The Evolution of Mammals

Mammals are vertebrates that have a backbone enclosed in a covering of nerves. The backbone and the network of these nerves lead to the brain of the mammal. Most of the mammals have four limbs that end into a peculiar Pentadactyl (a combination of 5 fingers and toes). This body structure is a peculiar description of all mammals, except the whale and the dolphin. The evolution to this body form is not only very complex, but, also very ancient.

Fish are one of the most ancient animals. Fish appeared in the sea around 510 million years ago. During this period land masses comprised of gigantic forests that included many insects, as well as micro-organisms. The 'game' of insects and plants on land was totally unused and hence fish living near the shoreline began getting attracted towards land. Many of these fish eventually developed their fins into a leg-like structure, and also developed lungs for land breathing. This breed 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 the amphibians. Shortly later, some of these reptiles, who loved to venture on land, began to develop into strange creatures, who had both mammal-like and reptile-like 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 geologic period. The evolution of the therapsids was rather fast and they quickly differentiated into various species. By the end of the Permian geologic period, the dominance of the therapsids on land had declined, as the dinosaurs began to appear. The therapsids were eventually wiped out due to the changing atmospheric conditions.

Creatures similar to the therapsids appeared about 200 million years ago. These creatures were scientifically known as the Morganucodon. The most dominant among these species was a mammal who was scientifically termed as the Morganucodon watsoni. Fossils of the Morganucodon watsoni, have been unearthed in Wales, Bristol, China, India, some parts of North America, South Africa and Central Europe. From the fossils that have been found, scientists have concluded that the Morganucodon watsoni was an inch long mammal who shared characteristics similar to those of a weasel.

In 1994, a single tooth of a similar species named as the Gondwanadon tapani, that shared an appearance with the 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 there is still a debate if the Gondwanadon tapani is in fact older than the Morganucodon watsoni.

The Morganucodon watsoni was a small, hairy creature. Scientists predict that it was a warm blooded animal, thrived on a diet of insects, and was nocturnal. It is evident from archaeological findings that the mammals-cum-reptiles like the Morganucodon watsoni and the Gondwanadon tapani survived the Jurassic period. It is believed that these creatures probably 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 were not considered as total 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. There were 8 known families of mammals surviving by the end of the Jurassic period, and the number increased to 15 by the end of the Cretaceous period.

The dinosaurs were extinct by the end of the Cretaceous period and the 10 mammal families who survived the change that killed the dinosaurs, started evolving and exploring the world without the fear of them.

The Eocene period was the golden period in the evolution of mammals because 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 the modern mammals, and eventually, to the evolution of man and civilization.