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The theory of evolution, this is an extremely popular and widely renowned theory, proposed by two people, Alfred Wallace, and more notably, Charles Darwin. These two men had extensive knowledge of nature and how it progressed, and these observations were a vital key to how an idea evolved into a brilliantly devised theory 
The image to the right is very commonly used to portray the evolution of man, as it shows a monkey (Left) slowly evolving over time to become what we are today as humans (Right) 
Evolution is how a species becomes more complex, and better suited (or adapted) to their environment, through the process of ‘Natural Selection’. This means that a pair of a species, with superior genetics, are likely to reproduce (as they are more capable in surviving the conditions they live in), combining their genes resulting in a slight change in offspring. Over numerous generations, these changes become more apparent and this is evolution. As generations go on, the genetic material within DNA changes, mutates, (sometimes enough to allow a generation to branch off into a completely new species!) and realistically improves from the last generation (as an inferior set of genes is less likely to be passed through numerous generations successfully) this results in the species becoming better suited to their environment, through the process of evolution 
To ensure survival of a species, a member of that species must be capable of surviving long enough to find a mate, allowing them to successfully reproduce, by avoiding being killed, and finding sufficient nutrition. Commonly this must the fought for, as there will be competition from many other members of that species looking for a mate, and even more other species hunting for food, and food may even be the species in question!
Not only must the creature survive, but it must protect its offspring during their early years to avoid them being killed. Some organisms manage all of these successfully; others may have a smaller number of offspring, while some may die before they can reproduce. The members of the species who do manage to reproduce, are superior to those who don’t, meaning those individuals are more ‘fit’ for survival (this may also be known as survival of the fittest) than the others. The genetics, adaptations and mental and physical traits of the individual, all resulted in the ‘fitness’ that was superior to that of other members of the species. These characteristics are then likely to be ‘inherited’ by its offspring, making them more likely to survive and reproduce and this continues over many generations to result in the evolution of the species. This process is called ‘Natural Selection’ 
A good example of how genetic mutations and natural selection occur is by looking at how finches adapted to make use of their mutations. Charles Darwin studied this, and how finches evolved and mutated to have different types of beaks, and how the resulting characteristics benefited them in survival. Through surviving the process of natural selection, and evolving over generations to allow their mutations to develop, the species of the finch was provided a great advantage over others 
Different beaks allowed the finch to feed differently, not only does this benefit the individual to survive, as they are better adapted to where they live, but also helps the species as a whole, as it means there is reduced competition for food against other members of the same species 
This is a brilliant display of how evolution occurs through the process of natural selection, and how this helps benefit the species further to allow new mutations to improve the species to a greater extent 
Survival of the fittest is the reason natural selection exists, put very simply, it is the idea that those better suited, or ‘Adapted’ to their environment, are much more likely to survive and pass down their genetics through reproduction, in contrast to those that are not as well developed 
As these superior genetics are passed down upon generations, some will mutate, sometimes benefiting the species, sometimes hindering them. Survival of the fittest means that those who mutate in a positive way, will have a benefit when competing for food, shelter, and a mate, and will therefore be more likely to survive, allowing these genetics to be passed on. Those who mutate in a negative way, will not be able to perform the before mentioned tasks as efficiently, and are then more likely to be killed, or starve. (Image left )
These mutations and adaptations don’t happen overnight, through a single generation, or even ten! For genetic mutations to have a noticeable impact, a particular set of genes will have to be copied and passed down thousands of generations, slowly changing that particular family tree of a species, either for the better or for the worse. This also means that sometimes a single species will branch off into many others. Looking at Darwin’s finches (above) shows how a single species can have different sets of mutations, some that are less beneficial than others, will eventually die due to natural selection (survival of the fittest). An example of this is shown by the diagram on the left; it shows how the human species has evolved over millions of years, and how it has branched off into others over time.
