FEATURES OF EARLY HOMININS (HOMINIDS) AND HUMANS: BIG BRAINS, LACK OF HAIR AND PENIS SPINES

HOMININS, THE MISSING LINK AND THE EVOLUTION OF MAN

Hominins are defined as creatures that stand upright and walk and run primarily on two legs, while apes are creatures that hunch over and, although capable of walking on two legs, prefer to use their arms when moving on the ground. Before the mid 2000s, scientists referred to hominins as hominids.

Scientists have found fossils of 5,000 individual hominins as far back as 4.4 million years, perhaps 7 million years. The earliest hominins, the genus Australopethecus, possessed long arms, short legs, a large small brain and a large face. These creatures would appear to us today as more ape-like than human-like. So far the earliest hominins species have been discovered only in eastern, northern and south Africa. Scientists describe Africa as the "cradle of mankind" because all of the oldest hominin remains have been found there.

There are two major groups, or genera of hominins: “Australopithecus” , which lived between 4 million and 1 million years ago and includes a number of species; and “Homo” , which appeared around 2.5 million years ago and includes, “Homo habilis” , “Homo erectus “ and “Homo sapiens” (modern humans).

Modern humans (us) appeared at least 300,000 years ago, and are believed to have evolved from: 1) Australopithecus aferenis (3.8 to 3 million-years-ago); 2) Australopithecus africanus (3 to 2 million-years-ago); 3) Homo habilis (2 to 1.5 million-years-ago), and 4) Homo erectus (1.8 to .5 million-years-ago). Most Scientists believe that the other ancient man species led to dead ends. There is a lot of controversy and debate in this field.

The study of early man has often been posed as a quest for “the missing link.” Most scientists bristle at the term. Tim White, a paleoanthropologist at the University of California, Berkeley, told National Geographic, “The term is wrong in so many ways, it’s hard to know where to begin. Worst all is the implication that at some point there existed something halfway between chimp and human. That’s a popular misconception that has plagued evolutionary science from the beginning.”

Websites and Resources on Hominins and Human Origins: Smithsonian Human Origins Program humanorigins.si.edu ; Institute of Human Origins iho.asu.edu ; Becoming Human University of Arizona site becominghuman.org ; Hall of Human Origins American Museum of Natural History amnh.org/exhibitions ; The Bradshaw Foundation bradshawfoundation.com ; Britannica Human Evolution britannica.com ; Human Evolution handprint.com ; University of California Museum of Anthropology ucmp.berkeley.edu; John Hawks' Anthropology Weblog johnhawks.net/ ; New Scientist: Human Evolution newscientist.com/article-topic/human-evolution

Genetic Changes That May Have Helped Humans Become Humans

Genetic comparisons with chimps suggests that losing chunks of DNA – including one associated with penis spines and facial whiskers – played a crucial role in making humans what they are, according to an article published in Nature by U.S. scientists in March 2011. Ian Sample wrote in The Guardian: “Scientists have identified a clutch of subtle genetic changes that have shaped our minds and bodies into the unique form that sets humans apart from chimpanzees and the rest of the animal kingdom... The findings offer up the humbling conclusion that the secret of human success may owe more to what we lost along the path of evolution, rather than anything we gained. [Source: Ian Sample, The Guardian, March 9, 2011 |=|]

“When the human genome was first deciphered more than a decade ago, some scientists expected to find extra genes that explained why humans had an intellectual edge over their closest living relatives and other species. But since diverging from chimpanzees around seven million years ago, it turns out that our human ancestors lost several hundred snippets of DNA, which together led to traits that are uniquely human, the researchers claim. In ditching these chunks of DNA, our ancient ancestors lost facial whiskers and short, tactile spines on their penises. The latter development is thought to have paved the way for more intimate sex and monogamous relationships. The loss of other DNA may have been crucial in allowing humans to grow larger brains. |=| “Intriguingly, hardly any of the lost DNA was from genes, which make the proteins that are the building blocks of life. Instead, the missing DNA came from areas of the genome that regulate where and when certain genes are active. "Like someone looking for their keys under a lamp post, the genes were the easiest place to look for differences between humans and chimpanzees, and in many respects those have been studied pretty well," said Philip Reno, a co-author on the study at Penn State University. "But there is a larger unknown in the form of these other regions of DNA, and in those we are only just beginning to find ways to pull out the differences between humans and chimpanzees." |=|

