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The mammals, as such, emerged from Class Reptilia in the Triassic Period of the Mesozoic Era. They differentiated themselves from the reptiles by evolving adaptations for being active at night.
The first vertebrates to come out onto land were fish. Waddling fish, like Tiktaalik, inhabited swamps and would sometimes come out of the water and up onto the soggy landscape. About 370 million years ago, in the Devonian Period, some of those creatures evolved adaptations that enabled them to spend long periods of time on dry land before returning to the water. Thus was born the first of Class Amphibia. The animals were drawn out of the water in a way that promoted the necessary mutations, the draw being the abundance of insects already living on the land. That abundance, spreading far from permanent bodies of water, exerted a further draw that promoted in a salamander-like creature the mutations, such as a dry, scaly skin and the cleidoic egg, that enabled it to live far from permanent bodies of water. Thus, 312 million years ago, in the Carboniferous Period, was born Class Reptilia. Then in the Triassic Period, roughly 225 million years ago, certain reptiles evolved into the first mammals.
What environmental factor would take a scaly-skinned creature that lays eggs and feeds its young, if it does, the same food that it eats and transform it into a hairy-skinned creature that gives birth to live young and feeds them with secretions from its own body? What ecological niche opened up in the Triassic Period and promoted the mutations that turned certain reptiles into mammals?
Answering those questions begins with asking what traits differentiate mammals from reptiles and how might those traits have evolved.
Hair is the first feature unique to mammals that we notice and, because mammals evolved from reptiles, we know that it must have begun as scales attached to the skin. The first evolutionary step loosened the scales and turned them into overlapping plates, such as the ones we see on the pangolin. Over time the plates became narrower and more numerous, eventually becoming quills, like the ones we see on porcupines and hedgehogs. Finally the quills became thinner and denser, thereby becoming pelts of hair, which is what we see on most mammals today.
Both scales and hair act as armor, protecting the wearers from cuts and scrapes that would let disease into their bodies. Hair also works well as thermal insulation, while scales, plates, and quills donít work as well. Hair follicles have the potential to evolve into sweat glands and that potential has been realized in some mammals. Our own ancestors, living on the expanding East African savannah, lost most of their body hair (they didnít need the insulation) and became the sweatiest creatures on the planet. The abundance of sweat glands on humans gives us a cooling system that enables us, given enough of a head start, to outrun anything on the savannah.
The evolution of loose plates into hair likely followed the evolution of endothermy in mammals. The increase in the number of mitochondria in the animalsí cells increased the rate of heat production, enabling the animals to keep themselves warm and stay active at night. It may have been the attraction of a nocturnal lifestyle that promoted the mutations that gave mammals their warm-bloodedness. A creature that can stay active in the cold, instead of going into torpor, has a reproductive advantage that will promote the necessary mutations when they occur.
Another trait characteristic of mammals is the fact that they give live birth and nurture their young with secretions from their bodies. It started with the monotremes, like the platypus. The creatures still laid eggs, but when the baby hatched, it fed by licking modified sweat from the motherís belly. At first the extra-nutritious fluid would have augmented what the mother was feeding the baby, but over time, the fluid, evolving into milk, would replace the feeding until the baby was weaned. It gave the mother a more efficient way of feeding her baby, providing food for the baby when she could not obtain any. The advantages of providing milk to their babies promoted in mammals the mutations that brought the altered sweat glands together into a cluster with a single duct passing through a fleshy nub, a nipple.
Mammals are born unfinished and one serious threat to them is dehydration. If babies can obtain water from their mothers, that threat will be abated: perspiration, suitably modified, can do the job of keeping the baby hydrated. A female who sweats in response to being licked will be more likely to raise viable offspring, so natural selection will promote any mutations that enhance that response. Thus began the evolution of the feature for which mammals are named - the mammary gland.
Why do mammals give birth to unfinished babies and not lay eggs? What advantage lies in that? Ending the need to make an eggshell ends a significant, if temporary, drain on the motherís supply of calcium. How that fact provides an evolutionary advantage, we shall have to see.
The evolution of mammals also changed the sensor pod, the head. In particular, natural selection changed the mouth, nose, and ears. Much of that change came through changes in the jaws.
In the late Carboniferous Period, over 300 million years ago, two amniote groups split off of Class Amphibia - the sauropsids (which led to the dinosaurs, modern reptiles, and birds) and the synapsids (which evolved into the mammals). About 265 million years ago, in the middle of the Permian Period, a synapsid group called pelycosaurs begat the therapsids, which then evolved a bony secondary palate to separate the nasal cavity from the mouth. That change refined the creaturesí sense of smell by diminishing interference from odors originating in the mouth.
The sense of hearing is also important for creatures that hunt in the dark, so natural selection promoted any mutations that improved mammal ears. One result was the sound gathering apparatus, the pinna or ear horn.
