We think of young animals as miniature versions of adults they are to become. For the vast majority of animals, though, this is not the rule. Think caterpillars, which probably undergo one of the most radical changes in body plans. Butterflies carry the genes to make the caterpillar body plan, and vice-versa. So do tadpoles. But in an axolotl, the adult phase of life is completely dropped. Unlike fellow amphibians which lose their gills and “grow up” to live on land (such as frogs and salamanders), axolotls spend all their lives in water, and don’t lose their gills. At the same time, they also possess sex organs which allow them to breed. This mutation may have served some ancient salamander (to which axolotls are related) to survive a severe food shortage, as tadpoles require fewer calories than “adults”.
An experiment conducted by the German scientist, Laufberger proved that the axolotl was an “adult larva”. He showed it by injecting an axolotl specimen with a growth hormone, which created a species never before seen. This new axolotl-salamander had lost its gills and even closely resembled a tiger salamander – its closest living relative – in colouration, but had elongated toes and other minor differences – enough to classify it as a separate species. This experiment was again famously repeated by the British biologist, Julian Huxley, who didn’t know that it had already been done before.
Paedomorphosis is one of those powerful ideas you start seeing everywhere you look, once you’ve got the hang of it. What does an adult ostrich look like? Most Africans, on seeing an galloping adult ostrich for the first time yell, “Big chicken!” They are most probably right. Ostriches are unique, flightless, and they possess weak downy feathers (for which they are ruthlessly hunted to make pillow-stuffing). How did they get rid of the wings and the large, strong feathers that help in lift-off? You guessed it – paedomorphosis. The genes that influence the growth of wings and flight feathers in ostriches never switch on, along with a host of other genes. This gives the adult ostrich such a comical “chick” like appearance.
Is it a coincidence that baby apes look more humanoid than adult chimps and orangutans? Most scientists now believe that our own “beautiful” appearance: the reduced eyebrow ridge, our smooth, (relatively) hairless skin, is nothing more than a mutation that allowed ancestral humans to retain childlike characteristics. Some even go so far as to say most characteristics that make us human such as our penchant for learning (and play) even in our later stages of life, our large memories and retention capabilities and even our bipedality are all due to neoteny. Although difficult to prove very conclusively, they are powerful arguments. Take for example the recently evolved tolerance to milk among adult humans (most adult mammals are lactose intolerant). Early humans couldn’t digest milk as adults – they had genes which carried orders to prevent the production of lactase (the enzyme which digests milk sugars) after a certain age. But over time, these genes either switched themselves off or mutated, allowing even adult humans to enjoy milkshakes and ice-cream that only kids could, a few thousand years ago.
This idea that humans are nothing more than juvenile apes was the central theme to an Aldous Huxley (famous for his other dystopian science-fiction novel Brave New World) novel After Many a Summer. Aldous is the brother of Julian Huxley, whom we met earlier, and was obviously acquainted with his brother’s work on axolotls. In Summer, an American Millionaire is paranoid about death and hires a mad British Scientist to fix him a cure. When some 200 year old journals of the Earl of Gonister are discovered by the scientist, the entries in it indicate that the Earl may still be alive as he has made himself immortal by eating barrelsful of rotting fish guts. Intrigued, the scientist and millionaire rush to the Earl’s residence to actually find the Earl alive, but as a grotesque ape. The Earl has finally “grown up” and now spends all his time scratching his quadrupedal and hairy body, and urinating on the floor, barely able to communicate. The novel ends on a disconcerting note, as the millionaire is ready to eat the fish guts and become an ape, just to live for two hundred years.
If you think about it, neoteny is the underlying factor in so many evolutionary changes. If humans are juvenile apes, dogs are neotenised wolves. While breeding for dogs, humans applied artificial selection pressures on wolves which allowed them to retain some of their puppy like characteristics which we have come to like about dogs. The floppy ears, the playful nature, even the odd dog-fur colours: golden, black and spotted (wolves are almost uniformly “wolf-gray”), are due to neoteny. Dogs, therefore, are eternal puppies; they lack the need to dominate and are readily subservient to their human masters – the main features which distinguishes them from wolves. These neotenic characteristics are seen in the extreme in toy dogs such as chihuahuas or the pekinese, where their large eyes and squashed muzzles are meant to imitate (supposedly) human babies.
One of the most important “natural neotenic accidents” (from the human point of view at least), is the evolution of chordates itself. The nearest living relatives of the chordates are marine creatures called sea squirts. A typical sea squirt is a bag anchored to the sea bottom, with two siphons at either end which pump sea water, serving as the circulatory system, a rudimentary gut and reproductive organs. A sea squirt doesn’t resemble a fish or other chordates such as lampreys in anyway. An adult sea squirt that is. Larval sea squirts are so like the tadpoles of amphibians, they are called ‘tadpole larva’. Like many larvae of sea creatures, these tadpole larvae live among plankton, freely swimming with tails that move exactly like the tails of axolotls. But unlike axolotls, these tadpole larvae undergo metamorphosis by attaching to the sea bottom, mouth first, and recycle all their larval organ systems: the nervous, digestive and even their tails to lead the simple life of adult sea squirts, leading to the popular myth that sea squirts become adults by “eating their brains”. (Maybe a future Aldous Huxley will base a story around how future humans finally “grew up” by attaching themselves to their couches and digesting their brains while watching television, or surfing the internet.)
Returning finally to the axolotl, whose tale this was, scientists now think that the axolotl may have actually undergone many paedomorphic and reverse-paedomorphic evolutions throughout history. Maybe evolving animals in general are continually, perhaps less dramatically than the axolotl, moving one way or another along an axis of paedomorphosis and reverse paedomorphosis. It certainly is an interesting idea.
Maitrey Deshpande (Mechanical)
Illustration by Jayanth Vadyala (Architecture)