Is there are evolutionary explanation for why humans and primates are ticklish? How might it have evolved?

Is there are evolutionary explanation for why humans and primates are ticklish? How might it have evolved?

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Tickling is a rather interesting phenomenon: When humans or apes are touched in certain areas like the armpits or sides, we respond with laughter AND frantic attempts to stop the assault. Obviously our ticklish areas tend to be very vulnerable, so it makes sense that we would want to protect ourselves, but why do we respond with laughter? How might both of these reactions to tickling have evolved? Do they have the same evolutionary purpose or different ones?

Tickling probably evolved from a defense mechanism but then gradually changed into a more social action, as explained in Provine, 2005 (PDF):

The neurological mechanism of tickling probably evolved from a reflex defense mechanism that protects our body's surface from external, moving stimuli, probably predators or parasites. Our response to tickle is more varied and complex than the typical reflex, but it has some stereotypic, reflex-like properties (i.e., we laugh when tickled, struggle to escape the tickler, huddle, fend off the tickling hand). Although you can be tickled to laughter by a machine (Harris, 1999) (PDF), most everyday tickle is yet another social context for laughter and a form of communication.

Why Did People Become White?

Humans come in a rainbow of hues, from dark chocolate browns to nearly translucent whites.

This full kaleidoscope of skin colors was a relatively recent evolutionary development, according to biologists, occuring alongside the migration of modern humans out of Africa between 100,000 and 50,000 years ago.

The consensus among scientists has always been that lower levels of vitamin D at higher latitudes — where the sun is less intense — caused the lightening effect when modern humans, who began darker-skinned, first migrated north.

But other factors might be at work, a new study suggests. From the varying effects of frostbite to the sexual preferences of early men, a host of theories have been reviewed.

Vitamin iDea

Vitamin D plays an important role in bone growth and the body's natural protection against certain diseases, and the inability to absorb enough in areas of less-powerful sunlight would have decreased life expectancies in our African ancestors. The further north they trekked, the more vitamin D they needed and the lighter they got over the generations, due to natural selection.

This explanation accounts for the world's gradients of skin color traveling south to north, the prevalence of vitamin D deficiency among African immigrants to higher latitudes, as well as the relatively darker skin of Canada's Inuit peoples, who have good levels of vitamin D despite living in the Arctic, due to their diet rich in oily fish.

Sounds about right . right?

In fact, there might have been a number of concurrent evolutionary pressures at work that contributed to the development of lighter skin, according to a new study published in the August issue of the Journal of Photochemistry and Photobiology B: Biology.

&ldquoIn our opinion the vitamin D hypothesis is one of the most likely hypotheses responsible for skin lightening, although there still is no consensus about it,&rdquo said study author Asta Juzeniene of the Oslo University Hospital in Oslo, Norway.

A number of competing theories were explained and evaluated by Juzeniene and her team, reopening a debate that remains one of the most interesting and controversial in biology.

Paling in comparison

Sexual selection may have played a role, for one, with males preferring paler skin in northern latitudes, the researchers surmised.

&ldquoOne of the hypotheses is that men seem to prefer women with a light skin color, which can be regarded as a sign of youth and fertility,&rdquo Juzeniene told LiveScience. &ldquoBecause light skin characterizes the early infant stage of primates, it may have become a visual cue that triggers appropriate adult behavior toward infants, i.e. decreased aggressiveness and increased desire to provide care and protection,&rdquo she said.

As lighter skin became associated with increased health in northerly latitudes, men may have preferred mates with lighter skin and produced ever-paler generations. Fertility and health statistics at different latitudes from a few thousand years ago aren't available, Juzeniene cautioned, however, so the theory is difficult to test.

Frostbite was another causal effect investigated by the researchers.

Some reports from American soldiers serving in the Korean War and elsewhere have indicated that dark skin is more prone to frostbite than white because it emits more heat. In colder climates, evolution could have negatively selected for paler skin if frostbite was significant enough to perhaps kill darker-skinned children.

Despite the anecdotal evidence, there is not enough scientific data to support frostbite as a strong enough single factor to lighten skin in places such as Europe, the researchers said.

On the farm

Another possibility noted was the switch from subsistence-based economies to agriculture approximately 10,000 years ago, which eliminated vitamin D-rich food sources from the diet. This would have had an especially potent effect in northern Europe, according to Juzeniene and her team.

&ldquoDevelopment of agriculture has occurred in several places, and did not necessarily lead to skin lightening if the ambient UVB [ultraviolet light from the sun] level was sufficiently high to allow adequate vitamin D synthesis. Cold climates and high latitudes would speed up the need for skin lightening,&rdquo however, if people were relying mainly on grains as a food source, the researchers wrote.

The main problem with this agriculture theory is that the switch from gathering to farming occurred relatively recently, and scientists question whether all of the evolutionary changes associated with skin color could have happened so quickly.

Skin lightening could also have been accelerated by something as simple as genetic drift, making it &ldquoeasier&rdquo for a pale skin mutations to succeed in northern latitudes.

Though other elements may have come into play and need to be examined further, vitamin D remains the most likely explanation, Juzeniene stressed, especially given its role in overall health.

&ldquoIf we assume that vitamin D does not play any role in the development of human skin color, neither white nor dark, many people in the world would suffer from vitamin D deficiency,&rdquo she said.

While people of all skin types have the ability to produce the same amount of vitamin D in their systems, &ldquohighly pigmented people will need to stay in the sun around 6 times longer than light people in order to synthesize the same amount of vitamin D,&rdquo Juzeniene said, and a lack of the vitamin — something occurring among many American children right now, partly because they don't get out much — can make humans more susceptible to everything from heart disease to internal cancers.

Primate origins

Humans appeared in southern Africa between 200,000-350,000 years ago. We know we come from Africa because our genetic diversity is highest there, and there are lots of fossils of primitive humans there.

Our closest relatives, chimps and gorillas, are also native to Africa, alongside baboons and monkeys. But primates’ closest living relatives – flying lemurs, tree shrews and rodents – all inhabit Asia or, in the case of rodents, evolved there. Fossils provide somewhat conflicting evidence, but they also suggest primates arose outside of Africa.