“In the years since the human genome project was completed it has become clear that humans and chimps share around 96 percent of their DNA. Of the three billion pairs of "letters" that make up the human genetic code, genes account for less than 2 percent. The US team compared the complete human genome with sequences from the chimp, macaque and mouse. They found that humans lack 510 short sections of DNA that are present in the other animals. Intriguingly, only one missing piece of DNA affected a human gene directly. The vast majority of lost DNA disrupted parts of the genome that control how genes are expressed. One missing section of DNA was found to block a gene that, in other animals, stifles the growth of brain cells. Losing that DNA may have been a pivotal moment in human development, as it allowed parts of the human brain to expand into the most complex organ known When the scientists checked their genetic discoveries against the Neanderthal genome, they found the same chunks of DNA were missing, meaning the DNA was lost more than 800,000 years ago, which is when our human ancestors split from the Neanderthal lineage.” |=|

Christine Dell'Amore wrote in National Geographic News: “In total, the scientists found at least 510 DNA "deletions" that have occurred during human evolution by comparing the human genome with those of a range of modern and extinct species—including chimps and Neanderthals. These deletions have brought about various changes to our bodies, for instance, boosting our brain sizes and stripping our faces of sensory whiskers. The DNA deletions — actually the result of rare cell mutations — do not hinder the overall function of a gene, the study authors noted. "Think about it as a lightbulb that's controlled by lots of different switches," said study co-author David Kingsley, also of Stanford."If you smash the lightbulb, the lights go out," he said. But "if you just remove an individual switch, [you've] altered the response to one particular input. That may produce a big effect for that little circuit, but it still preserves lots of other things." Co-author Bejerano emphasized that one of the paper's key messages is that sometimes small changes to DNA sequences can actually lead to larger, more elaborate structures in organisms, such as more complex brains. University of Sheffield's Rhonda Snook, who was not involved in the new research. said, Some "might think [the switches] are marginal," said Snook, But these small tweaks "could have potentially really profound effects on the way that organisms evolve from their ancestors." [Source: Christine Dell'Amore, National Geographic News, March 11, 2011 =]

gorilla, orangutan skulls

Early Man and Brain Development

Brain development was a key component to the development of man and one of the primary measures of an advanced brain is size. Scientists are now studying four genes that appear to control brain size. Mutations in these geneses cause microcephaly, a birth defect characterized by a small head and mental retardation. Scientists are also taking a look at another gene — MYH16 — that codes proteins that are active in muscles in the head, especially those involved in chewing, in chimpanzees and other primates. Humans possess this gene but is is turned off. Some scientists have theorized that this occurred to free the head from having powerful jaw muscles, allowing more room for a larger brain.

Scientists have also found genes and specific proteins within genes in greater numbers in humans than chimpanzees that appear to be involved in cortex development and cognitive processes. Among these are the DUF1220 protein and segments of genes that have undergone changes at a particularly rapid rate known as “human accelerated regions” or HARs. HAR1 is a gene associated with brain development that is at least 310 million years old and is found in chickens as well as chimpanzees and humans. Before chimpanzees and humans split — for over 300 million years — only two of its 118 chemical “letters” changed. Since chimpanzees and humans split 5 to 6 million year ago 18 letters have changed — indicating that a high degree of change and evolution has occurred in hominins.

Not only is the brain of modern humans much bigger than that of chimpanzees but the area of the cortex involved in learning has greatly expanded. A human brain does much of its growing after birth. Scientists using 3-D reconstruction software have determined that the estimated size of the skull capacity for some early hominins may have been overestimated by 15 percent.

