Take a quick look over your shoulder and you'll notice that, unlike the majority of primates, you're tail-less.
It's a tremendously good thing too. We really ought to be grateful for the change.
But what if it all went so very differently? Aside from needing a hole in the back of your jeans, here's how the world might look had our ancestors kept their tails.
Wait, don't some people actually have tails?
Technically speaking, we all had one – briefly – long before we were born.
Short tails are a feature of human development, temporarily emerging by around the sixth week of gestation. This tiny extension of the spinal column even contains up to a dozen vertebrae. Within a fortnight half of them are reabsorbed, with the other half fusing into the bone called the coccyx or tailbone.
Because nature loves a bit of variety, some folks – mostly males, for some reason – are born with the tip of this embryonic 'tail' still in place. The appendage comes complete with enough blood vessels to keep it healthy, and even muscles that can in some instances make it move. What you won't find in them are any of the original vertebrae.
In contrast to protrusions caused by various lesions or cancers, these vestigial 'true tails' are so rare in human births you'd be hard-pressed to find more than several dozen of them in the medical literature.
Still, they do occur. Most are easily removed surgically soon after delivery, being too small to serve much purpose and potentially attracting unwanted attention.
If we all had a long tail, though, it'd be quite a different story.
Wait, why don't we have tails?
Within 10 million years of a comet pounding dinosaurs into oblivion roughly 66 million years ago, small mammals with the physiology of the primate made an appearance.
Much like today's new and old-world monkeys, lemurs, tarsiers, and bush-babies, these animals had an extensive tail, probably to help them keep their balance while striding through the tree-tops.
Over time some of these tails evolved to act as a kind of extra thumb, gaining a level of dexterity most of us would just love to have. In fact, it was so handy it occurred in two groups of primates on separate occasions.
Unfortunately for us, around 20 million years ago, a group of primates appeared without tails. Their descendants include gorillas, chimpanzees, bonobos, and orangutans. Not to mention we humans, of course.
A recent study pinpointed the cause of this loss; an itinerant bit of genetic code called an Alu sequence jumped smack-bang into a gene critical for building tails, interfering with its ability to do its job.
It was uncovered by New York University graduate student Bo Xia, a researcher in stem cell biology. Xia compared the genomes of six ape species with those from nine species of tailed monkeys, looking for mutations that could explain why the apes had no tail.
This was where he chanced upon a gene called the T-box transcription factor T (TBXT). Nearly a century ago, experiments initiated by a Russian geneticist by the name of Nadezhda Dobrovolskaya-Zavadskaya showed mutations in this gene produced mice with shortened tails.
A closer look at the Alu element within the TBXT gene made it unlikely to be all that disruptive. But Xia noticed there were actually two of the jumping genes present in close proximity.
Together, they could mess up the gene's expression good and proper, explaining why apes like us have nothing dangling from our bums. Over time, changes in other genes helped cement the change, creating a clear rear and a flat tailbone perfect for sitting, crouching, and striding about on two legs.
Though this might explain the dynamics, it doesn't tell us why a chance ruining of the TBXT gene in an ape ancestor spread so widely across a range of hominid species all those years ago.
Okay, so tell me – what if our ancestors had kept their tails?
We can only speculate why one of our ancestors fared so much better when its tail refused to grow.
The whole event is even more surprising when we consider the change has put us at increased risk of developing neural-tube deformities that expose the spinal cord after birth, such as spina bifida. So it must have been a significant advantage to let the tail end go.
More than a mere extension hanging from the end of the spine, tails are anchored in some serious anatomical structures surrounding the hip area. Together, these bones, ligaments, and muscles pull the body into alignment, permitting balance, and – in the case of some monkeys – an ability to grasp and manipulate.
So if humans had tails, they'd also need the hips and muscles to make use of them. Otherwise, they'd be little better than a cumbersome length of sausage dragging along the ground. And nobody would want that.
This would be no small change. Tailed primates tend to have longer spines with a few more vertebrae, to give them the flexibility to slip deftly through the branches.
Apes, on the other hand, have musculoskeletal specializations with shorter lumbar sections that stiffen their spines. More robust backs can take the brunt of a fall or a jump better, potentially opening the way for bulkier bodies or rapid leaps into low branches and descents to the ground.
As for those tail muscles, an old hypothesis suggests they never went to waste. Instead, they were co-opted as a rigid structure that lends support to our abdominal organs such as the bladder and intestines.
It's what allows us to put intense pressure on our gut and still keep all the meaty bits in place, while also lending a hand to hold back any urine and feces that could unwittingly be squeezed out.
With that in mind, if humans had monkey-like tails there would have to be some adjustments to the rest of our bodies.
Not even considering the potential brain-space we'd need to allow for to sense and move our clever new limb, we'd need to put those pelvic floor muscles back to lend support to those extra muscles and bones. This could raise the risk of a whole lot of hernias and perhaps a bit of incontinence, if not require giving up bipedalism altogether.
To what extent the reduction of a tail helped our ancestors stand on their own two feet is up for debate, as is the reason why it vanished in the first place.
But had our ancestors held onto their tails, it's possible humans might never have even evolved at all.
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