Primates have differentiated over tens of millions of years. Nicholas R. Longrich/Wikimedia

The oldest primate relative, Purgatorius, lived 65 million years ago, just after the dinosaurs disappeared. It’s from Montana.

The oldest true primates also occur outside Africa. Teilhardina, related to monkeys and apes, lived 55 million years ago, throughout Asia, North America, and Europe. Primates arrived in Africa later. Lemur-like fossils appear there 50 million years ago, and monkey-like fossils around 40 million years ago.

But Africa split from South America and became an island 100 million years ago, and only connected with Asia 20 million years ago. If primates colonised Africa during the 80 million years the continent spent isolated, then they needed to cross water.

The continents 50 million years ago, when primates colonised Africa. Deeptimemaps , Author provided (no reuse)

Why Are Humans Primates?

Humans share many traits with primates, such as these Barbary macaques, including excellent vision and great dexterity. Image: markhsal/Flickr

I’m a primate. You’re a primate. Everyone reading this blog is a primate. That’s not news. We hear it all he time: Humans are primates. But what does that really mean? What do we have in common with a baboon? Or a creepy aye-aye? Or even our closest living relative, the chimpanzee?

These are simple questions to answer from a genetic perspective—humans share more DNA with lemurs, monkeys and apes than they do with other mammals. Genetic research of the last few decades suggests that humans and all living primates evolved from a common ancestor that split from the rest of the mammals at least 65 million years ago. But even before DNA analyses, scientists knew humans belong in the primate order. Carl Linnaeus classified humans with monkeys, apes and other primates in his 18th-century taxonomic system. Even the ancient Greeks recognized similarities between people and primates. Today, anthropologists recognize several physical and behavioral traits that tie humans to primates.

Primates have nimble hands and forward-facing eyes, as this capuchin monkey demonstrates. Image: Tambako the Jaguar/Flickr

First, primates have excellent vision. They have forward-facing eyes that sit close together, which allows the eyes’ fields of view to overlap and create stereoscopic, or 3-D, vision. (In contrast, for example, a cow or giraffe has widely spaced eyes and therefore poor depth perception.) Related to this great eyesight is the presence of a post-orbital bar, a ring of bone that surrounds the eyeball. Many primates also have a completely bony socket that encloses the eye. This bone probably protects the eye from contractions of chewing muscles that run down the side of the face, from the jaw to the top of the head. Many mammals that rely less on vision don’t have a post-orbital bar. If you poked a dog in the side of its head near the temple, you would feel muscle and the eye but no bone (and you would probably be bitten, so please don’t do that). Because primates depend on their vision so much, they generally have a reduced sense of smell relative to other mammals.

Primates are also very dexterous. They can manipulate objects with great skill because they have opposable thumbs and/or big toes, tactile finger pads and nails instead of claws (although some primates have evolved so-called grooming claws on some of their toes). Primates also generally have five fingers/toes on each hand/foot. This is actually a very ancient trait. The earliest mammals had five digits, and over time, many mammalian lineages lost a few fingers and toes while primates kept all of them. Primates also retain collar bones, which allow for greater mobility in the shoulder mammals that strictly walk on all fours, such as horses, lack collar bones so their limbs are more stable and don’t slip to the side while running.

And in general, primates tend to have larger brains than other mammals of a similar size. They also have smaller litters—often just one baby at a time—and longer periods of gestation and childhood.

Scientists are still trying to understand why primates’ unique set of features evolved. Some researchers think the earliest primates lived in trees, so good vision and dexterity would have been helpful in judging distances between branches or for climbing around. Others, such as Boston University’s Matt Cartmill, have suggested that these traits emerged because early primates might have been insect predators and needed clear eyesight and quick hands to grab prey. Both factors, as well as many others, could have played a role.

Our primate ancestors have been laughing for 10m years

The first hoots of laughter from an ancient ancestor of humans rippled across the land at least 10 million years ago, according to a study of giggling primates.

Researchers used recordings of apes and babies being tickled to trace the origins of laughter back to the last common ancestor that humans shared with the modern great apes, which include chimpanzees, gorillas and orang-utans.

The finding challenges the view that laughter is a uniquely human trait, suggesting instead that it emerged long before humans split from the evolutionary path that led to our primate cousins, between 10m and 16m years ago.

"In humans, laughing is a complex and intriguing expression. It can be the strongest way of expressing how much we are enjoying ourselves, but it can also be used in other contexts, like mocking," said Marina Davila Ross, a psychologist at Portsmouth University. "I was interested in whether laughing had a pre-human basis, whether it emerged earlier on than we did."

Davila Ross travelled to seven zoos around Europe and visited a wildlife reserve in Sabah, Borneo, to record baby and juvenile apes while their caretakers tickled them. Great apes are known to make noises that are similar to laughter when they are excited and while they are playing with each other.

"The caretakers play with the apes all the time and tickling is a very important part of that. There are certain body parts that are more ticklish than others, depending on the individual. Some were tickled on their necks or armpits, while others offered their feet to be tickled," said Davila Ross.

In total, Davila Ross collected recordings of mirth from 21 chimps, gorillas, orang-utans and bonobos and added recordings of three babies that were tickled to make them laugh.

To analyse the recordings, the team fed them into a computer program that arranged them on an "evolutionary tree" based on how related to each other they seemed to be. Remarkably, the laughter recorded from different primates linked together in a way that matched the evolutionary tree linking all of the species to one common ancestor.

"Our evolutionary tree based on these acoustic recordings alone showed that humans were closest to chimps and bonobos, but furthest from orang-utans, with gorillas somewhere intermediate. And that is what you see in the well-established evolutionary tree of great apes," said Davila Ross. "What this shows is strong evidence to suggest that laughing comes from a common primate ancestor."
Writing in the journal Current Biology, the researchers describe how the earliest laughter-like sounds were shorter and noisier, but with time became longer and clearer as the great apes evolved.

Human laughter sounds very different from the noises produced by great apes. The differences are thought to have arisen when certain acoustic features became exaggerated in early humans after they split from ancestors they shared with chimps and bonobos around 5.5m years ago.

