THE EVOLUTION OF MAMMALS
The changes from a reptilian to a mammal's lifestyle is significant. Reptiles are cold-blooded, relying on external heat sources to raise their body temperature to a point necessary for activity. During cold periods and at night, the reptile must accept substantial down time, awaiting the warmth of the next day to get started again. This leads to a lifestyle of relatively short bursts of energy needed to catch its prey and long periods of inactivity while the body processes the food. Mammals, on the other hand, are warm-blooded, generating their heat internally, retaining it by insulation of fur and/or fat. Having a more consistent body temperature means a more continuous activity level, which means more fuel (food) is required to supply the energy necessary for this active lifestyle. A more efficient metabolism is required which includes greater oxygen intake abilities and nutrient uptake. Larger lungs, diaphragm, a four-chambered heart, and improved circulatory systems were the result. A more efficient method of removing metabolic wastes was also required, resulting in improvements to the kidneys and the separation of the urinary and fecal excretory tracts (which are still combined in the monotremes, which include the platypus and echidna, the opossums and some species of shrews today).
Additionally, reptiles relied on giving birth to eggs. Eggs are basically left to hatch and fend for themselves. The new mammals evolved a system of reproduction which leaves the young to develop more fully inside the mother. After birth, the young are afforded extended parental care by the the reliable provision of the mother's mammary glands. Thus, the young can spend its energy in further physiological growth, rather than having to be immediately able to hunt and defend itself. Both these advances allowed more time for the development of a more complex organism and the cerebral system in the mammalian ancestry.
While we recognize that the "Age of Mammals" begins after the demise of the dinosaurs some 65 million years ago, the story of mammalian evolution begins well before their reign. One has to go back to a period 250 million years ago when the transition to mammals began in the form of mammal-like reptiles.
Mammals evolved from a group of reptiles called the synapsids. These reptiles arose during the Pennsylvanian Period (310 to 275 million years ago). A branch of the synapsids called the therapsids appeared by the middle of the Permian Period (275 to 225 million years ago). It was over millions of years that some of these therapsids would evolve many features that would later be associated with mammals.
It's impossible to know from which of the reptiles the first mammals evolved. The hallmarks of today's mammals - hair, warm blood and milk-producing glands - do not fossilize. We know that some dinosaurs, such as the Stegosaurus and Dimetrodon, supported large plates or sails of skin, developed to function as solar panels, absorbing the sun's heat for quick mobilization. However, during the period of dinosaurs, some of the sail-backed pelycosaurs (which includes the Dimetrodon) lost their sails. Even acknowledging a warming period ensued, it would be improbable that such an effective heating mechanism would be lost through evolution unless a more efficient method of heat control had evolved to replace it. It is therefore reasonable to supposed that the pelycosaurs and their successors, the therapsids, were to some degree endothermic. The therapsids, being only three feet long, would need some form of body insulation if it was to be effective. Thus, it is reasonable to conclude that some of these creatures were covered in hair, or fur.
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.
Early in the evolution of the therapsids arose a group called the cynodonts. These early mammal-like reptiles changed their teeth from being designed for catching and holding prey and then swallowing whole, to adding specialized teeth, including molars, designed for better mastication of food allowing for quicker digestion. In reptiles, all the teeth are alike, being replaced alternately along the jaw in waves from the back of the jaw to the front. Larger, but similar teeth replace the earlier teeth. As the cynodonts progressed toward today's mammals, this process was replace by the characteristic pattern of one set of deciduous teeth followed by one set of permanent teeth. The permanent nature of teeth enabled a specialization of teeth to serve specific functions. Additionally, the jaw of the cynodonts reduced the number of jaw bones. This freed up the superfluous bones to evolve into an entirely new function, becoming the tympanic, malleus and incus of the mammal's inner ear. Improved hearing gave these creatures a better awareness of their environment and, in turn, this increasing sensitivity called for a greater capacity for processing the auditory information in the brain.
The cynodonts dominated the world of 250 million years ago. At this time, all the land masses were joined together in one super-continent, called Pangaea, consisting of the northern Laurasia (Europe, Asia and North America) and the southern Gondwanaland (South America, Africa, Antarctica and Australia). By about 200 million years ago, the dynasty of the cynodonts suddenly was ended by the appearance of dinosaurs. Both the numbers of species and the size of the remaining species was markedly reduced. For the next 135 million years, these early mammals played a very small role in the world (both literally and figuratively), often termed the 'Dark Ages' of mammalian evolution. However, the evolution of the remaining mammals did not stop.
