Evidence for Evolution of Life (HL) (HL IB Biology)

Evidence for a last universal common ancestor

• During the evolution of life, evidence suggests that different species have evolved from a common ancestor
  • These species will share some similar characteristics with their common ancestor
  • For example, the bone structure of the vertebrate forelimb is similar in all species of vertebrates which indicates that they inherited this structure from a common ancestor
• DNA provides a useful tool to determine the evolutionary relationships between species
  • Organisms with similar DNA sequences are more closely related than those with very different DNA sequences
• All life on Earth is thought to have evolved from an ancient common ancestor, believed to have existed about 4 billion years ago
• This organism is known as the "Last Universal Common Ancestor", or LUCA
  • In a phylogenetic tree of life, LUCA would be the organism at the very base of the tree

The phylogenetic tree of life diagram

The evolutionary tree of life, indicating the evolution of life from the last universal common ancestor (shown in purple)

• Evidence for a common ancestry shared by all living organisms include:
  • Same biochemistry in all organisms
  • Same DNA bases and genetic code shared by organisms
  • Same shared amino acids forming protein molecules in organisms
• Researchers found several genes that are shared by eubacteria and archaea, indicating that these genes were inherited from LUCA
• It is possible that other organisms may have evolved at the same time as LUCA, but became extinct due to competition for shared resources
• The descendants of LUCA would have outcompeted other species and gone on to shape the tree of life as we know it

Estimating the timescale for the origin of life

• Fossils can provide evidence about the history of life on Earth and are often used to determine the timescale across which evolutionary events have occurred
• These timescales can be established using techniques to date the fossils or the rocks in which they are found
  • Carbon dating of the isotope carbon-14 for samples up to approx. 60 000 years old
  • Radiometric dating
    • It measures the relative proportions of certain radioactive substances (such as carbon-13 to carbon-12) in a sample
• Older rocks would be expected to contain evidence of more ancient forms of life, so accurately dating these would indicate when life may have originated
• Another technique by which the age of an organism can be determined is by analysing its genome
  • DNA changes as mutations occur and accumulate over time
  • By estimating the average time for DNA mutations to occur, the relative date when species branched from a common ancestor can be determined based on the number of mutations that have occurred between them
    • This would also apply to changes in the amino acid composition of proteins since any changes in the DNA will translate into a different protein structure and composition
  • On the assumption that these changes occur at a constant rate, this forms the basis of a molecular clock
  • This molecular clock can be used to determine the date of when life on Earth originated

Evidence for the evolution of LUCA near hydrothermal vents

• It is possible that LUCA evolved in hydrothermal vents deep in the ocean
• The conditions near hydrothermal vents provide opportunities for organisms to generate energy by chemosynthesis 

By NOAA, Public domain, Wikimedia

Life is believed to have originated near hydrothermal vents

• Scientists have found fossilised structures in the sedimentary rocks near deep-sea hydrothermal vents in Quebec, Canada
  • These structures are similar to those produced by modern prokaryotes found near hydrothermal vents
  • The fossils are at least 3.77 billion years old, but could be more than 4 billion years old; one of the oldest forms of life ever found
  • These fossil structures are small tubes made of haematite, which is the mineral form of iron(III) oxide
    • The presence of carbonate and other carbonaceous material in the sedimentary rocks indicates that oxidation and other biological activities may have occurred there
    • It indicates that these ancient bacteria had a similar biochemistry to modern iron-oxidising bacterial communities that live near hydrothermal vents
• Analysis of sequence data from modern species that live near hydrothermal vents indicates that they all share a common ancestor
  • Based on the properties and functions amino acid sequence data from these organisms, LUCA may have had the following characteristics:
    • Anaerobic, therefore able to survive in the absence of oxygen
    • Converted carbon dioxide into glucose
    • Used hydrogen as an energy source, instead of sunlight
    • Converted nitrogen into ammonia for the synthesis of amino acids
    • Survived in environments of very high temperature (thermophilic)
• Fossil evidence and genetic analysis indicates that LUCA may have been an autotrophic extremophile that lived in hydrothermal vents, in an environment with an abundance of hydrogen, carbon dioxide and iron
• Note that this is not the only hypothesis for the origin of life; scientists will continue to gather and an analyse data that may support or refute existing theories