pelvis in apes and man
Bipedalism (moving on two legs) is one of the key characteristics that defines hominins and humans. A hominin after all is defined as a creature that stands upright and walks and runs primarily on two legs. It is thought that bipedalism developed in hominins between 4 million and 8 million years ago. No one is sure how or why some apes, who were for the most part arboreal at that time, dropped out of the trees and began walking upright. A popular theory is that it first evolved to free the hands to carry food.

Of more than 250 species of primates, only one — humans — walks around on two legs. Walking is an activity in which the legs move the body forward and act like pendulums balancing the body as it moves along. Key to this movement is: 1) the ability to fully extend the knees to create the pendulum effect; 2) the forward curve of the back and the inward sloping of the thighbones to create a center of gravity over the feet; and 3) the gluteal abductor muscles attached to the pelvis that keeps the body from toppling over sideways when our weight is on a single foot.

Developing these features took some time and its sometimes hard to imagine how it could happen in a step-by-step evolutionary fashion without creating a bunch of awkward beasts that were neither very good at climbing trees or walking. Running is thought to have developed about 2 million years ago. It is a totally different activity than walking. It is more of a pogo-stick-like motion using tendons in the legs as elastic springs.

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

Significance of Bipedalism

Bipedalism is regarded as the first major step in the development of our human ancestors. The second major step is toolmaking, which occurred about 2.6 million years ago. In addition to creating a more efficient locomotion mechanism, bipedalism also freed the hands to do other things such as develop tools. These developments in turn lead to the formation of a larger brain, the main thing that separates modern man from apes today.

Bipedalism saved energy and water. It required less energy knuckle-walking. An erect body is higher off the hot ground and has less area exposed to the sun, creating the need for cooling water. Bipedalism also made it easier to reach fruit and berries off of low bushes in grasslands and stand up and look for predators on the horizon.

In a study published in the Proceedings of the National Academy of Sciences, researchers concluded that humans use one forth the energy walking on two legs that chimpanzee use when knuckle-walking based on how humans and chimpanzees perform on treadmill test. This finding provides evidence to the theory that early hominins began engaging in bipedalism because it is more efficient.

Bipedal Body Features

20120201-apes man russia.jpg
apes and man
Humans have a long curved spine, femurs (thighbones) that angle inward to pushed the center of gravity of the torso over the feet while chimpanzees have a short and stiff spine and straight femurs. Humans also have a broader pelvis and specialized hip joints and muscle that prevent them from swaying like chimps when they walk on two feet. Human spines are shaped like an “s” which is an effective design for maintaining balance, assisting locomotion and bearing the weight of the upper body.

Chimpanzees can not lock their legs straight or extend their knees. Instead the have to use muscles to support their body weight when they stand and energy is expended rocking back and forth when they walk. Two-legged walking is a temporary, transitory method of mobility for them.

Humans also have concave socket-like condyles (rounded prominence at the end of a bone, apes are convex), tibias with flared buttresses, femurs designed for carrying more weight, and bone enlargements that act like shock absorbers. All these help humans walk and support their weight. The human foot has an arch and aligned toes and a large, puffy heel bone with lattice-like structure and covered with paper-thin layers of bone — all of which are conducive walking. Chimpanzees have a thumb-like big toe that is useful for grasping and is an impediment to walking.

Thomas Greiner, a physical anthropologist at the New York Chiropractic College, has suggested that the fact that humans have bigger butts than apes may have played a big role in effectively walking upright. Apes have small butt muscles and can't stay upright for long. The large human butt muscle provide stability for walking or standing.

First Hominins, Sex, Motherhood, Big Toes and Bipedalism

Scientists estimate that around six million years ago, female hominins switched from being sexually receptive during brief periods of estrus to being sexually receptive year round. Scientists estimate this change took place around the same time hominins learned to walk and their pelvis became narrower.

