Age of Animals

Contents

Vendian Period, 600-540 MYA

easy to interpret and may belong to extinct phyla. But besides the fossils of soft bodies, Vendian rocks contain trace fossils, probably made by wormlike animals slithering over mud. The Vendian rocks thus give us a good look at the first animals to live on Earth. The Ediacaran hey-day predates by a distinct interval of perhaps 20 million years or more, the so-called "Cambrian Explosion". Although some scientists believe that many of these Ediacara fauna might have survived into the Cambrian period, they had vanished without a trace from later fossil records. Other scientists have suggested that the Ediacaran fauna were "failed experiments" in the evolution of multicellular animals. Unlike the Cambrian organisms, these odd designs left no descendants. A novel explanation suggests that the Ediacaran fossils weren't animals at all. Rather, they were probably lichens. Whatever the interpretation, it seems that the appearance of the Ediacaran fauna and the Cambrian biota are two separate events, and both flourished suddenly in a "complete state".

Figure 01a Ediacara Fauna [view large image]

For much of the past 20 years the debate has been polarized between those who believe that the Ediacarans were a dead-end experiment in evolution and those who maintain that the Ediacarans are the "long fuse" of the Cambrian explosion. As more fossils were discovered in Newfoundland, ... (Avalon assemblage - the oldest), the White Sea region of Russia (White Sea assemblage including those from Ediacara Hills, ...), and Namibia, ... (Nama assemblage), it turns out that both camps are, to some extent, right. As shown in Figure 01b, the Avalon assemblage consists of primitive type of animal living in deep sea with fungus-like traits that left no descendants. The other group from the White Sea and Nama assemblages lived in shallow-water. One of these, Parvancorina, bears a close resemblance to a recently discovered early Cambrian arthropod. Another, Arkarua, looks a lot like a Cambrian echinoderm. It is now thought that a handful of Ediacarans did cross over into the early Cambrian. The overwhelming majority did not make it, though; the few that did vanished within 5 million years. The first experiment in complex, multicellular life was over. But it laid the foundation for everything that followed. It is suggested that the sudden precambrian boom was triggered by massive

Figure 01b Ediacarans [view large image]

increase in deep-sea oxygen levels, and plenty of organic matter from the melting glaciers. The experimental method was to create large body from small units through fractal repetition.

Recent measurement of oxygen level over the past 600 million years suggests that oxygen may be the driving force for evolution. Figure 01c shows that periods of lower oxygen level have coincided with all the major mass extinctions, whereas land colonisation occurred with rising levels. The importance of oxygen can be illustrated by the lack of it. It is well known that animals need to feed, drink, reproduce and respire. The first three requirements can usually be put off for days or even years, but for the vast majority of animals respiration can be put off only for a few minutes. Evolution is prodded by natural selection, which is an euphemism for variable rates of death. And nothing kills quicker than lack of oxygen.

Figure 01c Oxygen Level [view large image]

Cambrian Period, 540-500 MYA (new timescale)

FA of exoskeletal material - It really is a crust secreted by the skin (a laminated crust that in most living arthropods is made up of three layers). FA of many invertebrate phyla - Life took off in the Cambrian period (the Cambrian Explosion). By its end, all the main lines of animal types, whose descendants fill the world today, had been established. All of this diversity flourished in the sea; there was still no life on land. FA of vertebrates1 (jawless fish, ostracoderm also known as Agnatha) - Fish without jaws were the first vertebrates. Just like the molluscs and arthropods, the early fish had a hard outer covering. This armor plating around their front ends was made of bone. They had probably ate by sucking in mud through their mouths. They filtered out particles of food as the water left through their gills. Some jawless fish

Figure 01d Cambrian Period [view large image]

still survive today. They are the lampreys and hagfish.

They include virtually all the groups known from the Burgess Shale and other Middle Cambrian localities, thus compressing the available time for the morphological diversification of metazoans, known as the Cambrian Explosion, to just 10 Million years or so. These extraordinary fossil deposits, where organisms are so well preserved that even their soft parts remain as carbon films, are referred to as Lagerstätten, a German word that means "resting places", only recently borrowed by geologists. A lagerstatte is a spectacular rarity, and a few dozen of them are scattered through the Earth's geologic record like gems.

