The Evolution of Human Locomotion (Bipedalism)

2.9 to 3.9 million years ago, during the Neogene period of the Cenozoic era, around the Pliocene to Pleistocene epoch, the species Australopithecus Afarensis (meaning Southern Ape and referring to the location in which the fossils were found) lived amongst other animals like the ground sloths and glyptodonts. (Wikipedia, 2019) The Australopithecus Afarensis is the earliest and best-known human species. After its near extinction came many more, Neanderthals, homo habilis, homo ergaster, etc… So now if we revert to modern society you can see we have homosapiens.

 

Homo is the genus that emerged from the otherwise extinct genus Australopithecus. This genus encompasses several other extinct species classified as either ancestral to or closely related to modern humans, most notably Homo erectus and Homo neanderthalensis. 

(Wikipedia, 2019) Since then, humans have gone through numerous changes, constantly adapting to their environment through means of evolution.

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What exactly is bipedalism?

 

Essentially, bipedalism is an organism’s ability for locomotion by its two rear limbs.

Some of the morphological characteristics of bipedalism include the

  • Presence of a bicondylar angle, or valgus knee (which brings feet in line with the body’s center of gravity),

  • A more inferiorly placed foramen magnum (the structure which connects the skull to the spinal cord and serves as a passage for the central nervous system),

  • Big toes (which provides additional leverage to the foot when walking, running or pedaling, and helps the body balance in conjunction with the little toe), a higher arch on the foot,

  • A more posterior orientation of the anterior portion of the iliac blade (which is basically the uppermost and largest part of the hipbone), and more. 

(eFossils, 2016) Our upright walking gait, as a result of our bipedalism, has been proven to considerably improve the rate of oxygen and energy consumed per distance covered during walking, compared to non-human apes like chimps. The other reasons why bipedalism provides us with such an advantage is due to its energetic efficiency, the allowance of a wider range of vision, and freedom of the hands for carrying items.  (Cradle Of Humankind, 2019)

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Our Environment

(Robinson, 2017) Late Pliocene climate and environmental change are hypothesized to have influenced human and mammalian evolution in eastern Africa, but data for the latest Pliocene are scarce. Research in 2017 provides us with stable carbon isotope data from the late Pliocene mammalian fauna from Ledi-Geraru, in lower Awash Valley (LAV for short). This data combined with pedogenic carbon stable isotopes indicate that the two regions in the Plio-Pleistocene boundary were largely similar but that important environmental differences existed during the emergence of homo around 2.8 million years ago. The mid-Pliocene interval in the LAV was characterized by increasingly C4 dominated, arid and seasonal environments.

 

The early homo mandible LD 350-1’s teeth enamel has a carbon isotope value similar to that of earlier Australopithecus from the LAV that the emergence of homo from Australopithecus did not involve dietary shift compared to other species at the time. To put it simply, overall, the compilation of faunal enamel and paleosol carbonate isotope records and community analyses indicate broad regional similarity through much of the Plio-Pleistocene in the LAV and Turkana Basin, although important differences existed during the late Pliocene when there was an emergence of the genus homo.

 

The overall regional trend is towards more open, arid and seasonal environments. Basically, grasslands. This pulsed shift towards aridity and a concomitant transition to grassland-dominated ecosystems is thought to have caused a forced adaptation to bipedal locomotion.

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The benefits of Bipedalism

Bipedal locomotion is very likely an adaptation to survive in a mixed woodland and grassland environment as it has made it much easier to see long distances over the tall grass. This may have benefitted in scavenging for food or avoiding large predators that hid in the grass.

 

Furthermore, these structures potentially help dissipate excess body heat and reduce the absorption of heat from the sun because less of our skin has direct exposure to ultraviolet radiation during the hottest times of the day.  As mentioned, bipedalism allows us to expend less energy between our longer strides, which in turn allows us to walk greater distances. (Encyclopedia Brittanica, 2019) This would be useful for scavenging for food in vast areas.

 

However, since the legs of bipedal animals need to be sturdy enough to support at least 2.5 times their body weight while running, over many generations, early hominin legs grew longer and much stronger than their arms. Their feet became longer and developed arches for more efficient to support their bodies. In addition, their ability to carry and manipulate objects such as tools and food increased. It also made it easier to hold babies and to tend to their needs. These adaptations to walking bipedally on the ground made it increasingly difficult to travel through the canopies of trees.

 

However, they obviously provided many other natural selection advantages.

By Sarisa Techasukij, Grad 24 Bangkok Patana student

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Bibliography

  • eFossils, 2016. eFossils. [Online] 

       Available at: http://efossils.org/book/anatomical-evidence-bipedalism[Accessed 7 October 2019].

  • Wikipedia, 2019. Australopithecus Afarensis. [Online] 

       Available at: https://en.wikipedia.org/wiki/Australopithecus_afarensis[Accessed 7 October 2019].

  • Wikipedia, 2019. Homo. [Online] 

       Available at: https://en.wikipedia.org/wiki/Homo[Accessed 7 October 2019].