Have you ever heard about babies born with tails? In November 2022, a baby boy was born in Brazil with a 2-inch-long appendage with a ball on the end. Scientists dubbed it the "true human tail." It is not the first time this rare phenomenon has happened.
The question that will now arise in our minds is, "Did our forefathers have tails?" Research-based on evolution proves that all primates, including our ancestors, had tails millions of years ago. Hence, this is an example of atavism. Atavism is defined as the reappearance of a phenotypic trait that has been lost during evolution.
Having a tail is a trait that has been around since the earliest vertebrates on Earth. Humans are vertebrates, and the tail is present in the human embryo until 8 weeks of development, after which it will disappear by apoptosis and form triangular shaped coccyx or tailbone.
Researchers found that the genetic explanation for the disappearance of tails in humans could be a mutation in a gene related to tail growth called the jumping gene. Besides genetics, the reason for the disappearance of the tail could be the transition from four-legged walking to two-legged walking. This would have helped them walk upright. Several times over the course of evolution, tail reduction has happened initially in amphibians, then in rodents followed by great apes.
Tail in neonates is also known as the 'caudal appendage' and is found in sacrococcygeal region.
True tail is composed of connective tissue, adipose tissue, muscles, nerves, and vessels.
The tail is called a pseudo tail if it is a lipoma, teratoma or a parasitic fetus. Pseudo-tail is also caused by an elongated coccyx or spina bifida.
Surgical excision is the treatment for both true and pseudo tail.
Other examples of atavism in humans include hypertrichosis, supernumerary nipples, large teeth, and colour blindness.
It is a minor congenital malformation where extra nipples are present on the milk line. It occurs due to the failure of apoptotic regression of mammary ridges during pregnancy. They can either be sporadic or appear in association with congenital conditions like cardiac arrhythmias and vertebral malformations.
Treatment is not required for supernumerary nipple but if the person wishes to remove it then surgical excision is done by surgeons.
Hyperdontia or large teeth
The exact reason for the appearance of Hyperdontia or supernumerary teeth is still unknown. The hyperactivity theory and atavism are the most accepted theories for hyperdontia.
It is an inherited condition where the affected individuals has decreased ability to see colour or differences in colour. If it is an atavistic feature, then what advantage it had in our ancestor? It made it easier for them to view brightly colored ripe fruit against a predominantly green woodland backdrop.
Werewolf syndrome or hypertrichosis is a rare skin condition which occurs due to excessive production of hair either all over the body or in localised body part. Studies suggest that our ancestors were covered with hair all over the body. Hence, hypertrichosis is considered as an atavistic trait.
Atavism and vestigial structures
Even though both atavism and vestigial structures are related to evolution, they are different. Vestigial structures are the rudimentary organs present in our body but lost their function during evolution. While atavistic structures are the disappeared structures or traits which reappear.
Atavism besides humans
Atavism can appear in both plants and animals. Some of the examples of atavism in animals are:
Dewclaws in dogs
Extra toes in horses and guinea pigs
The reappearance of wings on wingless insects
Presence of legs in snakes
Among plants, the return of sexual reproduction in Hieracium pilosella is one of the best examples for atavism.
Atavism plays a very important role in genetics and evolution. It is a major proof that we humans still possess a lot of genes for many traits that were present in our ancestors who lived thousands or millions years ago.