Owen Lovejoy of Ohio State University has theorized that hominin males that got around on two legs got more sex than tree-bound rivals and produced more offspring because walking freed their hands and this gave them an advantage in finding food and carrying it back home. Being freed of food gathering responsibilities allowed females to devote more time to rearing children that grew up healthy and strong.

Chimpanzees and other apes have opposable big toes that allow easily grasp tree and climb. They also allow baby chimps to tightly grip their mothers with four limbs, which allows a the mothers to forage, escape danger and travel while keeping the baby close. Lacking opposable big toes is an advancement that has made walking easier and has been found in advanced hominins but it also has made life more difficult for mothers who had to hold their babies and rely on others to gather food for them.

Costs of Bipedalism

Many of the aches and pains that humans experience with backs, knees and feet can be traced to bipedalism, walking and the pressure of gravity on an upright body. The spine, for example, was originally designed as a horizontal arch to carry weight along its length not a vertical column that carries weight above it. The upright design puts a lot of pressure on the lower regions of the spine, resulting in pressure and sometimes pain on the lower back. No other creatures experiences the kind of back pain that humans do. In the switch to bipedalism humans also gave up stability and speed and the ability to use the foot as a grabbing instrument.

One of the biggest consequences of bipedalism has been its impact on childbirth. A small hole in the pelvic bone is more advantageous for walking, running and supporting an upright body but is not so advantageous for giving birth to babies with relatively large brains and heads. The passage of the child through the bony canal of the human female pelvis is difficult and problematic. So narrow is the passage that the fetus most rotate as it moves through the canal and skull sometimes must compress by squeezing together, with the cranial bones that make up the skull overlapping. Getting the relatively broad shoulders of a human baby through the pelvic canal can also be difficult.

By contrast a chimpanzee infant exits its mother quickly and easily in a straight shot from the womb through the pelvic canal. The infant pops out face up so the mother and can pull it forward and immediately place the baby on her breast.

human versus chimpanzee foot

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.” |=|

Oldest Example of an Upright Ape — From 12 Million Years Ago

In November 2019, scientists announced that a previously unknown ape species Danuvius guggenmosi, whose fossils were discovered in southern Germany, provided the earliest known evidence of a primate standing upright. Associated Press reported: The remains of an ancient ape found in a Bavarian clay pit suggest that humans' ancestors began standing upright millions of years earlier than previously thought, scientists said. “An international team of researchers says the fossilized partial skeleton of a male ape that lived almost 12 million years ago in the humid forests of what is now southern Germany bears a striking resemblance to modern human bones. In a paper published by the journal Nature, they concluded that the previously unknown species — named Danuvius guggenmosi — could walk on two legs but also climb like an ape. [Source: Frank Jordans, Associated Press November Nov 7, 2019]

“The findings "raise fundamental questions about our previous understanding of the evolution of the great apes and humans," said Madelaine Boehme of the University of Tuebingen, Germany, who led the research. The question of when apes evolved bipedal motion has fascinated scientists since Charles Darwin first argued that they were the ancestors of humans. Previous fossil records of apes with an upright gait — found in Crete and Kenya — dated only as far back as 6 million years ago. Boehme, along with researchers from Bulgaria, Germany, Canada and the United States, examined more than 15,000 bones recovered from a trove of archaeological remains known as the Hammerschmiede, or Hammer Smithy, about 70 kilometers (44 miles) west of the Germany city of Munich.

“Among the remains they were able to piece together were primate fossils belonging to four individuals that lived 11.62 million years ago. The most complete, an adult male, likely stood about 1 meter (3 feet, 4 inches) tall, weighed 31 kilograms (68 pounds) and looked similar to modern-day bonobos, a species of chimpanzee. "It was astonishing for us to realize how similar certain bones are to humans, as opposed to great apes," Boehme said.

“Thanks to several well-preserved vertebra, limb, finger and toe bones, the scientists were able to reconstruct how Danuvius moved, concluding that while it would have been able to hang from branches by his arms, it could also straighten its legs to walk upright. "This changes our view of early human evolution, which is that it all happened in Africa," Boehme told The Associated Press in an interview.

