Understanding Evolution University of California Berkeley Lines of evidence: The science of evolution At the heart of evolutionary theory is the basic idea that life has existed for billions of years and has changed over time. Overwhelming evidence supports this fact. Scientists continue to argue about details of evolution, but the question of whether life has a long history or not was answered in the affirmative at least two centuries ago. The history of living things is documented through multiple lines of evidence that converge to tell the story of life through time. In this section, we will explore the lines of evidence that are used to reconstruct this story. These lines of evidence include: Fossil evidence Homologies Distribution in time and space Evidence by example Fossil evidence The fossil record provides snapshots of the past that, when assembled, illustrate a panorama of evolutionary change over the past four billion years. The picture may be smudged in places and may have bits missing, but fossil evidence clearly shows that life is old and has changed over time. Early fossil discoveries In the 17th century, Nicholas Steno shook the world of science, noting the similarity between shark teeth and the rocks commonly known as "tongue stones." This was our first understanding that fossils were a record of past life.
Two centuries later, Mary Ann Mantell picked up a tooth, which her husband Gideon thought to be of a large iguana, but it turned out to be the tooth of a dinosaur, Iguanodon. This discovery sent the powerful message that many fossils represented forms of life that are no longer with us today. Additional clues from fossils Today we may take fossils for granted, but we continue to learn from them. Each new fossil contains additional clues that increase our understanding of life's history and help us to answer questions about their evolutionary story. Examples include: Nicholas Steno's anatomical drawing of an extant shark (left) and a fossil shark tooth (right). Steno made the leap and declared that the fossil teeth indeed came from the mouths of onceliving sharks. Indication of interactions This ammonite fossil (see right) shows bite marks apparently caused by a predatory marine reptile called a mosasaur. Clues at the cellular level Fossils can tell us about growth patterns in ancient animals. The picture at right is a cross-section through a sub-adult thigh bone of the duckbill dinosaur Maiasaura. The white spaces show that there were lots of blood vessels running through the bone, which indicates that it was a fast-growing bone. The black wavy horizontal line in mid-picture is a growth line, reflecting a seasonal pause in the animal's growth. Transitional forms Fossils or organisms that show the intermediate states between an ancestral form and that of its descendants are referred to as transitional forms. There are numerous examples of transitional forms in the fossil record, providing an abundance of evidence for change over time.
Pakicetus (below left), is described as an early ancestor to modern whales. Although pakicetids were land mammals, it is clear that they are related to whales and dolphins based on a number of specializations of the ear, relating to hearing. The skull shown here displays nostrils at the front of the skull. A skull of the beluga whale that roams the seas today (below right) has its nostrils placed at the top of its skull. It would appear from these two specimens that the position of the nostril has changed over time and thus we would expect to see intermediate forms. Note that the nostril placement in Aetiocetus is intermediate between the ancestral form Pakicetus and the modern beluga an excellent example of a transitional form in the fossil record! Our understanding of the evolution of horse feet, so often depicted in textbooks, is derived from a scattered sampling of horse fossils within the multi-branched horse evolutionary tree. These fossil organisms represent branches on the tree and not a direct line of descent leading to modern horses. But, the standard diagram does clearly show transitional stages whereby the four-toed foot of Hyracotherium, otherwise known as Eohippus, became the single-toed foot of Equus. Fossils show that the transitional forms predicted by evolution did indeed exist. As you can see to the left, each branch tip on the tree of horse evolution indicates a different genus, though the feet of only a few genera are illustrated to show the reduction of toes through time.
Homologies Evolutionary theory predicts that related organisms will share similarities that are derived from common ancestors. Similar characteristics due to relatedness are known as homologies. Homologies can be revealed by comparing the anatomies of different living things, looking at cellular similarities and differences, studying embryological development, and studying vestigial structures within individual organisms. In the following photos of plants, the leaves are quite different from the "normal" leaves we envision. Each leaf has a very different shape and function, yet all are homologous structures, derived from a common ancestral form. The pitcher plant and Venus' flytrap use leaves to trap and digest insects. The bright red leaves of the poinsettia look like flower petals. The cactus leaves are modified into small spines which reduce water loss and can protect the cactus from herbivory. Another example of homology is the forelimb of tetrapods (vertebrates with legs).
Frogs, birds, rabbits and lizards all have different forelimbs, reflecting their different lifestyles. But those different forelimbs all share the same set of bones - the humerus, the radius, and the ulna. These are the same bones seen in fossils of the extinct transitional animal, Eusthenopteron, which demonstrates their common ancestry. Homologies: anatomy Individual organisms contain, within their bodies, abundant evidence of their histories. The existence of these features is best explained by evolution. Several animals, including pigs, cattle, deer, and dogs have reduced, nonfunctional digits, referred to as dewclaws. The foot of the pig has lost digit 1 completely, digits 2 and 5 have been greatly reduced, and only digits 3 and 4 support the body. Evolution best explains such vestigial features. They are the remnants of ancestors with a larger number of functional digits. People (and apes) have chests that are broader than they are deep, with the shoulder blades flat in back. This is because we, like apes, are descended from an ancestor who was able to suspend itself using the upper limbs. On the other hand, monkeys and other quadrupeds have a different form of locomotion. Quadrupeds have narrow, deep chests with shoulder blades on the sides.
