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 changes that have put us at increased risk of developing neural tube deformities that expose the spinal cord after birth, such as spina bifida.
So it’s definitely a significant advantage to let the tail end go.
More than just an extension hanging from the end of the spine, the tail is anchored to some serious anatomical structures around the hip area. Together, these bones, ligaments, and muscles pull the body into alignment, allowing for balance, and – in the case of some monkeys – the ability to perceive and manipulate.
So if humans have tails, they also need hips and muscles to use them. Otherwise, they’d be little better than long, complicated sausages shuffling on the ground. And nobody wants that.
It won’t be a small change. Tailed primates tend to have longer tails with multiple vertebrae, to give them flexibility.
Apes, on the other hand, have a musculoskeletal specialty with a short lumbar section extending from the tail. A stronger back can withstand the weight of a fall or jump better, potentially paving the way for a larger body or a quick jump onto a low branch and down to the ground.
As for the tail muscles, the old hypothesis suggests that they never go to waste. Instead, they are co-opted as rigid structures that provide support to our abdominal organs such as the bladder and intestines.
That’s what allows us to put strong pressure on our intestines and still retain all the chunks of fat.
With that in mind, if humans have tails like monkeys, there must be some adjustments throughout our bodies.
Even considering the potential brain space we need to feel and move our new intelligent limb, we need to restore the pelvic floor muscles to provide support for those extra muscles and bones. This can increase the risk of multiple hernias and possibly slight incontinence, or even, forgetting the mechanics of bipedalism (walking on two legs) altogether.
The extent to which tail reduction helped our ancestors stand on their own feet is debatable, as is the reason why it disappeared in the first place.
But if our ancestors held onto their tails, it’s possible that humans never evolved at all.
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