Fossils chapter 10

• Half-life: The time taken for half the amount of a radioisotope to break down into its products.
• radioisotope: An unstable atom of a specific element that breaks down into a predictable and stable product.
• sedimentary rock:  Rock that has formed through the accumulation of sediment and hardened under pressure.
• transitional fossil: A fossil that exhibits characteristics that are common to both its ancestor group and its descendant group
• radiocarbon dating: A process for determining the age of a fossil by measuring the changing ratio of two different carbon isotopes.
• geological timescales: A system for chronologically dating different sedimentary rock strata using known time frames, such as periods, eras, or eons.
• index fossil: A fossil that, due to the short existence and wide geographic distribution of its species, is used to define and quickly identify particular geographical timeframes.
• law of fossil succession: A key principle of the fossil record, states that fossils of the same age will be in the same layer of sedimentary rock, and fossils found in a higher or lower sedimentary layer will be younger or older respectively.
• permineralisation: A process of fossilisation where mineral deposits, typically carried by water, fill the spaces within organic tissue and form rock-like relics of an organism.
• The fossil record is arranged in chronological order and helps us map the history of life on Earth, placing species in the appropriate geologic time frame.

  • Fossils are the preserved remains or traces of an organism
  • Fossils can provide information about the structure, behaviour or evolution of that organism
  • The fossil record is incomplete
• Condition that promote fossilisation: 

  • Physical protection from scavengers and decomposers (fungi, bacteria)
  • Areas of rapid sediment accumulation
  • Constant cool temperatures
  • Low oxygen availability
  • Low light exposure
• Types of fossils

  • Mould: fossil formed when a living thing decomposes underneath sediment, creating a cavity in the shape of the dead organism
  • Cast: fossil formed when a mould fossil is filled with sediment 
  • Permineralised: fossil formed when mineral-rich groundwater deposits minerals like silica and calcite into organic material, creating a mineral relic
  • Trace fossil: fossil or structure indicating the presence of organisms, rather than the organism themselves. Example: nests, footprints, burrows.
• Formation of fossils:

  Sediment -> Compaction -> Cementation -> Sedimentary rock
• relative dating a dating technique used to determine the relative age of a fossil by comparing its position to other fossils or rock in surrounding rock strata (layers)
• The law of  fossil succession does not necessarily mean we need another fossil to determine the relative age of another fossil, rather we only need to determine the age pf the rock stratum in which the fossil was found. 

  Scientists are able to assign time periods for each layer of stratum, this is considered the geological timescale.
• Index fossils:

  • Help determine the relative age of a new fossil
  • Come from an organism that was geographically widespread and existed for a short, but precisely known time (preferably a fossil found in one layer of stratum)
  • Enable researchers to quickly and easily define the relative age of a target fossil
• Transitional fossils:

  • Assists relative dating by demonstrating the different forms of a particular genus that occurred along the genus’ evolutionary pathway.
• Homologous structures

  • Homologous structures are organs or skeletal elements of organisms that, due to their similarity, suggest their connection to a common ancestor. 
  • These structures do not have to look the same, or have the same function but must have similarities in the underlying structure
• Divergent evolution

  Divergent evolution is when two (or more) species evolve from a common ancestor.
• Analogous structure

  Analogous structures have the same function in distantly related organisms but do not share the same underlying structure. 

  The structures are similar because they evolved to do the same job, not because they were inherited from a common ancestor.

  Analogous structures are evidence of convergent evolution.
• Convergent evolution

  Convergent evolution is when different organisms independently evolve similar traits due to similar selection pressures.
• Analogous structures or Homologous structures

  A) analogous

  B) homologous

  
• Divergent vs convergent evolution

  A) divergent

  B) convergent

  
• Vestigial structures 

  Structures that have no apparent function and appear to be residual parts from a past ancestor are called vestigial structures. 

