The theory of evolution has been hotly debated over the years. The evidence supporting evolution is great, yet it is also lacking enough to not be a proven fact. The concept of evolution and the terminology involved can be daunting to understand. Basically the idea is that Earth was created and there were no life forms involved. Then a single celled organism developed and that all life forms on earth today developed from this single celled organism, of course over a long period of time. The evolution theory states that life began with a single celled organism which then evolved into multi-celled organisms then evolved into invertebrates(like jellyfish) then on to vertebrate creatures(fish) then into amphibians then on to reptiles, then birds, they then evolved into fur-bearing quadrupeds, then came the apes, and they then evolved into current day man.
The fossil record has been a corner stone in the evolution theory. Fossils are a rare occurrence, specific conditions are required to produce a fossil. The dead creature must be covered by dirt, sand, or rocks immediately. Usually when creatures die scavengers eat it or it simply decomposes. Our fossil record goes back to over 570 million years ago. Fossils are deposited into a part of the earth called the Sedimentary Rock, which forms in layers. Each layer, or Strata, represents a time period. Scientist can then start at the beginning and work their way up to current day, removing fossils and watching the changes species undergo. One of the best examples of evolution is that of the horse. In North America an almost complete fossil record beginning 54 million years ago has been found. This record shows the step by step evolutionary process at work. The gaps or "missing links" in the fossil record are extensive in some areas. Some gaps are minor and only missing a few steps or only contain partial skeletal fossils. Yet others gaps are huge like the gap between the Cambrian period and the Ordovician period. This gap spans an amazing 100 million years. Scientist believe that there are reasons behind these gaps, like: The probability of fossilization taking place is low. Some species are soft-bodied there for don't fossilize well. Some conditions just are not right for fossilization to occur. Many fossils have been destroyed due to erosion and tectonic movements. That is to name only a few theory's explaining the missing links in our long chain.
Comparative anatomy has been used by scientists all over the world to try and determine how closely one species is related to another. Scientists compare one species anatomy to another species to find out how closely they are related. Some structures of the anatomy of different species are very similar, yet serve a very different purpose in each, this is known as Homologous Structures. An example of this is the pattern or structure of the limbs of species. The Pent dactyl limb is relatively the same in species ranging from amphibians to mammals. The same also goes for insect mouthparts. All insects possess the same basic mouthparts but each species has modified them to suit their needs. Scientists believe that species evolved for different reasons. One reason is because of environmental changes, this is called Adaptive Evolution. The species had to evolved to adapt to its surroundings or perish. Some animals did not evolve to adapt to their changing surroundings and have gone extinct. Another type of evolution is Convergent Evolution, which is species not closely related developing similar structures or ways of life to suit the environment. These structures which are very similar and shared between very different animals are know as Analogous Structures. As an example, birds and insects are not closely related, yet both have developed wings to fly. Whales and lobsters both have tailfins that are very similar and they both use the tailfins for the same purpose, to propel themselves through the water. Some creatures of present day possess limbs that they have no use for. These unneeded limbs are called Vestigial Structures. Scientists believe these body parts are left over from the creatures pre-historic ancestors. Ostriches are a good example of Vestigial Structures. They have wings, yet they don't fly. Ostriches' ancestors flew at one time.
Scientist also gather evidence of evolution from the distribution of different species on the continents. You would think that animals found in South America would also be found in a similar habitat like Africa. Both South America and Africa have like climates and like environments, but animals found in Africa are not in South America. The modern mammal developed in the Northern Hemisphere and then migrated in three basic directions. The continents being connected at the time made it easy for land animals to get from one place to another. The Bering Strait connected Asia to North America and the Isthmus of Panama provided further passage into South America. The strait of Gibraltar connected Europe to Africa. Australia was reached via a land bridge from South Asia. The continents then began to break apart isolation the species. The camel is the best example of migration and isolation. The fossil record shows camels first developed in North America. They then migrated into Asia and further into Africa and South America. The modern camel evolved in Asia and Africa. The Llama evolved in South America.
Biochemical and comparative physiology have been useful in producing evidence to support evolution. With advances in science, we have learned that the genetic code is basically the same for almost every known organism. We all share the same basic DNA. Mutations in DNA occur regularly and are passed on to offspring. The study of these mutations show how closely species are related. By comparing DNA scientists have been able to determine our closest relative is the chimpanzee.
Evolution is not something that took place millions of years ago, it is still going on today. Bacteria have evolved enough to be resistant to our antibiotics. Plants and insects have also evolved, some are now resistant to pesticides. Evolution is an ongoing process, and we will continue to evolve. Some believe that the human race has evolved rapidly. If you look back a hundred years ago people did not have the technology of today. Scientists think that the human mind has evolved, and that it will continue to evolve.
The evidence of evolution is great, but a lot more needs to be uncovered.