Humans laugh as they exhale, but chimps can laugh as they breathe in as well. The human laugh is also produced by more regular vibrations of the vocal cords than in any of the apes.

Few studies have been carried out into the role of laughter in primates, but at least one study has suggested that it is important in expressing excitement and arousal. Laughing might also have been important for bonding within groups of animals.

Robert Provine, a psychologist and neuroscientist at the University of Maryland and author of the book Laughter: A Scientific Investigation, said students who took part in his own studies likened chimp "laughter" to a dog panting, an asthma attack or hyperventilation. Some even thought the noise was caused by someone sawing.

"The means of production of human and ape laughter are as different as the sound, with the ape vocalisation being produced during both inward and outward breaths, while the human parse an outward breath into 'ha-ha'," he said.

"The simplicity and stereotypy of laughter provides a valuble tool with which to trace vocal evolution, much as simpler systems of molecular biology are useful for investigating complex life processes," he added.

In March, reseachers reported that a chimp at a zoo in Sweden had started to challenge scientists' views about the unique nature of human behaviour.

The 31-year-old male, Santino, regularly displayed thuggish behaviour by preparing piles of rocks while the zoo was closed and then lobbing them at visitors when the gates opened. The chimp has since been castrated.

Zookeepers at the Smithsonian National Zoo in Washington DC have reported another human trait in one of its long-time residents, Bonnie, a 30-year-old orang-utan. Researchers believe Bonnie learned to whistle by copying the zookeepers. Although she is unable to hold a tune, other apes at the zoo have reportedly begun copying her.

The Evolutionary Biology of Altruism

I have been thinking a lot about these questions this Christmas Day and filtering my observations through the lens of all the exciting scientific research about the evolutionary biology of altruism reported this year.

Practicing love and kindness to others actually benefits you, your family, your social network, and your community at large. Even if you are feeling ‘selfish’, behaving selflessly may be the wisest ‘self-serving’ thing to do. If you want to have a competitive advantage in the long-run, science confirms that altruism, compassion and cooperation are all key ingredients for your success.

2012 was a hallmark year for scientific progress in understanding the evolutionary biology behind altruism, compassion and the importance of community. Neuroscientists have made huge progress in understanding our “social brain” which consists of structures and circuits that help us understand one anothers' intentions, beliefs, desires, and how to behave appropriately.

In this entry, I will connect the dots between all of this research and create a timeline that will hopefully be a resource as we try to find ways to create more loving-kindness in our society and less violence and bloodshed.

Christmas Day 2012

I woke up early this Christmas morning. While I was waiting for the water to boil I noticed a book called “Essays of E.B. White” on the kitchen table and started flipping through it. I stumbled on an essay called Unity which E.B. White wrote in 1960. I had been reading a lot of science articles about the evolutionary importance of community, cooperation, and empathy lately and the words from his essay hit home:

“Most people think of peace as a state of Nothing Bad Happening, or Nothing Much Happening. Yet if peace is to overtake us and make us the gift of serenity and well being, it will have to be the state of Something Good Happening. What is this good thing? I think it is the evolution of community.”

My mom has a December 24th tradition of spending the day with her good friend and next-door-neighbor at "The Haven” which is a local food bank. They distribute food to individuals and families in the community who are in need. Last night she came home with heartwarming (and heartbreaking) stories of various people who had come to the food bank that day. My mom doesn’t consider working at The Haven “volunteering”, or a sacrifice. Not because she’s saintly, or more altruistic than most. My mom realized a long time ago that it made her feel better around the holidays to connect with other people in the community from all walks of life than to sit at home all day by the fire with family, indulging. Scientists continue to confirm that her empirical findings and intuitions can be backed up in a laboratory or clinical studies.

The Evolutionary Biology of Altruism

In 1975, Harvard biologist E. O. Wilson published Sociobiology, which was viewed by most people at the time to be the most important evolutionary theory since On the Origin of Species. Darwin’s theory of Natural Selection and the “survival of the fittest” implied a machiavellian world in which individuals clawed their way to the top. Wilson offered a new perspective which was that certain types of social behaviors— including altruism—are often genetically programmed into a species to help them survive.

In the context of Darwin’s theory of 'every man for himself' Natural Selection, this kind of selflessness or altruism did not compute. E.O. Wilson resolved the paradox with a ‘one for all and all for one’ theory called “kin selection”.

According to the kin selection theory, altruistic individuals would prevail because the genes that they shared with kin would be passed on. Since the whole clan is included in the genetic victory of a few, the phenomenon of beneficial altruism came to be known as “inclusive fitness.” By the 1990s this had become a core concept of biology, sociology, even pop psychology.

As a gay person who came out in the 1980s, I always felt a very close ‘familial’ connection with my peers. The LGBT community was my clan and I was loyal to any member of my group who had the courage to come out. In the mid-80s I wrote a college paper about Sociobiology and Homosexuality. I always had a problem with E.O. Wilson's ideas of kin selection and altruism based on genetics. This was reconfirmed when I joined ACT-UP in the late 80s and witnessed fierce altruism in action with no genetic ties as we formed a coalition and took to the streets.

In 2010, E.O. Wilson announced that he no longer endorsed the kin selection theory he had developed for decades. This caused a big stir in evolutionary biologist circles. He acknowledged that according to kin theory, that altruism arises when the "giver" has a genetic stake in the game. But after a mathematical assessment of the natural world, Wilson and his colleagues at Harvard University decided that altruism evolved for the good of the community rather than for the good of individual genes. As Wilson put it, cooperating groups dominate groups who do not cooperate.

Wilson’s new research indicates that self-sacrifice to protect a relation’s genes does not drive evolution. In human terms, family is not so important after all altruism emerges to protect social groups whether they are kin or not. I think this is important for all of us to remember as we try to unite and bridge our differences. One caveat here, sticking too much with the group can be a bad thing, too.

When people compete against one other they are selfish, but when group selection becomes important, then the altruism characteristic of human societies kicks in, Wilson says. “We may be the only species intelligent enough to strike a balance between individual and group-level selection, but we are far from perfect at it. The conflict between the different levels may produce the great dramas of our species: the alliances, the love affairs, and the wars.”

Scientists confirm that we must cooperate to survive.