Figure 01e Chengjiang Fossils [view large image]

warned against too exclusive a reliance on natural selection. Close examination of the history of life shows that the change is not necessarily progressive; it is certainly not predictable. The earth's creatures have evolved through a series of contingent and fortuitous events such as the Cambrian explosion and the mass extinctions, which imparts a quirky and unpredictable character to life's evolutionary pathway. There is still much controversy over the significance of the Burgess and Chengjiang fossils. What is certain is that the transformation of life from single-celled organisms to multicellular organisms was swift, sudden and widespread. Another significant point is that if evolution was occurring at such a rapid rate, why are the Chengjiang fossils and the Burgess fossils so similar? During the 20 million year period between the two sites, evolution seems to have produced very little change. It seems that all of the diversity that was going to occur happened in a time period as short as 10 million years. Hardly an observation that supports a Darwinian view that life evolved by the slow accumulation of fortuitous mutations. Thus, there is suggestion that complex life came to earth (in the early Cambrian and probably Vendian) from elsewhere with many if not all of the biochemical processes in place. A possible fault with this kind of argument is the strong DNA linkage between the unicellular and mulitcellular organisms. It is highly improbable that the DNA structures of these organims are so closely related if the seed for multicellular organisms has another place of origin.

Figure 01f Natural Selection [view large image]

Research in 2004 attributed the complexity of multicellular organism to the use of RNA based regulatory signals. The Cambrian explosion was related to the abrupt addition of this genetic regulatory system. Figure 01g shows the complexity of eubacteria and archaea at low levels over the past billion years up to the present. While the complexity in eukeayote organisms advanced graudully up to a ceiling and then

Figure 01g Evolution of Complexity [view large image]

increased abruptly at the Cambrian explosion when a new regulatory system became available. (click here for detail). The proliferation of complex life forms some 20 million years prior to the Cambrian explosion might be just the initial trials to become multicellular.

Ordovician Period, 500-425 MYA

Shallow seas covered large area of the world, lichen-like plants had begun to adapt themselves to life on land. Post-Cambrian trilobites had diversified into many forms including the Cryptolithus shown in bottom of Figure 02. Echinoderms were characteristic inhabitants in this period. To the left is a primitive species that carried its bulging body on a short stalk. Another species (right) was also rounded or pear-shaped, but anchored itself by the tapering end of its body. The sea lilies (the red ones on extreme left) belong to the same group. Brachiopods (white shells, right) furnish the most abundant invertebrate remains in Ordovician sediments. Cephalopods (nautilus) were the early version of the present-day cuttlefish. They are active hunters with arms

Figure 02 Ordovician Period [view large image]

(tentacles) lie in front of the eyes and a parrot-like beaks. Their body is enclosed in shell. When they needs to move swiftly, the mantle cavity would be filled with water and squirts it out through a tube.
• At the end of the Ordovician, 85% of all species on Earth including many trilobites, brachiopods and nautiloids disappeared in what was the first major mass extinction.

Silurian Period, 425-408 MYA

Cephalopods diversified to a variety of shapes and sizes (see Figure 03). Eventually, a lineage progressed to ammonites in the late Silurian period. Also in the picture is a group of sea lilies (right); in front of them lie rounded clumps of flat corals and a few skeletons of horn-shaped (wrinkled) rugosa corals. FA of jawed fish - Placoderms (=plate skin) are armored by their extensive dermal skeleton. These dermal bones (or plates) form head and thoracic shields that are either articulated by distinctive joints or fused into a single unit. Pectoral fins are typically well developed. Bony shearing or crushing structures on the jaws substitute for true teeth, which are absent. The jaw joint is simple.

Figure 03 Silurian Period [view large image]

• At the end of the Silurian period, swamps and marshes beside the sea were already occupied by low vegetation composed of the most primitive types of vascular plants that reproduce like ferns.

