Column | Evolution and the loss of tail in us

A tail that is not used is just another limb that needs energy to grow.

Published - August 24, 2019 09:22 pm IST

A Squirrel Monkey tries to get food out of a frozen treat hung for him by zookeepers at the Phoenix Zoo, Tuesday, July 16, 2019, in Phoenix. The National Weather Service has issued an excessive heat warning to take effect until Wednesday night. The Phoenix Zoo use spraying, frozen treats and shaded area's to keep their animals cool. (AP Photo/Matt York)

A Squirrel Monkey tries to get food out of a frozen treat hung for him by zookeepers at the Phoenix Zoo, Tuesday, July 16, 2019, in Phoenix. The National Weather Service has issued an excessive heat warning to take effect until Wednesday night. The Phoenix Zoo use spraying, frozen treats and shaded area's to keep their animals cool. (AP Photo/Matt York)

A team of bioengineers for Keio University in Japan have come out with what looked first glance as a practical joke, but upon reading further, we find that they have devised a wearable robotic tail for needy people. A tail, and for humans? The details of the tail, named Arque and its use to assist the elderly with mobility, preventing them from falling are and to maintain balance while climbing stairs and so on, can be read in the article by Nancy Cohen in TechXplore.com of August 7, 2019, and the YouTube display of “Arque: Artificial Biomimicry-Inspired Tail for Extending Innate Body Functions” explains the uses of this man made tail; please see https://www.youtube.com/watch?v=Tr1-IhEhXYQ . Besides helping needy senior citizens, Arque can also help labourers while moving heavy loads, climbers who want to get on to high cliffs and so on.

If Arque type artificial tails become necessary as some of us age and have locomotor difficulty, why did nature not keep the tail as part of the anatomy of humans (and our close cousins such as the bonobos, chimpanzees and gorillas)? Most animals in sea as on land have tails -long or short- and the tails perform important functions. As Dr David Young of the University of Melbourne, Australia points out, the tail in an animal performs a variety of functions, apart from swatting flies and bugs. Fish uses its tail to steer along the water, crocodiles and camels store excess body fat in their tails as an energy reserve, and tails in mammals act as a counterbalance to the head and assist in running. Fast runners have long tails. Tree-dwelling monkeys have long tails in order to balance themselves.

Why did we lose it?

As we evolved from walking on four legs (quadripeds) to two feet (bipeds), the tail actually became more of a hindrance than a help. As Hannah Ashworth points out in the BBC Science Focus Magazine ( sciencefocus.com ), a tail that is not used is just another limb that needs energy to grow (and another thing for predators to grab. (Re: the last point, the lizard has learnt to simply drop its tail when it faces attack, to regrow it later at leisure). We bipeds walk straight, with our bodies aligned with a center of gravity that passes through our spine to our feet, without needing a tail to counterbalance the weight of our head. She further states: “natural selection would have favoured those of our ancestors that had smaller tails when they moved from the forest to the savannah.

Over the course of the next a few million years, it dwindled to nothing. (There are other distinct differences between us and the great apes. Dr. Kevin Hunt of Indiana University thinks humans' ancestors stood upright in order to reach vegetation in low-hanging tree branches. "When Africa started getting drier about 6.5 million years ago, our ancestors were stuck in the east part, where the habitat became driest. Trees in dry habitats are shorter and different than trees in forests. Thus, our ancestors stood up on both legs in the scrubby, dry areas of Africa, to pick up food from the trees. Chimps in the forests did not”. They walk both on two legs and when needed, on all fours. This habitat and bipedalism also made our noses different (stick up) from those of the great apes. Further, as Darwin pointed out how the simple act of standing up made all the difference in separating man from ape. One word: tools. "Once we became bipedal, we had hands to carry tools around. We started doing that only 1.5 million years after we became bipedal," Hunt explains). Moving from a quadruped into a biped has had such biostructural, energetic and functional consequences.

Actually, we still have a tail during the first four weeks of our lives in the womb, but then its gets absorbed by the 7th week and we are just left with the coccyx, at the base of our spine, which serves as a muscle attachment point)”. Apes notably such as chimpanzee, gorillas also have coccyx.

Not completely lost

What if this absorption goes wrong and the tail actually starts to form in infants? Fortunately this is a rare and so far these have lean no more than 40 cases reported to date across the world. The tail is no longer than 6 inches at best in Indian alone, there are just about 3 reports of babies born with tiny tails in the lumbar and several regions of the spine a total of 8 infants. The most recent case has been reported in 2017. All these neonates were operated with success, and have grown in health and are normal with no neurological or other effects. Analysis of the removed tail material showed it to be made of adipose tissues, collagen fibers, some nerve fibers, blood vessels and ganglion cells but no bones. Preliminary gene-based analysis of the tail material showed that the genes controlling tail development might be from the Wnt family of genes. Biological research – involving genetic analysis, cell biology and inhibitor search- on this “rare disorder” to find the cause and solutions will be of great value to the family.

dbala@lvpei.org

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