Fossils and Evolution

The fossil record defines the subsequent long-term record of life’s evolution on planet Earth. That record is more complete in younger layers.

But the record is not complete! More than 250,000 species of eukaryote (cells with a nucleus) fossils have been collected and identified to date! But the record is not complete! In the billions of years that life has existed – there may have been 5-50 billion species of life! Why is the record so incomplete?

1. Paleontologists have not come close to sampling every cubic centimeter of sedimentary rock exposed. 2. Not all organisms are represented in the rock record., because of their preservation potential 3. The sequences of sedimentary layers that exists does NOT account for every minute at every location since the formation of Earth.

By studying fossils in sequences of layers, paleontologists are able to observe progressive changes in: - species - An assemblage of species through time And can determine when some species die out and other species appear

But the question remains to the RATE at which evolution takes place. Originally, it was assumed that evolution happened at a slow, constant rate – gradualism.

More recently, punctuated equilibrium has been suggested. - where evolution occurs very slowly for quite a while, and then, during a short period of time, it takes place rapidly!

Factors that could cause a sudden pulse of evolution: - a geologic catastrophe that causes extinction

- a sudden change in the Earth’s climate

- The sudden formation of new environments

- The isolation of breeding population

Extinction: when the last members of a species die Some geologic factors that cause extinction: Global climate change: - Changing habitats, if species cannot adapt or migrate, it will disappear

2. Tectonic activity - modify the distribution and area of habitats 3. Asteroid or comet impact - impact would blot out the Sun, impacting food chains 4. Voluminous volcanic eruption - incredible volumes of ash and gas alter climate

5. Appearance of a new predator or competitor - researchers suggest that this phenomenon explains the mass extinction that occurred during the past 200,000 years - due to US (humans)!