Devonian Period, 408-362 MYA

FA of insects - The primitive (wingless) types still survive in springtails. Insects are the largest and most successful class of arthropods; in fact, they are the most successful group in the entire animal kingdom. The insect body is sharply divided into head, thorax, and abdomen. Six legs are attached to the thorax. Young, or larvae, may be aquatic and breathe with gills, but adults take air into tubes. Most adult insects also have hard, chitinous exoskeletons. FA of bony fishes - There are two groups, those with ray-like fins, ancestors of most fishes today (more than 90% of species) from carp to salmon, and sea-horse

Figure 04a Devonian Fish [view large image]

to tuna, and the lobefins. The ray-fins of the Devonian include cheirolepis, a fast-swimming predator with advanced jaw apparatus to swallow prey up to two-thirds of its own size. Lobe-finned fish is characterized by fins with fleshy bases, or lobes,

containing a jointed series of bones. It is from fish in this assemblage that the first tetrapods, the amphibians, developed. Holoptychius is one example, dipterus (a primitive lungfish) is another one that can breathe air. Figure 04a shows some of the fossilized Devonian fish from Yunnan, China. The one on upper left is the Youngolepis - a specimen in between the lobe-finned fish and the lungfish. Figure 04b summarizes the evolution of fish in a sequence from the jawless fish in the Cambrian, to the primitive jawed fish with amour

Figure 04b Evolution of Fish [view large image]

in the Silurian, and finally advanced to bony fish in the Devonian.

FA of sharks - Their skulls and skeletons were made up of cartilage (strong elastic tissue) and not bone. They have a series of separate gill slits that are not covered (see Figure 04c). FA of amphibians - Ichthyostega appeared on the scene about 362 MYA. They had four legs, but still retained fins on its tail. Members of this group spend part of their life in water and part on land. They often lay their eggs in water. When the tadpoles hatch out, they breathe with gills and are totally aquatic. Later, they undergo metamorphosis, gain lungs and legs, and are able to live on land.

Figure 04c Devonian Shark [view large image]

		A 365-million-year-old arm bone fossil was found in 2004 (see Figure 04d). It came from one of the first creatures able to do push-ups, an evolutionary step that was necessary for animals to move from the sea to dry land. This four-legged creature had a humerus, or upper arm bone. Such a bone, far different from the flipper bones of fish, gave the creature an important new ability - it could raise its upper body like an athlete doing push-ups. The defining moment has been captured by the drawing in Figure 04e. These are lobe-finned fish called Eusthenopterons, which were more than a fish but
Figure 04d Fossil [view large image]	Figure 04e First Land Animal [view large image]	less than a true amphibian. They are supposed to be the first creature that crawled onto land about 380 million years ago.

A popular scenario suggests that fish like Eusthenopteron, stranded under arid conditions, used their muscular appendages to drag themselves to a new body of water. Over time those fish able to cover more ground - and thus reach ever more distant water sources - were selected for, eventually leading to the origin of true limbs. Recent research in 2005 on the fossil of Acanthostega indicates that although this animal had four legs, they would not have been able to support its body on land. It seems that they may have initially functioned to help the animal in lifting its head out of oxygen-poor shallow water instead of moving on land. Only later did they find use ashore. Figure 04f shows the transformation of body structure from lobe-finned fish to modern reptile.

Figure 04f Tetrapod Trans-formation [view large image]

		reduction in size of the postparietal bones (green) and gradual reshaping of the skull. It also shows the pectoral, and distal fins gradually transformed into forelimbs and digits. A peculiarity of Tiktaalik is its poorly ossified vertebral column that seems to contain an unusually large number of vertebrae. The larger ribs may mean it was better able to support its body out of water. The longer snout suggests a shift from sucking food towards snapping up prey, whereas the loss of the gill cover bones probably
Figure 04g Tetrapod Transition [view large image]	Figure 04h Transition of Forelimbs [view large image]	correlates with reduced water flow through the gill chamber as the animal had become partially living on land. Figure 04h shows the transformation from fins to elbow and wrist-like structures as indicated by the parts in different colors.

