From Stardust to Life: The Formation of the Solar System, Earth's Habitability, and the Evolution of Life

Introduction:

     The planet we call home is Earth. It is the only planet in our solar system with a sustainable environment for the survival of organisms. But this environment did not appear instantly. There is a long chain of processes.  These are the results of cosmic collisions, chemical and biological evolution, and geological processes. The observable universe began approximately 13.8 billion years ago in an extremely hot, dense state. Over billions of years of cosmic evolution, matter eventually formed stars, planets, and asteroids.

The origins of matter: Before the solar system

The Big Bang theory:

    In 1927, Belgian cosmologist and Catholic priest Georges Lemaître focused on the expansion of the universe. Later in 1931, he proposed the "primeval atom hypothesis," which became the foundation of the Big Bang model. He theorized that the universe began in extremely hot, dense state. Later on, many scientists worked on this theory. 

• In 1929, Edwin Hubble discovered that the universe is expanding and galaxies are moving away from one another.
• In 1940, George Gamow proposed the idea of an early dense universe.
• In 1949, Fred Hoyle coined the term "big bang" for this theory, during a radio broadcast. Actually, he didn't believe it. He was just criticizing this theory.
• In 1964, Arno Penzias and Robert Wilson started an experiment. They tested an antenna at Bell Labs for satellite communications. They accidentally recorded background uniform cosmic noise. This "echo" of the universe's explosive birth is evidence of the"Big Bang" theory.
• Later in 1960, Robert Dicke and Jim Peebles laid the foundation of modern cosmology. They recognized the intense light and the afterglow of the Big Bang. When they heard about Arno Penzias and Robert Wilson, they instantly identified the Cosmic Microwave Background (CMB).
• In 1970, Stephen Hawking and Roger Penrose developed the "singularity theorem" by applying Einstein's theory of relativity to the universe as a whole.
• Then in 1980, Alan Guth introduced the theory of cosmic inflation. This theory suggests the rapid exponential expansion of a fraction in a second. It solved the mathematical flaws in the theory and explained why the universe appears isotropic, homogeneous, and remarkably flat.
• In 1989, John Mather and George Smoot discovered some tiny temperature differences in spots. Some were slightly cooler, and some were slightly warmer. The cooler spots show crowded and particle; otherwise, warmer spots show space. Crowded areas have more gravitational pull, which is used to build the modern universe.

Key concept of Big Bang theory:

    According to this theory, 13.8 billion years ago all matter, space, time, and energy were contained in a dense "singularity". The space stretches and expands according to this theory. It is explosion of space in itself . Over a million years, as they cool, gravitational pull tightens them together make cluster. As a result, eventually galaxies, asteroids, planets, and stars formed.

Formation of the solar system:

    There are nearly 10 hypotheses to explain the creation of the solar system, but the most widely accepted model is the "solar nebular hypothesis".

Solar Nebular hypothesis:

    According to the solar nebular hypothesis, approximately 4.6 billion years ago, a solar nebula (a massive rotating cloud of dust and gases) collapsed under its own gravity. Nearby, a supernova exploded and sent a shock wave through the cloud. This shockwave caused an explosion.

The birth of the Sun:

• The majority, about 99% of the nebula's mass, pulled into the center. The centre grew intensely hot and dense. Now this core is known as a protostar. 
• For millions of years, this material is kept in the centre due to gravity. 
• The intense heat (15 million °C) and immense pressure trigger nuclear fusion of hydrogen into helium. 
• This fusion releases a large amount of energy. This energy causes an outward push, which stabilizes the inner gravitational pull. 
• The gases are clear, and a shining, stable sun is formed.

Formation of planets:

• The planets are formed from dust and gases surrounding the Sun. 
• The microscopic grains collide and stick together. 
• This forms tiny rocks of about 1 kilometer. 
• Now they can create their own gravitational pull. 
• These rocks are called planetesimals. 
• Planetesimals combine together and form planets.

Formation of early Earth:

    The Earth is formed through different stages.

Earth's accretion:

    Accretion is the fundamental mechanism to build a whole planet. Over a large timescale, it is a continuous process where rocky debris, cosmic dust, and gas pull together due to gravitational force. At a microscopic level, it is an episodic impact event. 

• As discussed before, dust particles stick together and form asteroid-sized planetesimals. This sticking is the beginning of accretion.
• Planetesimals merged and formed a planetary embryo. 
• Then embryos merged to form Earth.
• Earth continued its growth around 70 to 120 million years after the solar system began.

