Bipedalism

Eduzendium article

A king penguin standing on its feet.

Bipedalism is the condition of having or using only two feet for locomotion. Throughout the course of evolutionary history, the use of bipedal movement came forward several times as an adaptation, including at what many consider to be an influential point in human evolution. The particular selective pressures that acted to bring bipedalism to the forefront time and time again are most likely diverse and unique to each group for which this type of locomotion was expressed. However, the prevalence of bipedal movement in animals both extant and extinct shows that this form of locomotion is sometimes advantageous.

Non-human species displaying bipedal locomotion

There are many species on Earth that use bipedal movement as their primary means of terrestrial locomotion, and also some that can use bipedal movement when pressed to escape danger.

Birds

In nearly every species of bird, a form of terrestrial bipedal movement shows itself. Because of their wings many birds do not use bipedalism as their primary means of locomotion in life, but when they land they hop or walk on two feet. Birds whose lives are spent in a mostly terrestrial capacity such as chickens, kiwi birds, and larger forms like ostriches and emus have reduced wings and larger, stronger legs. These birds are often highly-efficient bipeds, able to run at high speeds and relatively agile for their size. Ostriches have been known to run at speeds in excess of 40 mph (65 km/h)when chased by predators.

Birds are thought to be descendants of theropod dinosaurs, and are thought to have split from that lineage before the event that caused the extinction of much of land vertebrates 65 million years ago at the end of the Cretaceous period.

Dinosaurs

The type of bipedalism seen in birds is similar to that which would have been in bipedal dinosaurs, such as the Tyrannosaurus rex. A recent study that used chickens as analogs for dinosaur locomotion showed it was possible to change the weight-bearing bones in the legs to a more robust, non-avian dinosaur-like form through adding weights to the back of a chicken. The weight added was to symbolize a tail, which changed the form of the femur of the birds in the experiment.

Thus, dinosaur bipedal locomotion may be similar to that of birds, but the addition of the tail is a key morphological trait that changes it slightly.

Lizards

Several extant species of lizard walk or run on their hind legs when trying to escape predators. Lophognathus longirostris is an example that is closely related to the frill-necked lizards, and runs on its back legs to gain speed and agility over short distances. Physignathus lesueurii, another relative, is capable of running for short distances on the surface of water.

Anatomical correlates of bipedalism in humans and recent human ancestors

Through looking at the osteology of humans and their close relatives in the fossil record, a few key anatomical traits for the particular bipedalism associated with humans have come forward. A combination of these traits makes it possible for humans to walk upright and allows scientists to diagnose whether a fossil likely used bipedal movement or not.

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At left is a modern human femur, and at right is a femur of the common chimpanzee, Pan troglodytes.

Medially-angled femur

One trait that is often used to diagnose bipedal movement in hominids is the angle of the femur. In humans this bone angles toward the mid-line of the body, allowing for better balancing when walking or running and the load of the weight to be better distributed. The angle is created by the long neck of the femur in a human-like biped, which sabilizes the hipWhen a human femur is balanced on the distal (distant) end of the bone on the medial and lateral condyles, the angle created between the it and the flat surface it is balanced on is around 10 degrees. This bi-condylar angle is nearly 14 degrees in TM1513, an Australopithecine fossil found in 1938, as compared to the near-nonexistance of such an angle in chimpanzees. In the few fossilized femora that have been unearthed this angle has been helpful to show some form of bipedalism.

In addition, the medial condyle on the distal end of the femur of a human-like biped is considerably larger than in other extant apes. This is an adaptation that increases the stability of the knee joint and prevents the distal end of the femur from slipping out of alignment.

Sigmoid Spine Shape

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Two human vertebrae. The left is a robust lumbar vertebra and the right is a thoracic vertebra.

The "S" shape of the spines of humans and their closest bipedal relatives is another characteristic that helps diagnose a biped. This shape is the caused by progressive dorsal wedging. This morphological trait is the result of lumbar lordosis and thoracic kiphosis, both of which are different curves of the spine. Lumbar lordosis is the forward curve of the lower portion of the back, and thoracic kiphosis is the slight backward curve of the vertebrae to which the ribs attach. These shapes are likely developmental in nature and not genetic, as they develop in infants after they begin to walk upright and not before.

