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 ; Institute of Human Origins ; Becoming Human University of Arizona site ; Talk Origins Index ; Last updated 2006. Hall of Human Origins American Museum of Natural History ; Wikipedia article on Human Evolution Wikipedia ; Human Evolution Images; Hominin Species ; Paleoanthropology Links ; Britannica Human Evolution ; Human Evolution ; National Geographic Map of Human Migrations ; Humin Origins Washington State University ; University of California Museum of Anthropology; BBC The evolution of man"; "Bones, Stones and Genes: The Origin of Modern Humans" (Video lecture series). Howard Hughes Medical Institute.; Human Evolution Timeline ; Walking with Cavemen (BBC) ; PBS Evolution: Humans; PBS: Human Evolution Library; Human Evolution: you try it, from PBS; John Hawks' Anthropology Weblog ; New Scientist: Human Evolution; Fossil Sites and Organizations: The Paleoanthropology Society; Institute of Human Origins (Don Johanson's organization); The Leakey Foundation; The Stone Age Institute; The Bradshaw Foundation ; Turkana Basin Institute; Koobi Fora Research Project; Maropeng Cradle of Humankind, South Africa ; Blombus Cave Project; Journals: Journal of Human Evolution; American Journal of Physical Anthropology; Evolutionary Anthropology; Comptes Rendus Palevol ; PaleoAnthropology

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.

Bipedal Body Features

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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.

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.

First Hominins, Sex, Motherhood, Big Toes and Bipedalism

homo footprints
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

human versus chimpanzee foot

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.

Bipedalism Theories

No one really knows why hominins became bipedal. Maybe it because it helped them see threats and food options better. Or perhaps, it helped them reach up into trees for fruit or cool down in the hot African climate. [Source: Bob Yirka, December 13, 2011 =*=]

Some speculate that hominins rose up on their feet to get a better view over high grasses. Others say an upright stance exposed less body to direct sunlight in a hot climate than a body standing on all fours. According to one theory — first proposed in the middle of the 20th century that has largely been dismissed but still has its believers — when the Red Sea pored into northern Rift Valley of Africa 6 million years ago, patches of high ground became islands and vast areas became shallow lagoons. Under these conditions standing up and using one’s hands was the best way to open mollusks and tear apart crustaceans — as capuchin monkeys and crab-eating macaquess do — and walking upright was best suited for roaming the shallows of these lagoons loookung for shellfish, crsutaceans and small fish — food sources that were easy to exploit. [Source: David Attenburough]

Charles Q. Choi wrote in for Live Science: “Our ancestors evolved an upright posture well before our large brains or stone tools even appeared. The question, then: Why stand and walk on two legs when our ape cousins get by on four limbs? Walking as bipeds might actually use less energy than movement on all fours does. Freeing up the arms might also have enabled our ancestors to carry more food. Standing upright might even have helped them control their temperature better by reducing the amount of skin directly exposed to the sun.” [Source: Charles Q. Choi, Live Science, February 22, 2011]

The discovery of an Ardipithecus-like foot in the 2010s from 3.4-million-year-old deposits at Burtele, Ethiopia, further shows that at least two different forms of bipedalism coexisted in the Pliocene.

See Australopithecus sediba

Walking in the Trees and the Development of Bipedalism

Paleontologist Eric Delso argue that "human ancestors may have moved to the ground more to take advantage of opportunities on the open ground and less because they were forced to." Many scientists believe that bipedalism developed in the trees, where some apes, and perhaps early hominins, walked upright on large branches or stood on their leg to collect fruit overhead. Later they found bipedalism was more than efficient than walking on all fours or leaping from tree to trees when it came time for feeding on trees from the ground and moving from one tree to another in a less dense forests.

Scientists from the College de France believe that bipedalism developed when apes were in the trees. They say that tree branches are ideal place to learn to walk because there are branches that can be used for supports. Robin Compton of Liverpool University told the Observer, “Trees were an ideal nursery for the learning of human walking. they enable an animal to balance itself. They can reach out in any direction, above and below themselves, and find branches, Orangutans do just ths sort of thing.” The clam is backed by evidence of bipedalism in the bones of 6 million-year-old apes found in Kenya and early hominins found in Ethiopia.


In a June 2007 article in the journal Science, Susannah Thorpe and Roger Holder of the University of Birmingham and Robin Crompton of the University of Liverpool suggested that bipedalism arose much earlier than previously thought among arboreal apes — perhaps as early as between 17 million and 24 million years ago — based on the way wild orangutans navigate their way along fragile tree branches. Thorpe spent a year observing wild orangutans in the forest of Sumatra and saw them walk on two legs on fragile branches to reach fruit, using their arms to keep balance or grasp for fruit while using all four limbs on bigger branches. The finding is significant in that it shows bipedalism might have first evolved as a way to move around in the trees rather than on the ground.

