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Evolution of Life

Populations of organisms change over time. A process that began on earth 3.5 bya.

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Evolution of Life - Advanced

License: CC BY-NC 3.0

What is a Dinosaur?

A dinosaur is from a class of reptiles. They are diverse reptiles that first appeared during the Triassic period, approximately 230 million years ago, and were the dominant land vertebrates for 135 million years, from the beginning of the Jurassic period (about 200 million years ago) until they went extinct, at the end of the Cretaceous period, 65.5 million years ago. They are very strong evidence of evolution, the change in species over time. Dinosaurs went extinct because they could not adapt to a catastrophic environmental change. The ability to adapt to a changing environment is a key feature of natural selection, the process of evolution.

Evolution of Life

Evolution is the process by which populations of organisms change over time. It is a process that began on this planet well over 3.5 billion years ago and continues to this day, as populations of organisms continue to change.

Evolution occurs as organisms acquire and pass on new traits from one generation to the next generation. Its occurrence over large stretches of time explains the origin of new species and the great diversity of the biological world. Extant species are related to each other through common descent, and products of evolution over billions of years. Analysis of the DNA of different organisms indicates there is a similarity among very different organisms in the genetic code that help make proteins and other molecules. This genetic code is used by all known forms of life on Earth. The theory of evolution suggests that the genetic code was established very early in the history of life, and some studies suggest it was established soon after the formation of Earth. The timeline of the evolution of life, shown in Figure below, outlines the major events in the development of life.

Credit: Courtesy of the US Geological Survey
Source: http://commons.wikimedia.org/wiki/File:Geological_time_spiral.png
License: CC BY-NC 3.0

According to recent estimates, the Earth is about 4.5 billion years old. Most of the evidence for an ancient Earth is contained in the rocks that form the Earth's crust. The rock layers themselves, like pages in thick history book, record the surface shaping events of the past. Buried within them are traces of life, including the plants and animals that evolved from organic structures that existed perhaps as many as 3 to 3.5 billion years ago.[Figure2]

How do scientists know Earth is so old? The answer is in the rocks. Contained in rocks that were once molten, shown in Figure below, are chemical elements that act like an atomic clock. The atoms of different forms of elements (called isotopes) break down at different rates over time. Parent isotopes within these rocks decay at a predictable rate to form daughter isotopes. By determining the relative amounts of parent and daughter isotopes, the age of these rocks can be calculated—forming the so-called atomic clock.

Thus, the results of studies of rock layers (stratigraphy), and of fossils (paleontology), along with the ages of certain rocks as measured by atomic clocks (geochronology), indicate that the Earth is over 4.5 billion years old, with the oldest known rocks being 3.96 billion years old. More about the history of life on Earth will be discussed in History of Life: Introduction (Advanced) concept.

Credit: Courtesy of Jim D. Griggs, HVO, US Geological Survey
Source: http://commons.wikimedia.org/wiki/File:Pahoeoe_fountain_original.jpg
License: CC BY-NC 3.0

Molten rock, called lava, is expelled by a volcano during an eruption. The lava will eventually cool to become solid rock. When first expelled from a volcanic vent, it is a liquid at temperatures from 700 °C to 1,200 °C (1,300 °F to 2,200 °F). Not all types of rocks come from cooled lava, but many do.[Figure3]

Credit: Mariana Ruiz Villarreal (User:LadyofHats/Wikimedia Commons)
Source: http://commons.wikimedia.org/wiki/File:Timeline_evolution_of_life.svg
License: CC BY-NC 3.0

This timeline shows the history of life on Earth. In the entire span of the time, humans are a relatively new addition.[Figure4]

History of Evolutionary Thought

The theory of evolution by natural selection was proposed at about the same time by both Charles Darwin and Alfred Russel Wallace, shown in Figure below, and was set out in detail in Darwin's 1859 book On the Origin of Species. Natural selection is a process that causes heritable traits that are helpful for survival and reproduction to become more common, and harmful traits, or traits that are not helpful or advantageous for survival to become more rare in a population of organisms. This occurs because organisms with advantageous traits are more "fit" to survive in a particular environment and have "adapted" to the conditions of that environment. These individuals will have greater reproductive success than organisms less fit for survival in the environment. This will lead to an increase in the number of organisms with the advantageous trait(s) over time. Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of those variants best-suited for their environment. Natural selection is one of the cornerstones of modern biology.

