When we look at other primates, although the similarities are clear, there are also several stark contrasts. Most noticeable, is our gait – humans are the only truly bipedal (upright walking) primates. Bipedality appeared early in human evolution, and may have marked our divergence from Chimpanzees around 6 million years ago. Bipedality had a number of benefits, allowing us to adapt to a new habitat, and freeing up our hands for other tasks, but compromises also had to be made. Changes in the shape of our pelvis, which enabled us to walk upright, also made childbirth considerably more dangerous and complex. Nevertheless, bipedality is thought to have facilitated the marked advances in tool use and gestural communication that are hallmarks of the human condition.
There are numerous theories which try to explain what drove the evolution of bipedality in humans. The divergence of humans from other apes was associated with a move from forest to savannah, and from arboreal to terrestrial locomotion. These changes are believed to have occurred early in human evolution, around 5 -6 million years ago, and the hallmarks of bipedal locomotion are apparent in the skeleton of A.afarensis, around 3.5 million years ago. Fossilised footprints from around this time confirm that a bipedal hominid was present in Africa by 3.5million years ago. However, the move to bipedality was not binary, it a gradual change involving numerous skeletal changes, which occurred over millions of years.
What benefits did a bipedal stance offer to early humans? It has been suggested that standing upright in our new savannah environment improved our ability to see predators approaching through the tall grass. Another theory states that bipedality afforded us improved thermoregulation, by exposing the large surface area of our back and torso to the cooler, faster moving air found further from the ground. Some evidence suggests that sweating also evolved around this time, a thermoregulatory tactic that relies upon evaporation.
Sweating to Cool Down
Sweat as a means of regulating temperature is actually quite rare in the animal kingdom, and the human mechanism of sweating is unique. Sweat glands come in three types, sebaceous, aprocrine and eccrine; although only the later two are involved in thermoregulation. Aprocrine sweat glands are located deep in the skin under hair follicles. It is a highly effective cooling system for animals with thick fur, as it uses the hairs to conduct heat away from the body. Unique to humans is the eccrine sweat gland, which is more efficient at cooling a hairless body. The eccrine cooling system probably evolved in parallel with the move to bipedal locomotion and loss of hair in humans.
Others have suggested that bipedality was important because it enabled humans to carry food long distances, a benefit that may have been essential in a dry savannah habitat. In this environment, and particularly as the climate dried in Africa around 5 million years ago, food would have been more sparsely distributed, and walking the long distances between food patches may have been particularly difficult for females carrying young. It has been suggested that in order to solve this problem, males and females began forming monogamous pair bonds, with females remaining at home to raise their young and males gathering food that they carried home. The fact that humans, unlike some other primates, do not advertise female ovulation, has been suggested as a mechanism to ensure monogamy – males were forced to guard their mate as they could not be sure when she would be sexually receptive next. However, this idea has since been refuted as it is now thought that advertisement of sexual receptiveness, as in the exaggerated sexual swelling seen in chimpanzees is a recent invention in these lineages, which appeared after humans diverged.
Most of these theories rely on the coincidence of bipedal evolution with a move out of a forested environment. However there is good evidence that the appearance of bipedal locomotion predates this move, and early hominid ancestors appear to have been simultaneously arboreal climbers and terrestrial bipeds. One alternative possibility, which does not relate directly to habitat changes during human evolution, is that bipedality offered improved energy efficiency. Humans only exert 7% more energy in order to stand up rather than lie down. This is largely due to our lockable knees, meaning that muscles in our legs do not need to constantly tense in order to maintain a balanced upright stance. Other apes exert far more energy to stand up. However, early hominids were apes standing up – without the skeletal adaptations that came later during bipedal evolution, this energetic gain disappears and more likely becomes an energetic loss.
It is still not clear which factors were most important in driving human bipedal evolution. One experiment tested what provokes bipedality in facultatively bipedal species, chimpanzees and bonobos, and found support for the idea that bipedality evolved to enable humans to carry food, infants and tools, and to reach food items. When presented with a visual barrier, the apes did not generally respond by standing up. By contrast, they stood up more often when presented with objects to carry, or food which they needed to reach.
Bipedalism in Other Animals
Bipedality is not uniquely human. It has evolved independently in 5 different vertebrate lineages; within reptiles in the codonts and lizards and additionally in birds (bipedalism in this case is thought to have been ancestral to all birds, evolving in their dinosaur ancestor), and within mammals in the kangaroo, kangaroo mouse and of course, in humans. Bipedalism does make humans unique amongst primates, but the convergent evolution of bipedalism in a variety of different lineages indicates there must be some general benefit. Bipedalism is thought to have evolved in reptiles to facilitate speedy escape from predators, and in birds to enable flight. What caused it in humans? There still isn’t a concensus.
Compromises in Human Anatomical Evolution
Whatever the benefits of bipedality that lead to its appearance in the hominid lineage, there were also serious consequences. The human body had to undergo numerous changes in order to enable bipedal locomotion. Our pelvis became broader and shorter, our spine changed from an arch- to an s-shape and our toes became straighter and less mobile. The heal bone enlarged in order to bear a greater weight and two arches formed in the foot in order to improve leverage and provide shock-absorbance. Changes also had to happen to the circulatory system, in order to battle against gravity to maintain a supply of blood to the brain.