“Like humans, Danuvius had an S-shaped spine to hold its body upright while standing. Unlike humans, though, it had a powerful, opposable big toe that would have allowed it to grab branches with its foot and safely walk through the treetops. Fred Spoor, a paleontologist at the Natural History Museum in London, called the fossil finds "fantastic" but said they would likely be the subject of much debate, not least because they could challenge many existing ideas about evolution."I can see that there will be a lot of agonizing and re-analysis of what these fossils mean," said Spoor, who wasn't involved in the study.

Clues About How Humans Stood Upright Found in Tiny Fish

You’d think that the best way to study the evolution of human locomotion would be from studying feet but in 2016 scientists announced they had found a surprising new clue to the origins of human bipedalism in a commonplace, pinkie-size fish. Natalie Angier wrote in Smithsonian magazine: “Analyzing the DNA of the threespine stickleback, researchers led by David Kingsley, a biologist at Stanford University, identified a so-called genetic enhancer, a kind of volume control knob that works during body development to help sculpt the bony plates that cloak the stickleback in lieu of scales. The enhancer modulates the release of a bone-related protein known as GDF6, turning it up or down to alter the plates to suit the fish’s setting. For marine sticklebacks that live in open water with a multitude of toothy predators, the enhancer spins out enough GDF6 protein to help build hefty protective plates. But freshwater sticklebacks do better to dart off and hide, and so, through enhancer-driven twiddling of protein release, those fish end up with slimmer and more pliable plates. [Source: Natalie Angier, Smithsonian magazine, April 2016]

“A genetic toggler’s response varies from one setting to the next, while its target — the brick-and-mortar proteins — remains the same, lending evolution considerable flexibility. “It’s such a good mechanism for evolving traits that you see it used over and over,” Kingsley says. When the researchers explored the role of the GDF6 protein and its enhancers in shaping the bones of mammals, including the chimpanzee, our nearest genetic kin, they found an enhancer that affected development of rear limbs but not forelimbs. The gene’s greatest impact was on the length and curvature of the toes. In human DNA, however, the enhancer was deleted.

“That single genetic change could help explain important differences between a chimpanzee foot and our own — and how our ancestors gained the power to rise up and walk on two feet. A chimpanzee’s toes are long and splayed, and its big-toe equivalent pulls away from the other digits like a thumb: a prehensile foot designed for quick climbing. By contrast, in the human foot, the sole is enlarged while the bone of the big toe is thickened and aligned with the other, now-foreshortened toes: This is a sturdy platform, able to support an upright load in motion.

“Aside from showing that our big toe deserves far more respect than most of us know, the new finding demonstrates that minor alterations in DNA can have profound evolutionary impacts, and that nature is a tireless recycler and collage artist, mixing and matching a few favorite techniques to generate a seemingly bottomless diversity of forms. “Our shared history with fish,” says Neil Shubin, author of Your Inner Fish and a paleontologist, “makes them a wonderful arena for exploring the fundamentals of our own bodies.”

Australopithecus Afarensis’s (Lucy’s) Feet Indicate She Lived on the Ground 3 Million Years Ago

Australopithecus afarensis footprint

In 2011 AP reported, “A team of researchers who got a first look at a foot bone of Australopithecus afarensis found in Ethiopia recently concluded that the early hominin was fully comfortable with life on the ground,rather than in the trees. Carol Ward of the University of Missouri, and colleagues, reported in the journal Science that the discovery shows that ancient Australopithecus afarensis had feet similar to modern humans. [Source: Randolph E. Schmid, AP, February 10, 2011]

The study of Lucy’s bones showed she was able to stand upright. But no foot bones were found with her skeleton, so researchers have puzzled over whether she walked like modern people or was a blend of ground- and tree-dweller. The new discovery shows these relatives “were fully humanlike and committed to life on the ground,” Ward told AP. “It lays to rest the idea that they were a compromise.”