In November of 2012, Wilson’s theory was backed up by Michael Tomasello and researchers in the Department of Developmental and Comparative Psychology at the The Max Planck Institute for Evolutionary Anthropology. Their research, published by Current Anthropology offers an explanation why humans are much more inclined to cooperate than are their closest evolutionary relatives.

The prevailing wisdom about why this is true has long been focused on the idea of altruism: we go out of our way to do nice things for other people, sometimes even sacrificing personal success for the good of others. Modern theories of cooperative behavior suggest that acting selflessly in the moment provides a selective advantage to the altruist in the form of some kind of return benefit.

The authors of the study argue that humans developed cooperative skills because it was in their mutual interest to work well with others—practical circumstances often forced them to cooperate with others to obtain food. In other words, altruism isn't the reason we cooperate we must cooperate in order to survive, and we are altruistic to others because we need them for our survival.

Previous theories located the origin of cooperation in either small group settings or large, sophisticated societies. Based on results from cognitive and psychological experiments and research on human development, this study provides a comprehensive account of the evolution of cooperation as a two-step process, which begins in small hunter-gatherer groups and becomes more complex and culturally inscribed in larger societies later on.

The authors premise their theory of mutualistic cooperation on the principle of interdependence. They speculate that at some point in our evolution, it became necessary for humans to forage together, which meant that each individual had a direct stake in the welfare of his or her partners. Individuals who were able to coordinate well with their fellow foragers, and would pull their weight in the group, were more likely to succeed.

In this context of interdependence, humans evolved special cooperative abilities that other apes do not possess, including dividing the spoils fairly, communicating goals and strategies, and understanding one's role in the joint activity as equivalent to another's.

As societies grew in size and complexity, their members became even more dependent on one another. In what the authors of this study define as a second evolutionary step, these collaborative skills and impulses were developed on a larger scale as humans faced competition from other groups. People became more "group-minded," identifying with others in their society even if they did not know them personally. This new sense of belonging brought about cultural conventions, norms, and institutions that incentivized and structured feelings of social responsibility.

Our "Social Brain" may have a specific region hard-wired to share.

Research appearing in the December 24, 2012 journal Nature Neuroscience found that although a monkey would probably never agree that it is better to give than to receive, they do get some reward in a specific brain region from giving to another monkey.

The experiment consisted of a task in which rhesus macaques had control over whether they, or another monkey, would receive a squirt of fruit juice. Three distinct areas of the brain were found to be involved in weighing benefits to oneself against benefits to the other, according to a new research study by the Duke Institute for Brain Sciences and the Center for Cognitive Neuroscience. This research, led by Michael Platt, is another piece of the puzzle as neuroscientists search for the roots of charity, altruism and other social behaviors in our species and others.

There have been two schools of thought about how the social reward system is set up, Platt said. "One holds that there is generic circuitry for rewards that has been adapted to our social behavior because it helped humans and other social animals like monkeys thrive. Another school holds that social behavior is so important to humans and other highly social animals like monkeys that there may be some special circuits for it." This research is part of a new field of study into what neuroscientists are calling the Social Brain.

Using a computer screen to allocate juice rewards, the monkeys preferred to reward themselves first and foremost. But, they also chose to reward the other monkey if it meant no juice for either of them. Also, monkeys were more likely to give the reward to a monkey they knew over one they didn't. Interestingly, they preferred to give juice to lower status than higher status monkeys. And lastly, they had almost no interest in giving the juice to an inanimate object.

The team used sensitive electrodes to detect the activity of individual neurons as the animals weighed different scenarios, such as whether to reward themselves, the other monkey, or nobody at all. Three areas of the brain were seen to weigh the problem differently depending on the social context of the reward. When given the option either to drink juice from a tube themselves or to give the juice away to a neighbor, the test monkeys would mostly keep the drink. But when the choice was between giving the juice to the neighbor or neither monkey receiving it, the choosing monkey would frequently opt to give the drink to the other monkey.

Through the development of the specific part of the brain that experiences the reward of others, social decisions and empathy-like processes may have been favored during evolution in primates to allow altruistic behaviour. “This may have evolved originally to promote being nice to family, since they share genes, and later friends, for reciprocal benefits,” says Michael Platt.

Anterior cingulate gyrate (ACCg) in yellow

The authors suggest that the intricate balance between the signalling of neurons in these three brain regions may be crucial for normal social behavior in humans, and that disruption may contribute to various psychiatric conditions, including autistic spectrum disorders.

“This is the first time that we have had quite such a complete picture of the neuronal activity underlying a key aspect of social cognition. It is definitely a major achievement,” says Matthew Rushworth, a neuroscientist at the University of Oxford, UK.

Neuroscientists have discovered the seat of human compassion.

In September of 2012, an international team led by researchers at Mount Sinai School of Medicine in New York published research in the journal Brain declaring that: “one area of the brain, called the anterior insular cortex, is the activity center of human empathy, whereas other areas of the brain are not.” The insula is a hidden region folded and tucked away deep in the brain. It is an island within the cortex.

This most recent study firmly establishes that the anterior insular cortex is where feelings of empathy originate. "Now that we know the specific brain mechanisms associated with empathy, we can translate these findings into disease categories and learn why these empathic responses are deficient in neuropsychiatric illnesses, such as autism," said Patrick R. Hof, MD, a co-author of the study. "This will help direct neuropathologic investigations aiming to define the specific abnormalities in identifiable neuronal circuits in these conditions, bringing us one step closer to developing better models and eventually preventive or protective strategies."

According to Dr. Gu, another researcher on this study, this provides the first evidence suggesting that the empathy deficits in patients with brain damage to the anterior insular cortex are surprisingly similar to the empathy deficits found in several psychiatric diseases, including autism spectrum disorders, borderline personality disorder, schizophrenia, and conduct disorders, suggesting potentially common neural deficits in those psychiatric populations.

"Our findings provide strong evidence that empathy is mediated in a specific area of the brain," said Dr. Gu, who now works at University College London. "The findings have implications for a wide range of neuropsychiatric illnesses, such as autism and some forms of dementia, which are characterized by prominent deficits in higher-level social functioning."

