Because humans are the only primates that lack coats and have an abundance of eccrine glands, something must have happened since our hominid lineage diverged from the line leading to our closest living relative, the chimpanzee, that favored the emergence of naked, sweaty skin. Perhaps not surprisingly, the transformation seems to have begun with climate change.
By using fossils of animals and plants to reconstruct ancient ecological conditions, scientists have determined that starting around three million years ago the earth entered into a phase of global cooling that had a drying effect in East and Central Africa, where human ancestors lived. With this decline in regular rainfall, the wooded environments favored by early hominids gave way to open savanna grasslands, and the foods that our ancestors the australopithecines subsisted on—fruits, leaves, tubers and seeds—became scarcer, more patchily distributed and subject to seasonal availability, as did permanent sources of freshwater.
In response to this dwindling of resources, our forebearers would have had to abandon their relatively leisurely foraging habits for a much more consistently active way of life just to stay hydrated and obtain enough calories, traveling ever longer distances in search of water and edible plant foods. It is around this time that hominids also began incorporating meat into their diet, as revealed by the appearance of stone tools and butchered animal bones in the archaeological record around 2.
Animal foods are considerably richer in calories than are plant foods, but they are rarer on the landscape. Carnivorous animals therefore need to range farther and wider than their herbivorous counterparts to procure a sufficient amount of food. Prey animals are also moving targets, save for the occasional carcass, which means predators must expend that much more energy to obtain their meal. In the case of human hunters and scavengers, natural selection morphed the apelike proportions of the australopithecines, who still spent some time in the trees, into a long-legged body built for sustained striding and running.
This modern form also no doubt helped our ancestors avoid becoming dinner themselves when out in the open. But these elevated activity levels came at a price: a greatly increased risk of overheating.
Beginning in the s, Peter Wheeler of Liverpool John Moores University in England published a series of papers in which he simulated how hot ancestral humans would have become out on the savanna. Wheeler's work, together with research my colleagues and I published in , shows that the increase in walking and running, during which muscle activity builds up heat internally, would have required that hominids both enhance their eccrine sweating ability and lose their body hair to avoid overheating.
When did this metamorphosis occur? Although the human fossil record does not preserve skin, researchers do have a rough idea of when our forebearers began engaging in modern patterns of movement. Studies conducted independently by Lieberman and Christopher Ruff of Johns Hopkins University have shown that by about 1. Moreover, details of the joint surfaces of the ankle, knee and hip make clear that these hominids actually exerted themselves in this way.
Thus, according to the fossil evidence, the transition to naked skin and an eccrine-based sweating system must have been well under way by 1. Another clue to when hominids evolved naked skin has come from investigations into the genetics of skin color.
In an ingenious study published in , Alan R. Rogers of the University of Utah and his colleagues examined sequences of the human MC1R gene, which is among the genes responsible for producing skin pigmentation. The team showed that a specific gene variant always found in Africans with dark pigmentation originated as many as 1.
Early human ancestors are believed to have had pinkish skin covered with black fur, much as chimpanzees do, so the evolution of permanently dark skin was presumably a requisite evolutionary follow-up to the loss of our sun-shielding body hair. Rogers's estimate thus provides a minimum age for the dawn of nakedness.
Less certain than why and when we became naked is how hominids evolved bare flesh. The genetic evidence for the evolution of nakedness has been difficult to locate because many genes contribute to the appearance and function of our skin.
Comparison of the human and chimp genomes reveals that one of the most significant differences between chimp DNA and our own lies in the genes that code for proteins that control properties of the skin. The human versions of some of those genes encode proteins that help to make our skin particularly waterproof and scuff-resistant—critical properties, given the absence of protective fur. This finding implies that the advent of those gene variants contributed to the origin of nakedness by mitigating its consequences.
The outstanding barrier capabilities of our skin arise from the structure and makeup of its outermost layer, the stratum corneum SC of the epidermis. The SC has what has been described as a bricks-and-mortar composition. In this arrangement, multiple layers of flattened dead cells called corneocytes, which contain the protein keratin and other substances, are the bricks; ultrathin layers of lipids surrounding each of the corneocytes make up the mortar.
Most of the genes that direct the development of the SC are ancient, and their sequences are highly conserved among vertebrates. That the genes undergirding the human SC are so distinctive signifies, therefore, that the advent of those genes was important to survival.
These genes encode the production of a unique combination of proteins that occur only in the epidermis, including novel types of keratin and involucrin. A number of laboratories are currently attempting to unravel the precise mechanisms responsible for regulating the manufacture of these proteins.
Other researchers are looking at the evolution of keratins in body hair, with the aim of determining the mechanisms responsible for the sparseness and fineness of body hair on the surface of human skin. To that end, Roland Moll of Philipps University in Marburg, Germany, and his colleagues have shown that the keratins present in human body hair are extremely fragile, which is why these hairs break so easily compared with those of other animals.
This finding, detailed in a paper Moll published in , suggests that human hair keratins were not as important to survival as the hair keratins of other primates were over the course of evolution and thus became weak. Another question geneticists are eager to answer is how human skin came to contain such an abundance of eccrine glands.
