With help from meteorite samples, lunar samples and other evidence, scientist have come to the general consensus that Earth is approximately 4.5 billion years old which is a long time to develop the beginnings of humanity. It is logical to state that Earth may not have been able to support life forms as soon as it came to be; therefore life on Earth began as soon as Earth was stable enough to support it. Evidence, specifically the fossils of stromatolites (mainly the structure of these bacteria who had the ability to photosynthesise), shows that life on Earth began approximately 3.8 billion years before the present as these cyanobacteria had already developed cell walls to protect their protein producing DNA. Even then, no one knows exactly when, how or why life arose on our planet but there are many theories which can't be fully proved nor denied. 

 

Before heading in to the theories of the arising of life on planet Earth, it is important to remember that the Earth 4.5 billion years before the present was very much different to the one we know today. The climate was a mix between hot and humid weather, with electric storms being frequent if not normality and there was a great deal of volcanic activity. The atmosphere was believed to have been composed of methane, carbon monoxide, carbon dioxide, ammonia gas, nitrogen, hydrogen and water vapour. This belief was supported by the experiment conducted by Stanley Miller and Harold C. Unley which involved all the aforementioned gases being shut in an airtight container while being struck with electric currents to stimulate the electrical storms of early Earth. Miller and Unley found that in time, a reddish-brown layer formed on the outside of the container, which contained 11 of the 20 amino acids used on Earth today, nucleotides and ATP. They also found that these masses could polymerise to form larger masses, becoming the first simple proteins and nucleic acids. 

 

Of course, there is not right or wrong theory as to how life began on Earth; only that some have more evidence to support it than others. Here are a list of the theories and their explanations: 

 

• Distant Space- This theory claims that life never began on Earth but instead, somewhere else in distant space. Life forms just somehow made it to Earth. This theory is often added onto, with scientist and people claiming that life can here on the tails of meteorites that landed on Earth. 

• Multiple Risings- This isn't actually a discussed theory yet but there are statements that life may have arisen more than once on Earth, only that we just don't know it. ​

• Big Bang Theory- One of the most well known theories, the Big Bang Theory, suggests that there was a huge explosion that caused space to expand. No one is really sure how or why there was an explosion, but as the universe expanded, the resulting energy was converted into matter and anti-matter (more matter than anti-matter). The anti-matter cancelled out the matter but the remaining matter was scattered in all directions, due to the expansion, and began to cool. There has also been evidence that shows there was actually no real explosion but more of a sudden expansion (Mastin, 2015) but that is yet to be confirmed. Of course, The Big Bang Theory is only a theory but there has been sufficient evidence to support that the universe might have been the result of an explosion.

• Religious- God created Earth in 7 days and created life on days 3,5 and 6. 

• Panspermia- the theory that life exists throughout the universe and was distributed by meteoroids, asteroids and planetoids (much like the distant space theory but is more specific)'

• Spontaneous Generation- life arose spontaneously, there's no real explanation. Just that life soemhow came to be at a certain time period without no cause or precursor. 

• RNA World Hypothesis- RNA was the first most dominant molecule, responsible for creating life or guiding it through the stages. It continued to fulfil this role until the development of DNA and proteins, which were able to do the job in a much more efficient manner.

• Anthropic Principle- The universe consists of a billion billion planets, therefore the chances of life arising on one of them is highly likely. For us, it just happened to be Earth

• Primordial Soup of Organic Compounds- the belief that life began in the ocean, where the organic compounds found in the primordial soup were rich in ammonia and methane. These organic compounds were self-replicating and are thought to be the precursors to life on Earth. This theory is specific in the sense that it does not belive that the compounds formed WERE life itself, but were the first step in achieving life on Earth. 

 

Once again, it is important to note that none of theories are right or wrong. We don't know if we ever will find one correct explanation to satisfy our questions since the chances are unlikely, but we do know that no matter how we came to be, we will always be grateful for the lives we've been given!