Many scientists believe that all animal and plant life have developed from a single form of life that arose about 3 ½ billion years ago. This basic life form as we know it has gradually changed to throughout the past centuries, in which millions of different animals have come into being. Some kinds are still alive today. Others have been made extinct (no longer in existence). Some animals, whether living or extinct, are related to each other.
Theory of evolution suggests how species have changed over time. This theory is supported with a vast amount of hard evidence form many fields of study. Many scientists consider the occurrence of evolution to be a scientifically proven fact. Although, there is still a handful of groups who would strongly opposed to the idea of evolution. This read will use evolutionary theory as the basis of the ensuing discussion outlined below, describing how some animals originated and how species change
When did animals appear on earth? Many scientists suppose that the earth had formulated as a planet some 4 ½ billion years ago. The first life forms were simple, single cell organisms, in which appeared roughly 1 billion years later. Then more complex animals and plants developed from these single cells. Many groups of invertebrates developed about 650 million years ago, as the first vertebrates (fish) developed about 500 million years ago, and the first mammals appeared more than 200 million years ago.
How new species are formed: Most scientists today believe that groups of animals represent distinct specie groups, for instance, when they become so different that they cannot produce fertile offspring together i.e. a Rhino and a Tiger. Imagine a group of birds that lived only on one island. Then imagine that a few individuals got lost in a storm and landed on a different island. The two groups, now separated from each other, may gradually develop different traits as they adapt to their different environments. If they become so dissimilar, they could not produce fertile offspring together upon mating. They are then two different forms of species. This process can repeat itself over and over for millions of years, resulting in the development of more diverse groups of species.
How do species change? The types of particular animal or bird within any given species group is not the same. Some individuals are larger, darker; can tolerate heat better than others; as some will be stronger. Some of these individuals will have traits that make them better able than others, within their species, to reproduce and or survive. The offspring of the better suited species will most likely share some of the desirable traits inherent in their parents. For example, dark moths will be well hidden in a shady forest. More of their offspring will likely to survive longer than say a light moth, which can be easily spotted by predators. The aforementioned process which causes these traits is known as ‘natural selection’.