Advantages And Disadvantages Of Bipedalism Essay

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The Evolution of Endothermy

Abstract
This essay attempts to gather evidence, from several sources, for the reasons behind the appearence of endothermy, the necessary changes in the heart and circulatory system to accomodate endothermy, and during which time periods endothermy is likely to have arisen. It is proposed that endothermy has evolved multiple times: during the evolution of early mammals, and during the divergence of Triassic Theropods. The advantages and disadvantages to both endothermy and the contrasting ectothermy are suggested, and it is proposed that endothermy is not an advanced state, but simply an adaptation to a specific niche in a given ecosystem. To support this, the stem crocodilian Poposaurus gracilis is referenced as an example of an endothermic species that shares a strong evolutionary link with todays modern ectothermic crocodilians, seemingly adapting to an ectothermic lifestyle from a previously endothermic lifestyle. The formation of the heart during embryonic development is briefly considered, and a study is referenced that observed a gene, Tbx5, that has a role in forming the ventricular septation required to accomodate endothermy in an organism. Separation of blood in this manner allows for a multiple pressure system, vital to endothermy.

Introduction
Endothermy is the ability of an organism to maintain it’s internal temperature through it’s metabolism.¬†Endothermic organisms are able tolive in a greater range of habitats and temperatures as they can produce their own body heat and, unlike ectotherms, do not require a constant environmental temperature depedence. The usual source of internal heat comes from digestion of food and muscle contraction.

Endothermy has been found in both mammals and birds. Since both the mammal and avian classes share reptilian ancestors, it has been proposed that some reptiles were in fact endothermic, and that endothermy evolved during the Mesozoic on more than one occasion. Todays evidence suggests that endothermy was present in Mesozoic archosaurs, the ancestors of modern birds, though it is still in discussion. It could, however, be suggested that the evolution of endothermy was a single event, which would likely put its appearance in the Permian period, when the synapsids and the sauropsids appear to have formed distinct groups. This theory would suggest, however, that some sauropods would have reverted back to an ectothermic lifestyle.

Advantages and Disadvantages of Endothermy as opposed to Ectothermy
Both endothermy and ectothermy have their advantages, dependant on the organisms lifestyle. Endotherms, namely mammals and birds, tend to be more active than species of amphibians or reptiles. An endotherm can sustain a consistent body temperature in a large range of environments through internal heating. The heat is produced through digestion of food and muscle contraction, e.g. Shivering. The increase in energy would allow an organism to more readily escape predation, and would also increase stamina. However, such advantages do come at a cost. Endotherms require a lot more energy than endotherms, estimated to be about 4 to 10 times more.[1][2] Whereas an ectotherm can go a substantial period of time without eating, an endotherm needs to eat on a regular basis in order to sustain homeostasis.
Unlike endothermic species, ectothermic species regulate their body temperature by use of the environmental conditions. By moving into warmer or cooler areas, ectotherms can control their body temperature whilst using a lot less energy from metabolism. Some ectotherms can go for months without eating, whereas many endotherm species may show drastic declines during food shortages.
Both endothermy and ectothermy provide advantages in different niches. An ambush predator may be better suited to an ectothermic lifestyle, especially if it lives in a tropical region. Predators which are more active when hunting, like the Cheetah (Acinonyx jubatus), require more energy and a sustained body temperature, and would benefit more from being endothermic.

The Origin of Endothermy
As previously noted, the idea that endothermy has evolved on multiple occasions appears most likely. Mammals, including amniotes, marsupials and the monotremes, are all endothermic, as are the vast majority of extinct species. Similarly, all modern birds are endothermic, and it is widely accepted that theropods and closely-related archosaurs were also endothermic, seemingly before the appearance of Avian and Crocodilian ancestors (considering the Crocodilians apparant endothermic ancestry).
The change to endothermy requires some morphological changes, most noticeably the change from a 3-chambered heart (Seen in Amphibians and non-Crocodilian Reptiles) to a 4-chambered heart (Mammals, Birds and Crocodilians).[3] Hearts with distinct left/right ventricles are almost exclusive to modern endotherms, the exception being modern day Crocodilians, which have a complete septation of the ventricle, yet exhibit an ectothermic lifestyle. It is suggested by Seymour, R. S. et al, among others, that the ancestors to modern Crocodilians were endothermic. Such would explain the heart structure that is unique among vertebrates.[4] This is also suggested by the findings of Crocodilian ancestors such as Poposaurus gracilis, a bipedal archosaur.[5] (Were the Crocodiles to be descended from endothermic ancestors, as is most likely, it shows to me that the evolution of endothermy is not an advancement as suggested by others, but simply an adaptation to a different niche that requires an alternate lifestyle).
It is without doubt that the formation of the heart is a likely indicator of whether a species is ectothermic or endothermic, but why the need for this change, and how did it arise?
The evolution of new means of collecting, storing and using energy opened up new niches for endotherms. Were it not for endothermy, the long-sustained flight of birds would not be possible, nor would the hunting habits of many predatory mammals. Such a change would have presumably been followed by a large radiation of endothermic species into new niches and habitats. As is seen from todays wildlife, only endotherms can survive habitats such as Antarctica or the Arctic Circle. Being endothermic allowed for a greater distribution, which at the time would have been a great benefit.
A change from an ectothermic to an endothermic lifestyle required a different circulatory system.[3] Ventricular septation allowed for differing blood pressure around the body, and separated oxygenated and de-oxygenated blood.[4] Such features, it can be argued, are necessary for an endothermy lifestyle. The origin of the 4-chambered heart could be synchronous to the origin of endothermy.
A study by Koshiba-Takeuchi, K et al (2009) tracked the movement of transcription factor gene Tbx5 in the development of a mouse, turtle and anole heart. The results showed that Tbx5 was essential in ventricular septation during development.[3] This eventual change in heart formation allowed for the separation of blood required for endothermy.

