How humans lost their tails: Study isolates single genetic mutation responsible for loss of tails

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Humans do have a tail when they are embryos, but they lose it and it fuses into the vertebrae to form the coccyx (red), or tailbone


Humanity’s ancestors have all had tails, but researchers may have identified the genetic mutation responsible for humans and certain primates losing the appendage, a new study suggests.

A group of researchers from New York – including those at NYU Langone Health and NYU Tandon School of Engineering – have found that the TBXT gene (found in all bilateral animals) has a mutation that is present in apes and humans, but not in monkeys. 

The mutation on the TBXT gene consists of 300 genetic letters in the middle of it, according to the experts, including the study’s lead author, Bo Xia.

This mutation likely caused the tails of certain primates to disappear roughly 20 million years ago. 

Humans do have a tail when they are embryos, but they lose it and it fuses into the vertebrae to form the coccyx (red), or tailbone

Other hominoids (also known as great apes), including gorillas, orangutans, chimps, and bonobos do not have tails

Other hominoids (also known as great apes), including gorillas, orangutans, chimps, and bonobos do not have tails

Humans do have a tail when they are embryos, but they lose it and it fuses into the vertebrae to form the coccyx, or tailbone. 

‘Here, we present evidence that tail-loss evolution was mediated by the insertion of an individual Alu element into the genome of the hominoid ancestor. We demonstrate that this Alu element – inserted into an intron of the TBXT gene (also called T or Brachyury ) – pairs with a neighboring ancestral Alu element encoded in the reverse genomic orientation and leads to a hominoid-specific alternative splicing event.’

They continued: ‘We propose that selection for the loss of the tail along the hominoid lineage was associated with an adaptive cost of potential neural tube defects and that this ancient evolutionary trade-off may thus continue to affect human health today.’ 

It’s not certain that the TBXT mutation is definitively responsible for the loss of the tail, but Cornell University Cedric Feschotte, who was not involved in the study, told the New York Times ‘it’s as close to a smoking gun as one could hope for.’   

Famed naturalist Charles Darwin discovered the change in human and our ancestors’ anatomy in his 19th century book The Descent of Man, but could not be certain the coccyx was an ancient tail. 

‘I believe the Os coccyx gives attachment to certain muscles, but I cannot doubt that it is a rudimentary tail,’ Darwin wrote.

This mutation consists of 300 genetic letters in the middle of the TBXT gene

This mutation consists of 300 genetic letters in the middle of the TBXT gene

Other hominoids (also known as great apes), including gorillas, orangutans, chimps, and bonobos do not have tails.

It’s possible that this mutation and lack of a tail may impact human health, the researchers wrote in the study.  

‘While these genes and their relationships have been studied, the exact genetic changes that drove the evolution of tail-loss in hominoids remain unknown, preventing an understanding of how tail loss affected other human evolutionary events, such as bipedalism.

The oldest primate fossils, which went extinct around 66 million years ago, had tails, but the oldest known ape – known as Proconsul – did not, according to Michigan State University

It’s widely believed that Proconsul, which was first identified in 1909 by a partial jaw, went extinct approximately 15 million years ago.

Thirty-one genes have been identified in various species for developing tails. The researchers then used genetically engineered mice to see if the TBXT mutation is responsible for the tail. Most of the embryos failed to develop the tail, but one grew a short tail

Thirty-one genes have been identified in various species for developing tails. The researchers then used genetically engineered mice to see if the TBXT mutation is responsible for the tail. Most of the embryos failed to develop the tail, but one grew a short tail

Thirty-one genes have been identified in various species for developing tails.

When Xia and the other researchers compared six species of apes to nine species of tailed monkeys, they saw the mutation in apes and humans, but not in monkeys.

‘I nearly fell off my chair, because it is just a stunning result,’ Xia’s supervisor, Itai Yanai, told the Times.

The researchers then used genetically engineered mice to see if the TBXT mutation is responsible for the tail. 

Most of the embryos failed to develop the tail, but one grew a short tail.

The loss of a tail from hominoids and monkeys may be an adaption to their environment, with humans not only able to walk, but running as well. 

‘It has long been speculated that tail loss in hominoids has contributed to bipedal locomotion, whose evolutionary occurrence coincided with the loss of tail,’ the researchers wrote. 

‘Recent progress in developmental biology has led to the elucidation of the gene regulatory networks that underlie tail development.’

The study was published earlier this month on the preprint server bioRxiv

WHEN DID HUMAN ANCESTORS FIRST EMERGE?

The timeline of human evolution can be traced back millions of years. Experts estimate that the family tree goes as such:

55 million years ago – First primitive primates evolve

15 million years ago – Hominidae (great apes) evolve from the ancestors of the gibbon

7 million years ago – First gorillas evolve. Later, chimp and human lineages diverge

A recreation of a Neanderthal man is pictured 

A recreation of a Neanderthal man is pictured 

5.5 million years ago – Ardipithecus, early ‘proto-human’ shares traits with chimps and gorillas

4 million years ago – Ape like early humans, the Australopithecines appeared. They had brains no larger than a chimpanzee’s but other more human like features 

3.9-2.9 million years ago – Australoipithecus afarensis lived in Africa.  

2.7 million years ago – Paranthropus, lived in woods and had massive jaws for chewing  

2.6 million years ago – Hand axes become the first major technological innovation 

2.3 million years ago – Homo habilis first thought to have appeared in Africa

1.85 million years ago – First ‘modern’ hand emerges 

1.8 million years ago – Homo ergaster begins to appear in fossil record 

800,000 years ago – Early humans control fire and create hearths. Brain size increases rapidly

400,000 years ago – Neanderthals first begin to appear and spread across Europe and Asia

300,000 to 200,000 years ago – Homo sapiens – modern humans – appear in Africa

50,000 to 40,000 years ago – Modern humans reach Europe 



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