In the modern world, humans undoubtedly are the most successful species on earth. The apex predators who rule the world and evidently occupy the highest status in the food chain. And while there are hundreds of different reasons behind the success of our species including, increased brain powers, evolution of language and agriculture, civilisation of the population and much more; today, we will talk about the most basic traits of them all, the trait that distinguishes us from the rest of the animal world i.e Bipedalism.
What is Bipedalism?
In the strictest sense of the word, ‘Biped’ means ‘Two feet’, describing an organism who walks upright using it’s two rear legs. Although, primates along with a few other organisms may also show partial ability of bipedalism, it’s the Homo sapiens that had taken the full advantage of this ability. Thus, showing the characteristic striding of humans where the energy output of the body is reduced to a physiological minimum and shows a smooth undulating flow of progression.
Advantages of bipedalism:
The first hominids arrived here, when the world was going through a hard period with unsuitable weather, harsh life in wilderness and with a constant threat of predation. The bipedalism, however provided them with some of the best advantages in surviving in such a time and becoming a success in the history of evolution.
For one, the biped animals are characterized with an erect head that gave the early hominids – increased field of vision. A trait that helped them immensely in detection of danger as well as for finding food. For another, unlike the four legged animals the hominids now had their fore limbs or hands, as we now call them – free. Thus, they started using it for a wide array of other functions. This allowed them to manipulate the environment to their benefit in such a way that no other animal could.
Fossil record of Bipedalism:
The earliest evidence of bipedalism was found some 4.4 million years ago, which separated the first hominids from the rest of the apes thus, proving that the hominids achieved the feat of walking on two feet even before the evolution of brain started in them. Ardipithecus ramidus, is the earliest skeleton found till today which shows the perfect anatomy of an organism that can walk upright on two feet.
Similarly, anatomist Raymond Dart found a skull known as the Taung child in south Africa which dated 3 millions years old and while the skull size was small and showed most of the characteristics that defined a primitive ape, there was something that made Taung child stand out from the others. Raymond Dart found out during his inspection of the skull that the ‘foramen magnum’ – the hole through which spinal cord comes out of the head was positioned a bit too forward under the skull than it’s normally seen in apes. Thus, proving that Taung held it’s head upright – one of the basic characteristics of a biped.
In late 1930s and 1940s a number of Australopithecus skeletons were discovered which were older than that of the Neanderthal and Homo errectus, showing many primitive characters of apes like long curved fingers, short legs and toes equaling the size of arms -all the characters adapted for tree life– showing that although these organisms were able to walk on two feet, they didn’t really walked around like we do today. These Australopithecus were still very much ape like and spent most part of their lives hanging on the branches of tress.
One of the best known Australopithecus skeleton found till today is – Lucy, showing a clear cut anatomy of a biped with broad pelvis and thigh bones angled towards knee. So, when they would stand, their feet would come in line with the body’s center of gravity providing stability for walking on two feet. Later, some other skeletons, like that of Sahelanthropus tchudenesis and Orrorin tugenesis were discovered from Chad showing the clear cut signs of bipedalism in their anatomy but still very much like apes in their adaptations.
But it wasn’t until some 1.89 million years ago that the Homo errectus arrived. The first completely terrestrial hominid with an evolved big brain, long legs, straight fingers – that lived on land and walked much like we do today.
Theories on the evolution of bipedalism- Why did bipedalism evolve?
1. The postural feeding hypothesis:
This theory was proposed by Kevin Hunt of Indiana university, stating that bipedalism in early hominids came from the regular use of standing on two feet in order to obtain food and maintaining balance. According to postural feeding hypothesis – the convenience of obtaining food through standing eventually lead to the evolution of early form of bipedalism.
The modern day great apes like chimpanzees and orangutans can also be seen standing on their feet occasionally during feeding or while moving through branches.
2. Provisional hypothesis:
This hypothesis was proposed by Owen Lovejoy, stating that bipedalism evolved due to food provisioning and monogamous relationships. According to this theory, the male hominid collected food, while the female took care of the infant. Thus, creating a monogamous relationship.
In the recent years several studies have proved that chimpanzees could carry three times more food in bipedal posture than when walking on all four limbs.
3. Threat model:
This model suggested that bipedalism evolved as a mechanism of defense in the early hominids, who displayed aposenatism – a hostile display to intimidate predators. In such condition, their erect posture, tenus bipedal nature with long legs often helped and worked as a warning.
4. The thermoregulatory model:
This was proposed by Peter Wheeler, based on the upright posture of a bipedal hominid. It states that this posture would increase the surface area of the body, thus reducing the heat gained by them. Being upright with long legs also helped them to gain favorable wind speed and temperature, making it more comfortable for the organism. Such kind of a posture lead to reduction in the direct exposure to solar radiation and since, the wind speed is higher and air temperature lower, away from the ground, it leads to the faster heat dissipation from the body surface. This provided the early hominids with better comfort than those animals that walked on four feet.
5. Savanna based theory:
This theory states that the origin of bipedalism occurred when the hominids were forced to live in open savanna, after leaving the arboreal life on trees. The erect posture of a biped and its upright head gives them an advantage to look over tall grasses and become a better hunter to stay aware of it’s predators. But this theory had already been rejected because of the clear fossil records – showing that the early bipedal hominids were still adapted to live on tress and lead an arboreal life.
Conclusion:
The morphological and anatomical changes occurred through long period of evolution of bipedalism. But, it is not known why the origin of bipedalism occurred. It’s still unclear as to what driving force lead to this evolution. Even with a range of fossil records that has been discovered over time and the extensive studies that had been done on them, there are no answers to these questions.
None of the theories on evolution of bipedalism has been completely accepted. This is because while every one of these theories explain one part of the mystery, none of them are strong enough to answer all the queries regarding it. So, there can be more than one answer to the evolution of bipedalism. Perhaps, at different points of time there were different factors or driving force behind it. And maybe in the upcoming times, with the ever evolving technologies and discoveries of more fossils of early hominids and bipedal apes, we can someday solve this great mystery that lay surrounding one of the most distinctive trait of humans.