Carboniferous Period, 362-290 MYA

• The land surface was at this point widely covered by genuine forests, whose fallen logs and other plant material eventually formed many of the coal and oil deposits.

		A small group of creatures, cotylosaurs, gradually found new ways to sever their ties to rivers and ponds. From the cotylosaurs, an evolutionary intermediate between amphibians and reptiles, emerged the first true reptiles. These creatures were distinguished by their small size, agile limbs and the amniote egg. With its large shell and protective membranes, the amniote egg gave reptile the
Figure 05a Carboniferous Period [view large image]	Figure 05b Reptiles [view large image]	freedom to lay their eggs on land. Their descendants include all other reptiles, dinosaurs, birds and mammals (Figure 05b).

Hylonomus is the oldest vertebrate recognized as a reptile. Unlike most of the amphibians, these animals were generally not tied to water for reproduction, but laid eggs that were able to survive in a terrestrial environment. Hylonomus had an unbroken expanse of bone behind each eye opening. Other reptiles had one or more gaps, the temporal openings, occupying various positions and enclosed by various bones. Though these patterns vary in details, the gaps themselves - or the lack of them - are the basis for splitting the reptiles into four major groups as shown in Figure 05c.

Figure 05c Groups of Reptiles [view large image]

• FA of winged insects - Giant ancestral dragonflies (see Figure 05a), had wingspreads of 30 inches and bodies 15 inches in length. They were swift-flying predators with spiny legs used in capturing prey.



• FA of seed plants (gymnosperms).

Permian Period, 290-245 MYA

As the environment became drier and hotter, reptiles thrived at the expense of amphibians. Figure 06 shows many types of the reptiles living way up in the mountain. They are representatives of the groups of pelycosauria, "mammalian" reptiles from which the evolutionary line to the first mammals can be traced. They shared with mammals a particular type of skull. A single large opening in the wall of the skull behind the eye socket allowed highly efficient jaw-closing muscles to develop, greatly increasing the power of the jaws. The huge dorsal fan may have acted as temperature regulator. The amphibians with the tapering head were evidently the prey of the reptiles. Mass extinction of marine life near the end of the Permian period - Groups made extinct include trilobites, sea lilies, and rugose corals. Other marine invertebrates severely affected. Fish are generally unaffected. According to recent investigation, the disaster that killed off almost 90% of all life on Earth about 251 MYA, was likely caused by a huge asteroid or comet crashing into the planet.

Figure 06 Permian Period [view large image]

Triassic Period, 245-208 MYA

The continents reached their maximum phase of fusion (the Pangaea) with much of the land lay in the equatorial belt. The climates were hot and monsoonal. Following the massive extinction at the end of the Permian, the survivors underwent adaptive radiations as they diversified and began to reoccupy many of the now-vacated environmental roles. FA of dinosaurs - Out of the thecodonts in the Triassic came a most fantastic array of reptiles, the dinosaurs (terrible lizard). They were the stock from which the crocodiles and birds, as well as the dinosaurs and winged reptiles, developed. They evolved into two main groups in this period. Those shown on the right of Figure 07a are the meat-eating

Figure 07a Triassic Period

coelophysis, which walked and ran on their hind legs, captured prey with fore- limbs and jaws, and balanced their swaying bodies with stiffly extended tails.

shown in Figure 07b. The difference in the orientation of the pubis is related to the feeding habits and stance when walking. All ornithschians were herbivores and many were bipedal. As vegetarians they would have a large gut to allow the food to pass through sufficiently slowly to allow it to be digested, a process involving symbiotic bacteria. Thus, an erect ornithischian would have a "beer Belly", which has to hang between the legs with the pubis pointing backward. The bipedal saurischians were all carnivores so their guts would have been much smaller as meat is quickly digested.

Figure 07b Pelvic Structures [view large image]

• FA of mammals - Mammals also evolved during the Triassic, at about the same time as the dinosaurs. However, they were small and inconsequential components of the Triassic ecosystems. The megazostrodons are the most primitive mammals. These early mammals had become warm-blooded, and resembled living shrews.

