If you’ve ever seen the film Shallow Hal, you may remember the scene where Jason Alexander’s character reveals that he actually has a small tail hidden underneath his clothes. In the film this is used as a character explanation for his shallow and chauvinistic behaviour. But what you maybe didn’t realise is that this is actually a real phenomenon.
And it can actually it shed light on the very nature of our genes and our evolutionary heritage.
More People With Tails
In reality, it is very rare for a human to be born with a tail; but there are nevertheless plenty of examples of it happening in modern literature (numbering into the hundreds). In most cases, these are “pseudo tails” – tails that are malformed and generally useless.
But in about 1/3rd of examples these are full-blown tails that are capable of being moved and everything. These tails contain bundles of nerves, blood vessels and connective tissues and are covered in regular skin. If you’d never seen a human before you would have no reason to believe that the tails weren’t meant to be there!
And, when you think about it, it’s kind of an example of a real life superpower. One of many such examples, actually.
In the womb we actually have tails during one stage of our development, but those tails disappear before we are born. The question is why in some cases those tails remain and develop after birth.
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Atavism
The explanation for this is something known as “atavism”. Atavism is the term used to describe a physical genetic “throwback.” That is to say that, in this case, humans would once have had tails (it is widely believed that we resembled squirrels early in our evolutionary history) and as such some of the genetic code remains in our DNA.
All of us have the capacity to create tails but a few evolutionary twists and turns (successful genetic mutations) have resulted in them failing to appear. In cases of atavism, a small mutation undoes this more recent evolution thus setting back the clock and allowing those genes to express themselves, again.
From a survival-of-the-fittest standpoint, this is kind of good design. It let’s us check whether those traits might be more useful at a given point in history.
So, inside your DNA is a lot of unexpressed code and among that code is the ability to form a fully functional tail. In cases of human tails that code has been unleashed and the tail has formed.
Note that this is a different phenomenon than vestigiality – which describes cases where an organic structure has lost its original purpose but not yet been eliminated.
Other Examples of Atavism
If this explanation of human tails is accurate, then you might expect to see other examples of atavism in humans and throughout nature. And as it happens… you’d be right!
A popular illustration of atavism is the rare occasion where a whale or dolphin is found with legs. Both whales and dolphins are mammals, rather than fish, and thus they resemble us more closely in many ways (it’s why they have lungs and need to surface occasionally to breathe).
At some point in their evolutionary past, their ancestors must have decided that life was a bit more cushdy under the sea and thus begun to spend more time there. Eventually, they lost their legs and developed fins to get around. But, every now and then, a dolphin or a whale is born that still has stubby little legs.
And just as we humans have a coccyx (the vestigial remnants of a tail bone), whales still have scrawny little leg bones cramped at the back of their bodies. (Whales are the ultimate “don’t skip leg day…” cautionary tale.)
Whales also have bony ‘hands’ underneath their flippers!
Another example is horses that have been born with toes. This is a throwback that demonstrates their relation to the rhino. A horse’s hoof is actually just one giant “‘toe” with the others having withered and disappeared. Occasionally though, a horse will be born with a couple of “stragglers” sticking out of their feet.
Some women are even born with extra breasts in a vertical alignment just like other mammals – a condition known as “accessory breast syndrome”.
The mind boggles.
Dino Chicken!
Perhaps one of the coolest examples of atavism, though, is when a chicken was born with teeth. Why’s that so cool? Because it’s evidence that chickens descended from dinosaurs…
This wasn’t your typical case of atavism but rather an example of scientific meddling. In 2006, a researcher named Matthew Harris was experimenting with mutated chickens (as one does) when he noticed that one of the embryos had tiny teeth that were highly similar to those of an alligator. This chicken embryo was bearing a recessive trait from its evolutionary history, which demonstrated chickens still retain the ability to grow dinosaur-like teeth.
Unfortunately, this mutation was lethal meaning that the same mutation that allows the teeth to grow would kill the chicken – so don’t expect to stumble upon any examples of dino-chickens just yet.
Some researchers are considering a project to purposefully ‘de-evolve’ chickens as a means to essentially revive dinosaurs.
Dino… Man?
Most of these examples of atavism show people and animals re-growing limbs and toes that were lost relatively recently. Now you know why – extreme mutations that would send us back to the prehistoric ages tend to be lethal as they are simply no longer compatible with our modern bodies.
But every now and then a truly extreme example does occur. This was the case in the case for one 59 year old man who visited the doctor with heart pain that appeared to have no obvious cause. Eventually, he managed to get an x-ray, at which point he was given an explanation for his problem: he had the heart of a snake.
Wait, what?
Imagine being told that by the doctor…
Kind of creates more questions than it answers, right?
As you may recall from biology class, all creatures on the planet share a common family tree if you go back far enough. At one point, our ancestors lived under the sea like fish, before crawling out onto the land as reptiles. And in this remarkable example of atavism the patient survived a mutation that resulted in his heart more closely resembling that reptilian heart than that of a mammal with only three chambers.
The man survived but was put on medication for the rest of his life. Although this mutation wasn’t particularly useful, however, it does make you wonder what other abilities might be lying dormant in our DNA… and that’s something that scientists are particularly interested to find out.
It has been suggested that some day, we may be able to tap into these dormant traits as a means to enhance the human condition.
In a potential future where genetic manipulation is common place, this would make a lot of sense rather than attempting to introduce entirely new abilities; it would be much simpler to change one or two genes to bring back a tail or night vision.
I actually wrote this article over 10 years ago and am reviving it for the blog, today. Coming back to it now, I’m far less enthused at the idea of restoring long-lost traits like these. The body has evolved over billions of years and is the way it is for a reason. You can’t change a single aspect of the human body without having countless downstream reactions – which is why these mutations are usually fatal.
It’s arrogance to think we can just “give ourselves” cool new traits without significant consequences.
So, there you have it: atavism is a fascinating demonstration of the power of evolution. It can give us hints to our past and it might just even hold the key to our future – for better or worse.
And to dino chickens!