Some recent work on the lumbar vertebrae of female humans and our closest relatives proposes that lumbar lordosis is greater in females because they need to be able to carry the weight of a fetus when they are pregnant. Women appear to have one or two more lumbar vertebrae that are capable of slipping forward a bit and creating more of a curve in the lower back. Without this adaptation, women who became pregnant might fall over constantly (to the detriment of their fetus) because they would not be able to balance very well.

Low/Broad Illium in Os Coxae

The Os Coxae are formed of three fused bones that form one half of the pelvis. In chimpanzees, these bones are elongated and somewhat gracile due to the distribution of weight in a quadruped. An animal that moves like a human biped, on the other hand, places a lot of weight on the os coxae and therefore those bones have become short, thick, and broad. The illium in particular, the bone that is at the top of the pelvis, has truncated itself to allow stronger muscle attachments and more weight-bearing capacity. The bones need to be compact in order for the muscles of the legs to attach strongly to hold up a person as they walk.

In studies of biomechanics, there appears to be a balance between a need for width and more efficient narrow hip for a biped's pelvis. In modern human women, the pelvis is a compromise between the demands of walking, running, and jumping bipedally and the difficulty of delivering a fetus with a large brain size.

As hominid brains grew larger and they became more and more habitual bipeds, a compromise between a highly-efficient narrow pelvis and a clunkier, wider one became paramount in females. A narrower pelvic inlet makes for a better bipedal gait, but women with too small of one likely will die in childbirth because of the large size of human babies' brains. But for the expansion of the human brain, the female pelvis would likely be narrower and the inlet smaller.

Foramen Magnum

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The large hole on the bottom of the skull is the foramen magnun, the opening through which the spinal column passes. In a biped such as this human, this feature is directly on the bottom of the skull.

Upon finding the Taung Child in 1925, Raymond Dart faced harsh criticism in the attempt to prove that this was actually a bipedal human ancestor. As there were no post-cranial remains and only a partial juvenile skull, the only venue open to him was to try to infer the position of its foramen magnum. In human-like bipeds, this opening that the brain stem goes through is anteriorly placed. It is directly under the top of the skull, to allow the head to be held straight up and down. Dart inferred the position of the foramen magnum on the skull and believed it was placed anteriorly and that this was therefore a bipedal creature. With the lack of post-crania for Australopithecus africanus at the time, few accepted the view.

In fact, it was not until 1947 when Broom and Robinson found a partial skeleton, STS 14, that the world began to accept that A. africanus was indisputably bipedal.

Distinctly Arched Feet/Non-Opposable Hallux

In 1978, Mary Leakey and her team were looking for fossil hominids in the Laetoli region of Tanzania when they stumbled upon a find unlike any other. Over 100 footprints of many species, preserved in volcanic ash, captured a moment in time dated 3.7-3.5 million years ago. Probably accumulated over a period of hours, the footprints give invaluable information about behavior in Paleolithic animals, including elephants, sabertooth cats, and most stunningly of all, bipedal hominids.

The footprints at Laetoli show a pair of hominids walking side by side, possibly even arm in arm by some estimations. These footprints are clearly those of a biped, showing an arch in the foot and a non-divergent big toe. Both of these structures help bear the weight of the body when walking. These elements of the Laetoli prints are what diagnosed what was at the time some of the earliest direct evidence of human-like bipedalism known to science.

Other Features Used to Diagnose Bipedalism

The features above, when available for the fossil or bone in question, are preferred to diagnose bipedalism quickly and succinctly. However, to establish the locomotion of a partial skeleton or to make a diagnosis more iron-clad, there are many other features that can be indicators of bipedal movement. The list below is partial and many features exist.

• Large Semi-Lunar Canals in the ears-These canals are larger in a biped than in a quadruped, because they have a lot to do with maintaining balance while upright.
• Robust Talus and Calcaneus-The two bones that make up the ankle are large and flat in a biped, forming a platform that the bones of the lower leg rest on.
• Longer Leg Proportions-A biped's intramembral index falls between 50 and 80, indicating that the hind-limbs are significantly longer than the forelimbs. This adaptation allows for longer strides and a more efficient gait.

