WEIT Chap2

What is a fossil?

A fossil looks like a stone, but it is not really a stone. It is a memory of an organism that lived a long time ago. The addition of a time factor and chemical reactions formed it. We will explain precisely how a fossil is formed later.
Hence in fossils, we can see the body shape of the organism. This allows scientists to see how the ancestor of such organism looked like and in which way it evolved.
Fossils can be found nearly everywhere where there are stones.

What information can we find in fossils ?

Fossils differ in age. It is even the main criterion to classify them. The fossil record teaches us 3 things.

How can we determine the age of rocks ?

Since about 1945, scientists are able to determine the age of rocks. They use radioactivity. Indeed, in rocks there are radioactive elements (called radioisotopes) that disappear. The half-life is the name given for the time in which half of the isotopes disappear. This period of time can be known thanks to scientific methods. When we know the half-life of a rock, it is possible to compute its age.
For example, in old rocks there are often unranium-235. Rocks that have a half-life of 5,730 years are usually full of Carbon-14, because this element was very present in rocks at this period of time.

Gradual changes within lineages, splitting of lineages and the fact that transitional forms exist proves us that the theory of evolution is true. Transitional forms appear exactly where we expect them to appear,which means that we find them after their ancestors and before the actual forms of animals.In other words, they fill in the gap between what we see nowadays and what had lived so many years ago.

Evolution means taking the old and remodeling it into the new, because natural selection can only change what already exists. It cannot produce anything new.

Between myths and truth

During the Greek period, people didn't know what these tracks were. This probably gave rise to mythology.

Much later, in the nineteenth century, people thought fossils were made by supernatural forces, results of religious cataclysm or unknown species of far living animals.

How are fossils formed?

The formation of a fossil is called fossilization. It begins as the rest of a dead organism (animal or plant) falls in water or in a wet environment. The organism's body's decompose. As this happens, the rest of the animal is covered by sediments during ages.
The sediments and the bones are transformed in a kind of stone which wears the track of the organism. Erosion and extreme climatic phenomena reveals the fossil. That is why we can discover animals that were born thousands or millions of years ago!

Why are fossils important for Evolution study?

Fossils are the evidence of evolution. Without them, we would have only a "theoric" sketch of how species evolved throughout the centuries.

Two different case studies about evolution:

From the earth to the air...

The earlier species at the origin of birds were reptils. As we know, these animals walked and the it is after many steps that they began to fly. The first step of the transformaton was gliding. The arms grown bigger and bigger to allow some animals to jump from tree to tree. This was a first step that began to take place around 200 million years ago. Following this step, the beginning of the appearence of feathers on the arms of these gliding animals was a second step to achieved the process of flying. These feathers were asymetrical (as the wings of a plane) to allow the animals to begin like a flight. This step took a long time; the fisrt feathers that appeared were so small that it was not useful at all to fly but we know that every transformation and evolution is the result of a long effort.
Following the "feathers criteria", fossils show some "birdlike behaviour" in certain dinosaurs: some putting their heads under their arms while sleeping, other having nest of eggs.
In addition of these steps, natural selection made the rest. Reptils began to have wings that were uselful to run quicker to escape predators, then began to make like jumps thanks to their wings. Finally, flying was possible and that is how some reptils became birds!

From the earth to the water...

We have an excellent fossil record of whales, so it is one of our best examples of evolution.

Whales are mammals. They have vestigial traits that tell us that their ancestors lived on land. For instance, whales have hindlegs that are completely useless but show that their ancestors needed legs. In brief, the examination of fossils tells us that their ancestor was an animal which was terrestrial but probably went into the water in order to graze or flee its enemies. Little by little, the descendants spent more time into the water. There, mutations occurred: they acquired new features that allowed them to spend more time in the water and lost those which enable them to live on land (such as legs). Eventually, these animals became fully aquatic. There are several theories about why some animals went back into the water so many years after their ancestors moved from water to land. One of those theories is linked to the fact that since the dinosaurs that lived in the water, ate the aquatic mammals and competed with them for food disappeared, the water became less hostile (free from predators and full of food) and attracted terrestrial mammals.

Evolution in fossils

Evolution ca be observed in fossils with a gradual view. In fact, the different layers of sediments contains fossils in a way that allows us to see the different step of evolution. Because, actually, the oldest species remain in the deepers layers of earth. Let's take the example of the Globorotalia conoidea. Throughout the time, the number of chambers... back to Evolution_3BIbDF01-02_10/11
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