Option D1: Origins of Life On Earth

Biochemical Evolution: A series of chemical reactions created life. A theory was made that the conditions on earth were not the same as they were now. This is why we don't see any spontaneous appearances of life, whereas it could have been possible in the past.
It is suggested that early conditions on earth consisted of simpler molecules such as methane, ammonia, water, hydrogen sulfide, etc. The earth's surface was possibly hotter than today. Energy from storms or the sun was used to initiate chemical reactions between molecules in the air.
Hydrothermal vents emit hot water and dissolved minerals which serve as an ideal place for the formation of organic compounds. Similarly, volcanoes and extraterrestrial locations could have been optimal locations with the sufficient energy required to form a organic molecule.

Comets: Theories even suggest the deliverance of organic compounds from comets. Organic molecules could have been formed in space and brought to earth on comets or meteorites. About 4000 million years ago there was a heavy bombardment of meteorites that could have led to the synthesis of living organisms.

For life to spontaneously appear on earth, it would require four general processes:

1. synthesis of organic molecules from non-living molecules (the elements)
2. formation of organic molecules into more complicated polymers
3. ability for polymers to replicate
4. housing of these polymers within a membrane 

Miller Urey Experiment:

• Attempted to recreate the conditions of the past earth, but in a controlled environment. 
• Reduced oxygen, high radiation levels, high temperatures and electric charges
• Water vapour was combined with methane, ammonia and hydrogen and exposed to the electrical charge (lightning). After a week of cooling some simple molecules were found such as simple amino acids, ethanoic acid, lactic acid, urea and hydrocarbons 
• This formed the hypothesis of the possibility that organic molecules can be formed from inorganic elements
• Alterations to the experiment (used CO2) produced nucleotides, the building blocks for DNA.
• Although it shows that the synthesis of organic molecules was possible, the controlled environment represented by the lab seems unlikely in a primitive earth. 

From Monomers to Polymers: 

• Monomers that have been chemically created now have to become polymers 
• Condensation reactions without any enzymes (since they would not have existed yet) would have to occur for monomers to become polymers. 
• Polynucleotides would need to replicate without the help of enzymes and also as a single strand of RNA. 

RNA World Theory:

• RNA is proposed as the original molecule with the ability to carry information. It would have to be able to copy itself, to act as an enzyme for its own reaction. Ribozymes are ribosomes that mostly utilize the RNA to create proteins. 
• RNA can self-replicate, store information by sequencing four nucleotides and replicating these short sequences
• RNA as a catalyst can even be found nowadays to remove introns from mRNA and to synthesize new RNA molecules. Ribosomes have rRNA that facilitates the formation of peptide bonds.
• Only molecule that has these properties but DNA has superceded it due to it's chemical stability and ability to carry more data. Proteins can also carry more amino acids and therefore has more variety than RNA. 

1. RNA is formed from the nucleotides
2. Self replicate, then experiences recombination and mutation to develop new skills of enzymic activities 
3. Synthesis of proteins
4. DNA appears and overcomes RNA so that it becomes less useful

Protobionts: 

• preceded living cells in groups of abiotic molecules surrounded by membranes in order to separate themselves from the external environment 
• Protobionts can be shaken to form a molecular bilayer (liposome) when lipids are present. 
• Coacervates are examples of protobionts. Coacervates are formed when organic molecules found in water are separated into layers or droplets with a skin of water molecules surrounding it.
• Microsphere are made up of amino acids that polymerize due to heat. After cooling, they will form small sphere shells capable of growing,  for nutrients to diffuse into the membrane. 

Endosymbiotic Theory:

Cell organelles are theorized to be a living prokaryote living symbiotically inside another prokaryote. An organism such as an algae may enter the host cell as an undigested prey or parasite. They are allowed to survive, symbiotically exchanging molecules, food, or energy with each other.  

Mitochondria and chloroplasts are theorized to be endosymbiosis. These two organelles were thought to be parokaryotes on their own since they have their own DNA, ribosomes, double membrane and inner membrane.