Fossils: An Introduction

The Theory of Evolution by Natural Selection

• Evolution is a theory (set of related, well-supported hypotheses) that explains the diversity of living organisms alive today and those that have lived before.

• Natural selection is the mechanism by which that theory proposes species change over time, form new species, and sometimes go extinct.

• The support for evolution by natural selection includes 2 major lines of physical evidence:– Earth’s fossil record– Genetics

Fossils • Fossils are the preserved remains or traces of ancient organisms.

Types of Fossils

• Permineralization: the most common way fossils are made. Remains are covered by sediment and compressed and mineralized into rock.

Other Types of Fossils

• Amber• Ice• Trace fossils• Many

more…

A Taste of Geology• Fossils are found only in

sedimentary rocks.

• As soil and rocks are weathered, particles called sediment build up at the bottom of the water.

• Increased pressure crushes the particles together, making sedimentary rocks

Law of Superposition

• The oldest rocks are on the bottom of the column, the newest rocks are on top

Geological Time• Radioactive dating is used to

measure rocks, and fossils found in them:

• The mathematical expression that relates radioactive decay to geologic time, is:– D = D0 + N(eλt − 1) where– t is age of the sample, D is number of atoms of the

daughter isotope in the sample, D0 is number of atoms of the daughter isotope in the original composition, N is number of atoms of the parent isotope in the sample, and λ is the decay constant of the parent isotope, equal to the inverse of the radioactive half-life of the parent isotope[14] times the natural logarithm of 2.

• Earth’s geological history is long and complex… it is divided into hundreds of different eras, ages, epochs, etc.

So… we’ve got Fossils, and We Know How Old they Are

• We can combine these two things to figure out the history of life on Earth!!!

Phylogenetic Trees• Phylogenetic trees are used to represent the most probable evolutionary

relationships between a group of organisms.• They can be based on

– Genetics– Fossils– Morphology (structure)

Two types of Evolution are Found in the Fossil Record • Gradual

evolution is slow change over long periods.

• Punctuated equilibrium means big changes can happen fast (in geology, fast means 100,000s of years!)

How do species change over time?• Phyletic speciation: gradual changes in a single

species• Divergent Speciation: one species splits into two

or more.

• Both types appear in the fossil record