		As more mammalian fossils have been unearthed in the past few years, a very different picture of early mammals has emerged to replace the shrew-like description. Dinosaurs may have been the dominant creatures, but mammals were very much a part of their world. They invaded many more ecological niches and developed many more lifestyles than was previously thought possible before the extinction of the dinosaurs. Figure 07c presents a brief guide to early mammal evolution. According to this
Figure 07c Early Mammal Evolution [view large image]	Figure 07d Early Mammals [view large image]	diagram, mammals evolved from a group of "mammals-like" reptiles called cynodonts that prospered during the Triassic period. The larger members of this group went

• Castorocauda - It is also known as the "Jurassic beaver" because of its broad, flat 20-centimetre tail. At half a meter long and weighing 500 grams, Castorocauda was a large Jurassic mammal. It was part of a group called docodonts, which split from the other mammals early in the Jurassic and dies out about 100 million years ago. This significant fossil offers the first evidence that Mesozoic mammals as a whole had a much great ecological diversification than previously thought.
• Volaticotherium - These animals presumably used the flap to glide through the air, like a modern flying squirrel. They may have been in the air before birds, and were million years before bats. They died out about the same time as the Castorocauda. This fossil provides further evidence of mammalian diversity during the Mesozoic Era.
• Repenomamus - The "giganticus" version is the biggest mammal of the dinosaur age discovered so far. This predator is one meter long and weighed at least 12 kilograms with sharp front teeth. The species belonged to a group of mammals called the Eutriconodonts. These were the first mammals to hunt prey bigger than bugs. They evolved massive skulls and heavy jaw muscles that made them look like "pit bulls on steroids". The closely related "robustus" variety is about half the weight. It was caught (frozen in time) with remains of a juvenile dinosaur (about 14 cm long) in its stomach. The fossils are among the many early mammals (such as the Sinodelphys and Eomaia) found in the Yixian formation, Liaoning, China. These animals survived until the mass extinction at the end of Cretaceous.
• Fruitafossor - The chipmunk-size specimen is about 150 million years old. It has a powerful forelimbs, and peg-like teeth of an anteater or armadillo. It probably made a living by digging into nests of colonial insects. It is the earliest known burrowing mammal with an estimated ago of 150 million years.
• Sinodelphys - This 15 cm long specimen shares more wrist, ankle, and dental features with living marsupials than other mammals. In life, the slight, rodent-like marsupial would have measured 15 cm in length and weighed around an 30 gm. The animal was a proficient climber and likely spent its days scampering across the low branches of trees and bushes, and feeding on insects, worms, and other invertebrates.
• Eomaia - The fossil is 10 cm in length; an estimate of the body weight is between 20-25 gm. It was an early, primitive representative of the lineage of all placental mammals (Eutheria), including species of pig, horse, cat, dog, bat, mouse, rabbit, gorilla, chimpanzee, and human. The narrowness of the hips suggests an animal, which gave birth to live young, but the babies were not well developed. This strongly indicates there was no well-developed placenta. Eomaia ('climbing dawn mother') was adapted to scramble through bushes and trees - presumably to find food, but also to get away from predators, which could well have included small dinosaurs.

		of groups, including small predatory dinosaurs, the early mammals, and some crocodile relatives survived into the Jurassic. Yet large groups of archosaurs mysteriously vanished at the end of the Triassic (Figure 07f). It really isn't obvious why the non-dinosaurs get hammered the most. Anyway, the end-Triassic extinction pruned a number of dinosaurs, but the group as a whole marched on, and prospered in the Jurassic period.
Figure 07e Extinctions [view large image]	Figure 07f Triassic Extinction [view large image]

