pelvis in apes and man
Bipedalism (moving on two legs) is one of the key characteristics that defines hominids and humans. A hominid 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 hominids 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: Modern Human Origins ; Talk Origins Index ; Hall of Human Origins American Museum of Natural History ; Time Space Chart Hominid Fossils Pictures ; Smithsonian Human Origins Program ; Wikipedia article on Human Evolution Wikipedia ; Becoming Human University of Arizona site ; Human Evolution Images ;Hominid Species ; Institute of Human Origins ; Paleoanthropology Link ; Britannica Human Evolution ;Modern Human Origins ; Human Evolution ; Paleoanthropology and Evolution Links ;National Geographic Atlas of the Human Journey ; Yale Peabody Museum ; Humin Origins Washington State University ; Book: The Human Evolution Source Book

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

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.

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 hominids began engaging in bipedalism because it is more efficient.

First Hominids, Sex, Motherhood, Big Toes and Bipedalism

homo footprints
Scientists estimate that around six million years ago, female hominids 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 hominids learned to walk and their pelvis became narrower.

Owen Lovejoy of Ohio State University has theorized that hominid 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 hominids 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.

chimp and human thighbones

Climate Changes at the Time Hominids First Evolved

Some scientists theorize that hominids 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.

Ice Ages

Ice Ages are periods of time when huge continental glaciers (sheets of ice) crept down from the Arctic and covered much of North America and Europe. The Pleistocene Age, which lasted from 1,000,000 (or 500,000) to 10,000 years ago, is regarded as the period of ice ages, even though the first Ice Age glaciers began appearing around 2 million years ago and large glaciers had covered landmasses for millions of years before that.

There were four main ices ages defined by the warming and cooling of the climate and expanding and retreating of glaciers. Scientists refer to the ice ages as glaciations and the warmer periods in between them as interglacial phases. The ice ages lasted tens of thousands of years as the glaciers expanded, reached a peak and then retreated.

The four main ice ages are (the names refer to the southern limit of the glaciers in Europe and, in parentheses, the United States): 1) the Günz (known in the U.S. as the Nebraskan) occurred around 2 million years ago); 2) the Mindel (known in the U.S. as the Kansan) occurred around 1.25 million years ago); 3) the Riss (known in the U.S. as the Illinoisian) occurred around 500,000 years ago); and 4) the Würm (known in the U.S. as the Wisconsin) occurred around 100,000 years ago).

Ice Age Glaciers

During the Ice Age temperature were only 5 to 10 degrees colder than today. As recently as 20,000 years ago, glaciers hundreds and thousands of feet thick covered all of Canada, much of the United States ad most of northwestern Europe. There was less glaciation over Russia.

The glaciers became so large more as a result of cool summers that failed to melt the glaciers rather than cold winters (in really cold areas there is often less snow than slightly cold areas). Every year more and more snow accumulated and created massive glaciers.

Some glaciers were more than two miles thick. The weight of all this ice caused the Earth’s crust to sink between 300 and 800 feet. In many places the land is still in the process of springing back. So much water was frozen into the glaciers that the levels of the sea was lowers by dozens, even hundreds of feet.

Causes of the Ice Ages, Ocean Currents and Panama

The causes of the Ice Ages are still a mystery. Some scientists have suggested that they may have been caused by climate changes brought about the movement of the Earth in relation to the sun. Others have attributed them to volcanic eruptions or high levels of atmospheric gases or clouds that shut out the warmth of the sun.

Many scientists believe the ice ages were triggered by major changes in the ocean currents. They theorize that currents that brought warm water to the northern latitudes were somehow disrupted and the flow of warm water northward was somehow cut off and thus the climate in the northern hemisphere became colder and glaciers formed and expanded.

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 rock strata containing dust and pollen deposited in a coastal ocean-floor sediments.

20120201-HomoAsBipedalApes 2.gif

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-hominids 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 hominids 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.

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 hominid 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 hominids 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 hominid 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.


Walking in the Trees and Other Theories for 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 hominids, 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 hominids 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]

Study: Female Ancestors Wandered, Males Stayed Put

Seth Borenstein of AP wrote, “In modern times, men explored the New World. But 2 million years earlier, the men among our pre-human forerunners stayed put and it was the women who traveled to start new families, a study of fossil teeth from Africa suggests. The findings, published in the journal Nature, indicate females from two pre-human species seemed to move out of their birth homes and journey elsewhere, probably to prevent inbreeding, researchers said. Chimpanzees, our closest living primate relative, also have females that travel to mate and raise families. That's in contrast to lower primates and most mammals where it is the males that have the wanderlust. [Source: Seth Borenstein, AP, June 1, 2011]

Researchers studied 19 teeth, including eight from Australopithecus africanus individuals, a species considered a probable ancestor from about 2.2 million years ago. The other 11 were from Paranthropus robustus individuals, a dead-end species that was not our direct ancestors but more like prehistoric aunts and uncles from 1.8 million years ago.They looked for the mineral strontium in the teeth because that element varies by landscape. The idea was to see if they moved to different areas during various seasons. The research didn't show that, but something else popped up: The bigger teeth showed almost no mineral variation while more than half of the smaller teeth indicated they were from individuals that grew up elsewhere. So the researchers figure this was a male-female difference. Other scientists not involved in the research, though, said the tooth sample may be too small to draw that conclusion.

