Chapter 25 The History of Life on Earth
There is scientific evidence that states that in the early stages of our earth’s life, life could not be possible. The earth was still being bombarded with large chucks of rock, and huge pieces of ice but the ice would be vaporized on impact so oceans were still unable to form. All of this was going on about 3.9 million years ago, but after this time the bombardment started to calm down. We believe that the first atmosphere was very thick with water vapor and a series of other elements and other gases released by the volcanic eruptions that were still happening frequently.
It was in the 1920’s that a Russian scientist named A. I. Oparin and British scientists named J. B. S. Haldane hypothesized how organic molecules were created. Organic compounds are compounds that are carbon based, and on earth we all are carbon based life forms. Oparin and Haldane believed that if earth’s early environment was mimicked then organic compounds could be created. It was this hypothesis that save us a possible idea of how life formed on earth, the also hypothesized that the organic compounds were formed when molecules were exposed to lightning of high levels of UV radiation.
In 1953 Oparin and Haldane’s hypothesis was put to the test by Stanley Miller and Harold Urey at the University of Chicago. They created the conditions of early earth in a laboratory, and they were able to create amino acids and other organic compounds, there have also been many different experiments with different atmospheric components that have also created organic compounds; but there have also been experiments that have not. It is still unclear where exactly the first organic compounds came from, if they were created near a volcano, under water vent, or if they even came from space by a meteorite.
It is known that even if on early earth if amino acids were formed that is not all that is needed to create and support life. Cells need not just a very large amount of macromolecules like enzymes, proteins, and nucleic acids just to name a few to survive and reproduce. But scientists have been able to produce amino acid polymers by dripping amino acids clay, rock, and hot sand. So it may have been these amino acid polymers that may have helped the production of life on earth.
Now we need DNA and a metabolism to survive, but that is where the problem comes in you need one to have the other. To replicate a cell needs DNA, but to get the materials to replicate DNA needs elaborate enzymatic machinery and copious supply of nucleotide building blocks and both of those are created using a cell’s metabolism. It is possible that first forms of life were created using protobionts which are collections of abiotically produced molecules that are surrounded by a membrane. Liposomes could have been the first basic form of life because they can reproduce, they have a basic membrane, and they are able to stay chemically balanced.

When life started it is very unlikely that organisms used DNA, it is much more likely that they used RNA. The RNA played a big role in protein synthesis, so these RNA catalysts are called ribozymes. It would be much easier to start off with RNA because of it is singled stranded; also because of the errors made while replicating RNA this is the start of evolution and natural selection. It would be those errors that would start diversity, and then those errors would be passed down to the daughter cells, and eventually the offspring. Eventually the RNA would turn to DNA in an organism, then that organism would live the benefits of DNA such as it is more stable and replicates more accurately. Then RNA based organisms started the step aside to DNA organisms.

Fossil records are what give us a look back so we can actually see what life was like back before humans even existed. It is because of fossil records that we know certain animals existed; it also shows us the process of evolution. Looking at fossils records can show the slow changes over time that created the animals that we see today.
When fossils are dated they are not dated exactly, but they are dated to the general area of time that the organism lived. It is when we date the fossils that we can see the process of how evolution happened, such as what traits were useful and were passed down; and which ones disappeared over time. The way that fossils are dated is but using a technique called radiometric dating, and the way that radiometric dating is by measuring the half-lives. They measure the half-lives of the isotopes that are collected in the organism over its lifetime.
Fossils show us the process of evolution from the very small changes, to the changes that create new species. It is fossil records that show us how humans and chimpanzees had a common ancestor at one point, and how we both branched off and eventually became two different species.

Throughout life on earth there have been many different times that really changed everything on earth. Such as the arrival of the first single celled organisms, this was the first step of life, what got us started. This is what allowed all life on earth to be possible at all.

It was not just the creation of life that changed everything, now there was the creation of photosynthesis and the oxygen revolution. Photosynthesis allowed some organisms to create their own food so that they did not have to use energy to go and find energy. This was a huge step in the creation of plants in general. Second was the oxygen revolution this was when the levels of oxygen were so high that it actually started to attack the organisms.
Then was the emerging of the first eukaryotic cells, now the cells have many different characteristics that prokaryotic cells don’t have. Now is when cells can actually come together to make up one big organism instead of just one single celled organism. But it was not for a while until the eukaryotic cells can together to make a multicellular organism, but the earliest multicellular cells were recorded to be about 1.5 billion years ago. Now the bases of all multicellular organisms began, and evolution slowly changed it and created everything that you see today.
It was is the Cambrian period that animals started to eat each other, before this time there was real no predation but most animals were herbivores. It was during this point in time that claws and teeth were developed in the idea that they would be eating each other. It was right after this that animals started to find their ways onto land. Fins started to become feet, and gills became lungs. Not just like that, but very slowly over time.


Over the earth’s very long life it has not just been the living organisms that have change so has the world. It has changed in the way that the very ground that we are living on is moving. Over large periods of time the continents that we know today move to different positions around the world. At one point in the earth’s history all of the continents were together making one large land mass. Over many, many years the continents separated to leave us with what we see today. This did not just affect the world, but it affected the organisms that live on it. When the land masses were separated enough organisms would evolve differently to adapt to different situations. So animals that started out the same animal may have changed into two completely different animals because they were separated. When they were separated they had to adapt to the different situations it is not just the separation of the land on earth that can change the lining organisms on it, mass extinction is another way to change life on earth. Mass extinction will wipe the slate almost clean, this is when only the lucky ones survive and a huge change in the animals happens, Such as the change from the dinosaurs to mammals after that mass extinction.


Our genes are what make us who we are, and what we are; without genes we would literally be nothing. So when our genes change we change too, even though those errors in genes are rarely seen there are changes in our genes that will be seen. It is the smallest changes in our genes that can have the biggest change on the lives of animals in the future.


Evolution is not goal orientated it is all random changes that happen over millions and millions of years, evolution that works happens completely by chance. There is really no way that evolution could be goal orientated because evolution is based off of mutations that happen to us against our will, and if they work they it would be spread around; but if not it will either disappear or not make much of an impact. There are trends and patterns that come out of evolution, but it is not because the chance of all of us having eyes just happened. Trends in evolution shows that we all are related in some way we all have a common ancestor, but there may be some evolutionary chances that do happen more than once. We are all here by luck or chance, it is chance that we have everything that we have. Not just chance that a long time ago that an organism’s body was able to develop certain characteristics or traits, but also that those characteristics were actually useful and gave us another edge on life.