The Formation of Planet Earth
The Earth formed over 4.5 billion years ago from a cloud of gas and dust called the solar nebula. This nebula was the remnants of a supernova explosion of a giant star. Over billions of years, gravity caused the nebula to collapse and spin, forming the Sun at its center.
Around the Sun, the remaining gas and dust particles collided and stuck together into clumps called planetesimals. These planetesimals continued attracting more particles through gravity and colliding with each other. Some of the planetesimals grew large enough that their gravity shaped them into spheres, forming the planets.
The young Earth was very hot, with estimates ranging from over 7000°F to as high as 13000°F. It was mostly made of iron and rock with some lighter silicate minerals. Over millions of years, heavier iron sank into the center to form the core while lighter minerals floated up through the mantle. This differentiated the layers of the planet.
The Hadean Eon – The Violent Birth of Earth
The Hadean eon lasted from the formation of Earth around 4.6 billion years ago to around 4 billion years ago. The name Hadean comes from Hades, the Greek god of the underworld, referring to the hellish conditions of early Earth.
During this time, the planet was relentlessly bombarded by meteorites and asteroids, due to the large amount of debris still remaining from the solar system’s formation. These constant impacts generated extreme heat, vaporizing any oceans or atmosphere that tried to form. Larger impacts could have temporarily melted Earth’s entire surface into an ocean of magma.
However, meteorite bombardment may have contributed water to the young Earth. Carbonaceous chondrites are meteorites that contain water molecules and organic compounds. Early impacts from these space rocks may have delivered the original water to our planet.
Through this chaotic period, gravity pulled chunks of matter together into the Moon. Current theories propose that a Mars-sized object called Theia collided with the early Earth around 4.5 billion years ago. Debris from the impact was flung into orbit around the planet and eventually accreted into the Moon.
The Development of Earth’s Atmosphere and Oceans
As meteorite impacts declined around 4 billion years ago, Earth’s surface began cooling enough for the first oceans and atmosphere to form. The earliest atmosphere was likely composed mostly of nitrogen, carbon dioxide, water vapor, ammonia, and methane. These gases originated from volcanic outgassing and impacts from comets carrying ice.
With liquids present, rain began falling and filling basins to create oceans. Evidence suggests the oceans could have formed as early as 4.4 billion years ago. The famous Isua Greenstone Belt in Greenland contains some of the oldest known seafloor sediments at 3.8 billion years old.
The First Continents
The first protocontinents began forming through plate tectonics perhaps as early as 4.4 billion years ago. As Earth’s interior cooled, the hot, churning mantle created the first continental crustal rocks. However, this crust was likely small islands or microcontinents, not full continents.
The oldest confirmed continental crust is the Acasta Gneiss in northwestern Canada, which is 4.03 billion years old. Other ancient crust fragments include the 3.8 to 3.9 billion year old Isua Supracrustal Belt in Greenland and the 3.6 billion year old Pilbara Craton in Australia.
Modern plate tectonics likely started around 3 billion years ago as the mantle cooled further. Before this point, the crust was constantly recycled as new volcanic rocks formed and sank back into the mantle through subduction. The formation of large, stable continents required a drop in mantle temperature.
The Environment of Early Earth
Let’s explore what the surface of the planet would have been like over 4 billion years ago.
A Hot and Hellish Place
The Hadean eon represents the most violent and scorching period in Earth’s history. The planet was constantly bombarded by meteorites and comets. When larger objects like Theia impacted, they vaporized oceans and atmospheres and possibly melted the entire surface into a magma ocean.
Frequent volcanic activity also contributed to the intense heat. Radioactive elements were far more abundant, producing more internal heating and volcanic eruptions. Lava would have frequently covered the surface along with thick, noxious atmospheres.
Overall, the Hadean Earth would have resembled hell or the traditional visions of Hades. The surface was likely a barren, burning landscape hostile to any potential life.
