Why did humans lose their tail during evolution?

Question

Most mammals seem to have tails. Even human embryos start out with a tail. But here we are with nothing but a tail bone that hurts when we trip over backwards. What is it good for? Why don't humans have a tail to wag around?

Some science fiction (like Avatar) even portrays a tail as something sexy. And we all have experienced that a third arm of some kind would be very useful once in a while. Especially since humans stand up a lot of the time it could help us keep balance.

Is there any utilitarian evolutionary explanation for why we don't have tails any more? Is it only because we try to be different?

Answer 1

The tail serves a variety of complex balance tasks in locomotion, turning, jumping, aerial balance, hanging, and most mammals have a tail, save for other examples, i.e. rabbits, pigs, capibaras, manx cats, seals and bats, lemmings and hamsters have mini tails.

All the previous large apes related to humans lost their tail millions of years ago. The reason why that occured are multiple and mixed. Multiple good explanations have been suggested.

Gibbons are some of our most monkey-like ape cousins. If you see how they move, they run on two legs rather than 4. Monkeys with tails are less adapted to walking and their pelvis is angled differently. https://www.youtube.com/watch?v=b9Pt2qgSqJw

Large animals with small tails, such as caribou, hippos, elephants, deer, seem to keep some kind of tail, however with chimpanzee type animals, once the tail became small it didn't stay on as a mini-tail. So would it make sense for chimpanzees/humans to have small tails like hippos or very long ones? If a chimp can run on 2 legs, that leaves it two hands to catch the tail of another competing chimp.

The big apes move differently to small apes: big apes live on the ground more, they don't run in trees as much, they move differently, they are larger, they have completely different body ratios for arm and leg size, and there have been a wide variety of propositions for hundreds of years on this topic without a particularly clear reason being accepted for which great apes don't have a tail.

Answer 2

In 2021 an elementary genetic point mutation was found to coincide with the presence or absence of the tail about 25 million years ago, so it's not a complicated feature that takes millions of years, it's a basic random mutation that can happen in 1-2 generations in a population. It happened at the same time as there was a dramatic change in monkeys towards apes that have good bipedalism and inhabit a new ecological niche. The tail mutation could be a cause and/or a consequence. The corresponding animals are: Gibbons.

Gibbons are close to the oldest human ancestors that lost their tail. They branched off from monkeys ~20 million years ago and they have even more impressive climbing skills, they are entirely arboreal and run waving their arms around in a clumsy way. Gibbons don't have a quadrupedal running gait like big babboons and squirrels, who run like small dogs and cats.

If you study these videos of gibbons, think... Would a tail benefit or hinder these animals?

They are some of the most impressive climbers of all because their opposeable thumbs are well developed and their arms and legs are more ambidextrous than running animals.

So if you think back in time, for the point genetic mutation, potentially there was a family of monkeys living together that had individuals with tails and individuals without tails.

image of the latest primate phylogeny