All species evolve from pre-existing ones. It is strange but true that the first species of birds originally evolved from reptiles about 140 million years ago. The evolution of birds followed the same course as that of any other species — in a slow and gradual process. They originated from reptiles to develop to its present form passing through different stages. The first bird was called archaeopteryx and was of the size of a crow. Fossil records support the existence of this bird. But what are the distinct features that classified it as a bird? Though, in appearance, it was quite like a reptile and had teeth like a lizard and a long bony tail, yet in many other ways it had different characteristics from that of a reptile. Its head was balanced upon a long, thin and probably flexible neck. It had feathers instead of scales. These distinct features made it a bird and not a reptile and that is why archaeopteryx came to be known as the first bird on Earth. But it had several different features from the modern day bird. It could not fly properly for any considerable distance as it was not able to flap its wings very hard and probably used them for gliding as well as sailing through the air. It had a small poorly developed breastbone. It had claws on its wings and was a warm-blooded creature. There is no unanimity among the ornithologists over the issue of the origin and development of such intermediate form of creatures. The following are the two different views given by two different schools. One group believes that the first creatures that could fly were the tree-dwelling reptiles that jumped from branch to branch or from trees to ground in order to escape from enemies or in pursuit of food. The wings developed as a mechanism to aid in gliding or to help ease the fall as we see in the modern flying squirrels. The other group says that the ancestral reptiles were in ground dwelling form, lightly built and protected themselves by speeding away from the enemies. The development of feather arms and tail aided in lessening the forward resistance of wind and provided the lift in the initial stages, which together increased the speed of the bird. These processes gradually enabled them to fly with more perfection.