21 Theories of Evolution

Tabitha Panmei

Table Contents

1.      Introduction

2.      Brief History of Evolutionary thought

3.      Evidence of Evolution

a)      Fossil record

b)      Natural selection

c)      Field of Biological

4.      Mechanism of evolution

a)      Divergent evolution

b)      Convergent evolution

c)      Parallel evolution

5.      Difficulties on Theory

6.      Nineteenth-Century Evolutionary Theory

7.        Tenets of Evolutionary Theory

a)    Perpetual change

b)    Common descent

c)    Multiple of species

d)    Gradualism

e)    Natural selection

8.      Modern Evolutionary Theory

9.      How do scientists study human evolution?

Outcome of the study

After studying this module:

• By seeing this module is it important to you, to know about human evolution.
• After having read this module you might be understand how evolution occur.
• After seeing this module you personally see a conflict between scientific and religious explanations about theory of evolution.

1. Introduction

Physical anthropology is concerned with all expects of how humans came to be and how we adapt physiologically to the external environment, understanding the details of the evolutionary process is crucial. Therefore, it is beneficial to know how the mechanics of the process came to be discovered. To appreciate the nature of the controversy that still surrounds the issue, we need to see how social and political events influenced the discovery of evolutionary principles. Darwin saw evolution as the gradual in folding of new varieties of life from previous forms over long periods of time. And this is indeed one result of the evolutionary process. But these long term effects can come about only through the accumulation of many small genetic changes occur our generations and today. Evolution is a change in the frequency of alleles from one generation to next. Inquisitive thinkers of the ancient time speculated about the origin of life as well as appearance of man on earth. One such speculation, which is as old as human thought, is commonly known as the theory of spontaneous generation of life. It was believed that life originated spontaneously from inorganic components of the environment that existed since the formation of our planet. Evolutionary theory aims to explain the origin of life and all its present variety of forms: that is, the whole range of individual organisms normally categorized into species, genus, family and kingdom. It postulates the emergence of complex living organisms from non-living matter by way of a much smaller number of less complex ancestors. It claims that all life on earth can be traced back to one primitive organism, developing spontaneously and by chance, probably from a primordial soup of electrified chemicals.

This account has a certain intuitive appeal. After all, it is a matter of common observation that offspring are rarely if ever exactly the same as their parents. Variations on species-type naturally occur, some of them quite radical, a few of them perhaps making the individual stronger or fitter or cleverer. A variation that survive or reproduction in a generally hostile, competitive environment is more likely to be passed on to the next generation. One can imagine those variations gradually, over long periods of time, resulting in the emergence of new strains.

The precise mechanism by which variations occur and may be passed on from one generation to next was not known to Darwin. The discovery of chromosomes and the development of the science of genetics seemed at first to supply that need. However, gaps in the fossil record remains troubling to some throughout the 19th and 20th centuries. Some evolutionists suggested that the various species we know today did not emerge slowly but rapidly, in jumps that took place over thousands or perhaps even just hundreds of years, rather than the millions hypothesized by Darwin. The original theory was therefore adapted by suggesting ways in which variations could be brought about more rapidly through genetic mutation and/or environmental pressure (the inheritance of acquired characteristics being ruled out by the lack of an appropriate mechanism). Stephen Jay Gould has been the best-known exponent of this theory of “punctuated equilibrium”. As for the attempts to replicate the creation of life under laboratory conditions, these have not so far been successful.

The best known opponents of the evolutionary account are the biblical creationists, who believe that God created the various separate species in a sequence of distinct acts described in the Book of Genesis, perhaps even in six days of twenty-four hours each. This does away with the difficulty of explaining incredible variations between different forms of life in terms of environmental pressures working on spontaneous genetic variation, but introduces a whole series of other problems, not least the existence of the fossil record. The extreme creationist view is not taken seriously even by most Christians; and it has to be said that the methods and arguments of “creation science” do little to allay the fears of irrationalism which haunt this debate. It has been rightly said that a simplistic creationism “is the best thing that could have happened to Darwinism, the caricature of religion that has seemed to justify Darwinist contempt for the whole of religion.

1.   Brief History of Evolutionary thought

The discovery of evolutionary principles first took place in Western Europe and was made possible by advances in scientific thinking that date back to sixteenth century. Intellectual in cultures and in ancient Greece had notions of biological evolution, but never formulated them into a cohesive theory. Charles Darwin was the first person to explain the basic mechanics of the evolutionary process. A Scottish naturalist named Alfred Russel Wallece independently reached the conclusion like Darwin theory of natural selection.Throughout the middle Ages, one predominant features of the European world view was that all aspects of nature, including all forms of life and their relationships to one another, never changed. This view was partly shaped by a feudal society that was itself a hierarchical, rigid class system that hadn’t changed much for centuries. It was also influenced by an extremely powerful religious system, and the teachings of Christianity were taken literally. It was generally accepted that all life on earth had been created by God exactly as it existed in the present, and the belief that life forms couldn’tchange came to be known as fixity of species. Until the concepts of fixity and time were fundamentally altered, it was impossible to conceive of evolution by means of natural selection.