Bipedalism in Early Human Ancestry

The fossil record was notoriously silent on the bipedal relatives of humans in the past. In fact, Georges Cuvier, the father of vertebrate paleontology wrote in 1812, "L'homme fossile n'existe pas" ("fossil man does not exist). Since then, the fossil record has given up some of its secrets about the bipedal primates our own human line is descended from, and in recent years the discoveries of several new and ancient specimens have challenged traditional ideas about bipedalism and its origins.

The use of bipedal movement is thought to be a key element in human evolutionary history, as it is part of what separates humans and their ancestors from the other Great Apes. By virtue of similar musculoskeletal organization of human and ape bodies it is possible to trace common ancestry. Humans today bear many anatomical resemblances to their closest relatives. Residual anatomical traits that include but are not limited to dorsally-placed scapulae and a wide range of motion in the shoulder, arm, and wrist point to an origin of bipedal locomotion that is very recent in evolutionary history of the Genus Homo. These residual traits suggest suspensory adaptations in the forelimbs of a primarily terrestrial group of animals that usually bear the bulk of its mass on their hindlimbs.

In an extensive review, anthropologist Daniel Schmitt discusses the data pointing to a recent evolutionary history of specific locomotor traits in humans:

Experimental data collected on humans and nonhuman primates suggest that early hominid bipedalism evolved in an arboreal, climbing primate. The earliest mode of bipedalism included many aspects of locomotion seen in modern humans, but probably did not involve inverted pendulum-like mechanics [When modern humans walk, we vault over relatively stiff lower limbs in such a way that our center of mass is at its lowest point at heel-strike and rises to its highest point at midstance]. This difference in locomotor styles between early hominids and modern humans appears to be associated with small, gracile and poorly stabilized hindlimbs in our earliest ancestors (Stern and Susman, 1983). It seems likely that the shift to a more robust modern skeleton seen in early members of the genus Homo reflected the adoption of a relatively stiff-legged gait. This perspective on the evolution of bipedalism from a relatively compliant to a relatively stiff-legged style changes our understanding of locomotor adaptations in the genus Homo. The data described above strongly suggest that a relatively stiff-legged bipedal gait and associated physiological and musculoskeletal adaptations are not inherited from prebipedal ancestors or even from the earliest upright bipeds. These features are instead, specialized characters that evolved relatively recently.

A. africanus and A. afarensis

In one of the most stunning finds in the previous century, a near complete skeleton of a female Australopithecus emerged from the Hadar region of Eastern Africa in 1974. Nicknamed Lucy, this fossil was a skeleton of a bipedal female primate, and was dated to around 3.7 million years ago. The find cemented the place of the species A. afarensis. The remarkable preservation of post-cranial bones helped firmly establish Australopithecines as upright-walkers. Likewise, post-cranial remains from the more Southern A. africanus with STS 14 in 1947 showed what Raymond Dart had believed to be true about the locomotion of these unique animals.

A. afarensis is probably the better studied of these two hominids, but it has also caused more controversy. Some believe that there are too many similarities in the morphology of the species to the africanus specimens to call them separate species. Others argue the opposite direction and claim the size differences in the species are too large to be attributed to sexual dimorphism, and that the specimens from the Hadar region and Tanzania are therefore two taxa and not one.

There is also significant controversy over how much time these species spent on the ground versus in trees, as they retain more suspensory adaptations in the shoulders and arms. Some scientists have argued that these features suggest the Australopithecines lived in densely-forested environments in which bipedal movement was useful for moving in trees. Others claim it is merely evolutionary left overs.

Australopithecus anamensis, Ardipithecus ramidus and Ardipithecus ramidus kadabba

Paleoanthrpologists often thought in the past that the hallmark walking of hominids was something that had evolved relatively recently in human evolutionary history. With the discovery of Lucy and other Australopithecines, the date that our ancestors began walking was thought to be around 3.6 million years ago, based on the dates for the specimens. Molecular genetic evidence suggested a much earlier date for the divergence of our line from that of chimpanzees, however. For any time before 3.6 million years ago, there were precious few fossils, and none that could conclusively support bipedal walking earlier than these species. The so-called "Black Box" time period extended from 3.6 to 12 million years ago, and left a large gap in the understandning of hominid evolution.