This study suggests that behavioral and cognitive therapies can be developed to compensate for deficits in the anterior insular cortex and its related functions such as empathy in patients. These findings can also inform future research evaluating the cellular and molecular mechanisms underlying complex social functions in the anterior insular cortex and develop possible pharmacological treatments for patients.

We’re all in this together. We have not evolved for millennia to be isolated behind digital screens, connected only via text message and social media, or to grow up playing violent video games in windowless basements.

Science proves that our genes and our brains have evolved to be compassionate, to cooperate, and to foster community. This is common sense. Hopefully, the science presented here reinforces what we already know intuitively. Being altruistic and kind to one another benefits us all.

Why Did Humans Lose Their Fur?

Millions of modern humans ask themselves the same question every morning while looking in the mirror: Why am I so hairy? As a society, we spend millions of dollars per year on lip waxing, eyebrow threading, laser hair removal, and face and leg shaving, not to mention the cash we hand over to Supercuts or the neighborhood salon. But it turns out we are asking the wrong question—at least according to scientists who study human genetics and evolution. For them, the big mystery is why we are so hairless.

Evolutionary theorists have put forth numerous hypotheses for why humans became the naked mole rats of the primate world. Did we adapt to semi-aquatic environments? Does bare skin help us sweat to keep cool while hunting during the heat of the day? Did losing our fur allow us to read each other's emotional responses such as fuming or blushing? Scientists aren't exactly sure, but biologists are beginning to understand the physical mechanism that makes humans the naked apes. In particular, a recent study in the journal Cell Reports has begun to depilate the mystery at the molecular and genetic level.

Sarah Millar, co-senior author of the new study and a dermatology professor at the University of Pennsylvania’s Perelman School of Medicine, explains that scientists are largely at a loss to explain why different hair patterns appear across human bodies. “We have really long hair on our scalps and short hair in other regions, and we’re hairless on our palms and the underside of our wrists and the soles of our feet,” she says. “No one understands really at all how these differences arise.”

In many mammals, an area known as the plantar skin, which is akin to the underside of the wrist in humans, is hairless, along with the footpads. But in a few species, including polar bears and rabbits, the plantar area is covered in fur. A researcher studying the plantar region of rabbits noticed that an inhibitor protein, called Dickkopf 2 or Dkk2, was not present in high levels, giving the team the fist clue that Dkk2 may be fundamental to hair growth. When the team looked at the hairless plantar region of mice, they found that there were high levels of Dkk2, suggesting the protein might keep bits of skin hairless by blocking a signaling pathway called WNT, which is known to control hair growth.

To investigate, the team compared normally developing mice with a group that had a mutation which prevents Dkk2 from being produced. They found that the mutant mice had hair growing on their plantar skin, providing more evidence that the inhibitor plays a role in determining what’s furry and what’s not.

But Millar suspects that the Dkk2 protein is not the end of the story. The hair that developed on the plantar skin of the mice with the mutation was shorter, finer and less evenly spaced than the rest of the animals’ hair. “Dkk2 is enough to prevent hair from growing, but not to get rid of all control mechanisms. There’s a lot more to look at.”

Even without the full picture, the finding could be important in future research into conditions like baldness, since the WNT pathway is likely still present in chrome domes—it’s just being blocked by Dkk2 or similar inhibitors in humans. Millar says understanding the way the inhibitor system works could also help in research of other skin conditions like psoriasis and vitiligo, which causes a blotchy loss of coloration on the skin.

A reconstruction of the head of human ancestor Australopithecus afarensis, an extinct hominin that lived between about 3 and 4 million years ago. The famous Lucy skeleton belongs to the species Australopithecus afarensis. (Photo by Tim Evanson / Reconstruction by John Gurche / Flickr / CC BY-SA 2.0)

With a greater understanding of how skin is rendered hairless, the big question remaining is why humans became almost entirely hairless apes. Millar says there are some obvious reasons—for instance, having hair on our palms and wrists would make knapping stone tools or operating machinery rather difficult, and so human ancestors who lost this hair may have had an advantage. The reason the rest of our body lost its fur, however, has been up for debate for decades.

One popular idea that has gone in and out of favor since it was proposed is called the aquatic ape theory. The hypothesis suggests that human ancestors lived on the savannahs of Africa, gathering and hunting prey. But during the dry season, they would move to oases and lakesides and wade into shallow waters to collect aquatic tubers, shellfish or other food sources. The hypothesis suggests that, since hair is not a very good insulator in water, our species lost our fur and developed a layer of fat. The hypothesis even suggests that we might have developed bipedalism due to its advantages when wading into shallow water. But this idea, which has been around for decades, hasn’t received much support from the fossil record and isn’t taken seriously by most researchers.

A more widely accepted theory is that, when human ancestors moved from the cool shady forests into the savannah, they developed a new method of thermoregulation. Losing all that fur made it possible for hominins to hunt during the day in the hot grasslands without overheating. An increase in sweat glands, many more than other primates, also kept early humans on the cool side. The development of fire and clothing meant that humans could keep cool during the day and cozy up at night.

But these are not the only possibilities, and perhaps the loss of hair is due to a combination of factors. Evolutionary scientist Mark Pagel at the University of Reading has also proposed that going fur-less reduced the impact of lice and other parasites. Humans kept some patches of hair, like the stuff on our heads which protects from the sun and the stuff on our pubic regions which retains secreted pheromones. But the more hairless we got, Pagel says, the more attractive it became, and a stretch of hairless hide turned into a potent advertisement of a healthy, parasite-free mate.

One of the most intriguing theories is that the loss of hair on the face and some of the hair around the genitals may have helped with emotional communication. Mark Changizi, an evolutionary neurobiologist and director of human cognition at the research company 2AI, studies vision and color theory, and he says the reason for our hairless bodies may be in our eyes. While many animals have two types of cones, or the receptors in the eye that detect color, humans have three. Other animals that have three cones or more, like birds and reptiles, can see in a wide range of wavelengths in the visible light spectrum. But our third cone is unusual—it gives us a little extra power to detect hues right in the middle of the spectrum, allowing humans to pick out a vast range of shades that seem unnecessary for hunting or tracking.

