MYSTERIES of CURIOUS HUMAN TRAITS Felix Bast

FEATURE ARTICLE MYSTERIES OF CURIOUS HUMAN TRAITS Felix Bast

trait is a physical, physiological vestigial reflexes — the legacy of our purposes – expulsion of foreign particles or behavioural attribute of an distant past in the tree of life and shared from the nasal passage, and clearing of A organism. For example, we among many animals. mucosal build up. A popular theory is eat, walk on two legs, yawn, sneeze, Perhaps the most famous mysteries that sneezing sends a strong blast of air to daydream, mate, and sleep at night. among the human traits has been why expel the particles, but research reveals that this might not be the case (Openshaw Human beings, like any other animals, we yawn. Yawning, which occurs 1991; Burke 2012). Sneezing is seen are indeed complex organisms with when bored or stressed, is conspicuous a huge repertoire of interconnected across vertebrate lineages, including and found in most vertebrate species, traits, many of them remain mysteries chickens, iguanas, dolphins, whales, and is highly contagious. Early to science. cats and dogs, and presumably developed scientists conjectured that yawning had very early in evolution. A number of human traits have physiological functions like increasing Some people sneeze in response evolutionary significance. But many of oxygen supply to the brain, arousal and to light (photic sneezing) and sexual our traits are mere ‘baggage’, byproducts thermoregulation (cooling the brain), but stimulation, apparent functions of of Darwinian selection that are known in empirical research revealed that these biology as spandrels. such traits remain mysteries. Sneezing claims lack evidence. New research might also be an adaptive strategy for (Guggisberg et al. 2010) reveals that infectious agents to spread infection Yawn, Sneeze, Cough… yawning is a social communication tool from host to host; i.e., infectious Let us first consider body reflexes — to transmit empathy, boredom and mild agents like rhinovirus somehow elicit the autonomous or semi-autonomous psychological stress. a human physiological response to involuntary actions in response to Sneezing is yet another reflex that is sneezing enabling them to spread in specific stimuli. Many of these are thought to be serving two physiological the population.

20 | Science Reporter | August 2019 Sneezing may be an adaptive strategy for infectious agents to spread infection from host to host

Yawning is a social communication tool to transmit empathy, boredom and mild Laughing evolved in our primate ancestors to psychological stress sustain larger social bonding

Cough is a similar reflex thought to entering the lungs of the fishes, while potent greenhouse gases (Johnson and have a similar function to expel foreign the newly evolved function is to remove Johnson 1995). particles from breathing passages. As air from stomachs of suckling infants Another, albeit socially annoying cough is a natural protective reflex, in mammals (Howes 2012). Hiccups way to remove gases is by farting. new research suggests that repressing somehow retained through adulthood Flatulence is thought to have evolved it, especially the productive cough that with no apparent functions, something early in mammals as a reflex to expel expels sputum, might have negative biologists refer as neoteny. gases from colon, similar in function impacts (Pavord and Chung 2008). A related reflex is belching/ to belching. Flatulence is surprisingly Cough reflex is thought to have evolved burping — the release of gas through common in animals, with this reflex early in mammals, as most mammals mouth, a common reflex evolved early reported from giraffes, zebras, cough in response to irritants such as in mammals. It is thought to have elephants, bats, seals, penguins, capsaicin (Canning 2008). evolved in ancient ruminants to expel snakes, frogs and so on. Hiccup reflex develops in the the methane produced by intestinal This reflex seems to have evolved womb even when the fetus is merely 2 methanogenic bacteria. very early in animal evolution, with months old. Two contrasting theories Humans have an organ called even insects like termites reported suggest that hiccup might prepare vermiform appendix in our intestines to be releasing a significant amount unborn babies for breathing later, or that serves no obvious functions today. of methane this way (Zimmerman help the babies to expel the amniotic It is thought that this organ is a vestigial et al. 1982). fluid from the lungs. organ that housed cellulose-digesting However, newer research on bacteria when our ancient ancestors Laugh, Blush hiccups suggests that this reflex evolved were ruminants. Newer research shows Laughing is thought to have evolved very early in animal evolution some that vermiform appendix serves as a in our primate ancestors alongside the 370 million years ago (at the time safe house for human commensal gut brain to sustain larger social bonding when the famous fossil Tiktaalik lived) microbes (Bollinger et al. 2007). Cows (Marijuán and Navarro 2010; Ross when the first tetrapods developed release approximately 600 litres of et al. 2009). However, a number from lungfishes. The original function methane through burping each day — a of tetrapods produce laughter-like would have been to gush water through matter of significance for climate change voices, including dogs, rats, dolphins gills while preventing the water from research as methane is one of the most and birds.

