1. Evolution: The Evidence

2. 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.

3. Lines of evidence: The science of evolution
The history of living things is documented through multiple lines of evidence that converge to tell the story of life through time.

4. Lines of evidence: The science of evolution
Fossil Evidence
Chemical Similarities
Homologies
Distribution in time and space

5. 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.

6. Fossil Evidence
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.

7. Transitional Forms
Pakicetus, 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.

8. Transitional Forms
A skull of the beluga whale that roams the seas today 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 (transitional form).

9. Transitional 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!

10. Chemical Similarities
Living things on earth are fundamentally similar in their basic chemical composition.

11. Chemical Similarities
All living things on earth share the ability to create complex molecules out of carbon and a few other elements. 
In fact, 99% of the proteins, carbohydrates, fats, and other molecules of living things are made from only 6 of the 92 most common elements. 
This is not a mere coincidence.

12. Chemical Similarities
All plants and animals receive their specific characteristics from their parents by inheriting particular combinations of genes. 
Molecular biologists have discovered that genes are, in fact, segments of DNA molecules in our cells.

13. Chemical Similarities
These segments of DNA contain chemically coded recipes for creating proteins by linking together particular amino acids in specific sequences.

14. Chemical Similarities
All of the tens of thousands of types of proteins in living things are made of only 20 kinds of amino acids. 
Despite the great diversity of life on our planet, the simple language of the DNA code is the same for all living things. 
This is evidence of the fundamental molecular unity of life.

15. 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 or homologous anatomic structures.
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.

16. Homologies
In the following photos of plants, the leaves are quite different from the "normal" leaves we envision.

17. Homologies
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.

18. Homologies
Another example of homology is the forelimb of tetrapods (vertebrates with legs).

19. Homologies
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.

20. Homologies: Embryonic development
The embryonic development of all vertebrates shows remarkable similarities as you can see from these drawings.
Gill Slits

21. 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.

22. Distribution in time and space
The age of the Earth and its inhabitants has been determined through two complementary lines of evidence:
relative dating
numerical (or radiometric) dating.

23. Relative 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.

24. Relative Dating
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.

25. Numerical Dating
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.

26. Numerical Dating
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.

27. 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 world-otherwise known as plate tectonics.

28. Geography
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?

29. Geography
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.
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.