The new bone, discovered with other A. afarensis bones at Hadar, Ethiopia, is a metatarsal, one of the long bones connecting the toes to the base of the foot. It shows that Lucy’s kin had arches stiffening their feet like modern people, as opposed to apes whose feet are more flexible for grasping tree branches. This was an important step in evolution, Ward explained. “This shows our early ancestor walked like we would walk. They were not shuffling, they were walking upright . which is a key feature of our branch of the family tree.

“Now that we know Lucy and her relatives had arches in their feet, this affects much of what we know about them, from where they lived to what they ate and how they avoided predators,” said Ward, a professor of integrative anatomy. “The development of arched feet was a fundamental shift toward the human condition, because it meant giving up the ability to use the big toe for grasping branches, signaling that our ancestors had finally abandoned life in the trees in favor of life on the ground.”

Richard Potts, director of the human origins program at the Smithsonian’s National Museum of Natural History, called the report “an impressive paper for just one bone.” “Every once in a while you do get one piece of the puzzle that helps you fill in something. This bone really fills in a missing piece,” said Potts. That doesn’t mean A. afarensis did not climb trees, he added. It was probably a very adaptable creature, using trees when they were available but being quite comfortable on the ground.A. afarensis still retained the well-muscled arms that would have been useful in trees, Potts noted.

Family That Walks on All Fours Today: Reverse Evolution?

Four sisters and a brother in a family living in Hatay in southern Turkey are unable to walk on two legs. Instead they walk on their hands and feet, Some scientists have speculated — erroneously it appears — that their predicament could be the result of a genetic problem that could linked to when human ancestors walked on all fours. Tim Newcomb wrote in Popular Mechanics: Five of the 19 children in the Ulas family have walked on all fours since infancy. But the affected siblings have intellectual disabilities and imbalance issues, suggesting their way of moving was more of an opportunity for them to more easily navigate their world. Üner Tan of Çukurova University Medical School in Adana, Turkey, infamously classified the family’s walking style as backwards evolution in 2006, using the term “Üner Tan Syndrome.” That led British scientists Nicholas Humphrey and John Skoyles and professor Roger Keynesto to get involved, along with a documentary crew. Instead of agreeing with Tan, the British scientists noted that the balance issues and an inherited congenital condition made walking upright difficult, so the five children continued walking on all fours to better get around. [Source: Tim Newcomb, Popular Mechanics, March 2, 2023]

Kurdish family that walks on all fours

Terrence McCoy wrote in the Washington Post: “In a village of dirt roads and stone dwellings just north of the Turkey-Syria border, a slight rain falls as a hunched man clad in black wobbles down the pavement. At first glance, as seen in a BBC documentary, he appears inebriated. He clings to a stone wall to his left and lurches uneasily. But then he slowly lowers his hands, encased in green slippers, to the muddy ground. Gingerly, he begins to walk away and out of the frame — on all fours. The man is one of five children in a religious family bedeviled by an unusual condition that has flummoxed and fascinated scientists since the scientific community first discovered them in 2005. The parents are normal. But five of their progeny are quadrupedal. They walk appendages down, bottom in the air. Earlier theories held the family’s gait signaled a devolution to our primate ancestry, but fresh research claims those earlier theories had it all wrong. It’s not devolution. It’s adaptation to an unforeseen and rare disorder. [Source: Terrence McCoy, Washington Post July 17, 2014 ^^^]

“What is undisputed: The five Kurdish siblings — four female, one male — are like few others on the planet. They’re impaired with something called Uner Tan Syndrome, named after the Turkish evolutionary biologist who first described them. Characterized by loss of balance, impaired cognitive abilities and a habitual quadrupedal gait, it’s a syndrome, Uner Tan theorized, that suggested “a backward stage in human evolution.” In other words, the siblings were thought to be walking proof that our evolutionary advances could — poof — vanish, and we’d be back to walking on all fours. “The idea of reverse evolution was just a flash, an ‘aha’ experience,” Tan told NeuroQuantology. “I suddenly realized they were exhibiting the walking style of our ape-like ancestors. … I was the scientist who first suggested the existence of reverse evolution in human beings.” ^^^

