Fossils and evolution

• Fossils


  Preserved body, impressions or traces of a dead organism
• Fossils


  • Provide evidence of how life has changed over time (evolution)
  • Help scientists infer how Earth's surface has changed and give clues to what past environments were like
• How fossils form


  1. Sediment (e.g. sand, dirt or crushed rock) quickly covers dead organism

  2. Organism is thus not exposed to oxygen and scavengers which would make it decay faster

  3. Over time, sediment may be compressed to form sedimentary rock

  4. Parts of organism may decay and parts may be replaced by minerals from groundwater

  5. Sometimes all of the organism decays but a cavity in the form of the body is formed in rock forming a mould fossil

  6. Mould could also be filled by sediment creating a cast fossil
• Fossilisation Process


  • Mould Fossil
  • Cast Fossil
  • Iron sulphide cast of a shell from the Jurassic Period (England)
• Parts & Types of Organisms that Form Fossils


  • To be preserved as fossil, dead organism must decay very slowly
  • Soft parts of organisms decay must faster than hard parts. Therefore, extremely rare for soft parts to be preserved
  • Hard objects such as skeletons, shells, teeth and wood most commonly found as fossils
  • E.g. Fossils of dinosaurs, crabs and trees more likely to be found than fossils of slugs, mosses and algae
  • Sometimes whole organism becomes trapped in a substance which prevents bacteria accessing them, therefore preventing decay
  • Substances such as amber / resin (tree sap), ice or tar will preserve organism in almost same condition it died in
• Conditions for Fossilisation


  • Fossils more likely to be found in areas of rapid sediment accumulation, constant cool temperatures, low oxygen availability, low light exposure
  • These conditions mostly occur at the bottom of ocean, lake or river
  • So, organisms that lived in water more likely to be fossilised than ones that lived on land
  • Fossils are found in sedimentary rock and never in igneous or metamorphic rocks
  • Heat and forces needed to form igneous and metamorphic rocks destroy any traces of organisms in them
• The Fossil Record


  • List of all organisms that fossil evidence has been found for, including their location and relative age
  • Timeline which maps history of life on Earth, placing species in chronological order of existence since Earth's formation 4.5 billion years ago
• Evolution Theory from Fossil Record


  1. All living things on Earth evolved from a single-celled prokaryote (bacteria) that existed around 3.8 billion years ago

  2. Some generations of offspring of the bacteria evolved ability to photosynthesise and produce oxygen for atmosphere around 2.4 billion years ago

  3. Oxygen allowed other organisms that evolved to survive

  4. First multicellular organisms evolved around 900 million years ago

  5. First animals on land evolved 530 – 400 million years ago

  6. Mammals evolved 251 million years ago

  7. Human ancestors evolved 2 million years ago
• Sources of Evidence for Evolution


  • The fossil record
  • Comparative anatomy
  • Genetics and biochemistry
  • Geographic distribution of species
  • Embryology
• Fossil Record Evidence for Evolution


  • Early fossils are of fairly simple organisms and later ones are increasing complex suggesting new alleles and genes were developed by mutation
  • Fossil record shows that an increasing number of species have lived on Earth over time (increase in Biodiversity)
  • Transitional Fossils which show features of an ancestral species and a descendant species have been found (E.g. Archaeopteryx and Anchiornis – features indicate changing from dinosaurs to birds)
• Fossil Record Evidence for Evolution


  • Feathered dinosaur Anchiornis
• Comparative Anatomy


  Science of comparing physical structures of a species with others
• Homologous features


  Features that have same basic structure due to shared / common ancestor
• Homologous features


  • Pentadactyl limbs (limbs with 5 digits) - Humans, cat, whales and bats all have 5 digits at end of their limbs
• Homologous structures do not necessarily have same function in all species that share it
• The more alike 2 organisms are, the more genes they share due to a shared / common ancestor
• Genetics Evidence: DNA Structure


  • All living cells have same basic DNA structure and use same genetic code (ATCG)
  • Organisms that have similar anatomy show more genes in common than organisms that are less alike
  • E.g. 96% of genes in humans and chimpanzees are identical suggesting a common ancestor
  • Harder to compare DNA between fossils as DNA will usually not last longer than 1.5 million years
• Genetics Evidence: DNA Structure


  • Only 3 nucleotides different (shown in red) between human & rat in this DNA sequence
  • 7 nucleotides different between human & Yeast
• Biochemistry Evidence: Protein structure


  • Proteins are made up of amino acids arranged in a sequence
  • Sequence amino acids are arranged in determine what protein they make
  • Instructions to make amino acid sequence come from DNA sequences in genes
  • Gene and Protein amino acid sequences are used to compare how closely related species are
• Biochemistry Evidence: Protein structure


  • No differences in amino acid sequence between humans and chimps suggest that they are more closely related
• Geographical Distribution


  • Places on Earth where a particular species is found
  • Many unique species occur on isolated places or islands
  • Suggests evolution as isolation is necessary before speciation can occur
  • When species arrive on island, they take advantage of unused habitats and lifestyles
  • Different selection pressures on the islands may eventually lead to natural selection and speciation
• Geographical Distribution Evidence


  • Finches of Galapagos Islands. Different islands had finches with different beaks according to different foods available on each island. DNA and protein investigation have shown that 14 species of finches in Galapagos are related to warbler finches from Ecuador in South America. Evidence suggests that one species arrived on the islands 2-3 million years ago and then evolved on different islands.
• Embryology


  • Study of development, structure and function of embryos
  • Comparison of vertebrate show striking similarities in early stages of development
  • All have a 'tail'
  • There is a time during embryonic development of fish, lizards and humans when all have brachial arches which seem to grow into homologous structures of endocrine glands and gills
  • Based on embryology, biologists proposed that fish evolved into amphibians, which evolved into reptiles, which evolved into birds