Roots Oceanic ;5187947 said:Monotremes are evolved from reptiles, emerging around 285 million years ago as mammal-like. These creatures were, like monotremes today, small, mostly nocturnal, and still laid eggs. These animals are represented today not only by the platypus, but the echidna as well.
We can go ahead and answer the question @waterproof has been pressing, transitioning nicely to the topic of mammals evolving from reptiles..
Reptiles are ectotherms (or better known as "cold blooded"). This means that they rely on outside sources for temperature control. This is why lizards lay in the sun for heat. This means that the environment has to be suitable, or the habitats of these creatures have to be tolerable to survive.
As reptiles evolved into mammals, what we call endotherms ("warm blooded"), they gained the ability to regulate body temperature by internal mechanisms as opposed to strictly relying on outside temperature.
So, as reptiles, animals do not need internal functions for temperature regulation because they are dependent on the environment, living where temperature is constantly warm. In freezing weather, they would not be able to survive.
Most likely due to change of temperatures, reptiles adapted by growing hair and evolving temperature regulating bodily functions to survive. From there, they evolved into mammals.
It is felt that hair was first evolved in early mammals to help regulate temperatures. The earliest mammals grew these sensory cones between the scales of the evolving reptiles, which, when brushed on objects, gave a stimulus to the brain. Certain remnant scales still exist on rats' tails, armadillo shells and the backs of pangolins. Animal whiskers are still sensory receptacles. Hair is mainly the protein keratin, a form of skin, which is the same material making up the epidermis, feathers, fingernails, horns, hooves and claws. Humans have about 100,000 hairs on our head, while sea otters have 170,000 to 1,000,0000 hairs in their underfur; only one of the otters’ three types of hair. Polar bear hairs have hollow cores. The cores scatter light, making the hairs seem white. Actually, they are colorless. Sunlight passes through the bear's long guard hairs to its black skin, which absorbs the radiation as heat. The inner insulating hair prevents the heat from being radiated back to the air (similar to glass of a greenhouse). Most mammals can cause their hair to become erect, thus increasing the air, (i.e., heat) retention. Although humans have lost most of their body hair, goose bumps are the results of our own relic muscles that still exist which used to control these hairs. White-tailed deer grow winter coats four times thicker than summer coats.
http://www.bobpickett.org/evolution_of_mammals.htm
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