“But there were some problems with Tan’s suggestion. British researchers pointed out in a separate study that the family’s walk differs from that of some primates in a crucial way. They put all their weight on their wrists. Not on their knuckles. And now, a new study published in PLOS One further debunked the notion that the siblings represent reverse evolution. They do not, as Tan earlier surmised, walk like primates. Primates walk in a diagonal sequence, in which they put a hand on one side and a foot on the other, repeating this pattern as they progress forward. These humans, meanwhile, walk laterally — similar to other quadrupedals. ^^^

“According to the researchers, their walk is a byproduct of a hereditary condition that causes cerebellar hypoplasia. This condition complicates their sense of balance — and to adapt, they have developed quadrupedalism. “I was determined to publish this and set the record straight, because these erroneous claims about the nature and cause of the quadrupedalism in these individuals have been published over and over again, without any actual analysis of the biomechanics of their gait, and by researchers who are not experts in primate locomotion,” lead researcher Liza J. Shapiro of the University of Texas told The Washington Post. ^^^

“Still, their agility on all fours is impressive. “Their preferred form of locomotion, even when climbing or descending steps, is on all fours,” stated another study. “They move in this way fluently and effectively, and seemingly without discomfort. This contrasts markedly with normal adult humans who find such a gait — if and when they try it — tiring and uncomfortable even after practice.” ^^^

“The syndrome has another price. The siblings are able to speak, but barely, and have developed their own language to communicate with one another. According to Tan’s original study, they use fewer than one hundred words and had difficulty answering some questions. “What is the year?” Tan said he asked one of the siblings. “Eighty,” one said. “Ninety,” another replied. “Animals,” said another. “July,” explained the fourth. “House,” the last said. “What is the season?” “Animals,” said one. “What is this?” he said, pointing to a red shoe. “Tomato,” one offered. ^^^

“The siblings have 14 brothers and sisters who are not affected by the condition. It’s a large family that has at times protected them. Teased by some of the greater community, researchers found the four sisters stay close to home and crochet with needle and thread. The man, meanwhile, is most adventuresome and “remarkably agile.” He wanders about the village collecting bottles and cans and places them inside a pouch made by his shirt, which he holds up with his teeth. “The Ulas family remains a mystery to the scientific community, and the controversy surrounding them continues,” wrote Turkish psychologist Defne Aruoba. “Every once in a while, a new scientist appears in the village and offers a new treatment or asks for the father’s permission to do more testing. He doesn’t say yes and he doesn’t say no. He is in complete surrender to what life brings. His only concern is the welfare of his disabled children after he dies.”“ ^^^

Laetoli footprints probably from Australopithecus Afarenis

When and How Was Walking Invented?

Walking, it is believed, is something that developed gradually over time roughly beginning roughly 4.4 million years ago, with Ardipithicus, about a million years before the first tools, and achieving human-like status about 2 million years ago when the first homo erectus appeared.

In response to the question of when and how was walking invented, Jan Simek wrote in The Conversation: The shape of a creature’s bones and the way they fit together can tell the story of how that body moved when it was alive. And anthropologists can find other evidence in the landscape that indicates how ancient people walked. [Source: Jan Simek, Professor of Anthropology, University of Tennessee, The Conversation, November 1, 2021]

In 1994, Ardipithecus ramidus, nicknamed “Ardi.”, was discovered in Ethiopia and the fossils were dated to between 4.2 million and 4.4 million years old. When scientists examined its bones, they identified certain characteristics that indicated bipedalism. The foot, for example, had a structure that allowed the kind of toe push-off that we have today, which four-legged apes do not have. The shape of the pelvic bones, how the legs were positioned under the pelvis and how the leg bones fit together all suggested upright walking too. It may be that Ardi did not walk exactly as we do today, but bipedalism as the normal way of movement does seem to be characteristic of these fossils from as early as 4.4 million years ago.