Large Brains

Mo Costandi wrote in The Guardian: “One of the things that makes our species unique is our exceptionally large brain relative to body size. Brain size more than tripled during the course of human evolution, and this size increase was accompanied by a significant reorganization of the cerebral cortex, the prominent convoluted structure responsible for complex mental functions, which accounts for something like 85 percent of total brain volume.” [Source: Mo Costandi, The Guardian May 7, 2012]

Charles Q. Choi wrote in Live Science: “There is no question that our large brains have provided humans an extraordinary advantage in the world. Still, the human brain is an incredibly expensive organ, taking up only about 2 percent of the body's mass yet using more than a fifth of the body's energy, and until about 2 million years ago none of our ancestors had a brain larger than an ape's when compared to body size. So what kicked off the push for a larger brain? One possibility is that increased smarts helped our ancestors make better tools. Another is that larger brains helped us interact better with each other. Perhaps radical changes in the environment also demanded that our ancestors deal with a shifting world. [Source: Charles Q. Choi, Live Science, February 22, 2011]

brain size of different hominins

Theories About Increased Brain Size

Mo Costandi wrote in The Guardian: “What evolutionary forces drove this dramatic increase in brain size? Many theories have been put forward over the years, a popular one being that our ancestors' brains expanded to accommodate the faculty of language. A fossilized skull fragment belonging to a human ancestor that lived several million years ago provides yet more clues. A new analysis of the skull suggests that human brain evolution may have been shaped by changes in the female reproductive system that occurred when our ancestors stood upright. [Source: Mo Costandi, The Guardian May 7, 2012 |=|]

“At some point in evolution, our ancestors switched from walking on all four limbs to just two, and this transition to bipedalism led to what is referred to as the obstetric dilemma. The switch involved a major reconfiguration of the birth canal, which became significantly narrower because of a change in the structure of the pelvis. At around the same time, however, the brain had begun to expand. |=|

“One adaptation that evolved to work around the problem was the emergence of openings in the skull called fontanelles. The anterior fontanelle enables the two frontal bones of the skull to slide past each other, much like the tectonic plates that make up the Earth's crust. This compresses the head during birth, facilitating its passage through the birth canal. A neuroscientist explains: the need for ‘empathetic citizens’ - podcast |=|

“In humans, the anterior fontanelle remains open for the first few years of life, allowing for the massive increase in brain size, which occurs largely during early life. The opening gets gradually smaller as new bone is laid down, and is completely closed by about two years of age, at which time the frontal bones have fused to form a structure called the metopic suture. In chimpanzees and bononbos, by contrast, brain growth occurs mostly in the womb, and the anterior fontanelle is closed at around the time of birth. |=|

“When this growth pattern appeared is one of the many unanswered questions about human brain evolution. The new study, led by Dean Falk of Florida State University, sought to address this. Working in collaboration with researchers from the Anthropological Institute and Museum at the University of Zürich, Falk compared the skulls of humans, chimps and bonobos of various ages to the fossilized skull of the so-called Taung Child. |=|

Brain Size, Bipedalism and the Pelvic Birth Canal

Mo Costandi wrote in The Guardian: “"There's a trade-off between walking bipedally in an optimal way, which narrows or constricts the birth canal, and evolving fat, big-brained babies which need a wide birth passage," says Zollikofer. "Bipedalism and big brains are independent evolutionary processes. Hominins started walking bipedally long before the brain expanded, but these trends collided at birth, and we believe this happened much earlier than previously thought." [Source: Mo Costandi, The Guardian May 7, 2012 |=|]

“Evolution is an opportunistic process - species change over time, but only some of these changes prove to be advantageous to an organism's survival. Some of them can prove advantageous in different and unrelated ways, and this seems to be the case for evolution of the human brain. Delayed fusion of the metopic suture apparently evolved to overcome the obstetric dilemma that arose when our ancestors stood upright, but had the added advantage of allowing for the pattern of modern human brain growth. |=|

“There are other ways in which bipedalism could have led to increased brain size. It would, for example, have freed up the forelimbs, and this would likely have led to the expansion and reorganization of the sensory and motor brain areas that process sensation and control movement. Similarly, standing upright would have led to big changes in what our ancestors saw, which may have led to an expansion of the visual areas at the back of the brain. |=|

