1. Organisms of the Past

2. A fossil is evidence that an organism lived in the past.
Scientists can trace the ancestors using skulls, teeth, and leg bones and compare it to similar animals living today.
Another thing that can be learned is the age of a fossil compared with other fossils.
You can study the rock layer in which a fossil is found.
The oldest fossils are found in the oldest rock layers, which are the bottom.
Younger fossils are found in the later, upper rocks.

3. Scientists can compare organism’s bones to compare what is similar and different about organisms.
It can also tell about the organism’s origin and ancestry.
Another way to identify similarities in organisms is to examine the embryos.
An embryo is an undeveloped animal or plant.
Organisms that are extinct are no longer alive on Earth.
To classify them, scientists can compare fossil embryos with each other and with modern embryos.
Leftover structures can also indicate relationships between organisms.
For example, human adults have a tailbone at the end of the spine.

4. There have been mass extinctions of many organisms.
It’s when many different species die out at the same time.
The theories of dinosaur extinction are:
a meteorite hitting the Earth
dust clouds from volcanic eruptions
spread of disease
insufficient food supply  

5. People are also to blame for extinction.
They use pesticides and chemicals to destroy animal habitats.
They hunt and fish which can deplete resources.
Species can become endangered, or in danger of being extinct.