Question:
Review the earliest evidence for life on Earth. What form does the evidence take and where is it found. Discuss the controversies relating to some of this evidence and give your conclusion on the earliest date at which we can be confident that life was present on Earth
Talking about Astronomy, one cannot miss about the plenty of evidence that has been found to prove that about the existence of life, since ages. Different scientists claim that the presence of life was present ages ago while others state that the life wasn’t present so long. There is plenty of evidence in the form of fossils and the bodies of few organisms aren’t preserved as they were present in a very cold weather. Apart from that, the atmosphere in the planet has changed completely and hence, this atmosphere is creating an impact on the way of life. All these things need to be considered so that the study related to astronomy can be completed.
Different types of records are found by the scientists and hence, existence of life is proven. Finding fossil records is one of the easiest ways by which the scientists are trying to gather proof with regards to the existence of life, in the earth. Fossils are normally formed on the sedimentary rocks and these sediments are normally found below the bed of water body. Very less organisms are preserved in the sediments as the body of the organisms decay before the fossil in the sedimentary rocks. Few of the organisms may be formed on the rocks and the scientists may get the complete body of the organisms. On the other end, few of the organisms may not be preserved but the scientists will find shells and bones of the organisms and they will understand about the traces of life present (Science n.d.).
It is very difficult to find fossils of organisms which were into existence long back. During the Cambrian period, organisms with shell and bones weren’t existent and hence, it was difficult for soft organisms to form fossils. Scientists don’t find any fossils because most of the organisms were in microbial form. Apart from that, with time, recycling process takes place and hence, the scientists will not be able to find evidence. When the earth’s decomposing takes place continuously, the earth’s plates gets deformed and hence, it is difficult to understand the era in which the particular organism was present.
Darwin’s Dilemma was a problem in finding the fossils before the Cambrian period. It is very difficult to find fossils as fossils weren’t available during the Pre-Cambrian period. Lack of fossils states that there weren’t organisms present before the Cambrian period. There are fossils which present but none of the fossils can be proved as something that was present before the Cambrian period. In the year 1965, two scientists stated that they have found fossils which are 2 billion years old. This was found near the Gunflint Chart near the Lake Superior. The fossils were present in the form of stromatolites and microfossils of the organisms. After this, few of the other scientists found fossils which were 3.5 billion years old. Talking of 3.5 years old fossils, those fossils belong to Pre-Cambrian period.
Stromatolites are structures formed as layers by the colonies of micro-organisms. This is built by cementing or trapping together multiple organisms and they are normally formed over the layers of rock. This is not present everywhere. It is present only shallow water bodies. Shark Bay in Western Australia is one of the most common places wherein stromatolites are present in plenty. Most of the stromatolites have disappeared and they are just found in hypersaline waters. If the scientists are into research of Stromatolites then they will see that it is commonly found during the proterozoic and Archean periods but finding stromatolites before that period is also difficult. Few of the scientists stated that stromatolites aren’t biological creatures and hence, it cannot be considered as a life but it was seen that stromatolites are found only in association with microfossils and hence, it is proven that it is a biological creation.
Looking at the history, it can be said that the presence of life is quite controversial. Different scientists have different opinions about the life that was prevalent before millions and billions of years. There are plenty of fossils which have been stated to be found some 3.45 billion years ago but different scientists have different opinion over that. Bill Schopf states those 3.45 billion years ago, microbial life had just started to flourish and it these fossils would have been widespread at that point of time. On the other end, Martin Brasier states that the ones which are found aren’t fossils; he believes that it is just formed by reaction between vein chert and volcanic glass. 3.43 Billion year old stromatolites are present in the rocks of Pilbara and these are considered to be the oldest form of stromatolites. They are believed to the stromatolites of reef which are found deep inside the water (The telegraph n.d.).
Understanding of geological period is one of the other ways by which the presence of life on earth can be found. The layer of rocks will describe about the era. The scientists will understand that each era might have had different types of organisms. With the help of studying the rocks, the scientists will understand the era in which the organism was into existence. Geological studies have been in existence since long and hence, the scientists can find fossils which are more than 545 million years old. Geological studies states that life has been into existence for more than 4.6 million years ago.
