Chapter 6Life on Earth: What do

Fossils Reveal?

Fossils

Fossils are the remains or traces of ancient life which have been preserved by natural causes in the Earth's crust.

Fossils include both the remains of organisms (such as bones or shells), and the traces of organisms (such as tracks, trails, and burrows - called trace fossils).

Baby mammoth preserved by freezing is weathering out of permafrost in Siberia.

Fossil Preservation To become preserved as a fossil, an organism must: • Have preservable parts. Bones, shells, teeth, wood

are more readily preserved than soft parts. • Be buried by sediment to protect the organism from

scavengers and decay. • Escape physical, chemical, and biological destruction

after burial (bioturbation, dissolution, metamorphism, or erosion).

Upper torso of man preserved in peat bog. Lower half destroyed by harvesting machinery.

Fossil Preservation

Organisms do not all have an equal chance of being preserved.

The organism must live in a suitable environment.

Marine and transitional environments are more favorable for fossil preservation. Higher rate of sediment deposition.

Types of Fossil Preservation

1. Chemical Alteration of Hard Parts2. Imprints of Hard Parts in Sediment3. Preservation of Unaltered Soft Parts4. Trace fossils or Ichnofossils5. Preservation of Unaltered Hard Parts

Crawling, resting, dwelling, grazing and feeding traces.

Preservation of Unaltered Hard Parts

The shells of invertebrates and single-celled organisms, vertebrate bones and teeth:

a. Calcite (echinoderms and forams) b. Aragonite (clams, snails, modern corals) c. Phosphate (bones, teeth, conodonts, fish scales) d. Silica (diatoms, radiolarians, some sponges) e. Organic matter (insects, pollen, spores, wood, fur)

Chemical Alteration of Hard Parts

a. Permineralization - filling of tiny pores b. Replacement - molecule-by-molecule substitution

of one mineral for another (silica or pyrite replacing calcite)

c. Recrystallization - aragonite alters to calcite

d. Carbonization - soft tissues preserved as a thin carbon film (ferns in shale)

Fossil wasp, victim of a volcano and buried in lake sediment 30 mya, Florissant, Colorado

Imprints of Hard Parts in SedimentThis shows an internal mold of a marine snail. An internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull.

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This shows an external mold and cast of a trilobite that formed in a nodule of calcareous shale.

Preservation of Unaltered Soft Parts

• Freezing• Desiccation• Preservation in amber • Preservation in tar• Preservation in peat bogs

Amber is a form of tree resin -- exuded as a protective mechanism against disease and insect infestation -- that has hardened and been preserved in the earth's crust for millions of years. Often regarded as a gem, amber is actually an organic substance whose structure has changed very little over time, unlike that of other fossilized material, in which organic matter is replaced with minerals.Because amber oxidizes and degrades when exposed to oxygen, it is preserved only under special conditions. Thus it is almost always found in dense, wet sediments, such as clay and sand that formed at the bottom of an ancient lagoon or a river delta.Amber has preserved ancient life to such infinitesimal detail that it even captures fragments of DNA of the organisms entrapped in it. A wide variety of creatures has been found in Dominican amber Scientists are able to reconstruct this ancient, 25 million year old ecosystem, with amazing intricacy.

Trace fossils or Ichnofossils

• Tracks• Trails • Burrows – in soft sediment• Borings – in hard material• Root marks

• Nests • Eggs• Coprolites • Bite marks

Markings in the sediment made by the activities of organisms

Trace fossils or Ichnofossils

Trace fossils provide information about ancient water depths, paleocurrents, availability of food, and sediment deposition rates.

Tracks can provide information on foot structure, number of legs, leg length, speed, herding behavior, and interactions.

Biological classification

A system of binomial nomenclature (i.e., two names) is used to name organisms.

The first of the two names is the genus and the second name is the species.

Genus and species names are underlined or italicized.

Genus is capitalized, but species is not.

The Species

A group of organisms that have structural, functional, and developmental similarities, and that are able to interbreed and produce fertile offspring.

The species is the fundamental unit of biological classification.

The Species

Different species do not interbreed under natural conditions. Reproductive barriers between species prevent interbreeding.

Closely related (but different) species, such as the horse and the donkey CAN interbreed, but the offspring are usually sterile (unable to reproduce).

Cells All organisms are composed of cells. • Eukaryotic cells have a nucleus (or nuclei) and

organelles. – Organisms with this type of cell are called eukaryotes

(Domain Eukarya). • Prokaryotic cells have no nucleus or organelles.

– Organisms with this type of cell are called prokaryotes (Domain Archaea and Domain Bacteria).

Taxonomy

Organisms are grouped based on their similarities into taxonomic groups or taxa. Domain Kingdom

Phylum (plural = phyla)ClassOrderFamilyGenus (plural = genera)Species (singular and plural)

Classification of the human Domain Eukarya

Kingdom AnimaliaPhylum ChordataClass MammaliaOrder PrimatesFamily HominidaeGenus HomoSpecies sapiens

There are six kingdoms of organisms, grouped into three domains

1. Domain Eukarya

2. Domain Bacteria

3. Domain Archaea

Domain Eukarya

Organisms with eukaryotic cells (cells with a nucleus) • Kingdom Animalia (animals) • Kingdom Plantae (plants) • Kingdom Fungi (mushrooms, fungus) • Kingdom Protista (single-celled organisms)

Domain Bacteria

Organisms with prokaryotic cells (cells without a nucleus)

• Kindgom Eubacteria (bacteria and cyanobacteria or blue-green algae)

Domain Archaea

Organisms with prokaryotic cells, but which are very unusual and quite different from Bacteria. Archaea tend to live under extreme conditions of heat, salinity, acidity.