Changizi proposes that the third cone allows us to communicate nonverbally by observing color changes in the face. “Having those two cones detecting wavelengths side by side is what you want if you want to be sensitive to oxygenation of hemoglobin under the skin to understand health or emotional changes,” he says. For instance, a baby whose skin looks a little green or blue can indicate illness, a pink blush might indicate sexual attraction, and a face flushing with red could indicate anger, even in people with darker skin tones. But the only way to see all of these emotional states is if humans lose their fur, especially on their faces.

In a 2006 paper in Biology Letters, Changizi found that primates with bare faces and sometimes bare rumps also tended to have three cones like humans, while fuzzy-faced monkeys lived their lives with just two cones. According to the paper, hairless faces and color vision seem to run together.

Millar says that it’s unlikely that her work will help us directly figure out whether humans are swimming apes, sweaty monkeys or blushing primates. But combining the new study’s molecular evidence of how hair grows with physical traits observed in humans will get us closer to the truth—or at least closer to a fuller, shinier head of hair.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

Did an Unlikely Ocean Crossing Give Rise to Human Evolution?

Published Apr 29, 2021 6:20 PM by The Conversation

Humans evolved in Africa, along with chimpanzees, gorillas and monkeys. But primates themselves appear to have evolved elsewhere &ndash likely in Asia &ndash before colonizing Africa. At the time, around 50 million years ago, Africa was an island isolated from the rest of the world by ocean &ndash so how did primates get there?

A land bridge is the obvious explanation, but the geological evidence currently argues against it. Instead, we&rsquore left with a far more unlikely scenario: early primates may have rafted to Africa, floating hundreds of miles across oceans on vegetation and debris.

Such oceanic dispersal was once seen as far-fetched and wildly speculative by many scientists. Some still support the land bridge theory, either disputing the geological evidence, or arguing that primate ancestors crossed into Africa long before the current fossil record suggests, before the continents broke up.

But there&rsquos an emerging consensus that oceanic dispersal is far more common than once supposed. Plants, insects, reptiles, rodents and primates have all been found to colonise island continents in this way &ndash including a remarkable Atlantic crossing that took monkeys from Africa to South America 35 million years ago. These events are incredibly rare but, given huge spans of time, such freak events inevitably influence evolution &ndash including our own origins.

Primate origins

Humans appeared in southern Africa between 200,000-350,000 years ago. We know we come from Africa because our genetic diversity is highest there, and there are lots of fossils of primitive humans there.

Our closest relatives, chimps and gorillas, are also native to Africa, alongside baboons and monkeys. But primates&rsquo closest living relatives &ndash flying lemurs, tree shrews and rodents &ndash all inhabit Asia or, in the case of rodents, evolved there. Fossils provide somewhat conflicting evidence, but they also suggest primates arose outside of Africa.

Primates have differentiated over tens of millions of years. Nicholas R. Longrich/Wikimedia

The oldest primate relative, Purgatorius, lived 65 million years ago, just after the dinosaurs disappeared. It&rsquos from Montana.

The oldest true primates also occur outside Africa. Teilhardina, related to monkeys and apes, lived 55 million years ago, throughout Asia, North America, and Europe. Primates arrived in Africa later. Lemur-like fossils appear there 50 million years ago, and monkey-like fossils around 40 million years ago.

But Africa split from South America and became an island 100 million years ago, and only connected with Asia 20 million years ago. If primates colonized Africa during the 80 million years the continent spent isolated, then they needed to cross water.

Ocean crossings

The idea of oceanic dispersal is central to the theory of evolution. Studying the Galapagos Islands, Darwin saw only a few tortoises, iguanas, snakes, and one small mammal, the rice rat. Further out to sea, on islands like Tahiti, were only little lizards.

Darwin reasoned that these patterns were hard to explain in terms of Creationism &ndash in which case, similar species should exist everywhere &ndash but they made sense if species crossed water to colonize islands, with fewer species surviving to colonize more distant islands.

He was right. Studies have found tortoises can survive weeks afloat without food or water &ndash they probably bobbed along until hitting the Galapagos. And in 1995, iguanas swept offshore by hurricanes washed up 300 kilometers away, very much alive, after riding on debris. Galapagos iguanas likely travelled this way.

The odds are against such crossings. A lucky combination of conditions &ndash a large raft of vegetation, the right currents and winds, a viable population, a well-timed landfall &ndash is needed for successful colonisation. Many animals swept offshore simply die of thirst or starvation before hitting islands. Most never make landfall they disappear at sea, food for sharks. That&rsquos why ocean islands, especially distant ones, have few species.

Rafting was once treated as an evolutionary novelty: a curious thing happening in obscure places like the Galapagos, but irrelevant to evolution on continents. But it&rsquos since emerged that rafts of vegetation or floating islands &ndash stands of trees swept out to sea &ndash may actually explain many animal distributions across the world.

Several primate rafting events are well established. Today, Madagascar has a diverse lemur fauna. Lemurs arrived from Africa around 20 million years ago. Since Madagascar has been an island since the time of the dinosaurs, they apparently rafted the 400 kilometer-wide Mozambique Channel. Remarkably, fossils suggest the strange aye-aye crossed to Madagascar separately from the other lemurs.

Even more extraordinary is the existence of monkeys in South America: howlers, spider monkeys and marmosets. They arrived 35 million years ago, again from Africa. They had to cross the Atlantic &ndash narrower then, but still 1,500 km wide. From South America, monkeys rafted again: to North America, then twice to the Caribbean.

But before any of this could happen, rafting events would first need to bring primates to Africa: one brought the ancestor of lemurs, another carried the ancestor of monkeys, apes, and ourselves. It may seem implausible &ndash and it&rsquos still not entirely clear where they came from &ndash but no other scenario fits the evidence.

Rafting explains how rodents colonised Africa, then South America. Rafting likely explains how Afrotheria, the group containing elephants and aardvarks, got to Africa. Marsupials, evolving in North America, probably rafted to South America, then Antarctica, and finally Australia. Other oceanic crossings include mice to Australia, and tenrecs, mongooses and hippos to Madagascar.