The first find that began to break the Black Box barrier was the discovery of two new species, Ardipithecus ramidus ramidus by Tim D. White's team and Australopithecus anamensis by Meave Leakey in the mid-1990s. The early date for both of these species and the possible link to later groups and even humans continue to be interesting.

Another recently-discovered species, Ardipitchecus ramidus kadabba, gives even better evidence for habitual bipedalism and is dated to between 5.2 and 5.8 million years old. Evidence in the feet of this group suggests that they walked by rolling through their feet, placing weight on their big toes much the way modern humans do today. The evolution of this species and those above continues to be a hotly-contested issue, as it may change the ideas that current anthropologists have about the ecological conditions under which bipedalism came to be.

Orrorin tugenensis

One of the first hominid fossils to break the four-million-year mark was the discovery of Orrorin tugenensis by Martin Pickford and Brigitte Senut. First announced in 2001, this creature is thought to have lived nearly six million years ago, and the full complement of the specimens known for the species is 19 (including jaw fragments, a few teeth, finger and arm bones, and partial femurs). This animal is one of the forerunners for the earliest current evidence of bipedalism in the fossil record. Like other early bipeds, its hands and arms display many adaptations for climbing and swinging in trees, but its hind-limb fragments suggest that when on the ground, this creature walked in a fashion very similar to other bipeds of the human line. There are several primitive characteristics that suggest that this animal had not evolved to display derived features like reduced canines.

Sahelanthropus tchadensis

In the summer of 2004, a new fossil burst onto the paleoanthropological scene and established itself as what its finders consider to be the very oldest fossilized hominid to date. In the arid desert of Chad, a nearly complete skull found was found by Michael Brunet and his team and is dated to an astonishing seven million years old. If the date is correct and this is eventually proven to be a true hominid, this would be the oldest known bipedal primate, and perhaps the ancestor of all those that came after it. As Brunet put it upon seeing the fossil, "...[this creature] could touch with his finger" the point at which our linage diverged from that of chimpanzees.

It is difficult at this point to establish whether the species was a true hominid and whether it had adaptations suited to habitual bipedal walking, as there are few to no post-cranial remains currently available to science. However if this is shown in coming years to be a bipedal primate, the ideas traditionally held by anthropologists on the origins of human ancestors would have to change radically. The species was found 2500 kilometers East of what was thought to be the cradle of humanity, the Great Rift Valley of Eastern Africa. If Brunet and his team have dated the fossil correctly and it turns out to be a hominid, the origins of humanity would not only be moved dramatically on the planet but also would be nearly a million years earlier than predicted by DNA hybridization studies and molecular genetics.

There is therefore considerable conflict over the validity of the find. Some dispute the date of the site, some argue that this is not relevant until more fossils are established as it is so ambiguous as to its locomotion, and some argue that the find is not even a hominid to begin with and therefore not important to the study of human origins. A few of the problems stem from the general trouble scientists have with defining hominids in the first place. Hopefully in the years to come, the definition will be aided by the discovery of more Sahelanthropus fossils and a better understanding of what it means to be human biologically.

Evolution of Bipedalism

As previously noted, until the discoveries of fossils older than four million years in recent years many believed that the discovery of Lucy and her kind was the earliest conclusive evidence for a human-like biped. Controversy generated by the most recent discoveries is not a new phenomenon, however, as the arguments over which fossils fit where in the human family and how exactly they walked has been raging for decades. There is a near-infinite number of theories as to how the particular type of walking shown in hominids evolved, and where, when, or why our ancestors took to the ground in the first place. Below is a discussion of some of the theories available currently, and is by no means meant to show all theories or ideas on the subject.