Conclusion
Since their origin endotherms have diversified greatly, and can now be said to be the dominant group in a majority of habitats. The ability to sustain ones own internal temperature opened up a great number of niches for exploitation. However, as shown by the Crocodilian lineage, endothermy is not necessarily an advancement, but simply an adaptation to a new role.
The evidence is beginning to show that endothermy has evolved on mulitple occasions, most obviously during the evolution of mammals, and again somewhere within the close ancestry of both birds and crocodilians. As yet there are no clear dates, but further studying of genes such as Tbx5, the development of the heart in embryos, fossil finds and DNA sequences may make clear the origins of endothermy.

References
[1] – http://www.elp.manchester.ac.uk/pub_projects/2003/MNZO0MLK/lecture14.htm Lecture 14: Evolution of Endothermy. The University of Manchester. 24-11-2011 18:13
[2] – http://www.me.berkeley.edu/ME212/endothermy.pdf The Evolution of Endothermy. University of California, Berkeley. 24-11-2011 18:16
[3] – Koshiba-Takeuchi, K. et al. Reptilian heart development and the molecular basis of cardiac chamber behaviour. Nature 461 95-98 (2009)
[4] – Seymour, R. S. et al. Evidence for endothermic ancestors of crocodiles at the stem of Archosaur evolution. Physiol Biochem Zool 77 (6) 1051-1067 (2004)
[5] – Gauthier, J. A. et al. The bipedal stem crocodilian Poposaurus gracilis: Inferring function in fossils and innovation in archosaur locomotion. Bulletin of the Peabody Museum of Natural History 52 107-126 (2011)

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Darwin  •  bipedalism

Why be bipedal?

01 Feb 2005

The skeletal adaptation to bipedalism is well documented in early hominids. What is less clear is what events led to this adaptation and its eventual success. Hypotheses about why bipedalism arose have been very common, but most lack the necessary evidence to test them. All apes can walk bipedally, so the behavior itself was within the capabilities of the common ancestors of hominids and chimpanzees. What is necessary is to explain how bipedalism became so essential that it provoked skeletal adaptations that made other forms of locomotion much more difficult.

One argument is efficiency. Human bipedalism is very efficient at normal walking speeds, because forward motion results from gravity swinging each leg forward like a pendulum. The walking biped recaptures this forward momentum by slowing the swinging leg before footfall. As a result, walking at normal speeds on level surfaces requires very little muscular activity, making bipedalism more efficient than knuckle-walking or quadrupedalism (McNeill Alexander 1985). Aside from its energetic efficiency, bipedalism also has the advantages of raising the head, and therefore allowing a wider range of vision in a grassland environment, and of freeing the hands for carrying items or for tool use.

Despite these advantages, bipedalism also has considerable disadvantages. The first is that it makes climbing considerably more difficult. Without the ability to grasp with the feet, hominids are less secure in an arboreal setting. There are many indications that climbing remained an important part of the behavior of early hominids, discussed below. The combination of features found in early hominids reflects a compromise adaptation to climbing, which is based on the presence of morphological adaptations to bipedalism in the pelvis and foot. Part of this compromise was structural, involving much more powerful arms and possibly human-proportioned hands for gripping branches rather than suspending from them. Another part of the compromise was behavioral. The loss of a grasping foot is also a serious problem for child-rearing. In chimpanzees and other primates, the young can use their hands and feet to grasp and cling to their mother's fur. For hominid infants, such clinging would have been much more difficult, if not entirely impossible. One of the adaptations to bipedalism must, then, have been a behavioral change toward carrying dependent offspring until they were old enough to walk.