Almost certainly this accumulation occurred through changes in the genes that determine the fate of epidermal stem cells, which are unspecialized, in the embryo. Early in development, groups of epidermal stem cells in specific locations interact with cells of the underlying dermis, and genetically driven chemical signals within these niches direct the differentiation of the stem cells into hair follicles, eccrine glands, apocrine glands, sebaceous glands or plain epidermis.
Many research groups are now investigating how epidermal stem cell niches are established and maintained, and this work should clarify what directs the fate of embryonic epidermal cells and how more of these cells become eccrine sweat glands in humans. However it was that we became naked apes, evolution did leave a few body parts covered. Any explanation of why humans lost their fur therefore must also account for why we retain it in some places.
Hair in the armpits and groin probably serves both to propagate pheromones chemicals that serve to elicit a behavioral response from other individuals and to help keep these areas lubricated during locomotion. As for hair on the head, it was most likely retained to help shield against excess heat on the top of the head.
That notion may sound paradoxical, but having dense hair on the head creates a barrier layer of air between the sweating scalp and the hot surface of the hair.
Thus, on a hot, sunny day the hair absorbs the heat while the barrier layer of air remains cooler, allowing sweat on the scalp to evaporate into that layer of air. Tightly curled hair provides the optimum head covering in this regard because it increases the thickness of the space between the surface of the hair and the scalp, allowing air to blow through. Much remains to be discovered about the evolution of human head hair, but it is possible that tightly curled hair was the original condition in modern humans and that other hair types evolved as humans dispersed out of tropical Africa.
With regard to our body hair, the question is why it is so variable. Hairless Animals — Hairless Horse — image: fuckyeahhorses. The first hairless horse to be recorded by science was found roaming feral amongst a herd of zebra in South Africa in The horse was captured, studied, and put on exhibit.
What ultimately happened to her is a mystery but what we do know is many more like her were discovered in other countries in both feral and domestic populations of horse. So far as I can tell none were bred purposely to be hairless and this is probably best as they are prone to sun burns and other skin conditions.
The most famous living hairless horse is ironically named Harry, though he was named this before he went completely bald. Hairless rats are produced by breeding different combinations of genes. On the other hand, hairless lab rats provide researchers with valuable data on compromised immune systems and genetic kidney diseases. Hairless Animals — Naked Mole Rat — image: www. Nudity might be the least bizarre trait of these oddball rodents.
They are also the only known eusocial mammals, meaning their social structure closely resembles that of insects like ants or bees. Thanks to his keepers, the bird managed to grow a feathery coat and was successfully re-introduced to its family. Hairless Animals — Walruses — image: youtube. Hair is used as an insulator for many mammals, but walruses, like a number of other semi-aquatic mammals, have forsaken this trait and replaced it with a cushy layer of subcutaneous fat.
Skinny pigs are the name given to a breed of hairless guinea pigs. They were originally bred in a lab, primarily for use in dermatology studies, but have since become part of the pet population. Skinnies are distinctive for their human-like skin, which could be either comforting or creepy. The African elephant is the world's largest land mammal, and it's also nearly hairless. This is because of an adaptation to the hot, dry climate in which it lives.
With such large bodies, dissipating heat is more important to elephants than retaining it. As they grow in size, elephants lose more and more hair. It is also possible that hair on African elephants used to have sensory or protective purposes. While that is no longer the case — except for possibly an elephant's trunk — the continued presence of hair may be residual from evolution. Hair is used as an insulator for many mammals, but walruses, like a number of other semi-aquatic mammals, have minimalized this trait and replaced it with a layer of subcutaneous fat.
Walrus blubber is so thick that hair is almost entirely unnecessary, but the animal is nonetheless covered in short, reddish-brown fur. This coat is barely noticeable, however, so if it wasn't for their characteristic whiskers, walrus' bodies would appear completely naked.
There are a number of hairless dog breeds out there, including the Chinese crested dog, the Mexican hairless dog, the American hairless terrier, and the Peruvian hairless dog pictured. But there are several other varieties of hairless dogs that have yet to be officially recognized. Hairless dogs are popular pets because they are both hypoallergenic and convenient — there's no shedding to worry about.
However, their lack of hair does mean that they need sunscreen in hot weather and jackets for warmth in the cold. Depending on who you ask, a Sphynx is either distinctive and adorable or ugly and creepy. This animal is not to be confused with a sphinx, the mythical creature with the head of a human and body of a lion after which the massive statue in Giza was modeled. Instead, these are hairless cats shaped by breeders — not mythology or evolution.
Of course, since Sphynx cats are mammals, they are not completely hairless. They are covered in fine, barely noticeable down that contributes to the soft feel of their skin. Despite their untraditional looks, Sphynxes are well-loved as pets. They are known for their extroverted personalities, high energy levels, curiosity, and affection.
Plus, like hairless dog breeds, there is no shedding to worry about. They do not differ much from the standard guinea pig you are familiar with except for the fact that they are nearly hairless. The small bit of fur they do have is found on their legs, feet, and muzzles.
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