August 2019 | Science Reporter | 21 Blushing is reddening of a person’s the reflex usually happens in response face in response to stress, embarrassment to the stress hormone adrenaline. or romance. Darwin devoted a whole Goosebumps also develop in chapter on blushing in his 1872 book response to music when dopamine where he referred blushing as “the activated pleasure response leads to most peculiar and most human of all this reflex (Salimpoor et al. 2011), or expressions” (Darwin and Prodger in response to low temperature, which 1998). is a physiological response due to the Blushing is thought to be an contraction of hair follicles. In our emotional response evolved for non- animal ancestors that had a thick fur verbal communication among primates coat, this reflex was indeed beneficial, (Crozier 2001; Changizi et al. 2006). A as it would make fur stand up, with One can get itchy by seeing others itch newer hypothesis links the evolution of lots of pockets between furs where air blushing that is found only among bare- would be retained providing thermal Itch faced primates and evolution of colour insulation. Of course, goosebumps have Itching, also called pruritus, is a vision; colour vision evolved to detect no obvious functionality in humans, yet response to wide varieties of cues, skin colour changes due to blushing have remained as relics of our distant (Changizi et al. 2006). In other words, ancestry with furry animals. including immune response (histamines), we got our colour camera eyes to see neuropeptides, pain relief (opioids), each other’s emotions expressed through prostaglandins and so on. Itching blushing, not for food! There are many other might be due to a number of diseases of the skin, but can also be a reflex human traits and that generates scratching or rubbing Tickle peculiarities that are response. Tickling, yet another trait like laughing, also found in many also evolved for social bonding among Like yawning, itching can also be primates (Provine 2001). Tickling has other animals yet the contagious; one can get itchy by seeing perplexed a number of great philosophers evolutionary function others itch, or by looking at itching videos. Many people develop this including Plato, Aristotle, Francis remains a mystery. A response just by thinking of itching, Bacon, and Charles Darwin. Of course, number of anatomical one cannot tickle oneself indicating its or while reading about it. While no obvious role in social bonding. traits either might have consensus exists about the evolutionary New research using fMRI scans of adaptive explanations or function of itching, most probably the brain during tickling suggest that this might be mere spandrels. original function would have been a reflex excites the hypothalamus — a survival mechanism. For example, when region that is involved with pain and Sweat a mosquito bites us or when our skin is ‘fight or flight’ response; the original Adrenaline release also results in exposed to an irritant substance (Poison evolutionary function of tickling might sweating response, especially sweaty Ivy for instance), through the itching had been an acknowledgement of defeat palms. Sweat is a physiological response sensation we know where the irritation and submissiveness (Carlsson et al. to thermoregulation when exposed is happening. 2000). This explains why one cannot to high temperature. Sweat helps to Scratching an itchy skin causes the tickle oneself, as the brain is aware that cool the body down, as energy for the brain to release serotonin — a feel-good there is no danger in this. evaporation of sweat droplets is absorbed hormone; the reason why scratching feels from the body and leads to a drop in body so good (Mochizuki and Kakigi 2015). Goosebumps temperature. The scratch reflex is thought to have Goosebumps are yet another vestigial While sweating is believed to have evolved when the earliest vertebrates reflex that happens in response to developed in the common ancestor of evolved, as several vertebrate animals intense emotional stimuli such as fear, primates, it is also found in the armpits including turtles, frogs, goats, dogs, euphoria or sexual arousal. Goosebumps of horses. Sweat glands where sweat cats and so on have this reflex. When occur in many mammalian species is produced is found in all mammals; wounds heal, the mechanical closure of in response to threat; for example however, non-primate mammals have the wound by pulling the nearby tissues in cats and porcupines. The original only very few of them in their skin. stimulates itch nerves nearby causing function would have been to threaten Dogs have a similar thermoregulatory itch sensation. Of course, itching during the predators by assuming a larger body mechanism of panting, while pigs and wound healing is not an adaptive trait (if size (goosebumps make an animal look hippopotamuses roll on to muddy water scratched, obviously the wound breaks much larger than in reality), therefore for the water to evaporate. again), rather a spandrel.

