Bio - Topic 4 - Origin of Life

Hypothesis for water on earth:

• Early Earth Conditions:

  • Lack of free oxygen and ozone

  • Higher concentration of carbon dioxide and methane → temperatures were extremely high causing any original water to evaporate as vapor. also higher levels of ultraviolet light penetration

  • Early gravity/accretion conditions failed to retain it

• Delivery Mechanism: As the planet cooled, water was brought to Earth via impacts from water-bearing specifically asteroids, meteorites, and comets, containing ice and other molecules

Why water stayed on earth?

• Sufficient Gravity: Earth's mass is large enough to exert a strong gravitational pull, holding both liquid oceans to the surface and water vapor in the atmosphere.

• The Goldilocks Zone: Earth orbits at an ideal distance from the Sun, not too hot to boil all water away into space, and not too cold to freeze all water permanently solid → allows liquid water to persist

• Temperatures low enough to condense water

Challenges of explaining the spontaneous origin of cells

• cells are highly complex structures that can currently only be produced by division of pre-existing cells

• conditions on earth were very different

• it is not possible to replicate with certainty the conditions that would have existed only on earth

• well preserved fossils are rare

• the methods used to estimate dates of living cells have ranges of uncertainty

Oparin + Haldanes proposed steps of life on earth

• Catalysis of simple organic molecules from inorganic compounds

  • Formation of biological monomers: amino acids, fatty acids, and simple sugars

• Polymerization of monomers into polymers

  • Monomers may have been able to spontaneously form polymers on early earth via anabolic reactions, creating proteins and RNA strands

• The formation of a polymer that can self replicate

• Emergence of compartmentalization of molecules

  • Membrane can exist as vesicles, micelles, and bilayers

  • Early membranes were believed to be made of fatty acids rather than phospholipids

Fatty acids as simple amphipathic lipid molecules

• Fatty acid molecules are a hydrocarbon chain with a carboxyl (COOH) group attached to one end

• Fatty acids coalesce to form micelles

  • spherical structure in a watery solution, amphipathic

  • at higher concentrations and undert the appropriate pH conditions, fatty acid micelles can form vesicles, capturing fluids and dissolved molecules

RNA as presumed first genetic material

• RNA can be replicated and has some catalytic activity → may have acted as genetic material and enzymes of the earliest cells.

Hypothesis suggests that;

• RNA was formed from inorganic sources

• RNA was able to replicate using ribozymes

• RNA was able to catalyse protein synthesis

• Membrane compartmentalization occured

• Inside the cell, RNA was able to produce both protein and DNA

• DNA took over as the main genetic material because it is more stable

• Proteins took over as the catalytic form (enzymes) because they are more capable of variability

polymerisation

• process in which relatively small monomers combine chemically to produce a large, chain-like molecule called a polymer

anabolic reaction

simple to complex molecule by condensation reactions

evidence to support the RNA first hypothesis

• Shot RNA sequences have been shown to be able to duplicate other molecules of RNA → it can self-replicate

• RNA has some catalytic activity so it may have initially acted as both genetic material and enzymes of the earliest cells

• Ribozymes in the ribosome are still used to catalyse peptide bond formation during protein synthesis

Endosymbiosis

• The theory of endosymbiosis states that eukaryotic organisms evolved when a common ancestor, a larger host cell, engulfed a smaller prokaryotic cell via endocytosis, and instead of digesting the smaller cell with enzymes, a mutually beneficial relationship formed.

• Mitochondria and chloroplasts evolved

Evidence supporting endosymbiosis in mitochondria

MADDRS

• Membranes (Double-membrane bound)

• Antibiotics (Susceptible to some antibiotics)

• DNA (Circular naked DNA)

• Divide by binary fission like prokaryotes

• Ribosomes (70S ribosomes)

• Size (Same size as a bacteria, 8micrometers)

Evidence supporting endosymbiosis in chloroplast

• Double membrane

• 70S ribosome

• Circular DNA

Multicellularity

• scientists think multicellularity has evolved repeatedly

  • Individual cells clustered together, more efficiently obtaining and sharing nutrients and benefitting from group protection from predators

  • Over time some cells were thought to have differentiated by different patterns of gene expression often triggered by changes in the environment

  • Allows a larger body size and cell specialization

evaluate miller urey experiment

• Direct observation and experimentation of early earth is not possible so a model is required, and Miller and Urey designed an apparatus to model a hypothetical and assumed early earth ocean-atmosphere system

• Miller and urey did not include every single possible variable that could have influenced the formation of carbon compounds on prebiotic earth - physical representation of simplified system

• tested only first step of oparin-haldane hypothesis, could not create life.

• did create amino acids which are the building blocks of proteins, a necessary part of life.

• uncertainty of gas composition of early earth and subsequently determined to be different to composition that miller-urey used

• given this information, the miller-urey experiment was limited in its validity.

The abundance of water…

The abundance of water over billions of years of Earth’s history has allowed life to evolve.

Conditions on early earth and the pre-biotic formation of carbon compounds

• Conditions on earth

  • Lack of free oxygen and ozone

  • Higher concentration of carbon dioxide and methane → temperatures were extremely high causing any original water to evaporate as vapor. also higher levels of ultraviolet light penetration

  • Early gravity/accretion conditions failed to retain it

• Conditions may have caused a variety of carbon compounds to form spontaneously by chemical processes that do not now occur

miller urey experiment

• Setup: Simulated early Earth conditions by sealing water, methane, ammonia, and hydrogen gas in a closed system, then applying electrical sparks (to simulate lightning) and heat.

• Results: The mixture yielded various organic molecules, most notably amino acids, the building blocks of proteins

• What it showed: Demonstrated that organic molecules (the building blocks of life) could be synthesized from inorganic molecules under conditions thought to represent early Earth.

spontaneous formation of vesicles

• spontaneous formation of vesicles through coalescence of fatty acids into spherical bilayers

• Process: Fatty acids are amphipathic molecules that spontaneously coalesce into spherical bilayers (vesicles) when mixed with water, with their hydrophilic heads facing outward and hydrophobic tails facing inward. Include micelles, liposomes, and bilayer sheets.

• Significance: These vesicles create an internal environment isolated from the external surroundings, serving as a crucial structural for primitive cell membranes and early cellular compartmentalization in the origin of life.