Mammals, to which humans also belong, appeared on Earth late, approximately 216 million years ago, which is almost 100 million years after the first reptiles. Therefore, reptiles have roamed the Earth for almost a million centuries without any competition. Dinosaurs were the largest reptiles when mammals appeared. Both reptiles and mammals are vertebrates – they have a spine. Reptiles are usually cold-blooded – their body temperature changes with the change of air temperature. Mammals are warm-blooded – their blood temperature is always the same. However, maybe the dinosaurs were warm-blooded, no one knows for sure.
How are mammals different from other animals? The answer is in the word mammal. In the tits, ie. mammary glands, milk is produced for baby mammals. Reptiles do not have mammary glands. There are other differences. Mammals are usually completely or partially overgrown with fur (e.g., dogs, cats, mice, gorillas, which are all mammals, as opposed to alligators, chameleons, rattlesnakes, which are reptiles). Mammals usually give birth to live young – they do not lay eggs. However, there is an exception. The Australian beak, which lives along the streams of Tasmania and Australia, has fingers connected by skin, fur, lays eggs – and is considered a mammal. The Australian beak is a good example of the possible development of mammals. He has fur, but lays eggs, combining the trait of a mammal with the trait of a reptile. Some scientists think that before the appearance of real mammals, there were such animals that, like the Australian beak, leave the impression of incompleteness – they are not actually reptiles, but they are not mammals either.
The first real mammals, which appeared 216 million years ago, looked like tiny shrews. They ate insects and plants and probably spent most of their time in the trees. Fur and other essential characteristics of mammals have evolved over millions of years. Evolution is based on random changes and natural selection. Animals born with fur can grow up and get cubs with fur. Through many generations, this fur can be shown as an advantage, because it preserves body heat and helps warm-blooded mammals to maintain a constant temperature in a cold climate. Animals with fur could look for food in cold as well as warm places, having an advantage over animals that could only survive where it was warm. Eventually, animals with all the necessary characteristics for mammals developed. As dinosaurs became extinct, mammals spread in all directions. Over millions of years, they have evolved into many families – cats, whales, humans.
Interesting fact: animals that could not be classified as reptiles or mammals survived for millions of years. Among them were therapsids, which are classified as reptiles, but had teeth like dogs. They may have had fur, they were probably warm-blooded, and they laid eggs like an Australian beak.