  Formation of the moon:

      There are four major historical hypotheses for the formation of the Moon. Before the Apollo mission, there was a big debate between scientists about which hypothesis is acceptable.
  

  Co-formation hypothesis:

      The co-formation hypothesis, also known as "The Sister theory," said that Earth and the Moon formed at the same time from the same cloud of dust. 
  
  But this hypothesis can not explained if Earth and the Moon are like siblings. So, why core of moon is small as compared to Earth?

  The passing stranger hypothesis:

      This hypothesis says the moon was formed else where in the solar system and attract toward Earth due to the gravitational field.
  While all planets and stars have their unique and distinct chemical signature,the Moon and Eath have the same identical isotopes. It's like they have the same DNA signature.
  If the Moon was just another strange planet, why is its core tiny? And why do Earth and moon has same chemical signature? These questions are unsolved in this hypothesis. 

  The spin-off hypothesis: 

      George Darwin introduced this theory. According to George, Earth spins very fast, about 2.5 hours a day. This rapid motion caused a massive chunk of Earth's outer mantle to tear away. Then it gathered in the form of the moon in orbit. It is also called the "fission theory"In Pacific ocean there was huge empty space was found. It was evidence for the spin-off theory.
  
  This hypothesis also has flaws. 
  • Earth can never move at this speed.
  • Moon is 4.5 billion years old; on the other hand, the Pacific Ocean is 200 million years old.
  • Our current moon-Earth rotation system should be very change, as it is today.  

  The giant-impact hypothesis:

      The giant-impact hypothesis is a more acceptable and leading model for the formation of the Moon. This hypothesis suggests that when Earth formed, a giant Mars-sized body named "Theia" collided with Earth. The leftover mantal stuck together grain by grain and form moon. The moon is formed during the final stage of accretion.

  

  
  
  

  Modern variations of giant-impact hypothesis:

      There are new modern theories about the formation of the Moon based on advanced computer simulations.
  
  • The synestia hypothesis
  • Multiple-impact hypothesis

  The synestia hypothesis:

      The synestia hypothesis suggests that when Theia collided with Earth, it didn't just break apart; both planets were completely vaporized. The Earth became a donut-shaped cloud of vaporized, boiling rock and mantle. This massive structure is known as synestia. These atoms of Earth mixed together during this continuous process. The outer edge of the donut cooled, reforming Earth, and other pebbles clumping together to form the Moon.
  

  Multiple-Impact hypothesis:

      This hypothesis suggests that instead of a large planet collision, there was bombard of many tiny celestial bodies hitting Earth. Each collision vaporized rock and mantle into orbit. They create acceration disc or debris. Debris clumped together and formed moonlets. Later, these moonlets merged together and formed a single moon.

Heating of early Earth:

    Collisions caused the transformation of kinetic energy into immense heat. Also, bombardment by asteroids and compression due to gravitational pull increased its temperature. Furthermore, radioactive isotops decay generate internal heat.  

Magma ocean:

    Earth's surface was widely covered by thick, semi-liquid rock also known as molten rock or magma. On early Earth, it was composed of heavy concentrations of iron, magnesium, and a small amount of silica. A large amount of gases such as water, carbon dioxide, sulfur, and methane was also present.

Cooling of Earth:

    Within approximately 10 million years, Earth cooled. It radiated heat into outer space. The ultramafic and mafic igneous rock layered the planet's interior by fractional crystallization. Trapped water vapor and carbon dioxide were released from molten rock. When the temperature dropped and atmospheric pressure, water vapor condensed and continued to pour down rain. This rain continued for thousands of years.

Planetary differentiation:

    Based on density, seperation of distinct layers of planets is called the planetary differntiation process. It shapes geological evolution. It forms Earth's core, mantle, and crust.

Formation of the Core, Mantle, and Crust:

Core formation:

    Core of Earth is formed by the process of iron catastrophe. In this process, semi-liquid iron and nickel sank into the centre of Earth. This material permanently collected at centre and made the core.

Formation of mantle and crust:

    Heavy metal sank downward and light materials like silicates move and floating upward. Silicate contains silica, oxygen, iron, and magnesium. As they cool and crystalize for the Earth's mantle and crust.

Formation of magnetic field:

    The Earth's magnetic field was generated by the geodynamo effect. This effect is the conversion of thermal convection and the Earth's rotation into an electric and magnetic field. 