Molecular record of life is the third type of life and this record tries to understand the existence of life by way of studying the molecules that is present in the DNA of the living organisms. The DNA of the organisms is compared so that the relatedness between two of the DNAs can be understood by the scientists. After comparing the DNAs of Chimpanzee and humans, it is seen that 98-99% of the DNA was matching and hence, it has been found that they are interconnected. Few of the genes is common for most of the life on earth and hence, it can be said that most of the organisms are formed by way of transcription, translation and replication of the DNAs which were present (Physorg 2013).
The tree of life can be used to describe the tree of life. This tree states that there have been three different domains. Eukarya, Bacteria and Archaea are three different domains of the tree and the whole concept of DNA is based on this tree. After some research, it was stated that the tree consists of five domains and not three but plants, animals and fungi was considered in the Eukarya kingdom itself. Bacteria and Archaea are simple and small cells and taking this form of life to the next level, it can be seen that Eukarya came into picture. Eukarya are complex and larger cells. These cells also have nucleus and other structures in it.
The formation of eukaryotic cells is done by way of endosymbiosis. Endomsymbiosis is a symbiotic relationship wherein one organism is present in the body of the other organism. Organelles in the eukaryotic cells are due to the symbiotic relationship with the eukaryote ancestor. Chloroplasts are another form of life which existed billions of years ago. The process of photosynthesis in the plants, takes place inside the chloroplasts. Chloroplasts have evolved with time from cyanobacteria. Cyanobacteria evolved due to the process of photosynthesis and this photosynthesis took place some billion years ago. Chloroplasts have formed due to the symbiotic relationship in the ancestor cells. This is known by way checking the DNA molecules. The DNA moles of Cyanobacteria and chloroplasts are exactly same.
Respiration is the next step of the organism. Respiration process takes place only in structures called mitochondria. Mitochondria are present in complex organisms like that of plants and animals. This is also descended from the symbiotic relationship of bacteria. The connectivity between Cyanobacteria and Mitochandria is answered with the help of the DNA analysis (Astrobiology 2013).
The atmosphere of the earth also has changed over the period of time. The earth’s atmosphere has played a very important role in the evolutionary process. Initially, the intensity of the sun was not as high as it is today. It was just 70% of what is today. If the sun would have just been the same today as it was in the past then the humans and other living organisms in the earth would have been frozen. The greenhouse heating would have been much more than what it is today. In the past, there was no oxygen but today, there is 20% oxygen. Many of the scientists may state that the change in the level of greenhouse gases has changed due to the way of life but that is not true completely.
Evidence states that oxygen started to build on earth only 2.4 billion years back. Oxygen came into picture only when the cyanobacteria were involved in the photosynthesis process. Oxygen was considered to be toxic some 2.4 billion years ago and it was a gas which would easily react with any toxic life. In fact, most of the species have become extinct due to the existence of oxygen (The Archean n.d.).
There is plenty of evidence in the form of fossils and the bodies of few organisms aren’t preserved as they were present in a very cold weather. Apart from that, the atmosphere in the planet has changed completely and hence, this atmosphere is creating an impact on the way of life. Very less organisms are preserved in the sediments as the body of the organisms decay before the fossil in the sedimentary rocks. In the year 1965, two scientists stated that they have found fossils which are 2 billion years old. This was found near the Gunflint Chart near the Lake Superior. The fossils were present in the form of stromatolites and microfossils of the organisms. Different scientists have different opinions about the life that was prevalent before millions and billions of years. Eukarya, Bacteria and Archaea are three different domains of the tree and the whole concept of DNA is based on this tree. Endomsymbiosis is a symbiotic relationship wherein one organism is present in the body of the other organism. Cyanobacteria evolved due to the process of photosynthesis and this photosynthesis took place some billion years ago. Chloroplasts have formed due to the symbiotic relationship in the ancestor cells. Respiration process takes place only in structures called mitochondria. Mitochondria are present in complex organisms like that of plants and animals. This is also descended from the symbiotic relationship of bacteria. Oxygen was considered to be toxic some 2.4 billion years ago and it was a gas which would easily react with any toxic life.
References
The Archean, The first life of earth. Viewed on January 15, 2015.
Astrobiology, 2013, The oldest signs of life on earth, Viewed on January 15, 2015.
Physorg, 2013, Earliest evidence of life found: 3.49 billion years ago, Viewed on January 15, 2015.
The telegraph, ‘Oldest signs of life on Earth found’, Viewed on January 15, 2014.
Science, Earliest life on Earth: scientists find evidence in WA rock sediments, Viewed on January 15, 2015.
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