Some babies are born with “tails,” but not for the reason you might think: ScienceAlert

It might sound like a big story, but on very rare occasions, humans can be born with boneless hind appendages, sometimes up to 18 centimeters long.

According to official records, about 40 babies have been born with “real tails,” which are soft, boneless, finger-like protrusions that can be easily removed through surgery.

Still, according to the researchers, the rare case studies tend to generate “an unusually high level of interest, excitement, and anxiety.” This is often because the “tails” are viewed as harmless, evolutionary remnants of a long-lost ancestor.

As it turns out, this is based on an outdated theory that has been controversial for decades. The reality for these children may be much grimmer, and they deserve medical attention, not our morbid fascination.

The limbs that some babies are born with were historically considered “real” or “rudimentary” tails. But that’s a bit misleading as they don’t actually resemble any other tail found in nature. They usually contain no bone, cartilage, or spinal cord. They just hang there and have no clear function.

However, that doesn’t mean these appendages are as harmless as scientists used to think.

The misconception about the origin of the tail begins with Charles Darwin himself. Over a century ago, Darwin proposed that human tail tails were evolutionary accidents or vestigial remnants of a primate ancestor that once wore a tail itself.

In the 1980s, scientists took up this theory and implemented it. They argued that a genetic mutation engineered by humans to obliterate our tails could sometimes revert to its ancestral state.

In 1985, groundbreaking work defined two different types of “tails” that human babies can be born with. The first, as mentioned earlier, is a holdover or real tail originally thought to have been inherited from our ancestors.

Another type of coccyx outgrowth that sometimes includes bones is called a “pseudotail.”

Historically, the pseudotail has been associated with birth defects and is therefore not considered a holdover.

As it turns out, both rare appendages are likely an incomplete spinal fusion, known as spinal dysraphism. This suggests that their formation is not a benign “regression” in the evolutionary process, but a worrying disruption in an embryo’s growth, most likely due to a mix of genetic and environmental factors.

When a human embryo reaches the developmental period of about five weeks, it develops a tail-like structure of neural tube and notochord that resembles an early spinal cord.

Reduction of the tail of the embryo
The tail reduction process in a human embryo. (Tojima et al., Journal of Anatomy2018)

By the eighth week of development, this tail is usually reinserted into the embryo’s body. If it persists until birth, it could indicate the presence of a major birth defect.

In fact, human babies born with tails tend to have serious neurological defects. For example, in 2008, an article argued that “true residual tails are not benign” as they may be associated with underlying dysraphism.

About half of the cases studied were associated with either a meningocele or spina bifida occulta.

This suggests that babies born with a tail may need more medical care than simple surgery. And it firmly contradicts the 1985 paper, which stated, “The true human tail is a benign condition unassociated with any underlying cause.” [spinal] umbilical cord malformation.”

In fact, as early as 1995, researchers argued that babies born with both “real” and “pseudo” tails should undergo imaging as well as surgery to ensure their development is proceeding as intended.

Since then, why have vestigial tails been reported in case studies as if they were innocent, undisputed consequences of our genetic heritage?

Part of the problem is that it is not yet known whether a true tail is directly derived from the embryonic tail, as some scientists suggest. There simply isn’t enough research into where the congenital anomaly lies – partly because these case studies are so rare.

Regardless of where a baby’s tail comes from, however, there is strong evidence that it is a congenital problem and not a harmless holdover.

For the life and health of these children, this is an important message that needs to be clarified once and for all.

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