READ: Evidence for Evolution

Lesson Objectives

• Understand that the scientific theory of biological evolution is based on extensive physical evidence and testing. This includes:
  • differences between fossils in different layers of rock
  • the age of rocks and fossils
  • vestigial structures
  • similarities between embryos of different organisms
  • the same DNA and RNA materials found in all organisms
  • similar genomes found in almost all organisms.

Vocabulary

embryo

An animal or plant in its earliest stages of development, before it is born or hatched.

embryology

The study of how organisms develop.

fossil

The preserved remains or traces of animals, plants, and other organisms from the distant past; examples include bones, teeth, impressions, and leaves.

fossil record

Fossils and the order in which fossils appear; provides important records of how species have evolved, divided and gone extinct.

genetics

The scientific study of heredity.

genome

All of the genes in an organism.

paleontologists

Scientists who study fossils to learn about life in the past.

radiometric dating

A method to determine the age of rocks and fossils in each layer of rock; measures the decay rate of radioactive materials in each rock layer.

vestigial structure

Body part that has lost its use through evolution, such as a whale’s pelvic bones.

Introduction

Though the idea of evolution had been proposed prior to Charles Darwin, most people think of Darwin’s name when they think of evolution. Unlike others before him who based their ideas on speculation, opinions, myths, or folklore, Darwin’s theories were based on a tremendous amount of scientific evidence.

In 1859, Charles Darwin and Alfred Russel Wallace first presented several forms of evidence of evolution. Their evidence included:

• fossils of extinct species from different eras
• similarities between the embryos of different species
• physical traits of different species
• the behavior of different species
• the distributions of different plant and animal species around the world.

Darwin and other 19^th century scientists came to the conclusions they did without knowing anything about molecular biology. Today, even more evidence of evolution by natural selection is coming from molecular biology and genetics. Genetics is also helping explain the mechanisms of how evolution occurs.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

The Fossil Record

Paleontologists are the scientists who study fossils to learn about life in the past. Fossils are the preserved remains or traces of animals, plants, and other organisms from the distant past. Examples of fossils include bones, teeth, impressions, and leaves. Paleontologists compare the features of species from different periods in history. With this information, they try to unravel how species have evolved over millions of years. This method works better with some species than others. For example, it is difficult to track the evolution of bacteria from fossils, because their single cells do not last well as fossils.

Until recently, fossils were the main source of evidence of evolution. The location of each fossil in layers of rocks provides clues to the age of the species and how species evolved in the past. Older materials and fossils are deeper in the earth; newer fossils and materials are closer to the surface.

Fossils and the rocks they are embedded in provide evidence of how life and environmental conditions have changed throughout Earth’s history. They also help us understand how the past and present distribution of life on Earth is affected by earthquakes, volcanoes, and shifting seas, and other movements of the continents.

The Age of Rock Layers and Fossils

The many layers of sedimentary rock provide evidence of the long history of Eearth and the order of life forms whose remains are found in the rocks. The youngest layers are not always found on top, because of folding, breaking, and uplifting of layers. If the layers of earth were tilted by earthquakes or volcanoes, geologists can determine which layers came from the deepest parts of the Earth.

The fossils and the order in which fossils appear is called the fossil record. This record provides important records of how species have evolved, divided and gone extinct. Methods used to date the age of rocks and fossils make it possible to determine when these events occurred.

Geologists use a method called radiometric dating to determine the age of rocks and fossils in each layer of rock. This technique measures the decay rate of radioactive materials in each rock layer.

Radiometric dating has been used to determine that the oldest known rocks on Earth are between 4 and 5 billion years old. The oldest fossils are between 3 and 4 billion years old.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Vestigial Structures

Millions of species of animals, plants and microorganisms are alive today. Even though two different species may not look similar, they may have similar internal structures, and chemical processes that indicate they can have a common ancestor.

Some of the most interesting kinds of evidence for evolution are body parts that have lost their use through evolution. Most birds need their wings to fly. But the wings on an ostrich have lost their original use. These are called vestigial structures. Penguins do not use their wings to fly in the air; however they do use them to "fly" in the water. A whale’s pelvic bones-which were once attached to legs- are also vestigial structures.

If you look at an x-ray of the bones in your back (called vertebrae), you will see several vertebrae that come under your hips. These are called your coccyx, or tailbone. We do not use these small vertebrae; they are further evidence of our evolution.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Embryological Evidence

Some of the oldest evidence of evolution comes from embryology, the study of how organisms develop. An embryo is an animal or plant in its earliest stages of development, before it is born or hatched.

Centuries ago, people recognized that the embryos of many different species have similar appearances. The embryos of some species are even difficult to tell apart. Many of these animals do not differ much in appearance until they develop further. Many traits of one type of animal appear in the embryo of another type of animal. For example, fish embryos and human embryos both have gill slits. In fish they develop into gills, but in humans they disappear before birth.

The similarities between embryos suggests that these animals are related and have common ancestors. For example, humans did not evolve from chimpanzees. But the similarities between the embryos of both species may be due to our development from an ancestor we have in common with chimpanzees. As our common ancestor evolved, humans and chimpanzees diverged and developed different traits.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Similarities Between Molecules and Genomes

Molecular Clocks

Arguably, some of the most significant evidence of evolution comes from examining the molecules and DNA found in all organisms. The field of molecular biology did not emerge until the 1940s and has since confirmed and extended the conclusions about evolution drawn from other forms of evidence. Molecular clocks are used in molecular evolution to relate the time that two species diverged to the number of differences measured between the species' DNA sequences or protein amino acid sequences. These clocks are sometimes called gene clocks or evolutionary clocks. The fewer the differences the less time since the divergence of the species. For example, a chicken and a gorilla will have more differences between their DNA and protein amino acid sequences then a gorilla and an orangutan. This provides additional evidence that the gorilla and orangutan are evolutionally closer related than the gorilla and the chicken.

Molecular clocks, combined with other forms of evidence, such as evidence from the fossil record, have provided considerable evidence to estimate how long ago various groups of organisms diverged evolutionarily from one another.

Molecular Genetics

The development of molecular genetics has revealed the record of evolution left in the genomes of all organisms. It also provides new information about the relationships among species and how evolution occurs.

Molecular genetics provides evidence of evolution such as:

• the same biochemical building blocks – such as amino acids and nucleotides - are responsible for life in all organisms, from bacteria to plants and animals
• DNA and RNA determine the development of all organisms
• the similarities and differences between the genomes, the gene sequences of each species, reveal patterns of evolution.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Lesson Summary

• Fossil evidence, depicted by the skeletal fragments, demonstrates evolutionary milestones.
• Fossils and the rocks they are embedded in provide evidence of how life and environmental conditions have changed throughout Earth’s history.
• The fossils and the order in which fossils appear is called the fossil record.
• Geologists use a method called radiometric dating to determine the age of rocks and fossils in each layer of rock.
• Radiometric dating has been used to determine that the oldest known rocks on Earth are between 4-5 billion years old. The oldest fossils are between 3-4 billion years old.
• Body parts that do not serve any function are called vestigial structures.
• Vestigial structures indicate that two species have a recent common ancestor.
• The similarities between embryos suggests that animals are related and have common ancestors.
• The same biochemical building blocks – such as amino acids and nucleotides - are responsible for life in all organisms, from bacteria to plants and animals.
• DNA and RNA determines the development of all organisms.
• The similarities and differences between the genomes, the gene sequences of each species, reveal patterns of evolution.