With the days dominated by the dinosaurs, these early mammals confined themselves to a nocturnal lifestyle or limited rocky or underground habitats. Such a lifestyle could only be possible with a warm-blooded creature, who could maintain a body temperature necessary for nocturnal activity. Being small, the physical requirements of getting around necessitated the development of agility and coordination. Also, being carnivorous added to need for augmented sensory perception. Such activities require a higher metabolism, meaning more food. Finally, being nocturnal resulted in eyes becoming relatively large, hearing more acute, the nose more sensitive, vocal cords more developed, and whiskers becoming more pronounced. All these factors combined to give the small early mammals an awareness of their environment which demanded constant fine-tuning of the integration between cerebral and physical equipment. This enabled an opportunity for increasing evolution of reproductive strategies, maternal behavior, parental care, communication between individuals and learning.
In a manner of speaking, the net result was that, while dinosaurs were getting larger and developing extensive defensive and offensive tools, they were really evolving primarily their physical hardware, while the evolution of the early mammals used its energy in evolving their brain and behavioral software.
During this period of dinosaur dominance, the three forms of mammals that still exist today appeared; the monotremes, the marsupials, and the placentals. All three of these groups are believed to have originated from a common ancestor. If one were to define the first mammal as the most recent common ancestor of these three forms of mammals, as some do, then an animal found in England (Phascolotherium bucklandi) lays claim to that honor, roaming some 165 million years ago during the middle Jurassic period. Others argue for a creature from Texas during the late Triassic, about 225 million years ago, with the name of Adelobasileus cromptoni. By the Jurassic Period (180 to 130 million years ago), mammals were shrew-like animals, being insectivorous carnivores.
The earliest mammals are generally agreed to have been the ancestors of todays' monotremes. Monotremes resemble reptiles and differ from all other mammals in that they lay shell-covered eggs that are incubated and hatched outside of the body of the mother. Like reptiles, the ducts of the excretory system and the genital ducts open into a single external opening known as the cloaca (thus, the ordinal name Monotremata). Like mammals, they have fur, a four-chambered heart, are warm-blooded, and nurse their young from specialized glands. However, as might be expected in these primitive mammals, the four-chambered heart is, to an extent, incomplete, the body temperature averages lower than that of other mammals, and their are no nipples; rather, the milk is excreted through skin glands, with the young suckling the body fur to receive the milk. The monotremes are represented today by two species of echidnas and one species of platypus.
These monotremes split from the early mammals around 130 million years ago, probably in Australia, while it was still joined to Antarctica, as part of Gondwanaland. Monotremes had dispersed through Antarctica into South America by the early Paleocene (65 million years ago), but are only found today in Australia, where they were safe from competition with the more advanced placental mammals.
About 120 million years ago, the mammalian line ceased laying eggs and began bearing live young. These forms of mammals were the first marsupials, who bore their young at a very early stage in their development and transferred them to a pouch where modified sweat glands secreted milk. It is generally accepted that the first marsupials arose in North America and spread to South America, then to Antarctica and Australia some time before the breakup of Pangaea near the end of the Cretaceous period, some 70 million years ago. Others argue that a southern continental origin is more probable.
By the the end of the Cretaceous (65 million years ago), Pangaea had begun to break apart. These early marsupials (and the limited number of monotremes) were the sole inhabitants of the southern land mass, Gondwanaland, and the marsupial lines radiated out to fill out every niche available. (Although later continental movements enabled carnivorous placental mammals to reach South America and Africa, the islands of Antarctica and Australia remained to be occupied solely by marsupials and monotremes. More about the invasion of South America by placental carnivores can be read at THE VELVET CLAW.)
Meanwhile, in Laurasia, another branch of mammals took the marsupial reproductive strategy one step further. As the early mammals evolved, they continued to expand their awareness of the environment reflected in improved senses of sight, smell, touch and hearing. These processes, and an increasing need for coordination, required constantly more cerebral capacity. The placental evolution best suited this need. In this system, the embryo remained in the uterus, receiving nutrients and oxygen from the mother for an extended amount of time, enabling the development of the brain. As the brain continued to expand, it brought the mammals greater ability to learn, continuing the cycle and speeding up the process well beyond the levels attained by marsupials. The splitting of the placentals from the marsupials date back to about 120 million years ago. The first living placental order is believed by most scientists to be the Insectivores (including shrews and moles), while others support the Xenarthra, which includes the armadillos, sloths, and anteaters as more primitive.
About 70 million years ago, placentals and marsupials arrived in South America; the placentals being the herbivores, and the marsupial, the carnivores.
With the lowering
ocean level creating the Panama land bridge about 3 million years ago, the
placental carnivores (including the saber-toothed tigers) migrated south from
North America and wiped out the marsupial carnivores.
This land bridge was responsible for the northward migration of opossum,
armadillos and porcupines and the southward migration of raccoons, weasel, dogs,
bear and cats (including the saber-toothed cat).