• Thermal convection currents are caused by density fluctuation due to temperature differences. Earth's core releases heat outwards. As temperature reaches the top, it cools and sink back. This loop makes a thermal convection current.
• Compositional convection is occur due to chemical makeup differences in fluids. Heavy metals sink to the core and light elements rise. It causes compositional convection.
• Liquid nickel is a good conductor of electricity. As convection currents generate electricity, it runs through nickel. According to the Ampere-Maxwell law, when electricity passes through a conductor, it generate its own magnetic field. 

If we apply this law on early Earth, as a result, these convection current makes Earth's magnetic field.

Late heavy bombardment:

    The late heavy bombardment is a hypothesized event approximately 4.1 to 3.8 billion years ago. This theory was suggested after studying the sample of moon from Apollo mission. During this event, many celestial bodies and asteroids fell into the solar system due to the gravitational field. This is widely debated among scientists that wether it was a short negligible event or more gradual in impacts.

Formation of first atmosphere:

     About 4.6 billion years ago, Earth's first atmosphere was only of the lightest gases, hydrogen and helium. These gases are too light and fast enough to drift off into space. The second atmosphere formed when volcanoes released gases like nitrogen, carbon dioxide, and water vapor. It formed a hot, oxygen-free atmosphere. 

Formation of ocean:

    There are two primary sources of water on early Earth. 

• The first source is water vapor released from volcanic eruptions. Cooling molten rocks release a massive amount of water vapor and steam.
• Second source could be asteroids. During bombardment, some asteroids may contain water and delivered to the Earth.

After cooling, condensed water vapors form cloud and rain. It fills the basin of the Earth and forms oceans.

Formation of tectonic plates:

    Irregular-shaped slabs on Earth's crust are called tectonic plates. Convection currents and heat rising creates tension, break lithosphere into giant, interlocking slabs. These plates continuously movedue to convection current. There are three types of tectonic plate boundaries.

• Divergent boundaries: These are boundaries where plates pull apart from each other. These plates move and create ocean crust and underwater mountain ranges. Magma bubbles rises up in the gap. 
•  Convergent boundaries: At this point plates collide into eachother. Plates may subduction (one plate slide under the other). They crumple upward to form mountains, and also form volcanos and ocean trenches.
• Transform boundaries: Plates past slide eachother horizontally. They stucks and cause earthquaks, due to pressure release.

Formation of continents:    

    Continents are formed over 4.0-4.4 billion years ago. After further cooling light elements like silicon and aluminium form landmasses. These landmasses drift, collide, apart, and merge again and again. Then a supercontinents Nuna and Rudinia formed. About 335 million years ago, a super continent Pangaea split into two small landmasses; Laurasia and Godwana. These two continents futher split into seven continents as we see today.

How Earth become habitable for life?

    After a long period, the formation of gravity and magnetic field, atmosphere and geological activities make Earth a habitable planet. The biggest factor is that Earth sits at Goldilocks zone (a place where the temperature is neither too hot nor too cold at a certain distance from star).

Chemical evolution:

The primordial soup:

    The early atmosphere was a mixture of inorganic compounds. These are oxygen-free, inorganic, volcanic gases. Due to the absence of oxygen, Earth lacked an ozone layer. UV radiation and cosmic rays enter Earth without any barrier. They break highly stable molecules into free radicals. Then, combine again to form building blocks of life.

The Oparin-Haldane hypothesis:

    In 1921, this hypothesis was proposed by J.B.S. Haldane and Alexander Oparin. According to this hypothesis, Earth's early atmosphere was a 'reducing' environment. The atmosphere was rich in hydrogen, methane, ammonia, and water vapor. These molecules react to form monomers. The monomers linked together and formed complex polymers. Organic molecules eventually gathered in membrane-bound spherical droplet, called coacervates. They grow as precursor to early living cells. These protobionts grew and developed into the first true living cells. The Miller-Urey experiment is key evidence of this hypothesis.

Miller-Urey experiment:

    In 1953, scientists Stanely Miller and Harold Urey successfully synthesized amino acid, by zapping inorganic gases with electricity. In this experiment, they created primitive atmospheric conditions and a water cycle. The steps of the experiment are as follows:

• The primordial ocean: The flask was filled with water. Heated the flask to form water vapor.
• Reducing environment: The other flask was filled with highly reactive gases, methane, ammonia, and hydrogen gas, to create a reducing environment.
• The lightning strike: Two electrodes were used to create electric energy that breaks stable chemical bonds of gases.
• The rainfall: The high-energy gas then cooled and condensed. It convert gases into liquid water droplets. 
• A U-shaped tube used to maintain cycle. 