Hoatzin chicks have claws on their wings, as do some chickens and ostriches. This reflects the fact that bird ancestors had clawed hands. Homologies: comparative anatomy Organisms that are closely related to one another share many anatomical similarities. Sometimes the similarities are conspicuous, as between crocodiles and alligators, but in other cases considerable study is needed for a full appreciation of relationships. Modification of the tetrapod skeleton Whales and hummingbirds have tetrapod skeletons inherited from a common ancestor. Their bodies have been modified and parts have been lost through natural selection, resulting in adaptation to their respective lifestyles over millions of years. On the surface, these animals look very different, but the relationship between them is easy to demonstrate. Except for those bones that have been lost over time, nearly every bone in each corresponds to an equivalent bone in the other.
Homologies: developmental biology Studying the embryological development of living things provides clues to the evolution of present-day organisms. During some stages of development, organisms exhibit ancestral features in whole or incomplete form. Snakes have legged ancestors. Some species of living snakes have hind limb-buds as early embryos but rapidly lose the buds and develop into legless adults. The study of developmental stages of snakes, combined with fossil evidence of snakes with hind limbs, supports the hypothesis that snakes evolved from a limbed ancestor. Above left, the Cretaceous snake Pachyrhachis problematicus clearly had small hindlimbs. The drawing at right shows a reconstruction of the pelvis and hindlimb of Pachyrhachis.
Baleen whales have toothed ancestors. Toothed whales have full sets of teeth throughout their lives. Baleen whales, however, only possess teeth in the early fetal stage and lose them before birth. The possession of teeth in fetal baleen whales provides evidence of common ancestry with toothed whales and other mammals. In addition, fossil evidence indicates that the late Oligocene whale Aetiocetus (left), from Oregon, which is considered to be the earliest example of baleen whales, also bore a full set of teeth. Again, these observations make most sense in an evolutionary framework where snakes have legged ancestors and whales have toothed ancestors. Homologies: cellular/molecular evidence All living things are fundamentally alike. At the cellular and molecular level living things are remarkably similar to each other. These fundamental similarities are most easily explained by evolutionary theory: life shares a common ancestor. The cellular level All organisms are made of cells, which consist of membranes filled with water containing genetic material, proteins, lipids, carbohydrates, salts and other substances. The cells of most living things use sugar for fuel while producing proteins as building blocks and messengers. Notice the similarity between the typical animal and plant cells pictured below only three structures are unique to one or the other.
The molecular level Different species share genetic homologies as well as anatomical ones. Roundworms, for example, share 25% of their genes with humans. These genes are slightly different in each species, but their striking similarities nevertheless reveal their common ancestry. In fact, the DNA code itself is a homology that links all life on Earth to a common ancestor. DNA and RNA possess a simple four-base code that provides the recipe for all living things. In some cases, if we were to transfer genetic material from the cell of one living thing to the cell of another, the recipient would follow the new instructions as if they were its own. These characteristics of life demonstrate the fundamental sameness of all living things on Earth and serve as the basis of today's efforts at genetic engineering. Distribution in time and space Understanding the history of life on Earth requires a grasp of the depth of time and breadth of space. We must keep in mind that the time involved is vast compared to a human lifetime and the space necessary for this to occur includes all the water and land surfaces of the world. Establishing chronologies, both relative and absolute, and geographic change over time are essential for viewing the motion picture that is the history of life on Earth. Chronology The age of the Earth and its inhabitants has been determined through two complementary lines of evidence: relative dating and numerical (or radiometric) dating.
Relative dating places fossils in a temporal sequence by noting their positions in layers of rocks, known as strata. As shown in the diagram, fossils found in lower strata were typically deposited first and are deemed to be older (this principle is known as superposition). Sometimes this method doesn't work, either because the layers weren't deposited horizontally to begin with, or because they have been overturned. If that's the case, we can use one of three other methods to date fossil-bearing layers relative to one another: faunal succession, crosscutting relationships, and inclusions. By studying and comparing strata from all over the world we can learn which came first and which came next, but we need further evidence to ascertain the specific, or numerical, ages of fossils. Numerical dating relies on the decay of radioactive elements, such as uranium, potassium, rubidium and carbon. Very old rocks must be dated using volcanic material. By dating volcanic ash layers both above and below a fossil-bearing layer, as shown in the diagram, you can determine "older than X, but younger than Y" dates for the fossils. Sedimentary rocks less than 50,000 years old can be dated as well, using their radioactive carbon content. Geologists have assembled a geological time scale on the basis of numerical dating of rocks from around the world. Geography The distribution of living things on the globe provides information about the past histories of both living things and the surface of the Earth. This evidence is consistent not just with the evolution of life, but also with the movement of continental plates around the worldotherwise known as plate tectonics. Marsupial mammals are found in the Americas as well as Australia and New Guinea, shown in brown on the map at right. They are not found swimming across the Pacific Ocean, nor have they been discovered wandering the Asian mainland. There appear to be no routes of migration between the two populations. How could marsupials have gotten from their place of origin to locations half a world away?
Fossils of marsupials have been found in the Antarctic as well as in South America and Australia. During the past few decades scientists have demonstrated that what is now called South America was part of a large land mass called Gondwana, which included Australia and Antarctica. Click on the map below for a short animation that shows how Gondwana split apart 160 to 90 million years ago. Marsupials didn't need a migration route from one part of the world to another; they rode the continents to their present positions. Evidence by example Although the history of life is always in the past, there are many ways we can look at present-day organisms, as well as recent history, to better understand what has occurred through deep time. Artificial selection in agriculture or laboratories provides a model for natural selection. Looking at interactions of organisms in ecosystems helps us to understand how populations adapt over time. Experiments demonstrate selection and adaptive advantage. And we can see nested hierarchies in taxonomies based on common descent.