Oceanic crossings aren&rsquot an evolutionary subplot they&rsquore central to the story. They explain the evolution of monkeys, elephants, kangaroos, rodents, lemurs &ndash and us. And they show that evolution isn&rsquot always driven by ordinary, everyday processes but also by bizarrely improbable events.


One of Darwin&rsquos great insights was the idea that everyday events &ndash small mutations, predation, competition &ndash could slowly change species, given time. But over millions or billions of years, rare, low-probability, high-impact events &ndash &ldquoblack swan&rdquo events &ndash also happen.

Some are immensely destructive, like asteroid impacts, volcanic eruptions, and ice ages &ndash or viruses jumping hosts. But others are creative, like genome duplications, gene transfer between multicellular species &ndash and rafting.

The role rafting played in our history shows how much evolution comes down to chance. Had anything gone differently &ndash the weather was bad, the seas rough, the raft washed up on a desert island, hungry predators waited on the beach, no males aboard &ndash colonisation would have failed. No monkeys, no apes &ndash no humans.

It seems our ancestors beat odds that make Powerball lotteries seem like a safe bet. Had anything had gone differently, the evolution of life might look rather different than it does. At a minimum, we wouldn&rsquot be here to wonder about it.

Nicholas R. Longrich is a Senior Lecturer in Evolutionary Biology and Paleontology at University of Bath.

This article appears courtesy of The Conversation and may be found in its original form here.

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.

The Human Evolution Blog

The ABCs of Vitamin Deficiency: Why Human Have Such a Needy Diet

It’s a truth we learn as young children in discussions about food groups, the food pyramid, a “balanced diet,” and so forth: humans need to eat a wide variety of food in order to be healthy. A little bit of this, a little bit of that, not too much of X, don’t forget Y. Broccoli is great for magnesium and several vitamins Bananas give us potassium Carrots are a good source of vitamin A we need some meat or nuts for protein Eggs are important for vitamin D gotta get that calcium from dairy, and so on. It’s no wonder that so many of us just give up on achieving the right balance in our diet and instead pop supplements (few of which live up to their labels).

A vegan food pyramid

We’re so accustomed to the neediness of our diet that it rarely registers just how strange this is compared to other species. Our companion animals eat a very simple diet day in and day out and yet they do not suffer malnutrition for it. If we tried to subsist solely on lamb and rice, like many dogs do, we would eventually succumb to one of several possible deficiencies of vitamins and minerals. The koala can be perfectly healthy living purely on eucalyptus leaves, which are not very nutritious by our standards.

What gives? Why do we have such demanding dietary needs? Is there something seriously wrong with our relationship to food? The short answer is probably yes. As I have found myself saying a lot lately, humans are a pretty flawed species. We have an incredible number of quirks and glitches that defy simple logic and thus call out for explanation. While that may seem depressing, it’s actually pretty uplifting because many of these flaws do have explanations that are deeply informative about our past, a past in which we have persevered against all odds. Our needy diet is a perfect example.

Take vitamin C. As I’ve written before, the dietary requirement for vitamin C (ascorbic acid) is actually an oddity. The vast majority of animals make this micronutrient for themselves right in their liver. Unfortunately, an ancestor of all primates lost this ability many millions of years ago and primates have been dependent on dietary vitamin C ever since. This has restricted primates to climates where vitamin C is readily obtainable and has rendered them sensitive to scurvy. With few exceptions, non-primates have no fear of scurvy.

Our need for vitamin D is also strange. As I explain for Discover Magazine, vitamin D is mainly needed for calcium homeostasis in our bodies and thus strong bones. We cannot absorb and retain calcium unless we have enough vitamin D. This may be poor design right off the bat because it means there are two ways to become calcium deficient, either not getting enough calcium or not getting enough vitamin D. But the oddity of vitamin D biology is much deeper than that.

Yes, we can synthesize vitamin D totally on our own, but it involves three steps in three different tissues and also requires activation by sunlight in our skin. The need for sunlight puts us at risk for both skin cancer and folate deficiency. (Because sunlight destroys folate in our skin, too much of it zaps our body of this important nutrient, especially problematic for pregnant women.) So the story of our need for vitamin D is a lethal tug-of-war between skin cancer and folate deficiency on one side and rickets or osteoporosis on the other. This is an almost purely human problem which explains the widespread deficiency in vitamin D, even in the developed world. (Read my article for a more detailed explanation why other mammals don’t have this problem.)

Next stop, vitamin B12. This is another really strange one. Most vegans are familiar with B12 because it is the one micronutrient that we simply cannot get from any plant sources. Only animal products provide vitamin B12 so vegans must supplement. B12 plays a role in some of our most basic biochemical reactions such as the synthesis of amino acids and fatty acids, which are the building blocks of proteins and lipids, respectively. This begs the question: if B12 is so important but only animal products provide it, how do the many herbivore animals survive?

This is where it gets really weird. It turns out that herbivore animals harbor bacteria in their intestines that make vitamin B12 for them. All they have to do is absorb it. So why don’t we have those same bacteria? We do! So why don’t they make vitamin B12 for us? They do! But these helpful bacteria are in our large intestines and for some weird reason, we can only absorb vitamin B12 in our small intestine. This is why we still need to get it in our diet, even though we have plenty of it already in our guts. (I muse about the possible reasons for this in this article.)

Vitamins C, D, and B12 are just three micronutrients that humans, for some reason, have a tougher time getting than most other animals, but they do not stand alone. We scramble to get enough of some minerals as well, such as iron, calcium, and iodine. I devote a whole chapter of my book Human Errors to this theme. It seems to me that there are so many separate nutrients that we struggle to get enough of that there must be some kind of larger explanation. In my book, I describe one possible explanation, summarized below.

If we trace the last 10-15 million years of human evolution, for nearly all of that time, our ancestors were living in sub-Saharan Africa, in either the lush rainforests or the nearby bountiful grasslands. This environment teems with a wide variety of nutritious food. Without going into all the details of our transition from vegetarians to omnivorous scavengers to successful hunters (though not strict carnivores), our history is marked by our efforts to find and make use of diverse food sources. Along the way, we seem to have lost our ability to make certain nutrients for ourselves.