Mastodons, tapirs, horses, llamas and deer were the hoofed emigrants
southward. Rabbits and rodents
(including the very successful mice) also went south.
Today, fully half of the land mammal genera of South America came by way
of this land bridge, while only three genera (the porcupine, opossum and
armadillo) having found permanent homes in North America from the south.
By the end of the Cretaceous period, some 65 million years ago, as the continents diverged, the climate became more erratic, the vegetation changed, and the dinosaurs disappeared, the 'Age of Mammals' finally gets under way.
So many changes were occurring. Of course, the removal of the dinosaurs opened the daylight world to the mammals. Flowering plants, which had quickly dominated the landscape over the coniferous gymnosperms, evolved symbiotic relationships with the mammals, bringing mutual benefit to both plants and animals; food for the animals and a method of fertilization for the plants, thus enabling a radiation of new species of both plants and animals. And the movement of the continental plates caused the formation of new mountains and new habitats. As the Atlantic basin widened, the Andes and the Rocky Mountains rose where the American continents independently butted up against the Pacific plate. India broke free from Africa and drifted north towards Asia, creating the Himalayan range. As Africa turned a little and pushed against Europe, both continental plates crumpled a little, creating the Alps and the Atlas Mountains. With these new habitats came new plants, insects, and soon, new mammals.
The first mammalian carnivores to evolve from (and to feed on) the ancestral insectivores are known as the creodonts, which split into two main groups about 55 million years ago; the felids and the canids, or as we know them, the cats and dogs. (More on this can be found by reading about THE VELVET CLAW.)
Next came the herbivores to feed on the plant material itself. The teeth became broad, extremely hard and high-crowned to grind the tough fibrous plant material. The digestive system became specially adapted for processing low-nutrient plants, and the unnecessary claws evolved into hooves, which in Latin is ungula, thus, the name of ungulates. Like the carnivores, these herbivores split into two main groups; the odd-toed perissodactyls and the even-toed artiodactyls.
As has been the case throughout the evolution of animals, where food resources will allow, gigantic forms arose, and where food was limited smaller forms predominated. The concept of convergent, or parallel, evolution led each continent to evolve similar types of cats, dogs, rodents, pigs, and rhinoceros to fill in similar ecological niches.
As temperatures moderated from the Paleocene into the Eocene (from 53 to 36 million years ago), tropical forests aided in the rise and spread of the flowering plants and the radiation of the mammals. But, throughout the Oligocene and into the Miocene (from 36 to 8 million years ago), the temperature dropped about 5 °F as a result of a shift in the axis of the Earth's rotation and resulting formation of the ice cap on Antarctica, which at this time was arriving at the south pole. This drop in temperature (and consequential drop in precipitation) caused a change in vegetation from forest to grasslands. With this gradual change in the environment, came an evolutionary adaptation among the animals. Grass replaced forest in many regions and with it a number of habitats for browsers, who primarily feed on woody plants, were lost.
However, certain animals were able to capitalize on the dense stands of grass vegetation which enabled larger animal populations than afforded by forest habitats. With the exploitation of grasses, teeth became a limiting factor in species diversity since the grasses contained crystal of silica, which quickly wore down teeth. As a result, new species with specially adapted teeth incorporating hard enamel evolved, feeding primarily on these grasses. These new animals are known as grazers.
The grass itself benefits from the presence of the herds for they trample and eat the seedling of bushes or trees that might take root on the plain and that would, were they to grow tall, rob the grass of light and eventually displace it. It seems likely therefore that the spread of the grasslands and the evolution of grazing animals proceeded together, step by step.
Grass is high in cellulose, a material hard to digest. One adaptation was the emergence of bacteria in the gut to help break down the grass. The lagomorphs, including rabbits and hares, furthered this adaptation by passing the grass through their digestive system twice. After excreting the food product, it is ingested again to extract as much of the nutrients as possible. This process is called coprophagy, and is also practiced by some mice and beaver.
Certain of the even-toed ungulates evolved an extra stomach (the rumen) which partially broke down the food before passing it back to the mouth for further chewing before being sent down to the true stomach for final digestion. This, of course, is the proverbial "chewing of the cud." These are known as the ruminants. The success of these more advanced ungulates can be shown by the number of existing species. There are 194 species of even-toed ungulates and only 16 odd-toed ungulates, of which the horse is the only exclusive grazer among them.
This cud chewing makes
for longer time for food to be processed (70 - 100 hours through the gut) than,
say, for a horse (30 -45 hours). The
central difference, then, is that the digestive system of the horse is less
efficient than that of the ruminant, but in compensation the horse eats greater
quantities of food; the emphasis in ruminants is on highly efficient digestion
and on more selective feeding, but not on high rates of food intake.
