Fossil Evidence and Comparative Evidence Notes

Lesson 1: Fossil Evidence

• A one-month-old woolly mammoth, approximately 42,000 years old, was preserved in sediments due to bacterial acids and permafrost.

• The discovery of the mammoth's intact body allowed scientists to identify its last meal.

• A fossil is defined as any preserved evidence of an organism, including plants, animals, fungi, protists, and prokaryotes, which can be entire organisms, parts, or traces like footprints.

• A fossil is the remains or trace of an organism that formed at least 10,000 years ago.

• Darwin's observation of glyptodont fossils in South America suggested a relationship between these extinct animals and living armadillos, based on shared features like hard coverings.

The Fossil Record

• The fossil record is an important source of evidence for evolution, providing a record of species that lived long ago and showing common ancestry among groups.

• Fossils illustrate similarities between ancient and current species and demonstrate that some species, like the horseshoe crab, have remained unchanged for millions of years.

• Paleontology is the study of fossils and other clues to past life; paleontologists infer information about diet, environment, behavior, and ancestral relationships from fossil evidence.

Fossil Formation

• Fossils typically form in sedimentary rock.

• The process involves an organism dying and being buried in sediments.

• Sediments build up over time, encasing the remains.

• Minerals replace organic matter or fill pore spaces.

• In some cases, the organism decays, leaving an impression.

• The sediments harden into rock, and erosion can expose the fossil.

• Fossils do not form in igneous or metamorphic rock due to the heat and pressure involved in their formation.

Variations in Fossilization

• Impressions in mud can leave a mold, which, when filled with sedimentary rock, becomes a cast.

• Petrification occurs as pores in bones are filled with minerals, or tissues are replaced with minerals (replacement).

• Trace fossils, such as footprints, can also form.

• Amber, hardened tree sap, can preserve entire small organisms.

• Extreme conditions like freezing temperatures or dry locations can prevent decay, preserving entire organisms like woolly mammoths in permafrost.

Incomplete Fossil Record

• The fossil record is incomplete; most species that have lived are now extinct, and only a small percentage are preserved as fossils.

• Most organisms decompose before fossilization can occur; rapid burial in sediment is essential for preservation.

• Aquatic organisms are more frequently preserved due to constant sediment settling in aquatic environments.

• Land organisms can be fossilized in river flood zones and deltas, where sediments are carried by water.

Factors Affecting Fossil Representation

• Soft-bodied animals are less likely to fossilize than those with hard skeletons or shells, as soft tissues decompose quickly.

• Fossils may be lost due to erosion or remain undiscovered because they are deeply buried.

• Geological conditions influence fossil formation and preservation, leading to variations in the fossil record, such as sudden appearances or long periods of stasis.

Transitional Fossils

• Transitional fossils exhibit intermediate forms between species, supporting evolutionary relationships.

• Archaeopteryx, one of the first birds, shows characteristics of both birds and dinosaurs.

• Derived traits are newly evolved features, while ancestral traits are primitive features that appear in ancestral forms.

Estimating Fossil Age

• Relative dating determines the age of fossils by comparing them with surrounding rocks and fossils.

• Absolute dating uses chemical testing to determine the specific time period in which organisms lived.

Relative Dating

• Relative dating involves comparing rock layers to determine the order of fossil formation.

• The principle of superposition states that younger rock layers are deposited on top of older layers, thus fossils in lower layers are older.

• Geologists use index fossils, which are distinctive and abundant, to date rock layers.

• For example, trilobites, a marine arthropod, indicate that a rock layer formed approximately 540 to 250 million years ago.

Absolute Dating

• Absolute dating uses chemical testing, such as radiometric dating, to measure the age of a rock by analyzing the decay of radioactive isotopes.

• An isotope is a form of an element with the same atomic number but a different mass number.

• Radiometric dating requires knowing the half-life of the isotope which is the time it takes for half of the original isotope to decay.

• Uranium 238 (U238) decays to Lead 206 (Pb206) with a half-life of 4510 million years.

• The ratio of U238 to Pb206 is used to determine the age of a sample.

• Radioactive isotopes are found in igneous and metamorphic rocks, which are used to date associated fossil-bearing sedimentary rocks.

Carbon Dating

• Carbon dating uses the decay of the weakly radioactive carbon-14 (C-14) to calculate the age of carbon-containing materials but is only accurate for materials less than 60,000 years old.

• Tree ring and ice core dating provide records of annual climate conditions, aiding in the interpretation of evolutionary changes.

Lesson 2: Comparative Evidence

• Oilbirds use echolocation, a trait shared with bats and dolphins, to navigate in dark caves, which highlights the concept of comparative evidence in evolution.

• Biogeography studies the distribution of plants and animals around the world, providing evidence of common ancestry.

• Darwin observed that South American animals were more similar to each other than to animals in similar environments in Europe, suggesting common ancestry.

• Migration patterns influence diversity in isolated islands, with plants more easily migrating than animals.

Characters

• Characters are inherited features that vary among species and can be anatomical, developmental, or molecular.
• Biologists look for homologies, which are similarities due to shared ancestry, when comparing characters.

Anatomical Characters

• Shared anatomical characters suggest common ancestry; for example, hawks and eagles share keen eyesight and taloned feet.

• Homologous structures are anatomical structures inherited from a common ancestor.

• Animal limbs are examples of homologous structures, showing similar construction despite different functions.

Vestigial Structures

• Vestigial structures have reduced or no function in an adult organism but were functional in related or ancestral organisms.

• Kiwi bird wings and the human tailbone are examples of vestigial structures.

Analogous Structures

• Analogous structures have the same function and superficial similarity but are not inherited from a common ancestor.

• Insect wings and bird wings are analogous structures.

• Homologous characters indicate anatomical similarity inherited from a common ancestor, while analogous characters do not indicate a close evolutionary relationship.