Jurassic Period, 208-145 MYA

Dinosaurs became dominant, reaching their largest size. The brontosaurus (thunder lizard) was a huge sauropod with length up to 80 feet and a total weight of 30 to 35 tons (see the gigantic beast in Figure 08a). The large size probably helped them to escape predation by carnivorous dinosaurs. In the same picture, the stegosaur protected itself by the elaborate armour. Its small brain was compensated by large ganglia (a mass of never cells) between the shoulders and another one above the hips; those are sometimes referred to as the second brain. The dinosaurs also diversified into water and air - Kuehneosaurus were the gliding reptile, pterosaurs were the flying one, while nothosaurs and ichthyosaurs returned to sea. FA of birds - Archaeopteryx (ancient wing) is the oldest known creature that had feathers. Except for the feathers and the

Figure 08a Jurassic Period

braincase, this crow-sized extinct animal is much more like a small running dinosaur.

It was believed that feathers evolved from scale for flight. New evidence from fossils and recent idea in developmental processes indicate that they evolved for some other purpose and were then exploited for a different use. Numerous functions of feathers are plausible, including insulation, water repellency, court-ship, camouflage and defense. The development of such feature can be traced back to the theropods in the Triassic Period. In essence, all feathers start from a tube produced by proliferating epidermis with the nourishing dermal pulp in

Figure 08b Feathers [view large image]

the center. The evolution involved many stages from an unbranched, hollow cylinder (like the pinfeather) to the asymmetrical flight feather (see Figure 08b).

		The consequence of recent fossil finds has prompted reconsideration of the biology and life history of the theropod dinosaurs. Birds - modern birds and the group that includes all species descnded from the most recent common ancestor of Archaeopteryx - used to be recongnized as the flying, feathered vertebrates. Now we have to consider them as a group of the feathered theropod dinosaurs that evolved the capacity of
Figure 08c Avian Evolution [view large image]	Figure 08d Archaeopteryx Traits [view large image]	powered flight (Figure 08c). Other dinosaurs are very likely to have had feathered skin but were not birds.

Figure 09d shows the characteristics of the Archaeopteryx. It indicates that Archaeopteryx is at the transitional stage between reptile and bird. The size of Archaeopteryx is about 45 cm, and it fed on insects. It has a long bony tail, three-fingered hands with claws, and jaws with teeth. The claws on its feet and hands suggest that Archaeopteryx could climb trees, and the wings are clearly those of an active flying animal. This bird could fly as well as most modern birds, and flying allowed it to catch prey that were not available to land-living relatives. In effect, it had explored a niche in the air. Figure 09e shows the first Archaeopteryx fossil from Bavaria, southern Germany, and an artist's renderings of the very first birds.

Figure 08e Archaeopteryx and Fossil [view large image]

suggests that venom evolved in a lizard ancestor before snakes appeared (Figure 08f). Even the supposedly harmless Colubrids such as those sold in pet stores have enough poison in their venom glands to kill a human. Fortunately for the would-be pet owners, they have no front fangs, leaving them with a rather crude venom-delivery system in the back teeth. Snakes such as boas may have lost their venom as they evolved to kill by constriction. It is also found that venom didn't evolve from ever more toxic saliva but from altering cells from other parts of the body including the brain, eye, lung, heart liver, muscle, ovary and testis. Over generations these proteins, usually involved in key biological processes such as blood clotting or regulating blood pressure, were mutated into more potent varieties and concentrated into catastrophic overdoses. The common ancestor had nine such toxins in its venom. Modern snakes have recruited 17 more.

Figure 08f Snakes [view large image]

	Report in 2007 purported to find the missing link between lizards and snakes. The 95 million years old fossil has greatly reduced forelimbs, a diminished supporting skeletal girdle and an elongated neck (see Figure 08g), as seen today in snakes including pythons and boas. But
Figure 08g Half Snake [view large image]	researchers still cannot conclude that snakes evolved directly from such lizards without other fossils to fill the evolutionary gaps.

