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Evolution is responsible for diversity of life on earth because:
The fossil record is the source of the majority of solid evidence for evolution; Charles Darwin had collected many fossils for his studies and this began the fossil records. (An example of a fossil is given on the right ) There are different ways the fossil records can be interpreted to backup the theory of evolution. The first way the fossil record does so, is by observing different subgroups of vertebrate, the main ones in this case, being: fish, amphibians, reptiles and mammals. By observing slight changes between these subgroups of species throughout the fossil record allow us to deduce that they evolved into each other in the before mentioned order 
This then allows us to observe that an amphibian, perhaps a frog for example, is in-between a fish and a mammal in evolutionary terms, and this again can be proved by fossils through observing the adjustments in the species over time. A frog such as a Californian newt is a good example of this, as it has gills, alike that of a fish, however boasts four legs as opposed to fins, alike mammals. This example allows us to deduce the order in which they evolved, as fish must have evolved into amphibians, and then mammals, as it is much more likely than gills being ‘lost’ in the evolutionary process, and then regained later on, as it would have had to have gone if fish evolved into mammals, and then amphibians 
The forms of modern vertebrates alone, therefore, enable us to deduce the order in which they evolved.
This conclusion can be tested against the fossil record, and the fossil record is capable of supporting it due to fish, amphibians, reptiles, and mammals all appearing within the fossil records in the exact order as they inferentially should have evolved.
This inference provides strong evidence for evolution, due to the fact that if fish, amphibians, reptiles, and mammals had been alternatively created, for example through creationism, then they would not appear in the fossil record in the exact order of their apparent evolution 
The chart (left) helps to present this data, through observing the anatomy of the modern forms of vertebrates. It gives an example of what order of evolution these observations suggest, and what it does not. It then proceeds to show a graph, presenting the order of evolution of different species and branches of species throughout different ages of time.
Another example to further strengthen the mantel, of which fossils have created for the evolutionary theory, would be from looking at the fully formed mammal, the rabbit (fossil to the right). The rabbit must have evolved through the apparent stages of evolution, and therefore shouldn’t appear in the fossil record millions of years before its ancestors… and it hasn’t. This simple argument has proved to be an effective staple for the theory of evolution as many of those who are against evolution have appreciated this argument 
Due to the fact that nobody is capable of obtaining such a fossil of a rabbit millions of years before its ancestors (leading us to assume that it does not exist), that deviates from the norm of the apparent appearance of subgroups of modern vertebrate. This means that all those that have been discovered must follow this trend, thus proving this theory. This gives us conclusive evidence that the fossil record does provide good evidence for evolution 
[Left] An image showing a fossil of a “Dickinsonia Annelid”, obtained from the White Sea in Russia. It is an example of a fossil from the ‘Precambrian’ period, around 600-550 Million years old.
The Dickinsonia Annelid was once thought to be an annelid, it may also be interpreted as a Cnidarian similar to the modern-day Fungia 
The following are other ways fossils have backed up evolution.
DNA is a collection of complex molecules, containing genetic information about its host; this is the case for every living creature. These complex molecules can also be used as evidence for the theory of evolution, through mutations, and genetic similarities between species.
