Why do we have a coccyx? The coccyx is a series of three, four or five vertebrae at the base of the spine. A coccyx is something we have evolved not to need, but it is a great reminder of our origins.
The coccyx is also called the tail bone because that is what it is. It is the remnant of where we used to have tails. The early ancestors of humans were tree dwelling animals. Tree dwelling animals almost all have tails which they use to hold branches or to balance. They have prehensile tails which they can wrap around branches, almost like an extra hand. Tails are also used for balance. Cats move their tails around to keep their center of balance and to flip their bodies over when they fall. Some animals have tails that are simply used to swat flies and to keep their nether regions warm, such as horses and cows. Several animals don’t have tails, gorillas being among them. Gorillas and us are the only primates without tails.
The ancestors of early humans started to get larger and come down from the trees more and more often about 25 million years ago. They spent less time in the trees, started to stand more upright, and started to run. Once they no longer needed the tail, it became more of a hindrance than an advantage. Running upright with a tail would be difficult because it would shift our center of gravity and offset our balance. Also, any extra limb would require energy that would necessitate us eating more. We didn’t need it, so evolution slowly removed it.
However, up until about 2 million years ago, early humans still used to sleep in trees. The reason why we stopped is not completely understood, but one of the strongest reasons might be the way that we sleep.
Out of all primates, humans sleep the least, yet have the most REM sleep. REM sleep is said to be the most valuable part of sleep and is incredibly valuable for the brain. This could be one of the reasons why we have developed intelligence.
Sleeping in trees might keep animals safe from land-based predators, but it is difficult to get good sleep because of the inherent instability of a tree branch. Some part of the animal must always be aware of its surroundings and ready to correct a fall. Once humans evolved to sleep less, they needed to get good sleep. This could only be had on the ground. So, humans came out of the trees and started to sleep on the ground. They would probably sleep in small groups and use fires to keep predators away.
Being able to get more quality sleep in a shorter time was also probably important in giving humans the time to develop tools and culture, and maybe even language.
Coming out of the trees and walking on two legs also gave us other advantages. We saved a lot of energy, and we were able to use our arms and hands for things like hunting and crafting tools. Hands can be used to collect and carry food to a den, making it easier to live as a family unit. The female could stay and look after the young if the male was able to go out and carry enough food back for everybody. There’s no way to say that walking on two legs is the reason we have evolved to the point where we are, or that it was our ability to get better sleep. They are probably all connected parts of the same cog. However, if we still lived in the trees, it is doubtful that we would have come this far. And the coccyx is a reminder of how far we have come.
Things like the coccyx are called vestiges. They are parts of the body that we have retained, but no longer have any use for us. Some examples are the appendix, goose bumps, and wisdom teeth, amongst several others. They all used to serve a purpose but no longer do. So, why don’t we lose them completely? Because there is no evolutionary advantage to it. There was an advantage to losing the tail, but there is no advantage to losing the last few bones of it. So, they stay.
Incidentally, the word coccyx comes from the ancient Greek word for cuckoo because the bones look like the beak of a cuckoo.
So, the coccyx is the starting point of the tail we used to need when our ancestors lived in trees. It helped us balance and grab branches. Once we started living on the ground, we didn’t need it anymore so we lost it. And that is what I learned today.
https://geneticliteracyproject.org/2020/02/12/why-do-humans-have-tailbones
https://evolution-outreach.biomedcentral.com/articles/10.1186/s12052-014-0012-5
https://www.popsci.com/story/science/human-tailbone-evolution