22 | Science Reporter | August 2019 Itching has psychological The gag reflex, or pharyngeal implications as well. As in the case reflex, refers to the contraction of of placebo (a substance that has no the back of the throat when the area therapeutic effect but used as a negative around the tonsils is touched. As this control, or to elicit the psychological reflex prevents objects from entering response of having treated), substances the throat other than that during normal that have no skin irritations, in reality, swallowing, it helps in the prevention can elicit itching sensation if presented of choking-blockage of air supply to the as a real irritant. For example, crushed lungs. The gag reflex is thought to have leaves of a non-irritant plant when evolved only in mammals. presented as if it were poison ivy when While the gag reflex during normal rubbed can elicit itching sensation. eating is common in children, many This psychological effect is called the adults too gag while eating sticky food Snoring is caused by obstruction in the “nocebo” response. such as bananas. Delayed introduction passage of air during sleeping of solid food to children is thought Pain, Gag to be associated with adult gag reflex up at a similar time, yet data on these Similar to itching, the pain reflex is also during normal eating (Gill Rapley MSc lines is non-existent. believed to be a survival mechanism RM 2011). enabling us to know that things are not There are many other human traits right and to take a treatment course, like Sigh, Snore and peculiarities that are also found in many other animals yet the evolutionary proper rest (Bonavita and De Simone Sigh, an audible deep exhalation is function remains a mystery. A number 2011). Pain in animals is a hotly contested yet another bodily reflex that arises in of anatomical traits either might have topic with ramifications in animal ethics, response to a host of emotional cues adaptive explanations or might be mere welfare and consciousness. including depression, relief, despair, spandrels. The list includes exterior The current consensus is that exhaustion, sadness and so on. A popular testes, phallus with no bone, hair, pubic this reflex evolved in the earliest hypothesis is that this function evolved hair and other body hair, nails, beard, mammals such that all mammals can to maintain lung integrity and function eyebrow, and eyelash. feel pain; it might have evolved in the at times of depression (Li et al. 2016). earliest vertebrate even. A number During sigh, persons inflate tiny sacs Similarly, physiological traits such of invertebrates show responses of lungs called alveoli that enable high as colour vision, five digits on limbs, analogous to that in pain; examples levels of gas exchange. While sigh is bipedalism, fever, vomiting and cancer include crustaceans (lobsters, crabs, commonly observed for cats and dogs, might have evolutionary explanations etc.) and cephalopods (octopuses etc.). empirical research on the existence of as well. Evolutionary perspectives of Switzerland banned the practice of sigh in other animals has not yet been traits concerning human behaviour and boiling lobsters live, a standard practice conducted till date. sexuality are also hotly contested topics. The list includes evolutionary purposes of cooking the lobsters in a number of Snoring is caused by the obstruction of right-handedness, daydreaming, countries, as new research indicated in the passage of air during sleeping and sleeping, procrastination, altruism, that the lobsters can feel pain (Elwood is caused by a number of reasons, ranging suicidal behaviour, depression, crying, et al. 2009). from common cold and obesity to sleep vegetarianism (or non-vegetarianism), apnea. Snoring reflex is documented female orgasm, permanently large in a number of animals as well, and is breasts, menopause (“grandmother thought to have evolved in the common hypothesis” is a famous explanation), ancestor of mammals. masturbation, homosexuality, Snoring is alternatively considered promiscuity, kiss, and so on. to be a disease manifestation as well, as Curiosity-driven research on snoring is known to cause a number of evolution continues to light up the associated problems including decreased evolutionary functions, if any, of these libido, decreased concentration, sleep traits. deprivation, irritability, and even ischemic heart diseases and stroke (Koskenvuo et al. 1987). Snoring might Dr Felix Bast is DST-INSPIRE Assistant also have an evolutionary advantage as a Professor of Plant Sciences at the Central University of Punjab, Bathinda. Address: social signal for sleeping (as in the case Department of Plant Sciences, Central Pain reflex is a survival mechanism enabling of yawning); it might enable the hunting University of Punjab, Bathinda-151001. us to know that things are not right pack to sleep together so that they wake Email: [email protected]

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