“The new findings suggest that further brain expansion, as well as reorganization of the prefrontal cortex, could have occurred as an indirect result of the pelvic modifications that followed the transition to bipedalism. All evolutionary changes are due to changes that occur at the genetic level, and the dramatic increase in brain size that occurred during human evolution is no exception. Numerous genes have been implicated in human brain evolution, but it is difficult to link any of them to specific changes in brain organization or structure. ...Evan Eichler and colleagues reported that a gene known to be involved in development of the cerebral cortex was duplicated multiple times, and that this occurred exclusively in humans. They also estimate that these duplications took place between two and three million years ago, so it is tempting to speculate that they are somehow linked to the changes that may have occurred as a result of bipedalism. |=|

brain size versus tooth size in different hominins

Large Brains: Result of a Gene Mutation?

Setting off a scientific debate, researchers in 2004 said they may have discovered a genetic mutation that produced smaller, weaker jaws and created the space for a larger brains, paving the way for the earliest humans to branch off from their apelike ancestors. Associated Press reported: “Smaller jaws would have fundamentally changed the structure of the skull, they contend, by eliminating thick muscles that worked like bungee cords to anchor a huge jaw to the crown of the head. The change would have allowed the cranium to grow larger and led to the development of a bigger brain capable of tool-making and language. [Source: Associated Press, March 25, 2004 /*/]

“The mutation is reported in the latest issue of the journal Nature, not by anthropologists, but by a team of biologists and plastic surgeons at the University of Pennsylvania and the Children's Hospital of Philadelphia. The report provoked strong reactions throughout the hotly contested field of human origins, with one scientist declaring it "counter to the fundamentals of evolution" and another pronouncing it "super." /*/

“The Pennsylvania researchers said their estimate of when this mutation first occurred — about 2.4 million years ago, in the grasslands of East Africa, the accepted cradle of humanity — generally overlaps with the first fossils of prehistoric humans featuring rounder skulls, flatter faces, smaller teeth and weaker jaws. And the remarkable genetic mutation persists to this day in every person, they said. Nonhuman primates — including Homo sapiens' closest animal relative, the chimpanzee — still carry the original big-jaw gene and the apparatus enabling them to bite and grind the toughest foods. /*/

University of Michigan biological anthropologist Milford Wolpoff called the research "just super.” “The other thing that was happening 2 1/4 million years ago is that people were beginning to make tools, which enabled them to prepare food outside their mouths," he said. "This is a confluence of genetic and fossil evidence." Other researchers strenuously disagreed that human evolution could literally hinge on a single mutation affecting jaw muscles, and that once those muscles were reduced, the brain suddenly could grow unfettered. "Such a claim is counter to the fundamentals of evolution," said C. Owen Lovejoy of Kent State University. "These kinds of mutations probably are of little consequence." Jaws have been a focus of evolutionary research since Charles Darwin, and the mutation offers a tantalizing theory. But it is unlikely that one mutation — even at a crucial evolutionary juncture — would make a person, some skeptics said.”

Is the Human Face a Defense Against a Pummeling

In 2014, two scientists at University of Utah proposed that the faces of early hominins evolved millions of years ago as a means of limiting facial injuries in fist fights between males. The theory, published in the journal Biological Reviews, contrast with the prevailing idea that changes facial shape were shaped more by diet such as the development of the jaw to deal with hard-to-crush foods such as nuts. “Studies of injuries resulting from fights show that when modern humans fight, the face is the primary target,” biologist David Carrier said. “The bones of the face that suffer the highest rates of fracture from fights are the bones that show the greatest increase in robusticity during the evolution of early bipedal apes, the australopiths.” These are also the bones that show the greatest difference between women and men in early human ancestors and modern humans, Carrier added.[Source: Will Dunham, Reuters, June 10, 2014]

Will Dunham of Reuters wrote: “In both apes and humans, males are much more violent than females, and most male violence is directed at other males, Carrier said. The violence underpinning the need for a more robust facial structure may have involved fist fights over females, resources and other disputes. Australopithecus was a lineage that preceded our genus, Homo, and it emerged more than 4 million years ago in Africa. Australopithecus was bipedal, smaller than modern people and possessed a combination of ape and human characteristics. “Comparing great apes such as chimps and gorillas to australopiths, what changed in the face was a reduction in the length of the jaws, a great increase in the robustness and strength of the jaws, molar teeth and jaw muscles, a substantial increase in the size and strength of the cheek bones, and an increase in the part of the face that surrounds the eyes,” Carrier said.

