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The evolutionary process has developed over millions of years. Lineages evolved and split and modifications occurred gradually in stages. Life has been in existence on Earth for at least 3.5 billion years but the first two stages of evolution took place in the space of a billion years! The earliest types of cells were believed to be 'prokaryotes', cells without a special centre or nucleus, which are free-living entities. They make proteins and reproduce without the need of a plant or animal host e.g., bacteria.
'Eukaryotes' appeared between one and two billion years ago when an oxygen-rich atmosphere was present. These were cells that contain a nucleus. Eukaryotes have their DNA structures within the nucleus which are 10 to 1000 times more than those found in prokaryotes.
Of course things have greatly speeded up since then in terms of geological time. Quite recently about 600 million years ago, there was a sudden increase in the distribution, number and variety of organisms at the beginning of what is known as the Cambrian period. Organisms with hard parts - shells, carapaces and skeletons appeared. The first fish and the first vertebrates appeared; plants, previously restricted to the oceans, began the colonization of the land; the first insect evolved, and its descendants became the pioneers in the colonization of the land by animals; winged insects arose together with the amphibians; the first trees and reptiles appeared; the dinosaurs evolved; the mammals emerged, and then the first birds; the first flowers appeared; the dinosaurs became extinct; the earliest cetaceans (ancestors to the dolphins and whales) arose and in the same period the primates (the ancestors of monkeys, apes and humans). Less than 10 million years ago, the first creatures to resemble human beings evolved, accompanied by a spectacular increase in brain size. And then just a few million years ago, the first true humans emerged.
The stages of evolution in species certainly did not happen by a single DNA change like the change of colour in the eye. Complex organs as multiple limbs or adaptation in the pentadactyl limb for different functions were created bit-by-bit over many, many, generations. There are many intermediary stages, which can be seen in evolution of different species. For example, the evolution of feathers was an adaptation by some reptiles to provide insulation in cold weather.
It became useful later for quite a different function (archaeopteryx). Feathers turned into the wing for flying and a separate evolutionary origin of reptiles developing into birds came about. Some species of dinosaurs had feathers, although they could not fly using the feathers. Birds seem to have later adapted the feathers to flight. This, of course, means that birds are very closely related to reptiles, since dinosaurs were reptiles! The analysis of the organ structure in fossils allows us to make estimates of how far back evolutionary relationships go. But can we find more recent examples of such a process?
As stated, the phylogeny tree of certain groups reconstructs the stages through which a species developed. By knowing the characters of the organisms at the tips of a phylogeny group or tree one can figure out the ancestral characters that were likely present. A simple instance is of the modern flies and moths. They have silk glands but beetles and wasps of the same phylogeny group do not have.
The simplest explanation is that silk glands evolved only once in the ancestor of flies and moths. That means that when one imagines the most recent common ancestor of flies and moths (the ancestor at the node between them), one should think of an insect with silk glands.
Another more modern way of tracing evolutionary relationships is to see the changes in DNA during reproduction as the basic events in evolution as was seen in the first two stages in life's evolution between 'prokaryotes', and 'Eukaryotes'. In a species DNA alteration at the gene-level and the chromosome-level occurs because of mutations and natural selections. The gene pool of a population also changes. Not only are variations created, they are also preserved and passed on from one generation to the next. Comparing the DNA of different species one can have a fairly reasonable approximation of how much the DNA has changed during the formation of these species. This method is a contemporary one and is being increasingly used to study evolutionary relationships.
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