Watch a dog or a cat for any length of time and you will notice that their ear horns frequently turn like radar antennae hunting for a target. Thatís not a bad analogy, because ear horns enable an animal to locate the sources of soft noises. All land mammals have ear horns, of one form or another, because their common ancestors in the Triassic Period hunted insects at night. In the dark natural selection promoted any mutation that improved the animalís hearing, both in the intensity of the sound it could hear and in determining the direction whence the sound came. It began with a mutation that caused a slight protrusion behind the ear. By funneling sound into the ear, the protrusion gave its owner a slight advantage and thereby promoted the mutation. Subsequent mutations made the ear horns larger and connected them to small muscles that could turn them and flatten them for protection.
One of the defining characteristics of the synapsids is a single hole on each side of the skull behind the eye sockets (if you put your finger on the front of your ear and press gently, you can feel the entrance to that hole). That hole originally served to provide extra anchoring for the jaw muscles and also provided a feedthrough enabling the ear, such as it was, to connect directly to the animalís brain. It was around the rear of this hole that the pinnae evolved, perhaps beginning as far back as 265 million years ago, when the first proto-mammals began to separate from the reptiles. By 208 million years ago mammals of Order Haramiyida, looking something like squirrels or lemurs, scampered through the late Triassic forests. Those ancestors of all modern mammals appear to have been fully functional in the ecological niche into which their ancestors had adapted.
That niche belonged to nocturnal insectivores, animals that were active at night and hunted insects. The primary adaptations that separated the mammals from the reptiles were adaptations for night activity, dealing with the cold and the dark. Endothermy and hair, along with improved smell and hearing, opened a whole new larder to the mammals. Just as the fish spun off Class Amphibia to exploit food sources on land and just as the amphibians spun off Class Reptilia to exploit food sources far from water, so, too, did the reptiles spin off Class Mammalia to exploit food sources after dark.
Other adaptations also made the mammals different from the reptiles, especially in the jaws. Two small bones, the quadrate and the articulate, which form the joint where reptile jaws meet, came loose from the jaws and became part of the middle ear (the quadrate, in the upper jaw, becoming the incus (anvil) and the articulate, in the lower jaw, becoming the malleus (hammer)) through the interplay of mutation and selection. This was happening by the middle of the Jurassic Period (about 170 - 160 million years ago) as some of the mammals began returning to daylight.
Teeth are especially important to an animal because they aid in preparing the animalís food for digestion. The knife-like teeth of reptiles are good at cutting and tearing the animalís food into easily-swallowed chunks and for an animal with low metabolism that is sufficient. As some reptiles began evolving into mammals and developing higher metabolisms they also evolved a new kind of teeth - molars - that enabled the animals to crush and grind their food and work saliva into it to hasten digestion. The need for mammals to chew their food promoted mutations that made the jaw joint more flexible by detaching two bones and allowing them to evolve into the middle ear.
In birds and warm-blooded dinosaurs a gizzard filled with stones grinds the food to release the nutrients for quick and easy digestion. Mammals changed their teeth instead and evolved larger salivary glands (which is why mammals generally slobber more copiously than birds and reptiles do) to do the same job (in some reptiles the saliva has evolved into venom, which starts pre-digesting the prey while killing it). We may ask which evolved first - the larger salivary glands or the molars? Most likely they evolved together in an evolutionary pas de deux. A certain group of proto-mammals may have suffered a mutation that put extra points on some of their teeth, creating cusps in which food would be partially crushed. Members of the group who produced more than normal saliva would have gained a nutritional boost from the crushed food. Or a group of proto-mammals might have suffered a mutation that made them produce excess saliva. If some members of the group had mis-shapened teeth that could crush food, then they would have gained a nutritional boost at little cost in extra chemical energy and would thus have gained a reproductive advantage over those members of the group that lacked those advantages. In either case the constant interplay between those changes created the modern mammal mouth.
One of the features characteristic of mammals comes from their evolution into night-active creatures. Warm-blooded animals, such as birds, have to keep their eggs warm: if they donít, the embryo dies. On a warm day the creature can leave its nest temporarily to hunt and then come back to sitting on the eggs, doing this until the eggs hatch. Nights get cold, so a night-active animal canít leave her eggs at all: she takes them with her.
As reptiles evolved into proto-mammals, not hunting at night but hunting progressively later in the evening or earlier in the morning, some females held their eggs a little longer before laying them. The embryos inside the eggs were a little more fully developed and thus not as vulnerable to a short exposure to cold. Natural selection acted on that advantage and promoted the mutations behind it. Eventually the females were laying eggs that were ready to hatch more or less immediately. The babies were producing their own body heat and, if properly fed, could survive on their own for a time. Then the eggshell became a disadvantage for the mother with no countervailing advantage for the babies, so natural selection promoted the mutations that made the eggshell go away. Mammals were giving live birth.
Thus we see how reptiles moving into the ecological niche represented by night-active insects evolved into a whole new class of animal.
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