A Toxic Atmosphere
When the planet cooled enough to have an atmosphere, it was far from breathable for humans or today’s animals.
The early atmosphere lacked oxygen almost entirely and was instead dominated by greenhouse gases like methane, ammonia, and carbon dioxide. These thick gases caused a powerful greenhouse effect that helped offset the weaker young Sun.
However, to any complex life forms it would have been highly toxic. Ammonia is extremely caustic to living tissues. Methane and carbon dioxide are both asphyxiants unable to support human breathing.
No Ozone Layer
The lack of oxygen in the primitive atmosphere meant there was no ozone layer. This protective shield wasn’t formed until photosynthetic life began producing O2.
Without an ozone layer, intense UV radiation from the Sun reached the surface. This high radiation would have presented an additional challenge to the development of life. Any unprotected organic matter would be prone to breaking down.
A Waterworld Ocean
After the late heavy bombardment ended around 4 billion years ago, the Earth’s surface finally cooled enough for water to accumulate into oceans. However, there were likely no continents at first.
Some models suggest the early Earth was a global ocean dotted with volcanic islands. Tidal forces from the nearer Moon caused higher tides, driving more rapid erosion of the islands. This would have impeded the formation of large landmasses.
While oceans covered the globe, the young seas were also more acidic due to the thick carbon dioxide atmosphere. The higher acidity corroded early sediments and continental rocks.
Extreme Climatic Conditions
The Hadean Earth experienced some incredibly extreme swings in climate. With no continents and few landmasses, there were no barriers for winds to moderate global temperatures.
When one hemisphere was pointed towards the Sun, it would boil while the opposite hemisphere completely froze. Volcanoes could also trigger a runaway greenhouse effect, pushing average temperatures over 200°F until the gases were reduced.
These violent cycles prevented long-term climate stability needed for life. Complex organisms require more mild, steady conditions to gradually evolve.
Possibilities for Early Life
Despite the hostile environments, some evidence raises the possibility primitive life could have already arisen during the Hadean period.
Chemical Evolution
Before the origin of life, smaller organic chemicals and compounds necessary for life had to form. Experiments like the Miller-Urey experiment show that sparks and heat can synthesize amino acids from inorganic gases. These building blocks may have assembled on early Earth.
Extraterrestrial inputs likely contributed many organics. Carbonaceous chondrite meteorites contain amino acids and nucleobases used in DNA and RNA. These organics survived impacts and rained down steadily on the early Earth.
Proposed Evidence of Life
In 2017, researchers from UCLA discovered potential fossils of microorganisms in hydrothermal vent precipitates in Quebec, Canada dated to around 4.28 billion years old. This raises the possibility of bacteria inhabiting the deep oceans during the Hadean.
However, other scientists are skeptical of evidence for life this early. They argue the forms are more likely irregular mineral structures that only resemble microfossils. The debate over Earth’s earliest traces of life continues.
Origin of Life Theories
If the right organic chemicals were present, life could have begun forming as early as 4.4 to 4.1 billion years ago.
One theory proposes deep-sea hydrothermal vents as the cradle for life. These mineral-rich vents form porous rock chimneys with gradients of pH, temperature and minerals that could catalyze chemical reactions. The turbulence and mineral interactions may have assembled proto-cells.
Other models hypothesize that UV radiation from the Sun provided an energy source. When blasted with UV, simple organic mixtures can self-organize into more complex molecules like RNA nucleobases. Repeated cycles of evaporation and irradiation concentrating organics, eventually forming replicating polymers.
Conclusion
The early Earth was one of the most violent chapters in our planet’s 4.5 billion year history. A brutal bombardment sculpted the surface into a fiery, hellish landscape inhospitable to life as we know it today.
Nonetheless, during this period our planet differentiated into layers and accumulated the building blocks necessary for life. After hundreds of millions of years, the chaos tempered. Oceans, land, and atmospheres slowly stabilized enough for early microorganisms to emerge, paving the way for the tremendous abundance and complexity of life on modern Earth.