2. Origin of life

It is still a challenge for scientists and philosophers to define the term “life”, since it must include all life we are familiar with, but also not exclude life forms that might be fundamentally different from life on earth (= Aliens).

Life can be defined as:

• A network of regulatory mechanisms (negative feedbacks) inferior to the potential of expansion and reproduction (positive feedbacks).

• Things with the capacity for metabolism and motion

• Matter that can reproduce itself and evolve as survival dictates

• A way to hydrogenate carbon dioxide

• Living things that are self organizing and autopoietic (produce and maintain without help)

NASAs definition: Life is a self sustained chemical system capable of undergoing Darwinian evolution.

On the one hand life can be seen as a property of an ecological system rather than a single organism, on the other hand the Gaia theory states that life, functions as a single organism which maintains the environmental conditions necessary for it’s survival.

98% of life is built on CHNOPS: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, Sulphur

• Water:

  • Reactant in many metabolic reactions

  • Starting point of photosynthesis

  • Habitat

  • Main component of all living beings

  • Very good solvent for polar compounds

• Carbon:

  • life is obtained from carbohydrates or photosynthesis

  • Structural elements for life (RNA, DNA, cellulose)

Terrestrial life is carbon based:

• 95% of the elements life is built on are CHNOPS

• Water is required as solvent for biochemical reactions

• Structural elements for life (ribos, DNA, RNA) are all based on carbohydrates

Yes, extraterrestrial life could be possible.

• Carbon based life: it is frequently assumed in astrobiology, that if life exists somewhere else it will also be carbon based, like earth.

• Sufficient quantities of carbon and the other major life-forming elements (Hydrogen Oxygen, Nitrogen, Phosphorus and Sulphur), along with water, may enable the formation of living organisms on other planets with a chemical make-up and temperature range similar to Earth.

• Terrestrial planets, such as Earth, are formed from "stardust" in a process that allows for the possibility of other planets having formed with compositions similar to Earth’s.

• Another possibility would be silicon based life, since silicon has many properties similar to carbon (not soluble below 1600 degrees Celsius, highly reactive with water, can’t form chemical bonds with many elements).

The habitable zone is the region around a star where conditions are just right for liquid water to exist on a planet’s surface, not too hot and not too cold.

This is important because liquid water is essential for life as we know it.

If a planet is:

• Too close to its star, it would be to hot and water would evaporate

• Too far from its star, it would be too cold and water would freeze

Abiogenesis explains how life could begin from chemistry alone, without invoking living organisms or biological processes at the start.

Possible energy sources:

• UV rays

• Ionizing radiation

• Heat from volcanic processes

• Electrical discharge

The Kepler Project was a NASA space mission launched in2009 to discover Earth-like planets by monotoring the brightness of stars and detecting planets through the transit method.

It revolutionized our understanding of exoplanets by identifying thousands of planets, many of them potentially habitable.

The experiment simulated the conditions of the early earth’s atmosphere to see if these conditions may have favoured the chemical reactions that synthesised amino acids and other live-forming compounds.

They created the conditions by circulating a mixture of methane, ammonia, hydrogen and water vapor through a closed glass apparatus.

• water was boiled to mimic the ocean

• Electrical sparks were discharged into the gas mixture to simulate lightning as an energy source.

• The gasses passed through a condenser, causing them to cool and return as liquid, creating a continuous cycle.

• Eventually the liquid contained amino acids and other organic molecules.

This demonstrated that essential biomolecules can arise spontaneously under plausible prebiotic conditions.

The experiment provided the first experimental support for chemical evolution and remains foundational to studies on the origin of life.

Life originated on early Earth around 4 billion years ago, soon after the planet formed.

The exact origin of DNA is unknown and involves a chicken-and-egg problem, because genetic material is needed to make proteins, but proteins are needed to replicate genetic material.

The most accepted explanation is the RNA world hypothesis, since RNA can both store information and self-replicate.

Experiments show that early Earth had organic molecules, nucleotides, and fatty acids, which form growing, stable vesicles acting as primitive cells. Inside these vesicles, faster-replicating RNA sequences would habe been naturally selected, forming the basis of all later life.

• 1665: Cell discovered (Robert Hooke)

• 1892: Virus discovered (Dimitri Ivanovsky)

• 1928: Penicillin discovered

• 1972: First DNA molecule created in Lab

Life might have originated from these vents, in which an early form of metabolism took place (a cycle of chemical reactions released energy that was able to be harnessed by other processes).

The synthesis of amino acids could have occurred deep in the earth’s crust, they were then brought up into cooler waters where the presence of clay minerals have lead to the formation of peptides and protocells.

The hypothesis is supported by the fact that, Methane (CH4) and Ammonia (NH3) were not present in the vents, in contrast to the earth’s primitive atmosphere.