How Life Started on Earth - Theories (College Board AP Biology)

Evidence for the evolution of LUCA

• The first life form on Earth has been named LUCA
  • LUCA stands for Last Universal Common Ancestor
  • 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 fossilized structures in the sedimentary rocks near deep sea hydrothermal vents in Québec, 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 hematite, 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 of 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 analyze data that may support or refute existing theories

• The Oparin-Haldine hypothesis is that, to create the original first cells from nonliving material, the following four stages occurred:
  1. Simple organic compounds needed to be synthesized from inorganic molecules (this was demonstrated by Stanley Miller and Harold Urey)
  2. Then assembled into polymers
  3. Some of these polymers (it is thought to be RNA) developed the ability to self replicate (which enables inheritance)
  4. Formation of membranes (by lipids) that surrounded the polymers creating packages with internal chemistry different from the surroundings

Life Arising from Non-living Materials Diagram

The key stages involved in life arising from non-living materials

Miller-Urey experiment

• Miller and Urey recreated the conditions thought to have existed on Earth prior to life, using a specific piece of apparatus
• The apparatus allowed them to:
  • Boil water to produce water vapor reflecting the early primordial soup evaporating in the high temperatures that existed on Earth
  • Mix the steam with a mixture of gases (including methane, hydrogen and ammonia) that recreated the atmosphere
  • Add electrical discharges to the gases to simulate lightning (one of the sources of energy available at the time)
  • Cool the mixture (representing the condensation of water in the atmosphere)
• After a week Miller and Urey analyzed the condensed mixture and found traces of simple organic molecules including amino acids

Miller and Urey's Experiment Diagram

The apparatus used by Miller and Urey

Alternative Theory: Organic Molecules Arrived on Earth from Space

• One theory is that organic molecules were not first synthesized on Earth
• Rather, they were thought to have arrived from another celestial body eg. a meteorite
• A meteorite, named Maribo, landed in Maribo, Denmark in 2009
• Analysis of the meteorite's chemical makeup showed it to contain some of the earliest organic matter in the known universe
• Whilst the molecules found are primitive, simple molecules, they are believed to have been incorporated into the Earth's biomass since meteorites have been landing on Earth for billions of years

Alternative Theory: The RNA World 

• This hypothesis suggests that RNA pre-dated DNA as the original molecule carrying genetic information
• RNA was thought to exist even before the first cells took their form
• According to this hypothesis, RNA played a dual role in primitive cells
  • It stored genetic information
  • It also catalyzed the chemical reactions within the cells
• Only later in evolutionary time did DNA take over as the principal store of genetic information, with proteins becoming the major catalyst and structural component of cells
• To support this view, RNA still catalyzes several important reactions in modern day cells
• RNA is present as a structural component of ribosomes, which are a feature of all cells
• RNA has been found to possess some catalytic properties, in particular in the area of being able to catalyze its own replication