Table 1.1 Precursors theory of evolution

2.      Evidence of Evolution

a) The fossil record: The most evidence that evolution has occurred is found in the fossil record. Today we have a far more complete understanding of this record than was available in Darwin’s time. Fossils are the preserved remains of once living organisms. The fossil record shows that there is not an orderly sequence leading from one form to another. Several groups lived at the same time and characteristics developed at different rates; therefore the human family tree suggests a long and complex past.

Fossils from several early Australopithecus species that lived between four million and two million years ago clearly demonstrate a variety of adaptations that mark the transition between ape to human. Prior to four million years ago, fossil remains are scarce and incomplete; where available, however, they do show a primitive combination of ape and human features. Most of the key characteristics that stand out as distinctly human are related to their bipedal stance. The Australopithecus had an S-shaped spine that allowed for balance when standing. The opening through which the spinal cord attached to the brain was positioned more forward, allowing for the head to be balanced over the upright spine. The pelvic bone was shorter and broader than in apes, giving the pelvis a bowl shape that supported the internal organs when standing or walking upright. The upper legs angled inward allowing the knees to support the body while standing or walking. Shorter and less flexible toes functioned as rigid levers for pushing off the ground with each step.

Most early species had small canine teeth, a projecting face, and a small brain. They weighed between 22 and 37 kilograms, and were 0.9 to 1.5 meters tall. Males were generally larger than females. Both had curved fingers and long thumbs with a wide range of movement. The apes, in comparison, have longer, more curved, and stronger fingers that make them well adapted for hanging and swinging from branches. Apes also have short thumbs, which limits their ability to manipulate small objects.

b)Natural Selection: Natural selection is the gradual process by which heritable biological traits become either more or less common in a populations a function of the effect of inherited traits on the differential reproductive success of organisms interacting with their environment. It is a key mechanism of evolution. The term “natural selection” was popularized by Charles Darwin, who intended it to be compared with artificial selection, now more commonly referred to as selective breeding. Darwin realized that natural selection was the key to evolution and Most of the evolutionary biology agreed the process responsible for most of the major evolutionary changes that have occurred through time. Natural population provides clear evidence of evolutionary change. In the struggle for existence, those individuals with favorable variations would survive and reproduce, but those with unfavorable variations would not. Darwin himself was most interested in explaining the pattern of distribution of living species of organisms. One of his best known studies noted that Islands of the Galapagos Archipelago were inhabited by species of finch different from the finch species found on the Ecuadorian mainland.

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d) Field of Biology: The field of molecular biology provides the most detailed and convincing evidence available for biological evolution. In its unveiling of the nature of DNA and the workings of organisms at the level of enzymes and other protein molecules, it has shown that these molecules hold information about an organism’s ancestry. This has made it possible to reconstruct evolutionary events that were previously unknown and to confirm and adjust the view of events already known. The precision with which these events can be reconstructed is one reason the evidence from molecular biology is so compelling. Another reason is that molecular evolution has shown all living organisms, from bacteria to humans, to be related by descent from common ancestors. Fossil of extinct species have never been discovered. Nonetheless, the fossil record is complete enough to allow a detailed understanding of the evolution of life through time. The evolution of the major vertebrate groups is quite well known. The anatomical record found much of the power of the theory of evolution is its ability to provide a sensible of evolution for understanding the diversity of life. Many observations from a wide variety of fields of biology simply cannot be understands in any meaningful way except as a result of evolution.

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3. Mechanisms of Evolution

a) Divergent Evolution: Divergent evolution is the accumulation of differences between groups which can lead to the formation of new species, usually a result of diffusion of the same species to different and isolated environments which blocks the gene flow among the distinct populations allowing differentiated fixation of characteristics through genetic drift and natural selection. Primarily diffusion is the basis of molecular division can be seen in some higher-level characters of structure and function that are readily observable in organisms. For example, the vertebrate limb is one example of divergent evolution. The limb in many different species has a common origin, but has diverged somewhat in overall structure and function when isolated populations of a species evolve independently. It occurs when geographic barriers separate population numbers or when a small group leaves an original population.

b) Convergent Evolution: Occurs when natural selection has produced analogous adaptations in response to similar environment between different species

c) Parallel Evolution: it is the independent evolution of similar traits, starting from a similar ancestral condition. This is the situation in more closely related lineages, where several species respond to similar challenges in a similar way.

