Option D.1 – Origin of Life on Earth
D.1.1 – Describe four processes needed for the spontaneous origin of life on Earth
The four processes are:
1. Non-living synthesis of simple organic molecules
- The molecules include amino acids
2. Assembly of these molecules into polymers
- Such as polypeptides and carbohydrates
3. Origin of self-replicating molecules that made inheritance possible
- Allowing for the inheritance of characteristics
4. Packaging of these molecules into membranes with an internal chemistry different from their surroundings
- The membranes form spherical droplets
These four processes lead to cell-like structures.
From that point, evolution and natural selection could work to bring about the diversity of species we have today.
D.1.2 – Outline the experiments of Miller and Urey into the origin of organic compounds
Miller and Urey performed their experiments in 1953, looking at the potential for spontaneous formation of organic compounds.
Within their apparatus, they recreated the probable conditions of the earth before life was present. They mixed ammonia, methane and hydrogen gases to create the ‘atmosphere’. They created electrical discharges and boiled water to simulate lightning and rainfall.
The simulation ran for a week, and analysis of the water showed that it now contained many organic compounds, including fifteen of the known amino acids.
The results suggest that the spontaneous chemical reactions in earth’s atmosphere and water could be enough for the origin of organic compounds.
D.1.3 – State that comets may have delivered organic compounds to Earth
Comets contain a variety of organic compounds. Heavy bombardment about 4000 million years ago may have delivered both organic compounds and water to the early Earth.
Some experiments performed at NASA have shown that organic compounds can form in cold interstellar space.
D.1.4 – Discuss possible locations where conditions would have allowed the synthesis of organic compounds
1. See D.1.2 for details of how organic compounds could have been synthesised in earth’s atmosphere and water 2. See D.1.3 for
2. See D.1.3 for origin in comets. Organic molecules may have also arrived with meteorites or interplanetary dust.
3. Hydrothermal vents – these are found deep within earth’s oceans. Chemicals well up in the rocks and there may have been spontaneous synthesis of organic molecules.
D.1.5 – Outline two properties of RNA that would have allowed it to play a role in the origin of life
Today, DNA is used to form proteins and enzymatic proteins are used to produce DNA. However, it would be very unlikely for both protein enzymes and DNA to spontaneously occur at the same time. It seems more likely that RNA was a precursor for the two materials.
1. Self-replication
- One RNA molecule can be a template for the formation of another molecule. This role has now been taken by DNA in most organisms.
2. Catalytic activity
- RNA can catalyse chemical reactions. Although the role is now filled by protein enzymes, RNA still catalyses peptide bond formation.
D.1.6 – State that living cells may have been preceded by protobionts, with an internal chemical environment different from their surroundings
Membranes are an essential feature of living organisms, as they provide the barrier that allows organisms to have a different internal chemistry. The cell membrane divides the cytoplasm and its metabolism from the outside fluid.
The phospholipid molecules which compose membranes naturally form bilayers in water, which then enclose into a spherical structure around a droplet of fluid. When water contains these microspheres, it has a viscous, cloudy appearance and is called coacervate.
The hydrophobic properties of phospholipid bilayers are critical for creating a different internal environment. The primitive cells themselves are known as protobionts and would have evolved into modern day cells once genetic mechanisms began to appear.
D.1.7 – Outline the contribution of prokaryotes to the creation of an oxygen-rich
atmosphere
In the early days (about 2 billion years ago), earth’s atmosphere was very low in oxygen. The oceans were rich in iron and the bacteria at the time were probably iron-metabolising. 
Thanks to photosynthesising cyanobacteria (prokaryotes), oxygen began to accumulate in the atmosphere and the iron precipitated into its oxidated form. The prokaryotes used up water during photosynthesis, releasing the oxygen as waste.
The concentration of atmospheric oxygen increased very rapidly. Once that occurred,
aerobic cell respiration could be used.
The bacteria found in hot springs and extreme environments today are probably the most similar to the early prokaryotes.
D.1.8 – Discuss the endosymbiotic theory for the origin of eukaryotes
Eukaryotic cells contain organelles like mitochondria and chloroplasts which are not found in prokaryotic cells.
The endosymbiotic theory is that these organelles were independent prokaryotic cells that were taken up by another heterotrophic cell by endocytosis:
The cell with a nucleus only respires anaerobically. The bacterium is taken up by endosymbiosis and evolves into the mitochondrion.
The theory is supported by several characteristics of mitochondria and chloroplasts:
- They have double membranes – one from the original cell and one from the vesicle
- They grow and divide like cells
- They have a naked loop of DNA like prokaryotes
- They synthesise some of their own proteins.
The organelles were not the only requirement for eukaryote evolution: the cells also needed chromosomes, meiosis and sexual reproduction.