The proportions of the hand that allowed for the formation of a fist and the great increases in the robustness of the face occurred early in our lineage, 4 million to 5 million years ago, at about the same time as the bipedal posture appeared, Carrier added. Carrier said anthropologists have thought the new facial traits in the first bipedal apes were the result of a diet that included very hard objects, and the biomechanics of eating such food can explain many of these features. But he said recent analyses of wear patterns in teeth suggest most of these creatures did not eat hard objects.

“The study by Carrier and Michael Morgan, a University of Utah physician, builds on their previous research highlighting the role they contend violence played in driving human evolution. “I think our science is sound and fills some longstanding gaps in the existing theories of why the musculoskeletal structures of our faces developed the way they did,” Morgan added.

Human Hands Also May Have 'Evolved for Fighting'

chimp and human hands

The Telegraph reported: “Compared with apes, humans have shorter palms and fingers and longer, stronger flexible thumbs. Experts have long assumed these features evolved to help our ancestors make and use tools. But new evidence from the US suggests it was not just dexterity that shaped the human hand, but violence also. Hands largely evolved through natural selection to form a punching fist, it is claimed. ''The role aggression has played in our evolution has not been adequately appreciated,'' said Professor David Carrier, from the University of Utah. ''There are people who do not like this idea but it is clear that compared with other mammals, great apes are a relatively aggressive group with lots of fighting and violence, and that includes us. We're the poster children for violence.'' [Source: The Telegraph, December 20, 2012 +/]

“The forces of natural selection that drove hands to become nimble-fingered also turned them into weapons, Prof Carrier believes. ''Individuals who could strike with a clenched fish could hit harder without injuring themselves, so they were better able to fight for mates and thus be more likely to reproduce,'' he said. ''If a fist posture does provide a performance advantage for punching, the proportions of our hands also may have evolved in response to selection for fighting ability, in addition to selection for dexterity.'' +/

“To test the theory Prof Carrier conducted experiments with volunteers aged 22 to 50 who had boxing or martial arts experience. In one, participants were asked to hit a punchbag as hard as possible from different directions with their hands in a range of shapes, from open palms to closed fists. The results, published in the Journal of Experimental Biology, show that tightly clenched fists are much more efficient weapons than open or loosely curled hands. +/

“A punch delivers up for three times more force to the same amount of surface area as a slap. And the buttressing provided by a clenched fist increases the stiffness of the knuckles fourfold, while doubling the ability of the fingers to deliver a punching force. ''Because the experiments show the proportions of the human hand provide a performance advantage when striking with a fist, we suggest that the proportions of our hands resulted, in part, from selection to improve fighting performance,'' said Prof Carrier. ''The standard argument is that once our ancestors came out of the trees, the selection for climbing was gone, so selection for manipulation became dominant. And that's what changed the shape of our ancestors' hands. ''Human-like hand proportions appear in the fossil record at the same time our ancestors started walking upright four million to five million years ago. An alternative possible explanation is that we stood up on two legs and evolved these hand proportions to beat each other.'' +/

“Manual dexterity could have evolved without the fingers and palms getting shorter, he said. But he added: ''There is only one way you can have a buttressed, clenched fist: the palms and fingers got shorter at the same time the thumb got longer.'' Prof Carrier cited other evidence pointing to the role of fighting in the evolution of human hands: 1) No ape other than humans hits with a clenched fist. 2) Humans use fists instinctively as threat displays. ''If you are angry, the reflexive response is to form a fist,'' said Prof Carrier. ''If you want to intimidate somebody, you wave your fist.'' 3) Sexual dimorphism, or the difference in body size between the sexes, tends to be greater among primates when there is more competition between males. In humans the difference is mainly in the upper body and arms, especially the hands. ''It's consistent with the hand being a weapon,'' said Prof Carrier. +/