Another theory, known among some scientists as "East Side Story," hypothesize that the sudden expansion and deepening of the Great Rift Valley led to the evolution of mankind. Chimpanzees and gorilla are found almost exclusively on the forested west side of the valley while fossils of ancient upright walking homonids have been almost exclusively on the grassy east side. [Source: John Noble Wilford, New York Times, May 17, 1994]

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.” |=|

Bipedalism: Result of Rugged Landscape?

A new study by archaeologists at the University of York challenges evolutionary theories behind the development of our earliest ancestors from tree dwelling quadrupeds to upright bipeds capable of walking and scrambling. According to the University of York: The researchers say our upright gait may have its origins in the rugged landscape of East and South Africa which was shaped during the Pliocene epoch by volcanoes and shifting tectonic plates. [Source: University of York,, May 24, 2013]

“Hominins, our early forebears, would have been attracted to the terrain of rocky outcrops and gorges because it offered shelter and opportunities to trap prey. But it also required more upright scrambling and climbing gaits, prompting the emergence of bipedalism. The York research challenges traditional hypotheses which suggest our early forebears were forced out of the trees and onto two feet when climate change reduced tree cover. The study — 'Complex Topography and Human Evolution: the Missing Link' — was developed in conjunction with researchers from the Institut de Physique du Globe in Paris and published in the journal Antiquity.

“Dr Isabelle Winder, from the Department of Archaeology at York and one of the paper's authors, said: "Our research shows that bipedalism may have developed as a response to the terrain, rather than a response to climatically-driven vegetation changes. "The broken, disrupted terrain offered benefits for hominins in terms of security and food, but it also proved a motivation to improve their locomotor skills by climbing, balancing, scrambling and moving swiftly over broken ground - types of movement encouraging a more upright gait."

“The research suggests that the hands and arms of upright hominins were then left free to develop increased manual dexterity and tool use, supporting a further key stage in the evolutionary story. The development of running adaptations to the skeleton and foot may have resulted from later excursions onto the surrounding flat plains in search of prey and new home ranges. Dr Winder said: "The varied terrain may also have contributed to improved cognitive skills such as navigation and communication abilities, accounting for the continued evolution of our brains and social functions such as co-operation and team work. "Our hypothesis offers a new, viable alternative to traditional vegetation or climate change hypotheses. It explains all the key processes in hominin evolution and offers a more convincing scenario than traditional hypotheses."

chimp and human thighbones

Transition From Trees to the Ground Was Gradual

The last common ancestor of humans and chimpanzees are believed to have had shoulders similar to those of modern African apes, a finding supports the theory that early humans moved away from life in trees gradually. Charles Q. Choi wrote in Live Science: “The human lineage diverged from that of chimpanzees, humanity's closest living relative, about 6 million or 7 million years ago. Knowing the characteristics of the last common ancestor of humans and chimps would shed light on how the anatomy and behavior of both lineages evolved over time, "but fossils from that time are rare," said lead author of the new study Nathan Young, an evolutionary biologist at the University of California, San Francisco. [Source: Charles Q. Choi, Live Science, September 8, 2015 +]

“There are currently at least two competing scenarios for what the last common ancestor might have looked like. One suggests that similarities seen in modern African apes, such as in chimps and gorillas, were inherited from the last common ancestor, meaning that modern African apes may reflect what the last common ancestor was like. "A lot of people use chimpanzees as a model for the last common ancestor," Young told Live Science. +\

“The other scenario suggests these similarities instead evolved independently in modern African apes, and that the last common ancestor may have possessed more-primitive traits than those seen in modern African apes. For instance, instead of knuckle-walking on the ground like chimps and gorillas do, the last common ancestor may have swung and hung from tree branches like orangutans, which are Asian apes. Humans aren't the only species that have evolved and changed over time — chimpanzees and gorillas have evolved and changed over time, too, so looking at their modern forms for insights into what the last common ancestor was like could be misleading in a lot of ways," Young said. +\

“The ancestral state of the shoulder is key to understanding human evolution, because the shoulder is linked to many important shifts in behavior in the human lineage. Shoulder evolution could help show when early human ancestors began using tools more, spent reduced time in trees and learned to throw weapons. However, the human shoulder possesses a unique combination of features that makes it difficult to reconstruct the body part's history. For instance, while humans are most closely related to knuckle-walking chimps, in some respects the human shoulder is more similar in shape to that of tree-dwelling orangutans. +\

