Part 2: The Evolutionary Emergence of Bi-pedal Primates

The theory of evolution explains how all organisms show special adaptations to their environment and lifestyle. From the simplest bacteria to the largest vertebrate, biologists have found time and again that many aspects of an organisms structure, metabolism, behavior, life cycle, developmental patterns etc. are specifically tailored for that organism's reproductive success in its natural habitat. Simply put, the theory states that this is a consequence of many generations of variation and natural selection. Bipedal primates, the group to which humans belong, are no exception to this.

The evolutionary process has altered the physical characteristics and behavioral characteristics of primates which led to the appearance of bi-pedal primates. There are several major physical differences between bipedal and non-bipedal primates:

Differences between bipedal and non-bipedal primates

A human skeleton (left) and a mountain gorilla skeleton (right)

In the illustration above, we see two skeletons: that of a human, and that of a mountain gorilla. Humans are obligate bipeds, meaning that they must walk on two legs, while the gorilla is a quadruped, walking on all fours (knuckle walking). The difference between these two modes of locomotion are reflected in the skeletal structure. Although the mountain gorilla is not the common ancestor of humans and the other great apes, we will use it's skeleton to make a general comparison between bipedal primates and non-bipedal primates.

Anatomical adaptations to bi-pedalism:

The foot:

The foot of bipedal primates is evolved to carry the entire weight of the body. You can see from the gorilla skeleton that it's foot is very different. It is much wider, with more opposable digits for grasping limbs. Quadrupedal primates had flat feet while bipedal primates evolved ones that were more arched, to help with the distribution of weight and balance. Bipedal feet are also longer to facilitate efficient walking. Elongated structure transfers energy from the leg for the forward locomotion of the foot.

The legs:

One of the most apparent differences between the legs of a bipedal primates and their quadrupedal relatives is length. Longer legs create a longer, more efficient stride. Quadrupedal legs are more bowed for clinging to trees. This shape is also better suited for squatting. Bipedal knees are enlarged to support more body weight.

Hips:

The hip socket of bipedal primates evolved to become larger, allowing it to bare more weight and have a wider range of motion. Their hips also moved closer to the spinal column to maintain upright balance with less effort.

Torso:

As these primates began to walk upright, their spinal cord developed an 's' shape to prevent slumped posture and to place the center of the body directly over the feet.

Skull: 

The skull of bipedal primates is set more directly on top of the spine where as the quadrupedal primates' spinal cord enters the back of the skull.

Correlation between bigger brain ratio and relationships

Behavioral adaptations to bipedalism

-Traveling long distances

The evolution of true bipedal locomotion in hominids allowed for the evolution of a wide variety of new behavioral traits. First, true bipedal locomotion allows for far greater efficiency in walking long distances. Chimpanzees and gorillas can walk upright, but only with great effort and awkwardness. Secondly, bipedal locomotion frees up the arms and hands to perform a huge variety of tasks which a hominid perform. From fighting and hunting, to carrying food and infants over long distances, the free use of hands while walking would have presented hominids with many opportunities for advantageous behaviors.

-moving objects from place to place

 The ability to carry objects over long distances could also have lead to a development of a new material culture. Non-bipedal hominoids would not have been able to move objects from one place to another with such ease. Freeing the hands during locomotion therefore allows for a greater development of material culture (the gathering of objects, collecting useful items or food etc.). This greater material culture, in turn, may have led to biocultural evolution.

Theories of the emergence of bipedalism in hominids

Paleontological evidence shows that bipedalism emerged very early in hominid evolution. Why some primates should have moved to bipedal locomotion is unclear. Paleoanthropologists have developed several theoretical scenarios that may have led hominids to evolve bipedal locomotion.

The first such theorist was none other than Charles Darwin. Darwin correctly assumed that the African apes were the closest living relatives to humans, and devised a scenario that he thought might explain the transition to bipedalism. Darwin's idea was that hominids were driven to evolve bipedal locomotion by a change in diet. Accordingly, early hominids changed to lifestyle of hunting with weapons and eating meat. This would have required a freeing of the hands to carry weapons and meat. This change to eating meat, according to Darwin, may have happened concurrently with a change in habitat - a loss of tree cover.

It appears that there is a basic problem with this scenario: paleontologists have found that early hominids all lived in areas that were partially or fully wooded. If bipedalism arose in a forested environment, then Darwin's model will not work. Other models have been proposed. One postulates that strong but optional bipedalism arose while our ancestors were still in the trees. In this scenario, hominid's ancestors were already walking horizontally along tree branches before they came out of the trees. One piece of evidence in favor of this hypothesis is the fact that our nearest living relatives, chimpanzees, bonobos, and gorillas, are all capable of bipedal locomotion for short distances.

Other scenarios have been proposed, including changes resulting from the use of weaponry in inter-hominid competition, climate change and other factors. One theory holds that early hominids evolved bipedalism as a result of a change in social behavior, in which males were travelling to gather food and bringing it to females. This arrangement, called 'vested provisioning', may have provided the selection pressure to evolve bipedal locomotion. However, the current state of evidence does not point to any of these narrow scenarios as being the one correct evolutionary path that led to bipedalism.

Part 1: Who Are We?

The Missing Links' Physical Anthropology ProjectProfessor Chipley Physical Anthropology 2301-010

The Missing Links are a research team made up of students at Austin Community College. Our group members include Sofia Herrera, an anthropology major; Cassie Fitzgerald, a government major; Kayla Mandell, a spanish major;Krishan Bhattacharya, a who-knows-what major.

This blog serves as a resource for any and all freshman college level students.