4.  Difficulties on Theory

Some points of difficulties on theory of evolution are

• Lack of intermediate forms in fossil record.
• Lack of a viable mechanism for producing high levels of complex and specified information. Related to this are problems with the Darwinian mechanism producing irreducibly complex feature deleterious intermediate stage.
• The failure of molecular biology to provide evidence for a grand “tree of life”.
• Natural selection is an extremely inefficient method of spreading traits in populations unless a trait has an extremely high selection coefficient.
• The problem that convergent evolution appears rampant at both the genetic and morphological levels, even though under Darwinian theory this is highly unlikely.
• The failure of chemistry to explain the origin of the genetic code.
• The failure of developmental biology to explain why vertebrate embryos diverge from the beginning of development.
• The failure of neo-Darwinian evolution to explain the biographical distribution of many species.
• A long history of inaccurate predictions inspired by Neo-Darwinism regarding vestigial organ or so called “Junk” DNA.
• Humans show many behavioral and cognitive traits and abilities that offer no apparent survival advantage.

6. Tenets of Evolutionary Theory

a)  Perpetual change- a species changes from generation to generation through genetic modification.

b) Common descent- as we follow individual back through generation we find that may have a common ancestor.

c) Multiplication of species- new species develop as a result of modification of older species that become reproductively isolated.

d) Gradualism – large difference between organisms is the result of incremental changes over long periods of time.

e) Natural selection- each species adapts to the environment in which it lives.

7. Nineteenth-Century Evolutionary Theory

• Natural selection acts on variation within species.
• Belief that human created in God’s image.
• Comparative anatomy-similarities and differences among living things.
• The Embryology.
• Classification of features or not related.

8. Modern Evolutionary Theory

By the beginning of the twentieth century, the foundations for evolutionary theory had already been developed. Darwin and Wallace had described natural selection 40 years earlier, and the rediscovery of Mendelian genetics in 1900 contributed the major component, a mechanism for inheritance. For the first 30 years of twenty century, some scientists argued that mutation was the main factor in evolution, while others emphasized natural selection. In the 1920s and 1930s, biologists realized that mutation and natural selection weren’t opposing processes and that both actually contributed to biological evolution. The two major foundations of the biological sciences were thus brought together in what a scientist named Julian Huxley called the modern synthesis. In twenty century perspective, we define evolution as a two –stage process:

a) The production and redistribution of variation (inherited differences among organism)

b) Natural selection acting on this variation, whereby inherited differences, or variation, among individuals differentially affect their ability to successfully reproduction.

Modern evolution theory contributions from population geneticists and natural historians

Evidence gathered by scientists who observe natural populations rather than artificial systems in the lab contributed the following ideas:

• Selection pushes recombination further. A greater variation and combination of traits is found than can be explained by normal rates of recombination.
• Natural populations are genetically variable.
• Populations of species in different locations may vary genetically. This observation further divides “species” into genetically distinct individual populations.
• Differences between species and populations can be experimentally shown to have a genetic component. Most of these differences are polygenic, supporting Darwin’s claim that evolution takes place in small steps rather than by individual mutations.
• Natural selection does occur in natural populations.
• Differences among populations of a species are often related to environmental differences and, thus, are adaptive.

9.    Opposition of Evolution theory

Almost 150 years after publication of “origin of species” the debate over evolution is far from over. For the vast majority of scientists today, evolution is indisputable. The genetic evidence for it is solid and accumulating daily. Those who appreciates and understands genetic mechanism cannot avoid the conclusion that population and species evolve.Most critism and denials of evolution have come from religions sources, rather than from the scientific community. Although many religions have accepted the occurrence of evolution, such as those advocating theistic evolution, there are some religious beliefs which reject evolutionary explanations in favor of creatinism, the belief that diety supernaturally created the world largely in its current form. The resultant U.S. centred creation- evolution controversy has been a focal point of recent conflict between religion and science. In 1925 banning of teaching evolution in public schools was passed in Tennessee. Creationists explain the existence of universe as the result of a sudden creation event that occurred no more than 10.000 years ago, they determined either to eliminate the teaching of evolution or to introduce antievolutionary material into public school classes. Anti-evolution feeling also remained strong among many politicians, particularly those with strong support from Christian fundamentalists.Most criticisms and denials of evolution have come from religious sources, rather than from the scientific community. Although many religions have accepted the occurrence of evolution, such as those advocating theistic evolution, there are some religious beliefs which reject evolutionary explanations in favor of creationism, the belief that a deity supernaturally created the world largely in its current form. The resultant U.S.-centered creation–evolution controversy has been a focal point of recent conflict between religion and science.

