated, external limbs with distinct digits; they generally move about as well aground as aswim.
While the polar bear feeds as well as reproduces on land or ice flows, its over-sized paws, thick blubber and insulating hair are as well adapted for the frigid Arctic Ocean as for life on ice and snow. Its body shape is more elongated than that of other bears and, as well as insulation, blubber provides it a personal flotation device, perhaps sea-going adaptations as well: polar bears have been seen in the open ocean as far as 200 miles from land. A seal-eating specialist, it couldn’t survive away from the sea in any case.
The sea otter is even more fully marine: it spends the vast majority of its time in the water, dives for invertebrates, even feeds aswim while floating on its back. Its hind feet are flattened and webbed and it uses its heavy, flattened tail extensively in swimming. It is as closely entwined with the sea urchin as are polar bears with seals. Where sea otters have been over-hunted for their rich, densely furred pelt (they don’t keep warm by accumulating blubber), the undersea kelp forest is threatened by uninhibited urchin reproduction.
The art of the possible
In common to all these marine mammals is a spine that flexes up and down: they were runners pushing off against gravity before they became swimmers. Consequently, they still hold their tails perpendicularly to the planes of their bodies and move them up and down to propel themselves through water. You couldn’t ask for a clearer demonstration that evolution is contingent; in fashioning new adaptations, it works with what’s at hand.
Fish, however, evolved directly from the first marine vertebrates. The backbones of their ancestors never adopted the dorsal/ventral articulation of land-dwellers. Instead, their spinal columns flex side-to-side, a more efficient motion for swimming which confers the added streamlining advantage of keeping the tail in the same plane as the body. (If I understand correctly, snakes are actually ‘caught’ somewhere in between. Their apparent side-to-side spinal flexion is actually back-to-belly, only torqued some degrees off the horizontal: another ‘vestige’–like remnant internal leg bones–of their origins as legged lizards,)
And mermaids? Look at almost any illustration, high art or low, of how a mermaid holds her tail; it’s perpendicular to her body. It would have to be; she has a mammalian spine. It’s not only evolutionary biology that rules out a half-human/half-fish; physics does as well. Can you really imagine a spine that articulated front-to-back for half its length then side-to-side for the rest?
Mermaids aren’t real, Animal Planet buffoonery notwithstanding, but the idea of mermaids is very real, longstanding and widespread. Where does it come from?
The world’s mythologies are full of part-human/part-animal creatures. Egypt gave us falcon-headed Horus and the half-human/half-leonine Sphinx. Greece imagined the human-horse hybrid Chiron the Centaur who taught medicine to the human race and the horned and goat-legged Pan, spirit of nature and sexuality. The Dogon of Mali in Central Africa explain their knowledge of astronomy as a gift from the Nommos, amphibious, fish-skinned humanoids who came to earth from the star Sirius and the Inuit of Eastern Canada and Greenland feared Adlet (Erqigdlat), a tall and savage dog-legged people who warred unremittingly against humans (I.e., Inuit) and who may have been of a wave Amer-Indian people who migrated across Canada long after Inuit settlement.
Although many peoples have imagined some kind of human/sea creature hybrid, the mermaid per se seems to be largely an Indo-European construct. The ancient Semitic fertility goddess Atargaris/Astarte/Ishtar, who was associated with the sea and sometimes portrayed as a woman-headed fish, was appropriated by the Greeks as an aspect of Aphrodite and seems to have provided the armature for mermaids.
Greece also provided the earliest known reference to a human female transformed into a sea creature. Thessalonike, a younger half-sister of Alexander the Great, was so grief-stricken at the word of his death; she tried to drown herself in the ocean. But Alexander had previously bathed her hair in immortalizing water–she did not drown but changed into a mermaid. She is said to have plagued Aegean sailors for centuries, demanding to know whether Alexander was really dead and attempting to drown those who failed to answer that he still lived and reigned.
European belief in mermaids appears to have been boosted by the discovery voyages of Christopher Columbus and Henry Hudson. Sailing off the coast of Hispaniola (Cuba) on his return voyage to Spain, Columbus wrote: “Sighted three Mermaids on Jan. 4, 1493. The creatures rose high out of the water, but were not as beautiful as I had expected.” These are generally thought to have been West Indian manatees (Trichechus manatus), but Columbus also reported mermaids off the coast of Africa, possibly the African manatee (T senegalensis).
In 1608, Hudson logged an even more detailed sighting. “This morning, one of our company looking overboard saw a mermaid, and calling up some of the company to see her, one more came up and by that time she was close to the ship’s side looking earnestly upon the men. A little after, a sea came up and overturned her. From the navel upward, her back and breasts were like a woman’s, her body as big as one of us; her skin very white, with long black hair hanging down her back. When the mermaid finally went down under the waves, her tail was observed, which was like that of a porpoise and speckled like a mackerel.”
Whatever Hudson’s mermaid might have been–and it displayed a porpoise tail–it certainly wasn’t a manatee. On 15 June, Hudson was far from tropical seas, searching for a passage through the Arctic ice off the eastern Siberian Peninsula. The only local marine mammals that come close (but not very) could have been an immature Beluga whale (Delphinapterus leucas) or a Spotted seal (Phoca largha) perhaps.
Were-seals or selkies
Seals contribute another dimension to European mermaid beliefs. Both Norse and Gaelic folklore tell of were-seals called variously selkies, silkies or selchies. Although there are male selkies, most of the tales focus on females-seal-wives-who are either drawn towards humans or tricked and captured by them. A selkie shape-changes into a human by taking off its sealskin: whoever possesses the skin controls the selkie. Sometimes they fulfill the mermaid role of alternately warning sailors and fisherman of approaching storms or shoals, or wickedly drawing mariners on to a watery grave. At any given time, however, these creatures physically appear as seals or as humans. However divided their spirits, they never appear as half and half.
Are mermaids sea-going yeti?
The three orders of marine mammals Cetacea, Sirenia and Pinnepedia began their evolutionary journeys back to the sea between 15 and 50 mya, about the same time the ancestors of modern apes and humans were beginning to diversify. While polar bears are believed to have diverged from other ursids about 5 mya and sea otters from the line of mustelid weasels about 2 mya, their ancestral lineages first became distinct among Carnivora 15-40 mya. Given all that elapsed time, a human/marine-mammal hybrid is just as evolutionarily impossible as a half-human/half-fish.
It is at least conceivable, however, that an ancient hominid (the lineage of apes and humans) abandoned the land and re-adapted to life at sea: there might just be enough evolutionary time for that to have been possible. The only problem is, beyond reported sightings, there is no evidence that such an event ever occurred: no fossils, no mermaid DNA to compare to ape or human. (A recent examination of the DNA of supposed skins of yeti (Tibet’s abominable snowmen) shows them to be mostly bears, interestingly, of an ancient polar bear type.) Given all the physical evidence available for even the rarest marine mammals–observations, photos and recordings, specimen–the solely sight-supported ‘records’ for mermaids seem a bit thin on the ground, Or, should that be, dilute in the water?