1-3-1-1 Introduction

0 Contents 1 Background 1-3 Biological 1-3-1 Evolution 1-3-1-1 Introduction

 Introduction 1-3-1-1

1-3-1-1-0 Evolution - Fossil Evidence - Overview

 Source http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_01

01 Fossil evidence
02 Transitional forms
03 Homologies *
04 Homologies: anatomy
05 Homologies: comparative anatomy
06 Homologies: developmental biology
07 Homologies: cellular/molecular evidence
08 Distribution in time and space
09 Chronology
10 Geography
11 Evidence by example
12 Artificial selection
13 Ecology
14 Experiments
15 Nested hierarchies

*"homology" has come to mean any similarity between characters that is due to their shared ancestry

00 Contents 01 Fossil evidence Transitional 02
Nicholas Steno's anatomical drawing of an extant shark and a fossil shark tooth 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 once-living sharks.

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:

Ammonite with bite marks
Indication of interactions This ammonite fossil (see right) shows punctures that some scientists have interpreted as the bite mark of a mosasaur, a type of predatory marine reptile that lived at the same time as the ammonite. Damage to the ammonite has been correlated to the shapes and capabilities of mosasaur teeth and jaws. Others have argued that the holes were created by limpets that attached to the ammonite. Researchers examine ammonite fossils, as well as mosasaur fossils and the behaviors of limpets, in order to explore these hypotheses.
Thin bone section
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.

01 Fossil 02 Transitions Homologies 03

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 gray 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 gray whale — an excellent example of a transitional form in the fossil record!

Horse evolution tree 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.

02 Transitional 03 Homologies anatomy 04


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.

homologous leaves

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).

Homology of tetrapod forelimbs

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.

03 Homologies 04 Homologies comparative 05

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.
Pig foot bones
  • 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.

Monkey/human chest comparison

  • Hoatzin chicks have claws on their wings, as do some chickens and ostriches. This reflects the fact that bird ancestors had clawed hands.
Claws in hoatzin chick and Archaeopteryx

04 anatomy

05 Homologies

developmental 06

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.

Whale/hummingbird comparison

05 Comparative

06 Homologies

Cellular/Molecular 07

Homologies: developmental biology

Aetiocetus skullStudying 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.
fossil snake hindlimb Fossil snake hindlimb bones labelled
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 (below), 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.

06 developmental

07 Homologies

Distribution 08

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.

Animal and plant cell comparison

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 similarites 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.

07 cellular/molecular

08 Homologies

Chronology 09

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.

Relative vs. absolute dating Splitting of southern continents

08 Distribution 09 Chronology Geography 10


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.

Relative vs. absolute dating

09 Chronology

10 Geography

by example 11


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 world-otherwise known as plate tectonics.

marsupial distribution worldwide  

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.

Gondwana split-up animation

10 Geography

11 Evidence

Artificial selection 12
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.

Checking dataChecking wheat for aluminum toleranceComputers in the dairy industry

11 Evidence

12 Selection

Ecology 13

Artificial selection

Artificial selection provides a model that helps us understand natural selection. People have been artificially selecting domesticated plants and animals for thousands of years. These activities have amounted to large, long-term, practical experiments that clearly demonstrate that species can change dramatically through selective breeding.


Broccoli and brussels sprouts bear little superficial resemblance to their wild mustard relatives (right).

If domesticated dogs were discovered today they would be classified as hundreds of different species and considered quite distinct from wolves. Although it is probable that various breeds of dogs were independently domesticated from distinct wild dog lineages, there are no wolf relatives anywhere in the world that look much like dachshunds or collies (below).

Wild mustard

Dog breeds

These observations demonstrate that selection has profound effects on populations and has the ability to modify forms and behaviors of living things to the point that they look and act very unlike their ancestors. Artificial selection provides a model that helps us understand natural selection. It is a small step to envision natural conditions acting selectively on populations and causing natural changes.

12 Artificial selection

13 Ecology

Experiments 14


House sparrows

The environment affects the evolution of living things. As predicted by evolutionary theory, populations evolve in response to their surroundings. In any ecosystem there are finite opportunities to make a living. Organisms either have the genetic tools to take advantage of those opportunities or they do not.


House sparrows arrived in North America from Europe in the nineteenth century. Since then, genetic variation within the population, and selection in various habitats, have allowed them to inhabit most of the continent. House sparrows in the north are larger and darker colored than those in the south. Darker colors absorb sunlight better than light colors and larger size allows less surface area per unit volume, thus reducing heat loss — both advantages in a cold climate. This is an example of natural selection acting upon a population, producing micro-evolution on a continental scale.

13 Ecology

14 Experiments

hierarchies 15


Guppy experiment

Experiments also show that populations can evolve. John Endler of the University of California has conducted experiments with the guppies of Trinidad that clearly show selection at work. The scenario: Female guppies prefer colorful males for mating purposes. Predatory fish also "prefer" colorful males, but for a less complimentary purpose — a source of food that is easy to spot. Some portions of the streams where guppies live have fewer predators than others and in these locations the males are more colorful (top frame). Not surprisingly, males in locations where there are more predators tend to be less colorful (bottom frame).


When Dr. Endler transferred predatory fish to the regions with brightly colored male guppies, selection acted rapidly to produce a population of duller males. This demonstrates that persistent variation within a population provides the raw material for rapid evolution when environmental conditions change.

14 Experiments

15 Hierarchies

Index 2.3.1

Nested hierarchies

Common ancestry is conspicuous. Evolution predicts that living things will be related to one another in what scientists refer to as nested hierarchies — rather like nested boxes. Groups of related organisms share suites of similar characteristics and the number of shared traits increases with relatedness. This is indeed what we observe in the living world and in the fossil record and these relationships can be illustrated as shown below.

cladogram illustration showing nested hierarchies

In this phylogeny, snakes and lizards share a large number of traits as they are more closely related to one another than to the other animals represented. The same can be said of crocodiles and birds, whales and camels, and humans and chimpanzees. However, at a more inclusive level, snakes, lizards, birds, crocodiles, whales, camels, chimpanzees and humans all share some common traits.

Humans and chimpanzees are united by many shared inherited traits (such as 98.7% of their DNA). But at a more inclusive level of life's hierarchy, we share a smaller set of inherited traits in common with all primates. More inclusive still, we share traits in common with other mammals, other vertebrates, other animals. At the most inclusive level, we sit alongside sponges, petunias, diatoms and bacteria in a very large "box" entitled: living organisms.