Results and conclusion of experiment:

    These conditions were made to see how actually building blocks of life was created. Within just a week, 20 types of amino acids were formed. However, it also has some defects, like the atmosphere was less "reducing" and mostly consisted of carbon dioxide and nitrogen, but its valid core conclusion was that life can arise spontaneously from non-living matter. 

RNA world hypothesis:

    In 1962, Alexander Rich first proposed the RNA world hypothesis. He hypothesized that life was formed from a single self-replicating RNA. He said that primitive RNA carried the hereditary code, also RNA itself became a catalyst by folding into a complex shape. Later on, more scientist work on this hypothesis. 

• Carl Woese indicate that RNA is a single-stranded molecule; it can fold itself and become a catalyst.
• Francis Crick suggest accurately that the original enzyme was RNA.
• Lesli Orgal suggest that if RNA replicates itself without any protein, there is a high possibility that RNA was the first genetic material.
• Walter Gilbert officially coined the term RNA world.

They argued that a single, versatile molecule must have existed first.

Arguments against RNA world hypothesis:

• RNA is a single-strand, highly fragile molecule. Its backbone easily breaks when exposed to heat, water, or radiation.
• Scientists used a very clean place and chemicals in RNA folding tests. But in reality, reactions yield a random, messy mix of molecules.
• The metabolism and energy cycle required to form complex molecule like RNA.

RNA-peptide co-evolution model:

    Modern science proposed RNA-peptide co-evolution hypothesis. It suggests that RNA and peptides didn't evolve independently. But they form together and stabilized each other. Although, it also has evidence and defects.

Evidences:

• RNA is fragile; positive charges in ancient peptides stabilize its structure.
• Specific amino acids attach to specific RNA. Their physical interaction shows that the genetic code is rooted in it.
• Modern ribosomes: genetic translation shows that it is decendants of an ancient RNA-peptide partner.

Defects:

• It is very difficult to form a complex molecule like RNA and peptide in such a prebiotic environment.
• Both molecules need different environmental conditions to synthesize. For example, RNA requires high concentrations of salt. On the other hand , it is harmful for peptides.
• It is difficult to explain which comes first. How could a genetic molecule originate without protein, and how could protein be synthesized without genetic information? It is a prebiotic "chicken-egg paradox".

Formation of protocell:

    Protocells are  biomolecules lack membrane or membrane-bound stepping stone between non-living chemical compounds and a true living cell. The hydrothermal vents hypothesis is one of the leading hypotheses for the formation of protocells and origin of true living cells.

Hydrothermal vents hypothesis:

    The hydrothermal vent hypothesis was first proposed in 1981 by oceanographers Jack Corliss, John Baross, and Sahra Hoffman. This hypothesis proposed that life first originated in hydrothermal vents (deep-sea hot springs). Vents provide ideal conditions, minerals, and heat continuously for the synthesis of biomolecules. In these vents, reactions occur and form a protocell. Life appeared approximately 3.5 billion years ago.

Hot spring hypothesis:

    Hot spring hypothesis suggests that life began on Earth's continents rather than in hydrothermal vents. It says that on land, the dry-wet-moisture cycle caused by rainfall improve structure ability to compete and evolve for a protocell. The steps of formation of a protocell on land are described below.

• According to this hypothesis, biomolecules such as fatty acids, RNA, and amino acids are formed by volcanic gases, lightning, or meteorites.
• Minerals on shorelines acted as catalysts, and building blocks attracted each other and formed a lipid-bond sphere.
• Vesicles were formed by evaporation of dilute organic molecules. Evaporation drove them to stability and double-layered vesicles.
• As the vesicles formed, they capture surrounding RNA in their membrane.
• In hot springs and geysers, the cooling and heating cycle helped lipids to divide, fuse, and trap chemical compounds.

The protocells compete with the extreme environment and evolve into a true living cells.

Conclusion:

    The Earth and life as we see today have a very long, vast history. There are many major and minor events that make Earth a habitable planet, which provide an environment for life. Each stage prepared conditions necessary for the formation of life. At every event, scientists are debate and suggest different hypotheses and theories to understand these events.