Why don’t humans have tails anymore?
Olivia, 12 , the Netherlands.
Great question, and it gets to the heart of what we are as humans.
Think about your own family – do you have cousins? If so, you and your cousins share grandparents and these are your common ancestors.
Now imagine going back further in time. You and your more distantly related relatives also share common ancestors from longer ago, which you can see on your family tree. And when you look around the world, all living things also share a single common ancestor, which lived between 3 and 4 billion years ago.
Curious Kids is a series by The Conversation that gives children the chance to have their questions about the world answered by experts. If you have a question you’d like an expert to answer, send it to [email protected] and make sure you include the asker’s first name, age and town or city. We won’t be able to answer every question, but we’ll do our very best.
Life on Earth is a really big family tree. That means dogs and cats are related, but also you and squirrels, you and fish, and you and the dinosaurs. Everything alive today and that ever was alive is descended from that same original ancestor.
Four billion years is not something we can really visualise in our heads. To give an idea of how long ago this was, a billion golf balls would fill a large train station. So think of four of these – that is a lot of golf balls, and a long time ago.
Zooming in to more recent times, we are apes. We share common ancestors with the other living apes – chimpanzees and gorillas, orangutans and gibbons. And while chimpanzees and gorillas have many features in common, chimpanzees and humans are sister species. This means we are more closely related to each other than any other living species.
This also means a lot has happened (evolution) in the human lineage since we shared this common ancestor. Our anatomy has changed substantially, allowing us to walk upright, use tools, speak, and other features that make us the successful species we are.
Farewell love/Shutterstock
However, all apes including us are united by several features. All apes have large brains, for example, though ours is the largest. And all apes have a body plan that allows us to take an upright posture – our chests are much more vertical than a dog’s or even a monkey’s.
We also have a particular pattern of grooves in our lower molar teeth – the five bumps you can feel there are arranged in a Y-shape (known as Y-5 pattern). This is only found in apes.
Finally, all apes climb trees and suspend themselves from branches. We still have features of our arms and shoulders that allow us to do this safely.
We have these because we descended from a single common ape ancestor, probably around between 20 and 30 million years ago. Using
our golf ball image, a million golf balls would fit into a large bedroom, so imagine 30 large bedrooms of golf balls and you get some idea of how long ago that was.
Our current best evidence of what this common ancestor of apes looked like is based on fossils – the remains of once-living creatures that have been transformed into rock.
When we look at this current best evidence – such as in the extinct species Ekembo heseloni from Africa – we see a species that is actually fairly monkey-like. It climbed trees but may not have swung below branches – instead walking on top of them. This is quite surprising, as all living apes share features that allow us to swing from branches.
However, we know it was an ape because of two main features. First, it has that distinctive Y-5 pattern in its lower molars, just like you do. Second, it lacks a tail. Lacking a tail is a distinctive feature of all apes.
Why do all apes lack a tail?
We only have hypotheses (informed guesses) as to why our common ancestor didn’t possess a tail. This is because most other primate groups, both living and extinct, do have a tail.
Nataliabiruk/Shutterstock
One suggestion is that when the earliest apes shifted to more upright postures and other changes in the way they moved around in trees, their tail became less helpful. So perhaps evolution caused the muscles that had previously been used for tail attachments to instead become part of the pelvic floor.
The pelvic floor is made of the muscles at the base of the spine that help your internal organs resist gravity – keeping them inside your body rather than falling out. That is a pretty important function, as you can imagine.
Another suggestion is that tails disappeared from the earliest apes due to a genetic mistake. When a single short stretch of DNA found in humans and other apes, but not in other primates, was added to mice in a 2024 study, it caused the mice to develop only minimal or no tails.
So, despite how amazing having a tail would be to us now, our ancestors may have lost it due to them no longer needing it – or simply because of a chance mistake.
Mark Grabowski does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
