THE ORIGIN AND DEVELOPMENT OF ANIMALS

Most scientists believe that all plant and animal species probably developed from a single form of life that arose about 31/2 billion years ago. The basic life form gradually changed so that through the centuries, millions of kinds of animals have come into being. Some kinds are still alive. Others are extinct (no longer living). All animals, whether living or extinct, are related to one another.

This set of ideas about how species change over time is called the theory of evolution. The theory is supported by a vast amount of evidence from many fields, and most scientists consider the occurrence of evolution to be a scientific fact. However, many people reject the concept of evolution because it conflicts with their religious beliefs. The Biblical account of the Creation, for example, says that God took only a few days to create all living things essentially as they exist today.

This section uses evolutionary theory as the basis of a discussion of when some animals originated and how species change. For a discussion of religion and evolution and for more information on evolution.

When animals appeared on Earth. Most scientists believe that Earth formed as a planet at least 4 1/2 billion years ago. The first life forms were simple, single-celled organisms that appeared about 1 billion years later. More complex animals and plants gradually evolved from these simple organisms. Many groups of invertebrates arose about 650 million years ago. The first vertebrates-fish-developed about 500 million years ago, and the first mammals appeared more than 200 million years ago.

Another way of looking at these times is to imagine the history of life on Earth in terms of a single year. Start with the formation of Earth on New Year's Day, January 1. Bacteria, the first types of living things, would not appear until March 22. Many invertebrates would not show up until November 9. Fish would evolve from their invertebrate ancestors about November 20. Mammals would appear on December 16. Monkeys and apes would not be found until December 28. Human beings would appear only a few minutes before the end of the year, on December 31.

How new species are formed. Scientists consider groups of animals to represent distinct species when they become so different that they cannot produce fertile offspring together. 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 different environments. If they become dissimilar enough, they cannot produce fertile offspring if they mate. They are then two separate species. This process can repeat itself many times over many millions of years, resulting in great numbers of species.

How species change. The individuals of any given species are not the same. Some individuals are larger, some are darker, some tolerate heat better, and some are stronger. Some individuals have traits that make them better able than others of their species to survive and reproduce in their environment. Over long periods, those animals will produce more young that survive than will individuals with less desirable traits. The offspring of the better-suited species will probably share some of the desirable traits of their parents. For example, dark moths will be well hidden in a shady forest. More of their offspring will probably survive than will those of lighter moths, which may be easily seen and eaten by hungry predators. In the next generation, more moths in the forest will be dark. This process, which causes the traits of animal groups in nature to change through time, is called natural selection.