“In their paper the professor and colleague Michael Morgan, a University of Utah medical student, ponder on the paradoxical nature of the human hand. ''It is arguably our most important anatomical weapon, used to threaten, beat and sometimes kill to resolve conflict. Yet it is also the part of our musculoskeletal system that crafts and uses delicate tools, plays musical instruments, produces art, conveys complex intentions and emotions, and nurtures,'' they write. ''More than any other part of our anatomy, the hand represents the identity of Homo sapiens. Ultimately, the evolutionary significance of the human hand may lie in its remarkable ability to serve two seemingly incompatible but intrinsically human functions.'' +/

Claim That Human Throwing Developed 2 Million Years Ago Greeted with Skepticism

In 2013, scientists said in a study published in Nature that around 2 million years our human ancestors first started throwing with some degree of accuracy and power. Malcolm Ritter of Associated Press wrote: “There's plenty of skepticism about their conclusion. But the new paper contends that this throwing ability probably helped our ancient ancestor Homo erectus hunt, allowing him to toss weapons — probably rocks and sharpened wooden spears. [Source: Malcolm Ritter, Associated Press. June 26, 2013 ***]

“The human throwing ability is unique. Not even a chimp, our closest living relative and a creature noted for strength, can throw nearly as fast as a 12-year-old Little Leaguer, says lead study author Neil Roach of George Washington University. To find out how humans developed this ability, Roach and co-authors analyzed the throwing motions of 20 collegiate baseball players. Sometimes the players wore braces to mimic the anatomy of human ancestors, to see how anatomical changes affected throwing ability. ***

“The human secret to throwing, the researchers propose, is that when the arm is cocked, it stores energy by stretching tendons, ligaments and muscles crossing the shoulder. It's like pulling back on a slingshot. Releasing that "elastic energy" makes the arm whip forward to make the throw. That trick, in turn, was made possible by three anatomical changes in human evolution that affected the waist, shoulders and arms, the researchers concluded. And Homo erectus, which appeared about 2 million years ago, is the first ancient relative to combine those three changes, they said. ***

“But others think the throwing ability must have appeared sometime later in human evolution. Susan Larson, an anatomist at Stony Brook University in New York who didn't participate in the study, said the paper is the first to claim that elastic energy storage occurs in arms, rather than just in legs. The bouncing gait of a kangaroo is due to that phenomenon, she said, and the human Achilles tendon stores energy to help people walk. ***

“The new analysis offers good evidence that the shoulder is storing elastic energy, even though the shoulder doesn't have the long tendons that do that job in legs, she said. So maybe other tissues can do it too, she said. But Larson, an expert on evolution of the human shoulder, said she does not think Homo erectus could throw like a modern human. She said she believes its shoulders were too narrow and that the orientation of the shoulder joint on the body would make overhand throwing "more or less impossible." Rick Potts, director of the human origins program at the Smithsonian Institution, said he is "not at all convinced" by the paper's argument about when and why throwing appeared. ***

“The authors did not present any data to counter Larson's published work that indicates the erectus shoulder was ill-suited for throwing, he said. And it is "a stretch" to say that throwing would give erectus an advantage in hunting, Potts said. Large animals have to be pierced in specific spots for a kill, which would seem to require more accuracy than one could expect erectus to achieve from a distance, he said. Potts noted that the earliest known spears, which date from about 400,000 years ago, were used for thrusting rather than throwing.” ***

Why Did Hominins Lose Their Hair?