The study's lead author, Sandi Copeland, a paleoanthropologist at the University of Colorado, said the switch from male to female dispersal may indicate the start of a sense community, which has its evolutionary advantages. And it continues in many societies to this day. In less evolved animals, it makes evolutionary sense for the male to wander and impregnate many females and show might. In this case, the female moving could show that males in a community have bonded and cooperated, maybe for common defense. So it makes sense for the men to settle, while the females disperse, Copeland said. "There must be an evolutionary benefit for females to disperse," Copeland said.

University of Oxford archaeologist Michael Petraglia, who wasn't part of the study, said the research was intriguing, showing that our forerunners' "social relations and mating patterns are more in line with ours than with gorillas." But like Richard Potts of the Smithsonian Institution, Petraglia worried that the research may be drawing too many conclusions from a small number of teeth from a large time period. Study co-author Darryl de Ruiter at Texas A&M University said researchers were limited in how many samples they could use because tests destroyed parts of the fossil and they only used the largest and smallest teeth.

Out of Africa Theory

There are various theories describing how migration patterns played a part in the development of early humans. The traditional, widely-accepted "Single Origin, Out of Africa Theory" of human evolution posits that earliest hominids evolved in Africa; that Australopithecus species evolved into Homo species in Africa; that early Homo species migrated to Asia and the Old World from Africa between a million and two million years ago; and that Homo sapiens also evolved in Africa.

According to the “Out of Africa" theory there were two migration of African-born Homo species. First, Homo erectus began slowly moving into the Middle East, Europe and Asia between a million and two million years ago. Homo erectus splintered into numerous colonies that developed separately from one another. None of the colones outside of Africa contributed to the development of Homo sapiens , which also originally evolved in Africa.


fossil sites in Africa

Out of Africa Theory and Homo Erectus

Proponents of the Out of Africa theory argue that Homo erectus spread across Africa during the first 800,000 years after first appearing 1.8 million years ago. It moved to Asia but for some reason stayed out Europe. They argue that all modern humans have evolved from African Homo species.

Recent dating of fossils in Georgia, China and Indonesia suggests that Homo erectus may have migrated into Asia as early as 2 million years ago and migrated across the continent very quickly. This conclusion is based on fossils found in Georgia, China and Indonesia that have been dated to be between 1.7 million and 1.9 million years old.

Scientists believe that the effects of climate on food sources and habitat played a major part in the migration. Richard Leakey thinks that Homo erectus migrated out of Africa along the Nile. Many think Homo erectus ventured through the Middle East to places such as Dmanisi in Geogia.

Hominids are thought to have most likely migrated across what is called the Levantine Corridor through what is now Israel, Jordan, and Lebanon. It embraces the Jordan River and the Dead Sea and a geologic zone that included the Red Sea which began opening up between the African and Arabian tectonic plates around 2 million years ago, the time the first migrants were thought to have emerged from Africa. At that time the Levantine Corridor was a verdant area with lakes and lots of animals and plants that could have been consumed as food.

Evidence for Out of Africa Theory

Evidence for the “Out of Africa” theory includes a trail of stone tools, including carefully-crafted hand axes, that appear to have originated in Africa. Scientists also point out that many sophisticated hand ax are found in Africa but none have been found in Asia.

Thousands of primitive 1.5-million- to 1.4-million-year-old hand axes have been found in Ubeidya, Israel in the Levantine Corridor. Ubeidya is a vanished lake just south of the Sea of Galilee. Stone tools and fossils of large mammals found near the tools have been dated to 1.4 million years ago. Hand axes similar to one found there have been found throughout Europe and Asia. Hand axes found north of Lake Tiberias suggests there was another round migration through the Levantine Corridor around 780,000 years ago.

Genetic studies of 28 of China's 56 ethnic groups, published by the Chinese Human Genae Diversity Project in 2000, indicate that the first Chinese descended from Africans who migrated their way along the Indian Ocean and made their way to China via Southeast Asia.

Image Sources: Wikimedia commons

Text Sources: Mostly from National Geographic articles. I’ve gone through them all since around 1963 or so. Also the New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Times of London, Natural History magazine, Archaeology magazine, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, World Religions edited by Geoffrey Parrinder (Facts on File Publications, New York); History of Warfare by John Keegan (Vintage Books); History of Art by H.W. Janson (Prentice Hall, Englewood Cliffs, N.J.), Compton’s Encyclopedia and various books and other publications.

Page Top

© 2009 Jeffrey Hays

Last updated March 2011

This site contains copyrighted material the use of which has not always been authorized by the copyright owner. Such material is made available in an effort to advance understanding of country or topic discussed in the article. This constitutes 'fair use' of any such copyrighted material as provided for in section 107 of the US Copyright Law. In accordance with Title 17 U.S.C. Section 107, the material on this site is distributed without profit. If you wish to use copyrighted material from this site for purposes of your own that go beyond 'fair use', you must obtain permission from the copyright owner. If you are the copyright owner and would like this content removed from, please contact me.