A small, accidental mutation changed the course of primate evolution 25 million years ago. It's the main reason your lower back doesn't have a muscular appendage.
(Also, there is a tiny injury-prone tailbone.
Researchers recently published fascinating research online that revealed an ancient mutation in a primate genetic code that eventually led to the loss or tails in humans, chimpanzees and gorillas. This mutation is not found in most monkeys with their serpentine tails.
Itai Yanai (director of the Institute for Computational Medicine, New York University) said that there is compelling evidence that one change could have enabled the results. He was also the author of the research.
This discovery helps to appease a common childhood question and, perhaps, a continuing dilemma for many curious adults. "It's been a question in my head ever since I was little: Why don't you have a tail?" Bo Xia was the NYU Grossman School of Medicine Ph.D. candidate who made the discovery. (Xia broke his tailbone a few years back, which rekindled his interest in his long-lost tail.
The mutation did not occur in a prominent place in the primate genome. Hopi Hoekstra of Harvard University, who studies adaptations and genetic changes in vertebrates, explained that this was a tiny mutation. Hoekstra was not involved in the research.
Xia compared the genomes of tailed primates with those without tails. She discovered that humans and apes had a unique piece of DNA inserted in the TBXT gene. This gene carries instructions for tail formation. Hoekstra said, "It's the beauty of comparing genomes from primates with tails or tailless primates." They found a mutation which knocked out part of the gene responsible for this trait.
The authors explain that a type of gene known as a "jumping genes" can insert itself randomly in other parts of the genome, creating this mutation. This insertion led to a new pattern in the expression of TBXT genes that had no tail or a smaller tail.
Orangutans don't have tails like humans. Credit: CHONG JIUN YIH / GETTY IMAGES
Xia and his colleagues experimented with laboratory mice to support their findings. These mice share many of the same genes as humans, but not all. The mice were genetically engineered to have the same TBXT gene expression patterns as humans. Many mice were born without tails or with short tails. Mice that were not genetically modified had normal tails.
This all adds up to strong evidence showing that a single gene mutation was responsible for the loss of tail in primates. Xia noted that other genes may also play a part, but the tail lengths of the mice were different. Charles Fenster, a South Dakota State University biologist who studies evolution, said that the authors provided compelling evidence that primates lose their tails. Fenster was not involved in the research.
Once this mutation was discovered in primates, evolution began to work. Millions of years later our tails have almost disappeared, with the exception of the tailbone.
Evolution
The question of what caused the loss of our tails is now answered confidently by this new research. It was most likely a small but powerful genetic mutation. The question of why the physical change, or loss of tails, occurred is still unanswered. Also, the question is why it was evolutionary advantageous for humans and apes to lose their tails.
Hoekstra said, "This is always an difficult question." This research is not limited to speculation. There are many intriguing ideas.
In principle, it must have been good for many primate species to lose their tails. Beneficial mutations spread. "Benign mutations will spread. Fenster explained that deleterious mutations in the population will be eradicated.
"It is always a matter cost-benefit."
The benefits of losing the tail or a shorter tale for primates outweighed their costs in the end. Hoekstra stated that it is always a matter cost-benefit. Even though the benefits are vastly outweighed, there may be some negative consequences that still affect people today. Some of the genetically modified mice found in the research had neural tube defects. These are spinal cord defects. Today, 1 in 1,000 babies is born with a similar defect. Yanai, NYU's director of research, said that "Maybe this remnant problem still exists." In other words, we paid the price of a defect in one of 1,000 births by losing our tails.
What are the biggest benefits to losing a tail? These are just a few ideas.
The earliest evidence of bipedal primates dates back to 4.4 million years ago. However, primates from earlier times may have spent more time in trees or on the ground for better foraging opportunities. This tail mutation may have made it more likely that primates with this tail mutation were more successful than their tailed counterparts. They are better able to stand straighter and can forage on the ground.
The genetic mutation would have been more common as the tailless primates became successful. The tail mutation may have been used to "predapt" primates who were more active on the ground, according to Jef Boeke (also a study coauthor, and director of the Institute for Systems Genetics, NYU Langone Medical Center). Boeke speculated that "it preadapts" you to do larger and more rewarding things. This allows us to move more easily on our two feet."
Hoekstra noted that for many primates, losing their tails may have been an easier option. A tail is not always cheap to make (more calories are needed to support a major appendage); it can also be damaged; or, "it's another tool for a predator" she stated.
In the end, it was a great decision to lose a tail for our highly successful but imperfect species. We were able to travel great distances and be experts at hunting and foraging thanks to our large size bipedalism. Our larger, more energy-intensive brains could be fed. We developed intricate language. We could one day eradicate scourges and plagues using vaccines and antibiotics, venture into outer space and create amazing music.
We continue to uncover and reveal the secrets that made us such intelligent, capable creatures. In an age of computer-dominated science, the genetic researcher Xia, rather than using specially programmed software, discovered a significant genetic mutation through human curiosity and human ability.
Yanai said, "He looked at the genome." Yanai said, "That's a testimony to the enduring power human-generated discoveries."
