Fossils

History of LIfe

•3.8bya – First life on Earth (prokaryotes)

•2.4bya – Oxygen rich

•2bya – first eukaryotes

•900mya – multicellular organisms (soft bodied)

•535mya – hard shelled organisms (Cambrian)

•400mya – first vertebrates

•230mya - dinosaurs

•2mya – genus Homo

•200,000 ya – Homo sapiens

What is fossil record?

provides information about the history of life on Earth.

Fossils

Preserved remains of the organisms or anything that shows (directly or indirectly)

How can fossils appear as

○Moulds

○Cast

○Mineralised

○Trace fossils

How are fossils formed?

1.Remnants of an organism is rapidly covered by sediment – limiting exposure to oxygen, microorganisms (bacteria) and scavengers

2.Over time the sedimentary layers build, increasing the pressure to form sedimentary rock.

3.Within the rock, the fossilised remains form

Conditions for fossilisation

•Rapid sediment cover

•Pressure from sediment forms sedimentary rock

•Removal of oxygen and bacteria for slow decay

•Cool temperatures, low light availability

•Undisturbed for many years from scavengers and decomposers

Fossil dating techniques

• relative dating

• absolute dating

Relative Dating

•dating technique used to determine the relative age of a fossil by comparing its position to other fossils or rock in surrounding rock layers (strata)

•Exact age is not provided

•Used if radioisotopes are reduced

Relative dating: Law of fossil succession

•Sedimentary rock is formed by the accumulation of sedimentary layers on top of each other

•The layer on the bottom will be older than those positioned above

Relative dating: Geographical timescales

•Scientists can use the stratum (layers) to determine a particular period of time

Relative dating: Index fossils

•Index fossils are a group of widespread fossils which existed for a short period and have a known age.

•They can be used as a reference to easily determine the age of known fossils

Characteristics of index fossils

•Abundant

•Existed in many geographical areas

•Lived within a known short period of time

•Easy to distinguish (physically distinctive)

Relative Dating: transitional fossils

•Transitional fossils are fossils that shows traits that are common to both its ancestral group and its descendant group.

•They demonstrate evolutionary pathway

Absolute dating/ Radiometric Dating

•Examines the amount of radioactive material left in a fossil by reheating the rock and measuring the amount of escaping gas

•Used to determine the age (in years) of the fossil or rock

•Radioactive isotope breakdown into a more stable product overtime

•The older the rock, the less radioactive material left

Half Life

1.Radioisotopes are unstable elements that will breakdown overtime into a more stable product.

2.Rate of breakdown is by calculating the half-life of that radioisotope

3.The half-life is the amount of time the mass of the radioisotope is broken down into a stable form (half remains).

Examples of Radioisotopes

Relatedness: Structural Morphology

○The study of body structure to give insight into the relationship between species

Relatedness: Molecular Homology

○The study of DNA and amino acid sequences to determine relatedness between species

Relatedness between Species

●If species have a similar set of proteins, chromosomes or DNA sequences, it is said that they shared a recent common ancestor

Structural Morphology: Homologous Structures

are features found on different species that may look and function differently but have derived from a common ancestor.

Structural Morphology: Divergent Evolution

●They provide evidence of divergent evolution (share a recent common ancestor)

●Enough genetic differences accumulate to be classified as a different species

●Unable to interbreed

●Result of adapting to different selection pressures or genetic drift.

Structural Morphology: Analogous Structures

●Structures that serve similar biological functions but are not derived from a common ancestor.

Structural Morphology: Convergent Evolution

●Provide evidence of convergent evolution (distantly related species evolve due to similar selection pressures).

●Species do not share a recent common ancestor

●Evolution due to similar selection pressures

Structural Morphology: Vestigial Structures

●Vestigial structures have no apparent function but can be used to determine ancient ancestors.

Molecular Homology: Using Amino Acid sequences to determine relatedness

●Comparing protein structure and amino acid sequence to determine relatedness

●The bigger the difference in amino acid sequence, the longer the period of time that divergence would have occurred.

Molecular Homology: Hemoglobin

●Hb is involved in carrying oxygen

●Made up of 4 polypeptide chains consisting of 146 amino acids.

●Can be used to determine relatedness

Molecular Homology: Cytochrome C

●Enzyme present in mitochondria made up of 104 amino acids

●Assists in carrying electrons in aerobic and anaerobic reactions.

Molecular Homology: Using DNA sequence to determine relatedness

●Nucleotide differences between organisms is used to determine relatedness

●The greater percentage of DNA sequence in common, the more closely related

Molecular Homology: Amino acid sequence VS DNA sequence

●Due to degeneracy, DNA sequence is used to identify silent mutations

●However, amino acid sequences are easier to interpret and therefore used to determine relatedness between distantly related species.

Phylogenetic Trees

●A phylogenetic tree is a diagram that shows the evolutionary relationships between species.

Structure of Phylogenetic Trees

●Root: a line at the origin representing the earliest common ancestor.

●Branch: line on the phylogenetic tree

●Node: a point where the branch splits representing divergence

Leaf: the end of a branch representing present-day or extinct species

Features of Phylogenetic Tree

●Time of divergence is shown

●If point of divergence is recent, then the two are said to share a more recent common ancestor.

●Extinction is represented if the line stops prior to recent time.

●Interbreeding is shown when a line connects two species after divergence