Given food in short supply, the ruminant will probably survive after the
horse has starved to death.
Within the ruminants,
antlers and horns are found in the most progressive families.
The antlered artiodactys include the deer, elk, caribou and moose.
Antlers are usually present and occur in males only, with the exception
of caribou, where both sexes have antlers.
Antlers differ from horns in being annually shed and regrown each year.
In the Giraffadae (giraffe and okapi), both male and female have fur
covered bone protuberances. In the
Antilocapridae (pronghorn sheep), both male and female have a boney spur with an
annually shed sheath. Finally, in
the Bovidae family (antelope, bison, sheep, goats, and cattle), usually both
male and female have horns. The
horns are never shed and in some species grow throughout the life of the animal. Bovid horns are never branched, but can be spectacularly
curved or spiraled. The domestication of bovids about 8,000 years ago was one of
the major factors responsible for the first civilization in the Tigris/Euphrates
River valley in Mesopotamia (annual grain crops, including wheat and barley were
another reason). A few bovids
reached the New World in the Pleistocene via the Bering Strait land bridge.
Because this avenue of dispersal was under the influence of severe boreal
climatic conditions at this time, it functioned as a filter bridge, and only
animals adapted to these conditions dispersed across the bridge.
Migrators to the New World included bighorn sheep, mountain goat, musk
ox, and the bison. Other Old World
bovids that couldn't make the cold crossing included antelopes and the gazelles.
Bovids inhabit grasslands, making their greatest stand currently in the
savannas and grasslands of Africa.
In addition to the ungulates, the rodents prospered as plant eaters. They, too, dealt with the tough plant material. Their evolution was to maintain open roots to their front gnawing teeth, the incisors, so that they continue to grow throughout the animal's life, compensating for wear. Like the ungulates, rodents also enlisted the help of bacterial life to further break down the cellulose in the guts of the animals.
And, of course, with the grazers coming out in the open grasslands, came the predators. Only the largest of the herbivores (elephants and rhinoceros) were safe from predation. The smaller rodents burrowed for protection. Others chose speed to outrun predators. Horses, for example, evolved in North America, running on its' toes, which, over time, were reduced from four toes to one. (Numerous toes, necessary for better traction in the soft soil of the forests, became unnecessary, and, in fact, a liability on the hard ground of the drier grasslands.) Legs increased in length for running and the skull elongated to enable the eyes to better see predators while they fed.
As part of the mixing
of continental members 3 million years ago, horses traveled over the Bering
Strait to the Old World and over the Panama isthmus to South America.
Dramatic climatic shifts and the arrival of man contributed to the
extinction of Equus in the Americas. Only
one genus world-wide exists in the Old World from their maximum in the Miocene.
This genus, includes all the world's horses; zebras, asses and onagers -
the ancestor of the domesticated horse.
Speed was the name of the game. While the ungulates evolved speed by running on their toes, the predators couldn't do this since they needed their claws for the catch. Instead, they effectively lengthened their limbs by making their spine extremely flexible, enabling their hind and front legs to overlap beneath the body. Or they hunted in groups, like the lions and dogs.
Back in the forests, some of the tiny insectivorous creatures from the days in the shadows of the dinosaurs, adapted a lifestyle in the trees. The foods available in the trees were rich and varied, including leaves, buds, fruit, insects, eggs and nestlings. As opposed to a terrestrial life, where the sense of smell was the most important sensory organ, life in the trees emphasized the development of vision. Eyesight evolved to include looking sideways, integrating the two fields of vision to give a stereoscopic image of the surrounding, with perhaps even the first tints of color. In addition, agility and coordination were critical. Use of the forehands to grasp branches and food resulted in opposable thumbs, and the rear legs became more the means of propulsion and static support, leading ultimately to the ability to sit with the back erect. In addition to these traits, an omnivorous diet required a range of teeth functions, not just the slicers of the carnivores nor the grinders of the grazers. Along with these developments, was the co-committal development of the cerebral capacity. From this beginning, arose the primates.
With the origin of the primates, some 80 million years ago, came certain features that would forever differentiate these mammals from all others. Among them, they have retained four kinds of teeth (incisors, canines, premolars and molars), opposable thumbs, two mammary glands, frontally directed eyes and a relatively large brain. It is from this stock that has lead to the evolution of the human being, another story in itself.
(Editors' note: With the preceding information derived from various sources and, thus, dates of publication, there are some discrepancies in established dates as new information is continually being made available. While I've done what is possible to rectify these differences, the state of the science precludes the total agreement of all dates to be achieved from a summary of numerous documents.)