As a team, we aim to learn more about ourselves through the study of our primate ancestors and how they evolved. We have collected a significant amount of information supporting the existence of extinct bipedal primates and extant bipedal primates. With this project, we hope to leave you with a better understanding of the appearance of modern Homo sapiens sapiens through the lineage of bipedal primates. 

Evolution has a history of being a controversial subject. Some believe that the theory should not be taught in schools because it conflicts with a theology that is deeply rooted in our society. However, as technology develops and more evidence is collected, our understanding of human origins is becoming increasingly popularly accepted.

Evolution is studied both as a process and a result. It is an ongoing biological process with precise genetic meaning, a change in genetic structure of a population from one generation to the next, and can also be identified as the accumulation of changes in allele frequency overtime. Evolution occurs through 5 primary mechanisms that include mutation, gene flow, genetic drift, non-random mating, and natural selection.

Mutation is a change in DNA caused by an organism's environment or a mistake during replication that alter its sequence in a single base pair or chromosome.This change is a random, accidental occurrence that can result in a permanent variation of an allele. Mutations can occur in replication errors, exposure to damaging chemicals, and radiation. The only source of new genetic material arises through mutations. Mutations can vary depending on cell type; mutations in somatic cells or non-sex cells, have no evolutionary effect, while a mutation in gametes can lead to evolutionary changes. 

Recombination
Recombination is the exchange of genetic material between homologus chromosomes during meiosis. Partner chromosomes form pairs of double stranded DNA which line up at the center of the cell. This joining guarntees that each new daughter cell recieves only one of each pair from the mother and father cell. Crossing over ensures the shuffling of genes which manifest in phenotypic and genotypic variation. Recombination between chromosomes increases the gene pool's uniquiness and produces new combinations of genetic information that natural selection can act upon.

Natural Selection

A powerful mechanism of evolutionary change is natural selection; it is the genetic change or changes in the frequencies of certain traits in populations due to differences in reproductive success between individuals. Charles Darwin supported his theory of evolution through countless observations and experiments. These observations led to the following scientific revelation of characteristics that compose natural selection: there is variation of traits within a species, these traits are inheritable, populations tend to produce more offspring than the environment can support, and those individuals with the traits that best adapt to the environment will produce more offspring. As the environment applies more pressure on a population, variation within species provides advantages to individuals with the suitable traits that will allow them to procure evolutionary innovations such as: avoiding predators, surviving climate change, and enhance food gathering methods. 

Gene flow is the exchange of genes between populations when individuals from different environments migrate to new areas and reproduce. As genetic information is being transported in and out of the two populations, the diversity in variation of alleles increases and the differences between the two populations reduces. Gene flow occurs because of the movement of individuals between habitats. For example, the image below shows two populations of deer on either side of a mountain range. If the western deer travel through the mountain range and end up in a habitat populated by deer with differing genes, there is an opportunity for gene flow to occur. If the deer are able to reproduce then there will be a much higher variation of alleles seen in the eastern deer population and vice versa if the eastern deer were to mix with the western deer population.

  

Genetic drift, an evolutionary process that occurs in small populations, involves a random change in allele frequency over time which can be wildly unpredictable. Over extended periods, genetic drift creates a reduction in variation that has the capability to effect natural selection negatively. This is because more genetically similar individuals have fewer differences for environmental factors to work upon. Allele frequencies fluctuate randomly between generation to generation, due to the ratio intake of alleles produced is not exact as alleles received since not all alleles are passed on to offspring. If the population is smaller in numbers, then genetic drift increases, causing more individuals to be homologous and can lead to a polarized gene pool. The reduction in variation of genes can result in the lost of certain alleles, as well as, set into motion other processes affected by variation such as the bottleneck effect and the founder effect.  

Bottleneck Effect: In Earth's history there is evidence that identifies catastrophic events which caused extreme changes in natural selection, as well as, the elimination of a vast number of species. Those individuals that survived were able to mate with other survivors and produce viable offspring. A severe repercussion resulting after a bottle-neck event is the significant reduction in genetic diversity that makes up a species. The reduction in genetic diversity can also effect the survival of the remaining few since the population may possibly not be able to adapt to the new pressures of the environment because genetic variation natural selection would act on has already drifted out of the gene pool.    

Founder Effect: 

Founder Effect is when a population is descended from a small number of ancestors thus causing the new population to have little variation. This phenomenon is similar to a genetic bottleneck, but it may occur in different ways. For example, suppose a large population of deer live in an enclosed region. If a small, temporary passage were to open up to a neighboring area, it is possible that a new population could be founded there. If only a few deer from the original region migrate to the new territory, the new population will have a gene pool constrained by the small initial contribution, and thus less genetic diversity.

Another mechanism involved with evolution is assortative or non-random mating. Non-random mating occurs when no classification of criteria of possible mates are considered. Mating is solely a matter of chance.  

Example of Evolution in Wild and Domesticated Animals


Rats evolved in the wild to be small in size, have a short breeding cycle, and to have large gnawing teeth that enable them to eat a wide variety of foods. These traits enabled them to exploit a wide variety of niches. The earliest known mammal, hadrocodium, was quite rat-like, and small rodents have continued to be quite successful in the wild—they make up the largest group of mammals. However, in the 18th century, people began to catch the wild brown rat for use in the popular sport of "rat-baiting," in which bets would be placed on how quickly a dog could kill a large group of rats. This soon led to particular traits being selected for use in the sport, and because of their extremely short reproductive cycle, large populations with particular phenotypic traits were easily bred. Hooded rats with colored fur on their heads became popular pets in Japan, while white rats that all descended from a single individual with an albino mutation became the first animal domesticated for purely scientific reasons.