How Does the Theory of Evolution Apply to Bipedal Primates?

We are using the theory of evolution as a statistical concept to measure changes in the gene pool of bipedal primates. Homo sapiens (modern day humans) share a common ancestor with chimpanzees and bonobos. This ancestor existed 6 million years ago. It was ape-sized in body and brain. It had long arms and a grasping foot to dwell in trees. Sexual dimorphism occurred in this species. They were quadrupedal and had a diet consisting of fruit and leaves. Over the last 6 million years, bipedal primates have adapted to their changing environment and created the taxonomic subfamily of Homininae. Several Hominin species have existed over this time but humans are the only remaining hominins today.

There are two main categories of Hominins:

-Australopithecus genus
-Homo genus

Australopithecus

The Australopithecus were first discovered to have existed around 4 million years ago in Kenya. They were a highly successful genus and persisted for about 3 million years. Over time they migrated to South Africa.

A few traits define this genus:

-brain size: 390-515cc

-shorter, more bowl-shaped pelvis

-bipedalism, but some arboreal living

-big toe curved to grasp branches

-square ankle joints, useful for climbing trees

-inhabited woodlands

-small canines, but larger, more flat molars

Homo Genus
The Homo genus began to appear around 2.4 million years ago in Tanzania, Indonesia, and Europe. (not all at the same time) They retained many of the same traits as the Australopithecus except for a few changes.

These are those changes:

-brain size increased to 1,250cc

-development of large knees, and hips

-stabilization of the trunk with thicker lumbar vertebrae

-loss of hair

-inhabited savannas

-lacked forelimb adaptation for climbing

-tool use, and forging of weapons to hunt

-smaller molars due to the richer, more diverse diet

The differences in the anatomical structure between these two genus', and the locations of the discoveries of their remains suggest that gene flow, as well as genetic drift, impacted the genetic mutations that began to manifest in these life forms. Having to learn to adapt in a new, more arid environment, natural selection pushed for the recurrence of three major traits to create the Hominin species.

These traits define the Hominin species:

-habitual bipedality

-reduced canine size

-increased brain size


Bipedalism
The biggest change in the development of early primates was the switch to bipedalism, or walking upright. It is unclear why early humans adapted to bipedality, but it is thought that the climate changed, causing trees to recede and develop into a savanna. Bipedality is beneficial in both terrestrial and arboreal environments, but standing upright on a savanna makes it easier to spot predators over the tall grass. Habitual bipedalism honed this species' body to be able to run, which is probably how they hunted for food. Being able to run and carry tools could have made them more formidable to smaller prey.

With the switch to bipedality came a lot of physical changes.

These include:

-A change in the center of gravity, moving from the center of the torso in quadrupeds to the pelvis in modern humans. When walking, the center of gravity shifts from one side of the pelvis to the other. The pelvis shape became shorter and more bowl shaped. The narrowing of the pelvis along with the increased brain size made birth difficult, which could be the reason why they developed the need for longer postnatal nurturing periods.

-A change in foot structure: bipedal primates have lengthened feet with arches to support standing. Though this helps with standing upright, it means that our feet can no longer grasp well.

-Weaker and shorter arms began to develop in bipedal primates since so much pressure was put on the use of legs. They also developed stronger, more dextrous hands with opposable thumbs. This allowed for better tool use and an increased ability to carry and tend to babies.

Reduced Canine Size
Throughout the development of these early Hominin species there were constant geographical changes taking place. Continental drift impacted the environments of these species and in turn affected their food sources, possibly causing the leafy vegetation and fruit sources to be unavailable. With this change in diet came the reduction in canine size and the thickening of the enamel of the teeth, especially in the molars. This thickening of the enamel was present in the Australopithecines, but eventually faded out in the Homo genus due to the much more varied diet.


Increased Brain Size
It is possible that varying terrain scenarios, as well as changing climates, pushed these species to find new ways to survive. The crucial change in environment occurred with the switch from a woodland surrounding to an arid savanna. The change in our setting caused our every day lives to become different, and in reaction to these stimuli our brain began to develop and grow. The change in brain size that occurred in the Hominin species suggests that these primates had the cognitive capacity to adapt and thrive in different environments, thus leading to their expansion.

Behavioral Changes

As we moved out onto the savanna we left behind our old way of foraging in the trees and using them for shelter. On this new terrain we developed and honed many skills that would lead us to eventually inhabit the globe. A few of these early behavioral changes were critical to our survival.

Becoming nomadic
The ability to run long distances and travel to new places in search of food was vital to the survival of early bipeds. We became nomadic in order to scavenge game killed by carnivorous predators and migrate to new vegetation supplies. By nomadic we mean that these species followed food sources, which could have been the driving force for them to leave Africa. The term should not be confused with the more modern definition of nomad, which is a person who herds livestock to different regions to graze dependent upon the seasons.

Tool use
The beginning of tool use and the transition from the Old World Olduwan tool set to the new, more versatile Achuelian set was an important behavioral change as well. With this new "Swiss army knife" of stone tools, early bipeds began to transport their tools with them to use in new environments. This foresight is something that had not be considered before.

Development of social groups
The final, most prominent behavioral change is the development of social groups. It proved advantageous to early bipedal primates to create social groups most importantly to insure the success of offspring. With the increase in brain size, and the narrowing of the pelvis, birth was no longer something that could be done alone. Also, maturation of offspring was a slow and intensive process since the brain was not able to fully develop in utero.

These behaviors developed at different times and places as a response to external and internal stimuli- as evolutionary answers to what life threw at them. Changes in this species' gene pool and repetitive selection to create a phenotype capable of coping with varying terrain and climate, created a more cognitive brain that gave bipeds the ability to change their behavior and learn from their parents.