Although Lamarck’s theory of use and disuse has been mostly rejected as a mechanism for evolution, the general principle was not far off the mark. It is a general trend that body parts that are no longer critical for success eventually go away. It is not from the disuse itself, as Lamarck imagined, but because the constant onslaught of random mutation (and subsequent genetic drift) leads to the degradation of everything that is not maintained through natural selection.

If we apply this principle to our diet, the corollary goes something like this: if a given animal lineage is constantly provided with a certain micronutrient in their diet, they may eventually lose the ability to manufacture that micronutrient for themselves. That’s an oversimplification, but you get the point. Our ancestors stopped making vitamin C because they suffered a mutation in their GULO gene. This was tolerated because they already had vitamin C in their diet. Our ancestors stopped absorbing bacteria-produced vitamin B12 in their large intestine, because scavenging was providing dietary B12 in their small intestine. And so on.

Prior to the dawn of agriculture 10 to 15 thousand years ago, the dietary insufficiencies we now suffer with might never have been an issue. However, farming brought a narrowing of our diet to a few main foods and heavy reliance on just one or two staples. Worse, those staples are rich with carbohydrates but poor in most of the micronutrients we need. Yes, agriculture allowed us to “feed the masses,” but not necessarily with the kind of food that we really need. While hunter-gather tribes often face starvation and undernourishment, they do not seem plagued by many of the vitamin deficiencies we now contend with.

It’s been long known that modern diets do not at all resemble that of our mesolithic ancestors, not to mention our paleolithic ones. This has led to a number of obstacles to healthy eating habits deficiencies of certain micronutrients is just one of them. Given how completely dependent we are on the various starchy staples that form the base of cuisines worldwide, there is no easy fix for this. Unless you’re up for gnawing on tough roots, tiny fleshy fruits, bitter leaves, slimy worms, and the occasional plate of bone marrow (usually raw, of course), a return to the true “paleo” diet isn’t really an option.

Sex Essential Reads

Parenting and Children’s Sociosexual Behavior

How to Talk to Black Girls about Sex

Trying to pigeonhole sex into one or two “real” explanations is fruitless and damaging. A better approach is to recognize the multiple influences on our actions, to recognize that there are many fruitful ways to successfully be human, and to try to understand how sex and sexuality emerge and play out for individuals, in societies, and across our species.

Evolutionary hypotheses, societal expectations, and personal experience all matter in explaining why we do what we do. Choosing one as more important than the others is not going to make sex any less complex, or easier to deal with. As the science writer Carl Zimmer tells us about the vinegar worm, even “in the simplest animal imaginable, sex can be wonderfully difficult to decipher.”

Early Human Evolution

Modern humans and chimpanzees evolved from a common hominoid ancestor that diverged approximately 6 million years ago.

Learning Objectives

List the evolved physical traits used to differentiate hominins from other hominoids

Key Takeaways

Key Points

  • Modern humans are classified as hominins, which also includes extinct bipedal human relatives, such as Australopithecusafricanus, Homohabilis, and Homo erectus.
  • Few very early (prior to 4 million years ago) hominin fossils have been found so determining the lines of hominin descent is extremely difficult.
  • Within the last 20 years, three new genera of hominoids were discovered: Sahelanthropus tchadensis, Orrorin tugenensis, and Ardipithecus ramidus and kadabba, but their status in regards to human ancestry is somewhat uncertain.

Key Terms

  • hominin: the evolutionary group that includes modern humans and now-extinct bipedal relatives
  • hominoid: any great ape (such as humans) belonging to the superfamily Hominoidea

Human Evolution

The family Hominidae of order Primates includes chimpanzees and humans. Evidence from the fossil record and from a comparison of human and chimpanzee DNA suggests that humans and chimpanzees diverged from a common hominoid ancestor approximately 6 million years ago. Several species evolved from the evolutionary branch that includes humans, although our species is the only surviving member. The term hominin (or hominid) is used to refer to those species that evolved after this split of the primate line, thereby designating species that are more closely related to humans than to chimpanzees. Hominins, who were bipedal in comparison to the other hominoids who were primarily quadrupedal, includes those groups that probably gave rise to our species: Australopithecus africanus, Homo habilis, and Homo erectus, along with non- ancestral groups such as Australopithecus boisei. Determining the true lines of descent in hominins is difficult. In years past, when relatively few hominin fossils had been recovered, some scientists believed that considering them in order, from oldest to youngest, would demonstrate the course of evolution from early hominins to modern humans. In the past several years, however, many new fossils have been found. It is possible that there were often more than one species alive at any one time and that many of the fossils found (and species named) represent hominin species that died out and are not ancestral to modern humans. However, it is also possible that too many new species have been named.

Evolution of modern humans: This chart shows the evolution of modern humans and includes the point of divergence that occurred between modern humans and the other great apes.

Very Early Hominins

There have been three species of very early hominoids which have made news in the past few years. The oldest of these, Sahelanthropus tchadensis, has been dated to nearly seven million years ago. There is a single specimen of this genus, a skull that was a surface find in Chad. The fossil, informally called “Toumai,” is a mosaic of primitive and evolved characteristics. To date, it is unclear how this fossil fits with the picture given by molecular data. The line leading to modern humans and modern chimpanzees apparently bifurcated (divided into branches) about six million years ago. It is not thought at this time that this species was an ancestor of modern humans. It may not have been a hominin.

A second, younger species (around 5.7 million years ago), Orrorin tugenensis, is also a relatively-recent discovery, found in 2000. There are several specimens of Orrorin. It is not known whether Orrorin was a human ancestor, but this possibility has not been ruled out. Some features of Orrorin are more similar to those of modern humans than are the australopiths, although Orrorin is much older.

A third genus, Ardipithecus ramidus (4.4 million years ago), was discovered in the 1990s. The scientists who discovered the first fossil found that some other scientists did not believe the organism to be a biped (thus, it would not be considered a hominid). In the intervening years, several more specimens of Ardipithecus, including a new species, Ardipithecus kadabba (5.6 million years ago), demonstrated that they were bipedal. Again, the status of this genus as a human ancestor is uncertain, but, given that it was bipedal, it was a hominin.