Objectives:
After studying this material you should be able to:
- Explain what a fossil is and how it is formed.
- Explain the methods used to determine the age of a rock or fossil.
- Describe what is meant by transitional fossils.
- Describe how comparative anatomy and embryology provide clues to evolutionary relationships among species.
- Describe how vestigial organs provide clues to the organism's origin.
- Describe how the comparative analysis of DNA sequences can be used to trace evolutionary relationships.
- Explain what a phylogeny is.
- Describe how DNA from dead and/or extinct organisms can be used to determine their evolutionary relationships.
- Explain why evolution is considered both a scientific fact and a scientific theory.
Key Terms:
| fossils | relative dating | radiometric dating |
| half-life | comparative anatomy | vestigial organs |
| comparative embryology | molecular evolution | comparative DNA sequencing |
| phylogeny | PCR | amber |
What is a fossil? How is it formed?
- Trilobite fossil (400 mya)
- Archaeopteryx fossil (140 mya)
- Australopithecus afarensis (Lucy) (3.6 mya)
- Homo erectus (1.6 million-35,000 years ago)
- Fossils and Evolution from DeKalb College, Georgia.
- A fossil is evidence of past life.
- An organism, or its presence (tracks, trails, footprints, burrows), is preserved in rock (as a fossil).
- Impressions and mineralization (the replacement of parts of organisms by minerals)
- Most fossils are the hard parts of organisms (bones, teeth, shells); soft parts are rarely preserved.
- To be adequately preserved, an organism must be in an environment where it is protected from oxidation and bacterial decay. An aquatic environment, particularly one with a high sedimentation rate (swamps, tar pits), is best to preserve fossils.
- Fossils, starting as from far back as 3 bya, indicate that life evolved through great stretches of time and diversified.
Determining the age of a rock
- Relative dating techniques
- In a "normal" horizontal sequence of rocks (e.g., marine sedimentary), the oldest rock types will be on the bottom with successively younger rocks on top. Sediments are deposited gradually in a flat layer and are spread over a large area. (May not be useful in the rock has been folded.)
- Stratigraphic Illustration from Berkeley
- Geologic Time Scale. Developed in the 1800's from the relative dating of rocks and the fossils each layer contained. Major boundaries indicate mass extinctions.
- Index fossils - an assemblage of fossils that characterize a particular rock unit. Organisms have evolved and gone extinct through time. Fossil content can be used to help determine age of rock, and to correlate rocks from different localities.
- In a "normal" horizontal sequence of rocks (e.g., marine sedimentary), the oldest rock types will be on the bottom with successively younger rocks on top. Sediments are deposited gradually in a flat layer and are spread over a large area. (May not be useful in the rock has been folded.)
- Radiometric (absolute) dating techniques Be sure to check-out this one
- This method uses naturally-occurring radioactive isotopes. Radioisotopes decay at a constant rate to form stable (or daughter) isotopes. This rate of decay is measured by half-life (how long it takes for one-half of the parent radioactive material to decay to a daughter product). The ratio of parent isotope to daughter isotope in the rock reveals the number of half-lives, or length of time in years, that has elapsed. Think of radioactive elements as "geologic clocks."
Half Lives for Radioactive Elements
Radioactive Parent Stable Daughter Half life Potassium 40 Argon 40 1.3 billion yrs Rubidium 87 Strontium 87 48.8 billion yrs Thorium 232 Lead 208 14 billion years Uranium 235 Lead 207 704 million years Uranium 238 Lead 206 4.5 billion years Carbon 14 Carbon 12 5710 years
- This method uses naturally-occurring radioactive isotopes. Radioisotopes decay at a constant rate to form stable (or daughter) isotopes. This rate of decay is measured by half-life (how long it takes for one-half of the parent radioactive material to decay to a daughter product). The ratio of parent isotope to daughter isotope in the rock reveals the number of half-lives, or length of time in years, that has elapsed. Think of radioactive elements as "geologic clocks."
- Not all rocks can be dated absolutely, so a combination of techniques is used.
Transitional Fossils
- Is one that looks intermediate between two species or higher lineages. Ideally, the transitional fossil should be found stratigraphically between ancestor and descendant lineages.
- Thousands of such fossils exist, such as transitions from land animal to whale and from early ape to human.
- Trilobites
- Brachiopods
- Archaeopteryx, a mixture of bird and reptile features.
- Transitional Fossils from the Talk.Origins Archive
Comparative anatomy and embryology
- Homologous structures: Similar structures in different organisms having a common evolutionary origin. Example: Similarity of embryos of vertebrates suggest common ancestry. (See Life Magazine, Nov. 1996) Note that early human embryos have gill slits and rudimentary tails.
- All vertebrate skeletons consist of basically the same parts (see p. 394, text).
- Hominid pelvises, femurs, and feet
- Are birds really dinosaurs?
Vestigial Organs
- A structure that seems not to have a function in an organism but resembles a functional organ in another type of organism. For example, whales have useless pelvic bones and, occasionally, rear feet resembling those in other mammals. Some snakes have leg bones.
Molecular Evolution
"We are the products of the genes of our ancestors." All life forms based on DNA and 20 amino acids.
- Individual genes from different species can be sequenced and compared.
- These genes can come from the nuclear, mitochondrial, or chloroplast genomes.
- Mitochondrial DNA is maternally inherited (and have been used to trace human origins).
- Basically, the more similar the DNA the closer the evolutionary relationship (chimp and human DNA sequences are 98.4% similar).
- Chromosome banding patterns and protein sequences can also be compared.
- Patterns of restriction fragments (RFLP's), separated by electrophoresis and visualized by radioactive DNA probes, can also be compared.
Phylogeny of 8 species based on DNA sequencing
Interpretation:
- All species are evolutionarily related and share a common ancestor.
- Species are related based on the presence of shared and uniquely-derived point mutations.
- Relationships can be inferred (e.g., Species E is more closely related to Species F than to any other species; Species group E and F is more closely related to species group G and H than it is to any other species group).
In actuality, thousands of DNA nucleotides can be compared and computers are used to analyze the data and construct the phylogeny. The DNA used can be from any organism, living or dead (and from fossils too).
A phylogeny is a diagram that depicts the lineages, or evolutionary relationships, among species. Comparative anatomical, embryological, molecular, behavioral, physiological, chemical, geographical, and fossil data can all be used, together or separately, to construct a phylogeny.
"Jurassic Park" and extinct species
- See text, p. 349, for termites in 25 my amber.
- Ancient Bee in Amber
- Jurassic Park Bacterium. How it's done.
- Ancient nucleic acids from amber preserved insects
- Could we really bring back the dinosaurs like in Jurassic Park?
- Be sure to check-out this one 17-20 MYO Magnolia DNA sequencing from Talk.origins
Need a refresher on the PCR technique?
Evolution as a fact and a theory
Evolution--the process by which the genetic composition of a population changes over time--is a FACT.
- This process is all that is required to produce the diversity and similarity of all life on this planet today.
- Evolution has occurred; it still is occurring; it has been directly observed, documented, demonstrated, and described. Supporting evidence for it is overwhelming (and obtained from a wide range of scientific fields).
The mechanisms by which evolution occurs (e.g., natural selection, mutation, genetic drift) are presented as SCIENTIFIC THEORIES.
- Several theories have been proposed and debated. It is far from clear how evolution proceeds in every detail.
In summary, Darwin established the FACT of evolution, and proposed a THEORY, natural selection, to explain the mechanism of evolution.
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