Homo naledi

Charles Q. Choi wrote in Live Science: “Humans are unique for looking naked compared to our hairier ape cousins. So why did this nakedness evolve? One suggestion is that our ancestors shed hairiness to keep cool when venturing across the hot savannahs of Africa. Another is that losing our fur coats helped free us parasite infestations and the diseases they can spread. One unorthodox idea even suggests human nakedness developed after our ancestors briefly adapted for a streamlined life in the water, although most aquatic mammals of roughly human size actually possess dense fur. [Source: Charles Q. Choi, Live Science, February 22, 2011] One model suggests that early humans lost fur after developing bipedalism. Bob Yirka wrote in Phys.org: “ Two of the most basic questions in the study of human evolution revolve around why early people started walking around on two feet instead of four and why they lost their fur, especially in light of the fact that most other primates have kept their coats, and still make extensive use of their hands in walking and especially so, when running. Some have postulated that early hominins began walking upright as a means of helping them keep cooler. [Source: Bob Yirka, Phys.org. December 13, 2011 =*=]

“A new model by Graeme Ruxton and David Wilkinson disagrees, writing in their paper published in the Proceedings of National Academy of Sciences, that research they’ve done shows that it was only after early humans began walking upright that they began to lose their fur coats. =*=

“Ruxton and Wilkinson don’t offer any new theories as to why early humans started walking upright but they say after studying mathematical models that describe how humans or animals lose body heat based on different postures during different times of the day, that as expected, it appears a crouched posture does conserve body heat more so than does a fully erect one, especially in direct sunlight, suggesting that walking erect would indeed seem to be a way to keep cooler. Unfortunately, they say, that doesn’t take into account the body heat that is created internally by the very act of moving around. Thus, they say, models that suggest humans started walking upright as a means to cool off aren’t looking at the whole picture because in addition to the metabolic heat generated, the cooling effect gained in losing body hair would be much more profound in a person walking upright, than would be the case for one who remained crouching.Thus, they argue, it was only after hominins started walking upright that they began losing their fur, though they do agree that the latter change came about as a means of keeping cooler.” =*=

Shedding Penis Spines Helped Humans Become Human?

penis spines

The loss of DNA that gave rise to both facial whiskers and penis spines — both of which are found in chimpanzees and other non-human primates but not humans — may have a key episode in human development. Ian Sample wrote in The Guardian: ““Penile spines – which make the penis more sensitive and speed ejaculation – are more common in animals that face intense competition for mates, and where females are likely to mate with many males in rapid succession. The loss of penile spines may have allowed our ancient ancestors to copulate for longer, a development thought to have nurtured monogamous couples and paved the way for more complex social structures. [Source: Ian Sample, The Guardian, March 9, 2011 |=|]

Christine Dell'Amore wrote in National Geographic News: “Penile spines, which are still present in several modern animals, are usually small barbs of keratin—a type of hard tissue—that line the outside of the organ. The prehistoric male enhancement existed in the common ancestor of chimpanzees and humans, which lived about six million years ago, according to the gene analysis. But the "penile spine enhancer" code disappeared from human genes before our common ancestor split into modern humans and Neanderthals about 700,000 years ago, said study co-author Gill Bejerano, a developmental biologist at Stanford University in California. [Source: Christine Dell'Amore, National Geographic News, March 11, 2011 =]

“In some modern animals, such as domestic cats, penile spines help males fertilize females when sperm competition inside the female is fierce. For instance, the spines can break through copulatory plugs, or coagulated secretions of fluids placed inside the female by other males to prevent different sperm from fertilizing the egg. But most women today are monogamous, and the males "are not just present during the competitive act of fertilization—they establish long-term relationships with females," Kingsley said. Snook, who studies the evolution of sperm form and function, said theories linking simpler genitalia to monogamy are still tenuous. For instance, a phylogenetic analysis—a study comparing the presence or absence of spiny penises in different organisms over time—would offer more insight.” =

Image Sources: Wikimedia Commons

Text Sources: National Geographic, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Nature, Scientific American. Live Science, Discover magazine, Discovery News, Times of London, Natural History magazine, Archaeology magazine, The New Yorker, Time, Newsweek, BBC, The Guardian, Reuters, AP, AFP and various books and other publications.

Last updated September 2018