Modern creationism is characterized by movements such as creation science, neocreationism, and intelligent design, which argue that the idea of life being directly designed by a god or intelligence is at least as scientific as evolutionary theory, and should therefore be taught in public education. Such arguments against evolution have become widespread and include objections to evolution’s evidence, methodology, plausibility, morality, and scientific acceptance. The scientific community, however, does not recognize such objections as valid, citing detractors’ misinterpretations of such things as the scientific method, evidence, and basicphysical laws.

10. How do scientists study human evolution?

Since Charles Darwin’s time, scientists have gathered a huge amount of evidence about human evolution. Using the scientific method they form and test ideas to build theories about human evolution. Ideas generally start with an observation, from which scientists form a hypothesis. A hypothesis is a testable statement about the world based on observations. From a hypothesis we can make a prediction about what would happen in the hypothesis was true or false, and scientist then gather evidence to test whether their hypothesis is true. If the hypothesis is supported by multiple lines of evidence, it becomes a theory. In science a theory doesn’t mean a guess or a hunch. It means a well-supported explanation for some aspect of the natural world that incorporates facts, laws, observations and testable hypotheses.

In the case of human evolution, Charles Darwin noted that humans and apes share many biological features. He hypothesized that humans and apes share a common ancestor. If this hypothesis is true then humans and apes should share a high percentage of their DNA, and there should be fossils with humans and ape-like features. Darwin wasn’t able to gather this sort of evidence to test his hypothesis in his time, but many studies since then have tested these predictions. Both DNA and fossil evidence support the hypothesis of humans and apes sharing a common ancestor, and the idea that humans evolved from ape-like creatures is now an accepted theory. Darwin presented evidence showing that natural species including humans have changed, or evolved, over long spans of time. He also argued that radically new forms of life develop from existing species. He noted that all organisms compete with one another for food, space, mates, and other things needed for survival and reproduction. The most successful individuals in this competition have the greatest chance of reproducing and passing these characteristics on to offspring. Over hundreds of thousands of generations, one form of life can evolve into one or more other forms. Darwin called this process natural selection.

Modern science now understands that the mechanism for evolutionary change resides in genes, the basic building block of heredity . Genes determine how the body, and often the behavior, of an organism will develop over the course of its life. Certain information in genes can change, and over time this genetic change can actually alter a species’ overall way of life.

In recent decades, biological and social scientists have made impressive strides in understanding our complex physical and cultural origins. Their research has revealed gradual alterations in our genetic structure, as well as shifts in culture and behavior, that have transformed humankind into the planet-dominant species.

Scientists estimate that our human ancestors began to diverge from the African primates between eight million and five million years ago. This figure is the result of studying the genetic makeup of humans and apes, and then calculating approximately how long it took for those differences to develop. Used similar methods of comparing genetic variation among human populations around the world, it is thought that all people living today share a common genetic ancestor.

Summary

• In introduction we discussed about how Physical anthropology is concerned with all expects of how humans came to be and how we adapt physiologically to the external environment, understanding the details of the evolutionary process is crucial.

Theories of Evolution

•  Study of beneficial to know how the mechanics of the process came to be discovered. To appreciate the nature of the controversy that still surrounds the issue.
• Darwin saw evolution as the gradual in folding of new varieties of life from previous forms over long periods of time. And this is indeed one result of the evolutionary process.
• The discovery of evolutionary principles first took place in Western Europe and was made possible by advances in scientific thinking that date back to sixteenth century. Intellectual in cultures and in ancient Greece had notions of biological evolution, but never formulated them into a cohesive theory.
• Charles Darwin was the first person to explain the basic mechanics of the evolutionary process. A Scottish naturalist named Alfred Russel Wallece independently reached the conclusion like Darwin theory of natural selection.
• Precursor’s theory of evolution given by different thought have discussed.
• Evidence of Evolution found in the form of fossil record, natural Selection and field of Biology have discussed.
• Mechanisms of Evolution in three ways:

Divergent Evolution: when isolated populations of a species evolve independently. It occurs when geographic barriers separate population numbers or when a small groups leaves an original population. Convergent Evolution: Occurs when natural selection has produced analogous adaptations in response to similar environment between different species

Parallel Evolution: Difficulties on Theory also occur due to lack of intermediate form in fossil record and other molecular biology to provide evidence. Differences of Nineteenth-Century Evolutionary Theory and Modern Evolutionary Theory are also mention. Opposition of Evolution seen after publication of “origin of species” the debate over evolution is far from over. For the vast majority of scientists today, evolution is indisputable. Many criticism and denials of evolution have come from religions sources, rather than from the scientific community.