1. Tracing the appearance and development of life

• Sedimentary rock layers are deposited one on top of another, so in an undisturbed sequence the deeper layers are older than the ones above.
• Organisms whose fossils are found in deeper layers appeared on Earth earlier than those found in higher layers.
• Example fossils of invertebrates are found in deeper layers than fossils of vertebrates, so most invertebrates appeared before vertebrates.

• A mass extinction is an event in which a huge number of species die out in a relatively short time.
• The fossil record shows that certain layers contain many kinds of fossils, but the layers above them lack those fossils.
• For example, about 66 million years ago a mass extinction wiped out about 75% of all species on Earth, including most dinosaurs.
• Hypotheses for mass extinctions:

1. cosmic catastrophes such as the impact of a large meteorite (asteroid);
2. severe climate change and changes in the composition of the atmosphere;
3. massive volcanic activity;
4. changes in sea level and the changing positions and shapes of continents.

• Cause and effect.
• Cosmic catastrophes, severe climate change or massive volcanic activity in ancient times (causes) led to the extinction of many organisms (effect).

• Palaeontology (the study of fossils) works with anatomy and genetics to link ancient organisms to those alive today and shows how species branched from common ancestors.
• Example the modern Asian elephant is the closest living relative of the extinct woolly mammoth, since both come from a common ancestor.

• Skeletons of primitive whales in Wadi Al-Hitan (Fayoum) show that the area was once covered by an ancient sea.
• The Petrified Forest in Cairo shows that the region was once a humid forest, and the climate there has changed from humid to dry.

• Worked reasoning example 1. Petrified tree trunks are found in a dry desert.
• Trees need water, so the region was once moist with plenty of rain and rivers (a forest).
• Since then the climate changed to dry.

• Worked reasoning example 2. Marine shells are found in rocks high on a mountain.
• Marine organisms live only in seas; so the rock formed on an ancient sea floor, and was later lifted up when plates collided.