All of this concludes that survival of the fittest, allows superior genetics to survive, and inferior ones to meet their demise, this is what allows natural selection to exist 
It is theorised that DNA once began as ‘a simple self replicating peptide’, of which would contain 32 amino acids, that were supposed to form in the Earth’s oceans. Once this self-replication had completed, the theory of natural selection would take over, so those that had evolved to protect themselves from danger, and that were able to adapt to their surroundings through evolution, were capable of surviving longer than others. An extended survival then resulted in more reproduction, thus spreading, allowing the molecule to travel throughout generations. Some molecules would have evolved to be contained within a lipid bubble, would have a better chance of survival, passing itself down the generations, leaving itself open to further evolution. After this process has prolonged, the lipid bubbles eventually turned into cell membranes, forming DNA molecules 
These DNA molecules are how they are as we know them now, a collection of complex molecules possessed by every living creature, the DNA containing genetic information about its host. After DNA itself evolved, over longer periods of time it also caused their hosts to evolve, through the form of natural selection, and through mutations within the DNA itself. This can be shown by looking at humans and chimps, the fact that we are very similar suggests (using the theory of evolution) that we share a common ancestor, which if true, would mean that we humans and chimps both branched off from our common ancestor due to different accumulations of mutations within the DNA 
Here is an example of a double-helix DNA molecule (left). It consists of amino acids, 4 of them, labelled: Adenine, Thymine, Guanine and Cytosine, and these 4 amino acids are responsible for the diversity of life on earth that we see today. All individuals possess these same four amino acids in their DNA, what makes us all different is due to the variation in how these amino acids are arranged and linked together by hydrogen bonds. The DNA backbone is made up of phosphates and sugars, its job is to simply hold up the molecules structure, its substance is also very flexible, which is necessary to allow the molecule to spiral into the double helix 
There are numerous types of creationism, and lots of other beliefs that are used in conjunction with these, it is a very widespread and popular belief, heavily renowned by Christians. Different forms of creationism can include: Raelian, Panspermia, Catastrophic Evolution, Islamic, Vedic, American Indian, and many other beliefs from other cultures, however the most common form of creationism is Christian creationism, which branches off into many different groups, and I will talk of a few of these briefly.
Christian creationism as largely followed in the US, due to Christianity being such a prevalent religion in that country, not all Christian creationists follow the exact same beliefs, and can be split into different groups. These groups of Christian creationists consist of the following: ‘Flat Earthers’ – this is the belief that the earth is indeed flat, this is taken from the Bible; suggesting that the earth is also covered by a dome, surrounded by water, this water being what is believed by these flat earthers, as to what created Noah's flood. The bible reading from where the belief originated is from a literal point of view of “The four corners of the earth”.
‘Geocentrists’ – As opposed the flat earthers, geocentrists accept the fact that the earth is spherical, however they do not accept the idea that the sun is in the centre of our solar system, or that the earth even moves at all. Alike flat earthers, this is reason for geocentrists to state what they caused Noah’s flood, this being a solid firmant containing water above the still earth. The basis for their belief is again a literal reading of the Bible. "It is not an interpretation at all; it is what the words say.” An example of the geocentric model is shown to the left , the diagram shows how geocentrists believe that planets orbit the earth.
‘Young Earth Creationists’ –Or YEC, is probably the most common kind of creationism to date, which is again based upon a literal reading from the Bible. These beliefs are that the earth is 6000-10000 years old, and that earth and life was created by God in 6 days, that death was resulted from Adam and Eve, and lastly that geology must be in the terms of Noah’s Flood. The idea of the earth being spherical and that we are in a heliocentric solar system is accepted.
Those few examples of creationism theories consist of either the most extreme views, or the most common views, basically contrasting each other, giving both ends of the spectrum. This shows that the belief of creationism, ironically alike evolution, has branched off into many different forms of creationism 
Other major religions also share views on their concept of creationism too, these religions being Muslims, Jews, and Hindus. The Islamic view on creationism is slightly different to that of other religions, in that they believe creation not to have been the ‘Big Bang’ as such, but rather, “the heavens and earth” joining together. Whilst the literal readings of the Koran do not promote the idea of the big bang, they do not dismiss it either. Further readings from the Koran also state that ‘Allah’ created everything, the planets and the sun, and even their orbital paths… it also suggests that he created ‘laws’. All of this, much alike the more widely accepted view on creationism, was done within 6 days according to the Koran 
As for Jewish views on creation, they believe once again in the 6 days of creation, and a 7th of rest by God. However these ‘days’ are not viewed as literal days, but instead viewed as ‘a specific stage of creation’, each ‘stage’ being the creation of something different. More specifically: Day 1, gasses; Day 2, water; Day 3, dry land and plants; Day 4, fish and birds; Day 5, animals; Day 6, humans. This is also accepted due to being similar to scientific evolution, and some even believe that the evolutionary process is based on this view!