• Kingdom Archaebacteria

Evolution

Evolution = change • Organic evolution refers to changes in

populations • In biology, evolution is the "great unifying

theory" for understanding the history of life.

Evolution

Plants and animals living today are different from their ancestors because of evolution. They differ in appearance, genetic characteristics, body chemistry, and in the way they function.

These differences appear to be a response to changes in the environment and competition for food.

Lamarck's Hypothesis of Evolution

Jean Baptiste Lamarck (1744-1829) observed lines of descent from older fossils to more recent ones, and to living forms.

He correctly concluded that all species are descended from other species.

Lamarck's Hypothesis of Evolution

Lamarck assumed that new structures in an organism appear because of the needs or " inner want " of the organism.

Structures acquired in this way were thought to be somehow inherited by later generations - inheritance of acquired traits.

The idea was challenged because there was no way to test for the presence of an "inner want."

Lamarck's Hypothesis of Evolution

Lamarck also suggested that unused body parts would not be inherited by succeeding generations.

The hypothesis was tested and rejected after an experiment in which the tails were cut from mice for twenty generations. The offspring still had tails.

Similarly, circumcision has been practiced for more than 4000 years with no change among newborn males.

Darwin's Natural Selection

Charles Darwin and Alfred Wallace were the first scientists to assemble a large body of convincing observational evidence in support of evolution.

They proposed a mechanism for evolution which Darwin called natural selection.

Darwin's Natural Selection Natural selection is based on the following observations: 1. More offspring are produced than can survive to

maturity. 2. Variations exist among the offspring. 3. Offspring must compete with one another for food,

habitat, and mates. 4. Offspring with the most favorable characteristics are

more likely to survive to reproduce. 5. Beneficial traits are passed on to the next generation.

Darwin's Natural Selection

Darwin's theory was unable to explain WHY offspring exhibited variability.

This was to come many years later, when scientists determined that genetics is the cause of these variations.

This principle can be stated as: " the survival of the fittest."

Inheritance, Genes, and DNA

Gregor Mendel (1822-1884) demonstrated the mechanism by which traits are passed to offspring through his experiments with garden peas. His findings were published in an obscure journal and not recognized by the scientific community until 1900.

Inheritance, Genes, and DNA

Mendel discovered that heredity in plants is determined by what we now call genes. Genes are recombined during fertilization.

Genes are linked together to form chromosomes.

Mendel is known as the "Father of Genetics."

Chromosomes and DNA

• Within the nucleus of each of our cells are chromosomes.

• Chromosomes consist of long DNA molecules (deoxyribonucleic acid).

• Genes are the parts of the DNA molecule that transmit hereditary traits.

The DNA molecule consists of two parallel strands, which resemble a twisted ladder.

The twisted strands are phosphate and sugar compounds, linked with nitrogenous bases (adenine, thimine, guanine, and cytosine).

DNA

The structure of the DNA molecule was discovered by Watson and Crick in 1953.

DNA carries chemically coded information from generation to generation, providing instructions for growth, development, and functioning.

Reproduction and Cell Division

Reproduction in organisms may be: • Sexual • Asexual • Alternation of sexual and asexual generations

All reproductive methods involve cell division.

Genetic Recombination

New combinations of chromosomes result through sexual reproduction. One of each pair of chromosomes is inherited from each parent.

This sexual genetic recombination leads to variability within the species.

Asexual reproduction• Binary fission - single-celled organisms that divide to

form two organisms• Budding – a bud forms on the parent that may:

– Separate to grow into an isolated individual, or– Remain attached to the parent (colonial organisms).– Budding occurs in some unicellular and some

multicellular organisms. • Spores shed by the parent (as in a seedless plant like

moss or ferns) that germinate and produce male and female sex cells (leading to alternation of sexual and asexual generations).

Diploid and Haploid Cells

In a human cell there are 23 pairs of chromosomes. One of these pairs determines the sex of the individual.

• Diploid cells - cells with paired chromosomes.

• Haploid cells - sex cells (or gametes) with only one half of a pair of chromosomes. Example: egg cells or sperm cells

Cell division

• Mitosis - Division of body cells of sexual organisms. Produces new diploid cells with identical chromosomes to the parent cells.

• Meiosis - Division of cells to form gametes or sex cells (haploid cells), with half of chromosomal set of the parent cell; occurs in a two-step process, producing four haploid gametes.

Recombination of Genes

• Fertilized egg forms when two gametes (egg and sperm) combine. Fertilized egg has paired chromosomes (diploid cell).

• Variation occurs because of the sexual recombination of genes.

• Genes are recombined in each successive generation.

Mutations

• Mutations are chemical changes to the DNA molecule.

• Mutations can be caused by: – Chemicals (including certain drugs), – Radiation (including cosmic radiation, ultraviolet

light, and gamma rays). • Mutations may also occur spontaneously

without a specific causative agent.

Mutations

Mutations may occur in any cell, but mutations in sex cells will be passed on to succeeding generations.

Mutations produce much of the variability on which natural selection operates.

Causes of Evolution

Evolution may involve change from three different sources:

• Mutations • Gene recombination as a result of sexual

reproduction • Natural selection