Australopithecus afarensis (“Lucy”) lived around 3 million years ago, also in Ethiopia. Lucy had a partial but well-preserved pelvis, which was how anthropologists knew she was female. The pelvis and upper leg bones fit together in a way that showed she walked upright on two legs. No feet bones were preserved, but later discoveries of A. afarensis do include feet and indicate bipedal walking as well.

“In addition to fossil remains, scientists found other remarkable evidence for how Lucy’s species moved at the Laetoli site in Tanzania. Beneath a layer of volcanic ash dating to 3.6 million years ago, anthropologists found 70 fossilized footprints that indicate the presence of at least three individuals walking upright on two feet. Given the presumed age, the makers were likely Australopithecus afarensis. The tracks prove that these hominins walked on two legs, but the gait seems to be a bit different from ours today.

“A hominin whose anatomy was so like our own that we can say it walked as we do did not appear in Africa until 1.8 million years ago. Homo erectus was the first to have the long legs and shorter arms that would have made it possible to walk, run and move about Earth’s landscapes as we do today. Homo erectus also had a much larger brain than did earlier bipedal hominins and made and used stone tools called Acheulean implements. Anthropologists consider Homo erectus our close relative and an early member of our own genus, Homo.

Endurance Running Key Part of Human Evolution

Many scientists believe large brains developed relatively rapidly hand in hand with scavenging and endurance runners. Our upright posture, relatively hairless skin with sweat glands allow us to keep cool in hot conditions. Our large buttocks muscles and elastic tendons allow us to run long distance more efficiently than other animals. [Source: Abraham Rinquist, Listverse, September 16, 2016]

According to the “endurance running hypothesis,” first proposed in the early 2000s, long-distance running played a critical role in the development of our current upright body form. Researchers have suggested that our early ancestors were good endurance runners — presumably using the skill to efficiently cover large distances in search of food, water and cover and maybe methodically chase down prey and — and this characteristic left an evolutionary mark on many parts of our bodies, including our leg joints and feet and even our heads and buttocks. [Source: Michael Hopkin, Nature, November 17, 2004 ||*||]

Michael Hopkin wrote in Nature: “Early humans may have taken up running around 2 million years ago, after our ancestors began standing upright on the African savannah, suggest Dennis Bramble of the University of Utah, Salt Lake City, and Daniel Lieberman of Harvard University in Cambridge, Massachusetts. As a result, evolution would have favoured certain body characteristics, such as wide, sturdy knee-joints. The theory may explain why, thousands of years later, so many people are able to cover the full 42 kilometres of a marathon, the researchers add. And it may provide an answer to the question of why other primates do not share this ability. ||*||

“Our poor sprinting prowess has given rise to the idea that our bodies are adapted for walking, not running, says Lieberman. Even the fastest sprinters reach speeds of only about 10 metres per second, compared with the 30 metres per second of a cheetah. But over longer distances our performance is much more respectable: horses galloping long distances average about 6 metres per second, which is slower than a top-class human runner. "Everyone says humans are bad runners, because when you think of running you tend to think of sprinting," he adds. "There's no question we're appalling sprinters, but we're quite good at endurance running."||*||

“How did we get so good at running? Scavenging is the best answer, Lieberman suggests. Our savannah ancestors would have been in competition with hyenas, who are also good long-distance runners, to get to the site of a big kill and pick over the remains. "You could see a flock of vultures on the horizon and just take off towards them," he says. Or perhaps early humans used their endurance simply to chase prey to exhaustion. ||*||

“The theory makes sense of a raft of human characteristics, Bramble and Lieberman write in this week's Nature1. Not only do we have springy Achilles tendons and stout leg-joints, our hairlessness and tendency to sweat make us very good at dissipating heat. Running may even have improved our balance, says Fred Spoor, who studies human evolution at University College London. "Running requires a lot of delicate coordination: your legs are off the ground and you need to coordinate your eyes to see where your foot will land," he says. ||*||