Climate Change

Charles Darwin himself theorized that proto-humans began walking when our ancestors came to live less in trees and more on the ground, likely due to a change in how they subsisted or changes in the climate of their homes The idea that our ancestors left the trees in search of food and as a result became more adjusted to terrestrial bipedalism is well-established and long-lived in anthropology. The idea surrounding this is sometimes based in the energetics of foraging. For example, if trees (and therefore food sources) are more spread out, an animal able to cover more distance would have an advantage in gathering food. Walking upright is energetically more efficient than quadrupedal locomotion when the animal in question is adapted to walking.

Another aspect of the climate hypotheses is that the upright stature of bipeds allows them to better regulate their body temperatures in direct sunlight. Human ancestors were thought to have evolved in a savannah environment, where the ability to remain cool during the day would allow an animal to travel farther distances and perhaps gather more food. Up to a 60% reduction in direct sunlight on the body helps contribute to a 40% reduction in water needs. A quadrupedal animal of the same size would have significant heat load because of the greater body surface area exposed to direct sunlight.

Provisioning the Family

The discoveries of the Ardipithecus and Orrorin fossils in the 1990s turned some of the theories of the origins of bipedalism on their heads. Ecological evidence from the sites suggests that these creatures lived in a much more closed forest environment than what was thought for human ancestors. How then, would bipedalism be an advantage to them?

One idea proposed by Owen Lovejoy is that the way we walk evolved because of the ability to carry things with the hands, specifically food. If an animal was able to carry food over long distances and gather more of it, it could provide better for its family and its reproductive success would be enhanced. This idea was originally conceived under the idea that a biped would have human-like social organization and have something of a nuclear family in which the male would provide food and the female would help raise the offspring. Clearly, this particular idea may have been formed under preconceived social ideas about what is acceptable in human ancestors. The backbone of the theory is sound, however, because indeed having the hands free because of walking most likely allowed for better provisioning (whether it was the male or female).

Carrying Hypotheses

Closely related to the provisioning hypothesis is a group of ideas that center on the availability of the hands to a habitual biped. Carrying infants, which would be very useful for an animal with a similar life history to humans in which infants are born at a highly-vulnerable and low-developmental stage. The Dikika Baby, a remarkably-complete ''Australopithecus afarensis'' fossil announced in 2006, gave a glimpse into the life-histories of our ancestors. Based on teething and bone patterns, it looked as thought the individual was about three when it died, and that its brain was developing at about the same rate as a child chimpanzee The amazing thing is that the bones of this specimen show that it was likely carried by an adult for long periods of its infancy.

Another carrying theory focuses on toolmaking or tool use as a means by which bipedal movement evolved in human ancestors. This is unlikely, as many anatomists have studied the bones of the earliest hominids and concluded they may not have been able to hold things with as precise a grip as toolmaking requires. However, the possibility that they were using tools that have long since rotted away like sticks or grass remains, as there is no evidence to the contrary. For the time being, the earliest conclusive evidence of tool use is younger than the origins of bipedalism as currently conceived by at least a million years.

Threat Displays or Secondary Sex Characteristics

Some scientists believe that the beginnings of walking upright were based in being able to show dominance by making oneself bigger. By standing on the hind-limbs, they argue, the ape ancestor of humans was able to become larger in the eyes of its peers and therefore may have had more reproductive opportunities. This is somewhat impractical as a theory, because it leaves several questions unanswered. For instance, if dominant males were using bipedalism to scare other males, what motivation would females have to adopt that style of walking?

Another theory suggests that the style of walking adopted by our ancestors displays the secondary sex characteristics that are unique to our species. Our ungainly parts are on display for all to see them, and it's thought that the sexual selection that followed favored not only the style of locomotion they had but also the characteristics that were displayed by it.

"Aquatic Ape" Theory

A relatively recently proposed theory on the origins of bipedalism is the idea that apes may have been living in areas with large expanses of water in the late Miocene period, and therefore may have had to walk on two feet to cross the flooded areas. This behavior is seen in current primate populations, and is somewhat plausible due to the water reducing the load on the bones and muscles, thereby making bipedal walking easier.

However, this theory is not well-supported by fossilized evidence. Like the ones on threat displays and secondary sex characteristics, the idea is limited by the fact that behavior does not fossilize. It is very well possible that the origins of bipedalism may lie in marshes or floodplains, but until the ecology of the paleo-environment is shown to be thus it cannot be a usable theory.