These changes also impacted on our health. Humans suffer a greater risk of knee and lower back problems due to changes in pressure on the spine and legs. Humans are also more likely to choke because of our lowered voice box. Perhaps most seriously of all, childbirth became increasingly dangerous and complicated with the skeletal adaptations to bipedality.
Being bipedal required a major change to the shape of the pelvis, which in turn lead to a complete reconfiguration of the birth canal, which became narrower and more convoluted. The matter was further complicated by continued increases in brain size during this period of human evolution. One adaptation to improve the situation was the development of openings in the skull called fontanelles, which allow the two frontal bones of the skull to slide past each other. This has the effect of compressing the skull, allowing a large brain to pass through a narrow birth canal. These openings are more commonly known as the ‘soft-spot’, which persists for sometime after birth. The orientation and movement of the foetus during childbirth also changed in response to our new bipedal stance. Ultimately, our pelvis placed an upper limit on the size of skull that we could give birth to, and further increases in brain size were achieved through increased brain development time after birth. Human babies are far less precocious than their counterparts in other apes; we simply cannot allow our children to develop any further prior to birth. The complexities of human childbirth may have necessitated the appearance of a midwife role, and thus placed increased importance on the social group for human survival.
Standing on two legs has a major impact on the other limbs – when we became bipedal we freed up our hands for other tasks. The human hand adapted for a variety of uses, including grasping, picking up and carrying objects of a range of sizes and eating, as well as secondary uses in gesture. Thus, bipedalism may have selected for increased brain size in order to expand sensory and motor regions of the brain. By allowing us to see farther, selection may also have acted on the visual system to improve our ability to process visual input and remember complex landscapes for navigation. If this is the case, it is ironic that the very skeletal changes that encouraged increases in brain size also placed restrictions on the extent of that expansion through the difficulties of childbirth.
Our Hands and Feet
Bipedalism led to a huge range of changes in the hand; including elongation of the thumb and individual control of the fingers, which enabled them to perform a wider variety of tasks and are ultimately believed to have provided the increased dexterity needed for complex tool use. Most noticeable of these changes is the opposable thumb. Although most primates have an opposable thumb, the human thumb is capable of moving further across the hand than in any other primate. The opposable thumb has doubtlessly been fundamental in the increased tool use seen during human evolution.
Recently, the standard view of human hand evolution as a consequence of bipedality has been challenged. Work by Dr Rollian and colleagues indicates that structure changes in hands and feet are generally strongly correlated because of the underlying genetic blueprint which controls their development. Thus, changes in the foot, which were necessary to facilitate bipedal locomotion, would have also caused analogous changes to the hands. Selection on foot structure is thought to have been stronger, as it impacted so much on locomotion, and therefore changes in the hand were driven by changes in the foot. Changes in the foot, such as elongation and straightening of the big toe, and shortening of the other toes, produced equivalent changes in our fingers which may have been beneficial for the development of stone tool technology. It is important to recognise, however, that this effect could have also acted as a restraint; potentially beneficial changes in the hand may not have been favoured because of their negative effects on the foot.
Although some improvement in dexterity can be explained by structural changes in the hand itself, the brain also plays an important role; in primates the cerebral cortex has complete control over the contraction of the hand muscles, and in humans the development of the central nervous system improved this further. The hands therefore, may be considered the direct tools of our consciousness, and this may explain their key role in gestural communication.
Making and Using Tools
Tool use is often thought of as being uniquely human, however basic tool use has now been demonstrated in a variety of species including chimpanzees, crows, octopuses and elephants. However, clearly humans have taken this talent further than any other species, in particular in constructing and assembling our own tools out of raw materials which often have to be laboriously collected and transported taking long periods of time.
The first stone tools appeared around 2.6 million years ago, associated with the so-called ‘handy man’ – Homo habilis. They were simple tools, made in a style known as Oldowan, and included flakes of quartz, obsidian and flint. They were not much more advanced than what modern Chimpanzees can construct. However, recent finds of animal bones covered in hack marks indicate that a much earlier human ancestor, A.afarensis (3.4 mya) was using tools to cut meat.
For another 1-2 million years, tool use persisted in much the same way. Tools were made as needed and then discarded. It wasn’t until 1.8 million years ago, with the appearance of H.heidelbergensis, that more advanced Acheulian tools started to appear. These tools were characterised by a shaped bifacial design. It is believed to have been developed directly from oldowan tools, and at some sites the two have been found side-by-side.
Although the new Acheulian technology was available at the time of the first human dispersal from Africa around 1.8 million years ago it was not part of this movement. The technology was not taken over to Europe and Asia until much later, around 1 million years ago. This suggests that at this time there were many groups of hominids which had different tool-making behaviours coexisting in Africa, but not transferring skills and knowledge between groups.
