Preserving fossils is not easy.

First, the organism must be well buried in a fine silt. Then that silt must harden while preserving the form of the organism in order to make a recognizable fossil. Once formed, the fossil must "survive" millions of years of geological history, so it must not be crushed beyond recognition, broken to bits or recycled in the Earth's interior. And finally, there has to be erosion at just the right time in order to expose the fossil to the searching scientist.
It takes luck to become a fossil, luck to stay a fossil and luck to find a fossil!

In spite of these difficulties a few lineages are well represented in the fossil records. These lineages provide important evidence for evolution and also are a great educational opportunity.
One example of a fairly well preserved lineage is that of the horse.

Thorough 58 million years of sediments we find a fossil record that shows a tiny dog-size, forest dwelling creature (called Hyracotherium) becoming a large, grasslands dwelling creature we call a modern horse (and scientists call Equus).

It takes the training and the keen eye of an experienced anatomist to identify and analyze fossils and we cannot go into that kind of detail here, but I'd like to show you two bits of evidence in horse evolution and explain them to you (briefly).

This image represents a hypothetical layer of fossil sediments (reconstructed from many different fossil sites). The deepest layers are the oldest and they are overlaid with more recent materials so prehistory is read from the bottom (oldest) to the top (most recent). It isn't important to learn the geological times listed along the side - they are just there to guide you. I've placed a representative fossil leg in each zone and the name of the fossil animal from which it came. (Again, don't worry about the names.) The legs are not drawn to scale because it would have taken up a lot of space. The Hyracotherium was a small animal with several toes on its foot like that of most animals and it lived in the forest. We know the habitats of fossil animals because fossil plants were buried with them and give us a clue as to the environment in which the fossils lived. Over the millions of years the ancestors of the Hyracotherium evolved into Miohippus and grew a little larger. (We know that Miohippus was an ancestor of Hyracotherium due to other fossil evidence that I don't want to go into.) Miohippus fossils are found among the fossils of forest and grassland plants. Moving from one habitat to another puts different demands on the animal and natural selection started to choose animals from among the population best suited to grasslands. By the time Miohippus had evolved into Merychippus it had started to get much bigger and had evolved the habit of running across the plains. This put selection pressures to bear on the animals legs. You need strong, sturdy legs to carry a big animal quickly across the grasslands.

By the time Merychippus had evolved into Pilohippus, it was very large and very fast. Along the way, through millions of years of selection pressure, those "horses" with mutations creating one strong toe to support their heavy bodies were more successful. Horses with feet poorly adapted to running, short legs with many toes, just couldn't keep up (evolutionary speaking) so their fossil remains are no longer found after the Miocene. In the last few million years Pliohippus evolved into today's modern horse (Equus).

This fossil record shows how a small forest dwelling animal evolved into a large animal adapted to its habitat. Take a look at the above image and notice the subtle changes in leg structure that occured during horse evolution.

Here's a similar example showing the evolution of horse molars (back teeth). These are drawn almost to scale. Notice how the teeth of Hyracotherium were small and with very few ridges. That's the teeth of an omnivore, an animal that eats a variety of food. As the "prehorse" evolved into a grassland creature it had to eat grass (surprise!) and its teeth evolved to best handle the coarse, gritty texture of grass. Today's modern horse has very large teeth well adapted to the heavy chewing required for an animal living on a diet of grass. Of course there is much more fossil evidence but I think we will leave it there. I hope I have convinced you that there is plenty of evidence to support Darwin's theory of evolution by means of Natural Selections. I hope you agree that Paley's theory of Natural Theology (a "designer") and Lamarck's theory of the Inheritance of Acquired Characteristics have been disproven by the evidence. Natural Selection is sufficient for explaining adaptations and a great deal more. You have completed Lesson 3. Read through you notes and, when you are ready, go to your self assessment. Alternatively, you can return to the beginning, take a break and take the self assesment later.