“To see what the shoulder of the last common ancestor might have looked like, researchers generated 3D shoulder models from museum specimens of modern humans, chimps, bonobos, gorillas, orangutans, gibbons and monkeys. The scientists compared these data with 3D models that other scientists previously generated of ancient, extinct relatives of modern humans, such as Australopithecus afarensis, Australopithecus sediba, Homo ergaster and Neanderthals. +\ Australopithecines such as Australopithecus afarensis and Australopithecus sediba are the leading candidates for direct ancestors of humans. "Recent data from the australopithecines helped us now test different models of human evolution," Young said. +\

“The scientists found the strongest model showed the human shoulder gradually evolving from an African apelike form to its modern state. "We found australopithecines were perfect intermediate forms between African apes and modern humans," Young said. This finding suggests the human lineage experienced a long, gradual shift out of the trees and increased reliance on tools as it became more terrestrial, he said. "These results pretty much confirm that the simplest explanation for how the human shoulder evolved is the most likely one," Young said. The scientists detailed their findings online Sept. 7, 2015 in the journal Proceedings of the National Academy of Sciences.” +\

Australopithecus Afarensis’s (Lucy’s) Feet Indicate She Lived on the Ground

Laetoli footprints probably from Australopithecus Afarenis

Australopithecus Afarenis had arched feet suggesting it had abandoned life in the trees and lived on the ground. Alok Jha wrote in The Guardian: “The ancestors of humans were walking upright more than 3 million years ago, according to an analysis of a fossilised foot bone found in Ethiopia. The fossil, the fourth metatarsal bone from the species Australopithecus afarensis, shows that this forerunner of early humans had a permanently arched foot like modern humans, a key requirement for an upright gait. Arches in human feet put a spring in our step: they are stiff enough to propel us forward but flexible enough to absorb the shock at the end of each stride. [Source: Alok Jha, The Guardian, February 10, 2011 |=|]

“Scientists already knew that A. afarensis could walk on two feet but were unsure whether the creatures climbed and grasped tree branches as well, much like their own ancestor species and modern nonhuman apes. The fourth metatarsal, described on Thursday in Science, shows that A. afarensis moved around more like modern humans. "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 Carol Ward, a professor of integrative anatomy at the University of Missouri-Columbia who led the analysis of the fossil. “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 favour of life on the ground." |=|

“The best-known example of A. afarensis is "Lucy", who lived in eastern Africa more than 3 million years ago. Before that, more than 4.4 million years ago, Ethiopia was populated by Ardipithecus ramidus, which seems to have been a part-time terrestrial biped, though its foot had many of the features of tree-dwelling primates, including a highly mobile big toe. |Unlike other primates, human feet have two arches, which stretch along the length of the foot and across it. Ape feet do not have these arches and are far more flexible, with a mobile large toe that is useful for climbing trees and holding onto branches. |=|

“These ape-like features are not present in the foot of A. afarensis, however. Given that its foot was more like that of modern humans, scientists think that A. afarensis no longer depended on the trees for refuge or resources 3 million years ago. "Arches in the feet are a key component of human-like walking because they absorb shock and also provide a stiff platform so that we can push off from our feet and move forward," said Ward. "People today with 'flat feet' who lack arches have a host of joint problems throughout their skeletons. Understanding that the arch appeared very early in our evolution shows that the unique structure of our feet is fundamental to human locomotion. "If we can understand what we were designed to do and the natural selection that shaped the human skeleton, we can gain insight into how our skeletons work today. Arches in our feet were just as important for our ancestors as they are for us." |=|

“Isabelle De Groote, a palaeontologist at the Natural History Museum in London, said: "These findings confirm that our human ancestors were walking on two legs by about 3.2 million years ago. |=| "Bipedal locomotion or two-legged walking is one of the hallmarks of the human species. Older human fossils still show adaptations to spending some of their time in the trees ... for feeding or nesting, but the evidence here suggests that by 3.2 million years ago one of our ancestors, Australopithecus afarensis, was fully committed to bipedal walking."” |=|

Climate Theory of Bipedalism

Some scientists theorize that it was an abrupt climate change that brought about the development of the upright walking style and a larger brain. The reasoning goes something like this: the cooler and drier climate that occurred in eastern Africa after the creation of the Panama isthmus transformed rain forests into savannahs, where food sources are more scattered and farther away. The changes forced proto-hominins out of the trees in the rain forests and onto savannah grasslands, where they needed a larger brain to develop more complicated food gathering tasks, locate scarce food and remember when they were in season.

The apes that lived in the new environment found that walking upright and moving about on two legs was a much better way for getting around on the ground than knuckle-walking style that chimpanzees and gorillas now employ. Walking upright was the most energy efficient way to cover distances and reach scattered food sources. It also freed the hands to gather a broad range of foods. The erect posture kept the body cool by exposing more skin to breezes and less skin to the sun.

