Earth was formed around 4.54 billion years ago through the gathering of mass from the accretion disc of the solar nebula during the formation of our solar system. In its early stages, earth was constantly under heavy bombardment from other floating bodies, leading to intense volcanic activity. After approximately a billion years of constant collisions Earth was eventually covered in water and hence set the stage for life to begin and prosper, with the earliest evidence showing that life may have begun around 3.7 billion years ago. The evolution of life on earth was serendipitous, a result of billions of years of atmospheric terraforming and persistent natural selection overcoming extinction events. This report will delve into how life on earth began and was prepared, the formation of the atmosphere and the evolution and persistence of life.
There has been a long-standing debate as to where the water on earth truly originated from with various proposals such as water from the solar nebula, comets and asteroids. The most likely of these according to research from scientists from the Arizona State University is that both a combination of asteroid impacts and water from the solar nebula have resulted in earth’s massive supply of water. Both water sources have similar deuterium (an isotope of hydrogen) to hydrogen ratio [D/H ratio] to earth’s oceans (Anderson, P. (2018) How did Earth get its water?). Comparatively, water from comets (despite them being made primarily of ice) has a higher D/H ratio making them less likely candidates. With water present, the planet cooling down and within the Goldilocks Zone of the sun (water can remain as a liquid), life could begin.
Similarly to how water first came to earth, the question of ‘How did life first begin on earth?’ is another debated topic. A commonly accepted hypothesis as to life’s origins are primordial unicellular organisms that were carried throughout space through asteroids, comets and other cosmic bodies. This is called Panspermia, the idea that life in the form of extremely resistant microbes (extremophiles) is in fact present everywhere and is carried serendipitously to their destinations (Than, K. (2019 ) How Did Life Arise on Earth?). Earth, was one of these supposed destinations and with water present, the microbes could proliferate and expand. This theory, however, does not come without some criticism, such as where did life begin in the first place? Though due to the nature of the hypothesis (life came from elsewhere), it is hard to experiment to either prove or disprove it.
The initial atmosphere of the earth most likely consisted of hydrogen and helium (main gases in the solar nebula). Scijinks.gov. (not dated) How did Earth's atmosphere form?) However, as these molecules are extremely fast, due to the heat, and small and so would have easily escaped earth’s gravity. Further on, due to rampant volcanic activity, huge amounts of carbon dioxide, ammonia and methane (which later diminished due to reacting
One of the largest steps in the evolution of life was the dawn of eukaryotic (cells with membrane-bound subunits, like human and animal cells) and prokaryotic cells (opposite of eukaryotic) and later on multicellular organisms. One such pivotal bacterium was Cyanobacteria or blue-green algae which arose around 2.7 billion years ago. These cyanobacteria synthesised oxygen from dissolved carbon dioxide, which was the predominant gas in the early ‘atmosphere’ using water as a medium and sunlight as the energy source (photosynthesis). Over hundreds of millions of years, bacteria that could photosynthesise eventually made enough oxygen equivalent to around 1% of the oxygen we have today (~21% of atmosphere). (Scijinks.gov. (not dated) How did Earth's atmosphere form?)
After billions of years of oxygen production, some organisms evolved to be multicellular and aerobic, or oxygen dependent being more efficient in the production of energy within the organism. This newfound use of oxygen naturally favoured organisms that could utilise oxygen and so various new species of life in the oceans emerged. Scientists believe that these environmental changes and the birth of new species may have resulted in the Cambrian Explosion (541-516 million years ago) which was an exponential increase in the diversity and magnitude of life in the oceans.
The introduction of multicellular organisms allowed for sexual reproduction and the passing down of traits through chromosomal inheritance.This allowed for both positive and negative mutations within species allowing for even greater genetic diversity which eventually led to the first terrestrial animals as it was favourable in their environment. Plants, due to the incredible levels of oxygen during this era also began to grow on land, becoming a food source for the early terrestrials. Furthermore, due to the continued synthesis of oxygen from carbon dioxide by cyanobacteria, atmospheric oxygen broken down by Ultraviolet radiation from the sun combined with gaseous oxygen to form O3, commonly known as ozone protecting the first terrestrials from deadly radiation. (Albany.edu. (not dated) Ozone)
With the stage set for the dawn of life on land, the first terrestrials in the palaeozoic era roamed the earth whilst consequently avoiding/were less affected the greatest extinction event in earth’s history - The Great Dying, the Permian-Triassic extinction event. This extinction event killed off around 95% of all life in the oceans and around 70% of all terrestrial vertebrates on the planet. (En.wikipedia.org. (not dated). Evolutionary history of life.) There were 5 mass extinction events in Earth's history, Ordovician-silurian, late Devonian, permian triassic, triassic-jurassic and Cretaceous-Paleogene extinctions. All of these extinction events served to “bottleneck” both gene pools within.
This brings us to now. Over 3.7 billion years of evolution, mass extinctions and environmental pressures has eventually led to the most intelligent and complex species in the planet. Humans. That evolved from early hominids, who evolved from chimpanzees, who evolved from early mammals after the Cretaceous-Paleogene extinction event, all the way back to the first unicellular life form on our planet.
The evolution of life on earth was incredibly lucky with water being provided to the planet and Earth being in the Goldilocks zone of the sun. Life had a slow beginning, but over time organisms began to change the planet eventually altering the composition of the atmosphere enough to support terrestrial animals. Despite the various extinction events and loss of diversity, life on earth continued to evolve and adapt to what it is today. Understanding the nature of life’s evolution on Earth may allow current and future scientists to understand how life may or may not occur on similar planetary bodies in the universe, perhaps helping to answer the age old question whether there could be intelligent life out there in the universe.
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How Did Life on Earth Begin? Essay.
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