Source: Darwin: http://commons.wikimedia.org/wiki/File:Charles_Darwin.jpg; Wallace: http://commons.wikimedia.org/wiki/File:Alfred_Russel_Wallace.jpg
License: CC BY-NC 3.0

Charles Darwin, left (1809-1882), and Alfred Russel Wallace, right (1823-1913). Both scientists proposed a process of evolution by natural selection at about the same time. However, Darwin is primarily associated with the theory of evolution by natural selection due to his abundance of data.[Figure5]

The theory of evolution encountered initial resistance from religious authorities who believed humans were divinely set apart from the animal kingdom. There was considerable concern about Darwin’s proposal of an entirely scientific explanation for the origin of humans. Many people found such an explanation to be in direct conflict with their religious beliefs. A caricature of Darwin as a monkey, shown in Figure below, reflects the controversy that arose over evolutionary theory. In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the basis of modern evolutionary theory.

Source: http://commons.wikimedia.org/wiki/File:Editorial_cartoon_depicting_Charles_Darwin_as_an_ape_%281871%29.jpg
License: CC BY-NC 3.0

An 1871 caricature portraying Darwin with an ape body and the bushy beard he grew in 1866. Such satire reflected the cultural backlash against evolution.[Figure6]

The identification of DNA as the genetic material by Oswald Avery and colleagues in the 1940s, as well as the publication of the structure of DNA by James Watson and Francis Crick in 1953, demonstrated the physical basis for inheritance. Since then, genetics and molecular biology have become core aspects of evolutionary biology.

Currently the study of evolutionary biology involves scientists from fields as diverse as biochemistry, ecology, genetics and physiology, and evolutionary concepts are used in even more distant disciplines such as psychology, medicine, philosophy and computer science.

Misconceptions About Evolution

The following list includes some common misconceptions about evolution.

  • The term evolution describes the changes that occur in populations of living organisms over time. Describing these changes does not address the origin of life. The two are commonly and mistakenly confused. Biological evolution likewise says nothing about cosmology, the Big Bang, or where the universe, galaxy, solar system, or Earth came from.
  • Humans did not evolve from chimpanzees or any other modern ape; instead they share a common ancestor that existed around 7 million years ago.
  • The process of evolution is not necessarily slow. Millions of years are not required to see evolution in action. Indeed, it has been observed multiple times under both controlled laboratory conditions and in nature.
  • Evolution is not a progression from "lower" to "higher" forms of life, and it does not increase in complexity. For example, bacteria have simpler structures and a smaller amount of genetic material than humans do. This does not mean however, that bacteria are “less evolved” than humans are. Bacteria have evolved over many millions of years and are well adapted to their own environments.

After Darwin

Since Darwin's time, scientists have gathered a more complete fossil record, including microorganisms and chemical fossils. These fossils have supported and added more information to Darwin's theories. However, the age of the Earth is now held to be much older than Darwin thought. Researchers have also uncovered some of the preliminary mysteries of the mechanism of heredity as carried out through genetics and DNA, which were areas unknown to Darwin. Another growing subject is the study of comparative anatomy, which looks at how different organisms have similar body structures. Molecular biology studies of slowly changing genes reveal an evolutionary history that is consistent with fossil and anatomical records.


  • Analysis of the DNA of different organisms indicate that there is a similarity in the genetic codes that help make proteins and other molecules in very different organisms.
  • The theory of evolution by natural selection is based on the concept of the survival of the fittest, where individuals with beneficial traits are better able to survive and reproduce in the environment in which they live.


  1. What is evolution and natural selection?
  2. Outline the formation of modern evolutionary theory.
  3. How have more recent scientific findings fit with evolutionary theory since Darwin’s time?
  4. What are the misconceptions about evolution?
  5. Large animals are more evolved than single-celled organisms such as bacteria. Do you agree with this statement? Explain your answer.

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Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ Credit: Courtesy of the US Geological Survey; Source: http://commons.wikimedia.org/wiki/File:Geological_time_spiral.png; License: CC BY-NC 3.0
  3. [3]^ Credit: Courtesy of Jim D. Griggs, HVO, US Geological Survey; Source: http://commons.wikimedia.org/wiki/File:Pahoeoe_fountain_original.jpg; License: CC BY-NC 3.0
  4. [4]^ Credit: Mariana Ruiz Villarreal (User:LadyofHats/Wikimedia Commons); Source: http://commons.wikimedia.org/wiki/File:Timeline_evolution_of_life.svg; License: CC BY-NC 3.0
  5. [5]^ Source: Darwin: http://commons.wikimedia.org/wiki/File:Charles_Darwin.jpg; Wallace: http://commons.wikimedia.org/wiki/File:Alfred_Russel_Wallace.jpg; License: CC BY-NC 3.0
  6. [6]^ Source: http://commons.wikimedia.org/wiki/File:Editorial_cartoon_depicting_Charles_Darwin_as_an_ape_%281871%29.jpg; License: CC BY-NC 3.0

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