Later, a larger brain helped hominins to develop strategies to hunt large animals and use tools which in turn requiring more intellectual reasoning and larger brain, speeding along the evolution process.

Climate Changes at the Time Hominins First Evolved

Some scientists theorize that hominins developed as a separate species during periods of climate change. Before 3.5 million years ago, North and South America were not connected and waters from the Atlantic and Pacific mixed and lowered salinity levels in the Atlantic, which meant that lighter water from the tropics was carried all the way to the Arctic Ocean.

When the isthmus of Panama was later created, water from the Atlantic and the Pacific no longer mixed, which increased the level of salinity in the North Atlantic Current, causing it to sink before it reached the Arctic, causing an icecap to form there. The changes also caused the northern diversion of the equatorial Atlantic Ocean current and the intensification of the Gulf Stream, which resulted in more snowfall in the north and the built up of glaciers.

These changes led to an ice age. Between 2.8 and 2.5 million years ago glaciers began creeping from the Arctic Ice cap down over much of the northern hemisphere and the climate in Africa became noticeable colder and drier. Evidence of rainfall and climate changes in the period is based on analysis of dust particles in rock strata and pollen deposited in a coastal ocean-floor sediments.

20120201-HomoAsBipedalApes 2.gif

Support and Criticism of Climate Theory of Bipedalism

Backing up this so-called climate theory of bipedalism are primate studies. Fruit eating primates, for example, generally have a larger brain than leaf-eating ones because it requires more sophisticated reasoning, scientists suggest, to find food which is only found in some areas at certain times of the year. The brain of the leaf-spider monkey, for instance, is only half the size of the fruit-eating howler monkey, even though occupy roughly the same terrain. [Source: Eugene Linden, National Geographic, March 1992]

Arctic ice-cores and the fossil record thus far seem to indicate that global climatic changes took place about the same time as major developments in human evolution. A cold, dry spell about 2.8 million years ago, for instance, associated with appearance of grasslands in Africa occurred at about the same time as the first homonids. Another cold period a million years coincides with extinction of the genus Australopithecus. During these same time periods forests antelopes were replaced by giant buffalo and other grazers.

There are lot of problems with the climatic theory and bipedalism. Some studies indicate the savannah grasslands in and around the Great Rift Valley, where almost all of remains of our earliest ancestors have been found, have remained pretty much unchanged for the last 15 million years. Bone samples, dated Between 2.8 and 2.5 million years ago, from Lake Turkana provide no evidence of abnormally rapid evolutionary activity at this time. This has led scientists to argue that "human evolution was much more a response to a prolonged series of climate fluctuations rather than any single shift."

Perhaps most damning of all is the time of climate changes is evidence that has come forth with fairly recent hominin discoveries. Discoveries of new Australopithecus and Ardipithecus species seem to indicate the timing of climate theory is off. These species began walking upright between 5 million and 3 million years ago long before the climate changes associated with the creation of the Panama isthmus,

Other evidence seems to indicate that the first hominins to walk upright did so in the forest rather than the savannah. Evidence found with remains of “ Australopithecus ramidus” , for example, seem to indicate it lived in a wooded environment rather than savannah grasslands.

Ardipithecus Ramidus Debunks Climate Theory

Scientists believe that “Ardipithecus kadabba” and “Ardipithecus ramidus” , who lived around three and four million years ago, lived in the forest because: 1) their teeth indicate they ate woodland foods: and 2) their remains were found among fossils of forest dwelling plants and monkeys.

If it is indeed true that “Ardipithecus” lived in woodlands and is bipedal it means he likely developed the ability to walk in woodlands which would debunk the “savannah hypotheses”—that man first walked up right to survive in a grassland habitat — and the “climate theory” — that changes in climate which caused woodlands to change to savannahs caused early hominin to walk upright.

Lovejoy has theorized that Ardipithecus came out trees for sex. Based on the fact that the canines of Ardipithecus males are small and similar to females — unlike male apes which have large canines and use them mainly in fights with other males — Ardipithecus won over females by coming down of the trees to collect high-protein, high-fat food given to the females offspring in return for sex and bipedalism developed as a way to carry back food.

Scientists have found some evidence (from soil samples and analysis of teeth) that “Ardipithecus” lived in a savannah environment not a wooded one. Most scientists agree that a lot of analysis and research still needs to be done to make any authoritative claims about Ardi and her kin.


Family That Walks on All Fours Today: Reverse Evolution?

Kurdish family that walks on all fours

Three 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. 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. [Source: Terrence McCoy, Washington Post July 17, 2014 ^^^]

“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.^^^

“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.”“ ^^^

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 September 2018

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