  Examples of vestigial structures include the human appendix, the pelvic bone of a snake, and the wings of flightless birds.
• Amino Acid sequence similarity

  Comparing amino acid sequences is another means of determining how related different organisms are.
• • Molecular homology is the study of the similarities between organisms at a DNA and amino acid level to determine their relatedness
  • The more similar the sequences are the more closely related the organisms are.
  • We analyse proteins from conserved genes which are found in a number of different species.
  • Conserved genes are genes that remain largely unchanged throughout evolution, and are found across the genome’s of different species and has a significant role in survival.
• Haemoglobin 

  (protein found in red blood cells that carries oxygen from the lungs to the cells around the body)

  Researchers can assess the number of number of amino acid differences between the chains of different organisms and conclude on their degree of relatedness.
• Cytochrome c 

  (an enzyme present in the mitochondria that consists of 104 amino acids which are encoded by a conserved gene in mitochondrial DNA)
• DNA sequence similarity

  DNA sequences of different species can be compared to determine the evolutionary relatedness between them.

  We can compare the differences in the DNA sequences of different species simply by looking at the order of these bases at corresponding gene regions.
• • A limitation to analysing amino acid sequences is that closely related species are likely to share similar sequences for certain proteins.
  • Therefore, scientists determine relatedness by comparing nucleotide sequences, looking for silent mutations that, due to the redundancy of the genetic code, may have accumulated without altering the amino acid sequence.
• A phylogenetic tree is a branching diagram that represents evolutionary relationships among different groups of organisms that have diverged from a common ancestor.

  Every phylogenetic tree is an hypothesis about relationships

  Phylogenetic trees are useful for showing:

  • Relatedness between groups
  • Timelines of species evolving from a common ancestor
  • Shared characteristics of different groups
• Interpreting phylogenetic trees

  • When interpreting a phylogenetic tree in relation to one taxon, trace where the node splits off, to determine the other closest related taxon
  • Phylogenetic trees can include a timescale to show the time points of divergence events - the branch length represents time.
• Uncertainty in phylogenetic trees

  • The lack of a node means the exact divergence point is unknown
  • The break means a possible ancestor but there is no evidence of a transitional fossil to support this
  • A short leaf that does not reach the end of the tree, meaning the species is extinct
  • Nodes splitting into multiple lineages indicates that it is unclear which species diverged first from the others - this can occur from insufficient evidence or if two speciation events occurred closely together (adaptive radiation)
• Using the fossil record: absolute dating

  • Used to calculate the absolute age of a fossil. Absolute age is an estimate of the age of a fossil or rock.
  • Radioactive dating involves comparing the ratio of radioactive isotopes (radioisotopes) inside the fossil to the relatively stable around in the atmosphere. 
• Radiometric dating is based on:

  1. Radioisotopes are unstable elements that will break down over time into a more stable product. For example, carbon-14 (a radioisotope) will break down into nitrogen-14.
  2. While these radioisotopes can break down at any point, on average the rate of breakdown is constant and can be modelled. We model this breakdown by calculating the half-life of the isotope
  3. Half-life describes the amount of time before half of the mass of a radioisotope is broken down into predictable and stable products.
• Scientists can use different radioisotopes to date a fossil. 

  Regardless of the radioisotope used, scientists will measure how much of it is present in the fossil versus how much of its breakdown product is present. 

  This ratio can be used to work out the time that would have needed to have passed to facilitate this amount of breakdown, which is the age of the fossil
• Radiocarbon dating: a form of absolute dating used to determine the age of a fossil my measuring the properties of radiocarbon (carbon-14 to nitrogen-14)
• The principle of carbon dating:

  • All living things contain carbon. Carbon exists in ratios of two isotopes - carbon-12 (stable) and carbon-14 (radioactive isotope). The ratio of these two isotopes will be the same as the ratio in the atmosphere, given that carbon is constantly being cycled between the organism and its environment while its alive.
  • When the organism died, its carbon-14 will decay and break down into nitrogen-14. Carbon in the dead organism will not be replaced therefore the amount of carbon-12 remains the same, while carbon-14 causes the ratio to change
• Principles of carbon dating

  • Scientists can measure the amount of carbon-14 present in the fossil and determine how long ago it died. By comparing c-14 : c-12 ratio in the fossil to the ratio of c-14 : c-12 in the atmosphere. 
  • The longer ago the organism died, the less carbon-14 will be present.