Both oldowan and Acheulian technologies were reductive: involving a sequence of actions to remove material, moving towards a clearly visible goal. More complex tools began to appear around 250,000 years ago and were produced only by Neanderthals and Modern humans. These tools were far more complex to construct, and in particular, required actions which did not obviously progress the tool towards the goal until much later in the construction. Therefore, it is thought that foresight was essential for the transition from Acheuleun to the more complex Levallois tools. Levallois are a part of a broader category of tools known as Mousterian, Levallois being a style of tool found in Europe. As well as foresight, the Mousterian tools may have also required language in order to enable the technique to be taught.
Levallois Tool Construction
Around 50,000 years ago further developments in tool use led to the appearance of the Aurignacian tools. These were characterised by the use of a variety of different blades, and this change is likely to have been facilitated by increased long-distance trade, providing a greater diversity of high-quality raw materials. Around this time, bone and antler were being utilised for tool construction in addition to stone, and it was possible to create fine needles for cloth making from these materials. Aurignacian technology is thought to have largely been restricted to modern humans. Although there is some evidence that European Neanderthals had this technology, they did not develop it across their geographic range, rather only in areas where they overlapped with Aurignacian-producing modern humans. It is therefore thought likely that the Neanderthals copied their neighbours, rather than independently developing the technology. Final developments occurred around 29,000 years ago to produce the Gravettian technology.
While tool use is certainly not unique to humans, what may be is a deeper understanding of the relationship between a tool and the result of its use. In support of the idea that humans process tool construction and use in a more complex way than other animals comes from fMRI imaging. This reveals three areas which are associated with tool use in all primates: the occipitotemporal, intraparietal and ventral premotor cortex. In humans, there is one more: the left inferior parietal lobe. The human brain holds knowledge about tools in two different types: semantic or conceptual knowledge about the tool, and the skills which are needed to use the tool. Planning and foresight are key to the construction of complex tools, and the development of tool use in the hominid lineage reflects the development of these abilities in early humans.
Bipedality evolved around or shortly after our divergence from Chimpanzees, and changed our way of life irrevocably. It enabled us to travel long distances, thermoregulate more effectively, see predators from far away, carry food home to our mates and improve communication through the use of gestures. However, it required major changes to our skeleton and circulatory system, and these changes were not without consequence. Changes to the pelvis, coupled with increases in brain size made childbirth increasingly painful, difficult and dangerous. Although certain adaptations such as the soft spot, increased post-birth development and positional changes of the foetus during birth, enabled us to cope with bipedal reproduction, mortality during child birth probably placed an upper limit on changes in our skeleton.
Freed up from the burden of locomotion hands were able to adapt to a variety of new tasks including tool use. However, changes in the hand were also driven by correlated changes in the foot. Tool use and bipedalism are thought to have appeared around the same time, and the unprecedented advances in human-tool use are doubtlessly related, at least in part, to the freedom of our hands to adapt primarily to this task. Continued brain evolution is mirrored by increasingly complex tool construction, requiring extensive foresight and planning. Neither bipedality nor tool use can be said to be uniquely human. Our gait is unique amongst primates, however, and our technologies unrivalled throughout the animal kingdom.
Articles in this Series:
- Intro: What Makes Us Human?
- Part One: A Brief History
- Part Two: Intelligence and Language
- Part Three: Anatomical Adaptations
- Part Four: Culture and Faith
Want to Know More?
- How the Rest of the Animal Kingdom Walks on Two Legs
- Crompton and Gunther (2004) Humans and other bipeds: the evolution of bipedality. Journal of Anatomy 204: 317 – 319
- Steudel (1996) Limb Morphology, Bipedal Gait and the Energetics of Hominid Locomotion. American Journal of Physical Anthropology 99: 345 – 355
- Folk, Holmes and Semken (1991) The evolution of sweat glands. Int J Biometeorol 35: 180 – 186
- Videan and McGrew (2002) Bipedality in chimpanzee (Pan troglodytes) and bonobo (Pan paniscus): Testing hypotheses on the evolution of bipedalism. American Journal of Physical Anthropology 118: 184-190
- Rolian, Lieberman and Hallgrimsson (2010) The coevolution of human hands and feet. Evolution 64: 1558 – 1568
- Human Evolution: The Origin of Tool use
- Peeters, Simone, Nelissen et al (2009) The representation of tool use in humans and monkeys: common and uniquely human features. The Journal of Neuroscience 29; 11523 – 11539
- McPherron (2010) Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia. Nature 466: 857 – 860
- Lepre et al (2011) An earlier origin for the Acheulian. Nature 477: 82 – 5
- Clayton and Emery (2005) Quick guide: corvid cognition. Current Biology, 15; R80 – R81
- Johnson-Frey (2004) The neural bases of complex tool use in humans. TREE 8: 71 – 78
- Rosenberg (1992) The Evolution of Modern Human Childbirth. Americal Journal of Anthropology, 35: 89 – 124
- Falk et al (2012) Metopic suture of Taung (Australopithecus africanus) and its implications for hominin brain evolution PNAS
Accessed December 2011.
Featured image used under a creative commons licence from Wikimedia commons. Original image by Tkgd2007.