Many animals keep their balance with the aid of semicircular canals in the inner ear, which are filled with fluid that acts as an acceleration detector. These structures are unusually large in both modern humans and our evolutionary cousin Homo erectus and this shows, says Spoor, that they might have helped primitive runners stay on their feet. In fact, running seems to be the only reason that we have prominent buttocks, says Lieberman. He has measured the activity of the gluteus maximus muscle in volunteers during a walk and a jog. "When they walk their glutes barely fire up," he says. "But when they run it goes like billy-o." It remains to be seen how the theory will be received, says Spoor. If correct, it means that the genus Homo is unique among primates in its running ability. But some experts maintain that there is nothing special about human locomotion, and what separates us from other apes is simply our outsized brains. “ ||*||

Anatomical Features That Made Humans Good Runners

Pigs are terrible runners. They lack the so-called nuchal ligament, an elastic band of tissue that runs from a ridge on the base of the skull to the spine. It keeps an animal’s head steady when it runs. Horses, dogs, cheetahs, and other good runners have such a ligament.William J. Cromie of the Harvard News wrote: “Traces of a nuchal ridge can be found in skulls millions of years old, so the next step was to check the fossils of early humans at the renowned Peabody Museum. It turns out that neither the earliest prehumans or the chimps that are their nearest relatives have a nuchal ridge. But some later-evolving hominins did. Known as Homo erectus, these tall, upright people were similar to modern humans. From the neck down, we would identify with them. [Source: William J. Cromie, Harvard News, November 18, 2004 ^=^]

“The meaning of this discrepancy struck Bramble and Lieberman in the head, so to speak. Chimps and the stooped predecessors of H. erectus, known as australopithecines, spent much of their time in the trees and had no good reason to run around much. With their long arms and more apelike anatomy, they walked or climbed around Africa from about 6 million to 2 million years ago. Two or 3 million years ago, when H. erectus came out of the trees and roamed the grassy savannas of Africa, running became a very handy thing for getting food. Four-legged animals can move like missiles, but tall, two-legged creatures move like pogo sticks. To be fast and steady, you need a head that oscillates up and down, but doesn’t pitch back and forth or bobble from side to side. ^=^

The nuchal ligament is one of several features that allowed early humans to run with steady heads held high. “As we started to think more about the nuchal ligament, we became more excited about other features of bones and muscles that might be specialized for running, rather than just walking upright,” Lieberman notes. One that comes immediately to mind is our shoulders. The burly, permanently hunched shoulders of chimps and australopithecines are connected to their skulls by muscles, the better to climb trees and swing from branches. The low, wide shoulders of modern humans are almost disconnected from our skulls, allowing us to run more efficiently but having nothing to do with walking.” Femur fossils of more recent hominins are stronger and larger than older ones, “a difference thought to have evolved to accommodate the added stress of running upright. ^=^

“Then there’s buns. “They are one of our most distinctive features,” Lieberman comments. “They are not just fat but huge muscles.” A quick look at a fossil australopithecine reveals that his pelvis, like that of a chimp, can only support a modest gluteus maximus, the major muscle that comprises a rear end. “These muscles are extensors of the hips,” Lieberman points out, “best used to push apes and australopithecines up the trunks of trees. Modern humans don’t need such a boost, and they don’t use their rear ends for walking. But the instant you start to break into a run, your gluteus maximus starts firing,” Lieberman notes. ^=^

“Such “firing” stabilizes your trunk as you lean forward in a run, that is, as the center of body mass moves in front of your hips. “A run is like a controlled fall,” Lieberman explains, “and your rear end helps you stay up.” Runners also get a lot of help from their Achilles tendons. (Sometimes a lot of trouble, too.) These tough, strong bands of tissue anchor our calf muscles to the heel bone. During a run, they act like springs that contract then uncoil to help push a runner ahead. But they’re not needed for walking. You can stroll across the African plains or city sidewalks without Achilles tendons.” ^=^

Image Sources: Wikimedia commons, except Kurdish family on all fours from Science Nordic

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 April 2024

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