Mammals are warm-blooded vertebrates with hair. There are many other characteristics of mammals, but this one defines the class like no other. Most have four limbs. This body structure is a peculiar description of all terrestrial mammals. The evolution into this body form is not only very complex, but, also quite old. Fish are one of the most ancient animals, and the first vertebrates. Fish appeared in seas around 510 million years ago. During this period, land masses were laden with gigantic forests that included many insects, as well as microorganisms. Insects and plants on land were totally unused, and hence fish living near the shoreline began getting attracted towards land. Many of these fish eventually developed their fins into leg-like structures, and also developed lungs for terrestrial breathing. These breeds of fish became amphibious, and lived partially on land and partially in water. About 10 million years after fish came into being, reptiles began to differentiate from amphibians. Shortly (in evolutionary terms!) afterwards, some of these reptiles began to develop into strange creatures, having both mammalian and reptilian characteristics. Scientifically, these creatures were known as the therapsids. The first fully evolved therapsids appeared about 285 million years ago. This was the beginning of the Permian geological period. The evolution of the therapsids was rather fast, and they quickly differentiated into various species. By the end of the Permian geological period, the dominance of the therapsids on land had declined, as reptilian dinosaurs began to appear. The therapsids were eventually wiped out due to changing atmospheric conditions. Creatures similar to the therapsids reappeared about 200 million years ago. These creatures were scientifically known as the Morganucodon. The most dominant among these species was a mammalian creature scientifically termed as the Morganucodon watsoni. Fossils of M. watsoni, have been unearthed in Wales, England, China, India, some parts of North America, southern Africa, and Central Europe. From the fossils that have been found, scientists have concluded that the M. watsoni was an one-inch-long mammal who had characteristics similar to those of a weasel. In 1994, a single tooth of a similar species named as the Gondwanadon tapani, superficially similar to Morganucodon watsoni, was found in India. Research on the basis of the tooth revealed that the Gondwanadon tapani, appeared about 225 million years ago, making it an even older creature than the Morganucodon watsoni. This fact has not been supported by any further evidence, and it is still debated whether the Gondwanadon tapani is in fact older than the Morganucodon watsoni. Morganucodon watsoni was a small, hairy creature. Scientists believe that it was a warm-blooded animal, thrived on a diet of insects, and was nocturnal. It is evident from archaeological findings that mammal-reptile hybrids like the Morganucodon watsoni and the Gondwanadon tapani survived the Jurassic period. It is believed that these creatures later evolved into four-legged mammals, or the marsupials and placentals. It has been deduced that during the Jurassic period, these mammals reproduced by laying eggs, and hence are not considered mammals. Around 30 million years ago, by the end of the Jurassic period, mammals which were known as multituberculates appeared. These mammals possessed the unique characteristic of giving birth to live young. There were 8 known families of mammals surviving by the end of the Jurassic period, and the number had increased to 15 by the end of the Cretaceous period. The dinosaurs were extinct by the end of the Cretaceous period, and the mammal families started evolving and exploring the world without fear. The Eocene period was the golden period in mammalian evolution, since during this period all the groups of mammals that exist in the modern era, appeared. This event is said to have occurred about 45 million years ago. Fossils that belong to the Eocene era show some of the earliest mammals. Common examples were the Indricotherium , which was tall-shouldered, and the Brontotherium, which was elephant-shaped. All these species contributed to the evolution of modern mammals, and eventually to the evolution of man.