Over time, scientists have devised many different theories to reconstruct the circumstances that led to the evolution of bipedalism. Charles Darwin himself correctly assumed that the African apes are the closest human relatives, and constructed a model for hominid origins that stressed the appearance of bipedalism from an ape-like ancestor. In Darwin's model, bipedalism is seen as the adaptation resulting when a quadrupedal ape is forced to assume a terrestrial adaptation. In Darwin's formulation, this adaptation was partly caused by the advent of a hunting subsistence pattern, where the hands needed to be free to carry weapons and meat. Additionally, Darwin thought that a change in habitat from woodland to savanna left early hominids without the refuge of trees, resulting in less importance of climbing and a greater need for efficient movement on the ground. Other later researchers picked up many of the themes of Darwin's model, stressing other important features of life on the savanna, such as the need to see over tall grass, and the need to adapt to intense solar radiation. Bipedalism has been suggested as an adaptation to both these factors, by placing the head high and upright, and decreasing the exposure of the trunk to direct light from overhead. This model came to be called the savanna model, or stressing the importance of hunting in the model, the killer-ape hypothesis.

Today, we have greater knowledge about the environments that early hominids occupied, and many aspects of the savanna model do not appear to describe the conditions under which bipedalism evolved. All of the sites before 3 million years ago seem to have been partially or fully wooded, and no early hominids are known from full savanna environments. Additionally, the fossil forelimb elements of early hominids demonstrate the continued importance of climbing. The importance of hunting has been questioned because chimpanzees hunt extensively without the adaptations of early hominids, and because no tools, weapons or adaptations to making tools are known from the earliest hominids. These observations imply that bipedalism was not the simple consequence of a single climatic change.

Lovejoy (1981) has suggested that social factors may have been principally responsible for the origin of bipedalism. In his hypothesis, food sharing was an important component of social behavior. Lovejoy speculates that males may have supplied food to females in order to gain mating access or to contribute to the parenting of their own offspring. This behavior would require the use of hands for provisioning. Such a hypothesis must be reconciled with the apparently high level of sexual dimorphism among early hominids, but may provide significant insights.

One reason for the proliferation of hypotheses to explain hominid origins is that we have almost no knowledge about the postcranial anatomy of the immediate ancestors of the hominids. Most hypotheses have assumed that the common ancestors of living African apes and hominids were essentially like chimpanzees, with suspensory locomotion in the trees and knuckle-walking on the ground. Whether hominids originally evolved from a knuckle-walking ape or not has been controversial. Some scientists, like Brian Richmond and David Strait (2000), argue that early hominids like Lucy bear anatomical features that indicate a knuckle-walking ancestry. In this formulation, the occasional bipedal locomotion of chimpanzees and gorillas is a model for how obligate bipedalism originated. The anatomical changes that characterize the known hominid fossils grow from a more intensive use of this basic hominoid behavior.

Other scientists point to the possibility that knuckle-walking evolved in parallel in chimpanzees and gorillas. The manner of arboreal locomotion in living and extinct apes seems to have been greatly influenced by body size. Known early hominids average slightly less than chimpanzees in body size, and it is possible that their common ancestor was small, rather than chimpanzee-sized. Wolpoff (1999) has suggested that the ancestors of hominids may have been small apes who often walked or ran bipedally above large branches, as well as on the ground. From this perspective, the knuckle-walking of chimpanzees and gorillas and the bipedalism of hominids represent different strategies for ground locomotion related to body size. Under this hypothesis, the large apes developed a suspensory adaptation in response to increases in body size, with locomotion on the ground occurring later than or secondary to this increase. In contrast, early hominids adapted more fully to the ground before their body size increased, resulting in an anatomical adaptation to bipedalism, with climbing secondary.

None of the factors here excludes any of the others, and probably the origin of hominid bipedalism involved a complex combination of these and possibly others. Until scientists have more knowledge about the anatomy of the first hominids and their ancestors, we will be unable to rigorously test these hypotheses. Nevertheless, even as the record of hominid evolution has been pushed back to six million years, bipedalism remains the hallmark adaptation of our lineage. For this reason, explanations for its origin remain one of the most important parts of paleoanthropology.

Bipedalism :: legs and feet

Bipedalism :: pelvis

References:

Lovejoy CO. 1981. The origin of man. Science 211:341-350. JSTOR

McNeill Alexander Ra. 1992. Human locomotion. In: Jones S, Martin R, Pilbeam D, editors, The Cambridge encyclopedia of human evolution. Cambridge: Cambridge University Press. p 80-85.

Richmond BG, Strait DS. 2000. Evidence that humans evolved from a knuckle-walking ancestor. Nature 404:382-385. PubMed

Wolpoff M. 1999. Paleoanthropology. Second edition. New York: McGraw-Hill.

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