DNA can provide evidence for the theory of evolution, is through observing genetic similarities between different species. A classic example of this would be through the fact that we humans share 96% of our DNA with a chimp, and through observations, the physical similarities can show. This genetic similarity suggests that we are descended from chimps, and therefore also descended from a common ancestor. Humans and chimps have many similarities related to there physical structure, so we would expect the same from there DNA, and due to the fact that we and chimps must have evolved from a common ancestor, we too must have evolved from chimps, thus giving another solid example of evolution through genetic similarities 
Another example of this would be between Homosapiens, and Neanderthals, according to fossil evidence, we share physical properties to Neanderthals, also suggesting a genetic link to them. The image to the right shows scans of Homosapien chromosomes (top), and Neanderthal chromosomes (bottom). The parts of the chromosomes highlighted red (directed by arrows), according to the key, are repetitive parts of DNA, and according to the image, we share a fair amount of DNA with Neanderthals, suggesting that we both have a common ancestors. This once again, strengthens the proof of evolution 
This great diversity of life is somewhat unusual if you consider the fact that the amino acids within DNA is the same for all living creatures, this provides the fundamental molecular unity of life with means of evidence. This can show in similarities between species though, for example every living creature has to obtain energy to grow and reproduce, this applies for both animals and plants. The similarities also continue into bodily structure, and this is more to the point, as a similar body structure between different species’ provides evidence that those species had a common ancestor, this is evidence that evolution exists as the species’ in question would have both began as this common ancestor, and through genetic mutations in there DNA, they split off into different species and evolved into what they are now 
MRSA is a living example of evolution, such as many other diseases. Its evolution is shown between two particular strains of disease, caused by mutations in there genetics during meiotic division.
The two forms of these bacteria are:
Methicillin-resistant Staphylococcus aureus (MRSA) also known as the epidemic MRSA against which Methicillin antibiotic has no effect.
Methillin Susceptible Staphylococcus aureus (MSSA). This form is still contained by the use of the antibiotic Methicillin.
Evolution is simply an accumulation of small genetic mutations spread over a large period of time, generally observed within a species of animal; however it is tougher to observe any major mutations due to the fact that it takes a long time for enough to accumulate. On the other hand, with a bacterium such as MRSA, genetic mutations show a greater effect, due to the fact that they are simply smaller, therefore smaller amounts of mutations have a larger effect on the bacterium, making these changes more noticeable. In many cases, the evolution of such bacterium causes it to split off into different strains of the disease, alike how a species of animal splits of into sub-species’.
These mutations in MRSA simply prove how a living organism can evolve, and change into a completely new creature over a particular period of time.
The process by which MRSA evolved occurred as follows. Antibiotics selectively kill susceptible forms of the bacteria, and the antibiotic is the selection pressure on the population of Staphylococcus aureus. The species of MRSA mutates randomly, producing a resistance in its genetics at low frequency in the population. This newly mutated gene can then be transferred via plasmids to other MRSA bacterium and frequent uses of antibiotics then puts this gene at an advantage of surviving over the MRSA that do not possess this gene, and therefore allows this gene to reproduce. The descendants of this MRSA bacterium will then also carry this gene and its resistance to the antibiotics increase in frequency and at this point the MRSA bacteria has evolved into two different forms.
MRSA is living prove of evolution through genetic mutations, and therefore strongly proves the theory of evolution, it may be on a much smaller scale, but it is still proof none the less 
Embryology can provide evidence for evolution, as it can show how something can change dramatically through evolution from an accumulation of smaller changes. This is achieved by switching certain genetics while an embryo is in development, which can in turn result in numerous different outcomes.
The image (Left ) shows a human embryo at its fifth week of development, and you can find that it looks incredibly similar to a fish embryo (Right ). The folds beneath what is to be the head of the human embryo and above the heart are ‘branchial arches’ and these develop into gills for fish. These branchial arches in humans perform their own changes, as they develop into bones, the larynx, carotid artery, neck muscles and cranial nerves. This shows that the embryo’s of a fish and a human begin as very similar organisms, suggesting that we both evolved from a common ancestor to attain these similar traits 
This however does not only apply for humans, but for all vertebrates, amphibians even begin as a foetus alike that of a fish. This would suggest that a fish is capable of evolving into an amphibian, and all of this applies for reptiles, they however evolve from a fish to amphibian, then again to a reptile, adding another stage to evolution. This follows on again by the same process to evolve into a mammal, the fact that this mirrors the same sequence of evolution is unlikely to be mere coincidence, and therefore embryology helps prove that evolution has occurred and that we have had a complex evolutionary history 
http://anthropologynet.files.wordpress.com/ 2007/06/neanderthal_skull_vs_homo_sapiens _skull.jpg