Cretaceous Period, 145-65 MYA

FA flowering plants (angiosperms) - The modern types of flowering plant became common some time in the middle of the Cretaceous. Hardwood trees slowly replaced the conifers as the dominant trees of the forests (see Figure 09). These new plants provided fruits, flowers and nectar as new sources of food, causing great changes in all life on land. Dinosaurs continued to dominate the land. But the fauna was very different from that of the late Jurassic. Large sauropods were rare, and the medium-sized coelurosaurs had been replaced by the ostrich-like ornithomimids, which included the duck-billed hadrosaurs and the armoured ankylosaurs and ceratopsians. The 30 feet long sea monsters called mosasaurs were the descendants of nothosaurs. Carnivorous mammals such as Repenomamus were beginning to come out of the shadows. A complete fossil of this animal was found at the base of the Yixian Formation in northeastern China. On its left side, under the ribs where a mammal's stomach might well have been, lies a fragmentary and disarticulated skeleton of a young dinosaur about 14 cm long. The devourer of this little dinosaur was more than a meter long, and is estimated to have weighed 4 - 6 kg.

Figure 09 Cretaceous Period [view large image]

Tertiary Period, 65-1.64 MYA

After the K-T impact, the previously obscure mammals proliferated to fill the environmental niche left by the dinosaurs. The landscape and its plants would be familiar to a visitor from the present day, although some of the creatures might look somewhat strange. A typical scenery is shown in Figure 10a. The big animals are the brontotherium that stood eight feet tall at the shoulder, belonged to a group of odd-toed ungulate (hoofed). The threatening carnivores are sabre-toothed tigers. While a leopard attacked an early horse.

Figure 10a Tertiary Period [view large image]

Mammal Characteristics and Comparison with other Vertebrates:



Reproduction - Mammals nurse their young (Figure 10b). Postnatal care of the young is unusual in lower vertebrates but common in birds and mammals. In addition, none but the most primitive of mammals lays eggs; all others bear their young alive, and in most there is a well-developed mechanism for the nourishment of the young embryo within the mother's body. All these features seem to be associated with a high degree of organization of the mammal body, for the development of which there is needed a considerable period of growth.

Figure 10b Maternal Care [view large image]

Warm Blood - Mammals are warm-blooded forms; a high body temperature is maintained. Hair and sweat glands are peculiar mammalian features associated with temperature regulation, and there is a highly developed breathing mechanism, which is in constant use. In addition there is a very efficient circulatory system with a complete separation of aerated and impure blood streams. The heart (Figure 10c) is a double-barreled, four-chambered organ, as in birds (but not in reptiles), with blood on one side in in transit from the body to the lungs, on the other, from the lungs to the body. Figure 10c is a much-simplified diagram to

Figure 10c Circulation Evolution [view large image]

show the evolution of the double circulation through the heart due to the substitution of lungs for gills in higher vertebrates. In the fish the blood from the gills flowed directly, via the arteries, to the body (systemic capillaries), and hence

the heart was a simple pump. The lungs, however, return their blood to the heart, where it tends to mix with "spent" blood from the body. Crosswise subdivision of the heart in birds and mammals separates the two streams.



Brain - While there has been little change in the nervous system of the trunk in vertebrates, that of the head region, the brain, has undergone a tremendous amount of evolution in vertebrate history. The brain of reptiles is a small structure tucked away in a small area at the base of the skull. The brain of mammals, even among the most primitive, has enlarged enormously. Most parts of the brain have, however, remained fairly constant in size. It is in the cerebral hemispheres, originally small structures dedicated to the sense of smell, that almost all the growth has taken place (Figure 10d). This is the region, where higher brain functions have arisen. It has placed the mammals

Figure 10d Brain Evolution [view large image]

as a group far above any other vertebrate stock in their degree of mental development.

Figure 10e shows the difference between the mammalian and avian brains. It looks similarly shaped but smaller, and it is much less furrowed. Given the well-known dictum that more convolution means higher cognitive function, most scientists have long assumed that birds have limited mental powers. Recent research suggests that the largest part of the avian brain, the pallium (corresponding to the cortex in mammal), works along with structures below it to control complex behaviors (see Figure 10e). Although the nervous systems of the two classes of animals are constructed very differently, they have functional similarities. Many parts of the brain are comparably connected by nerve pathways that have similar functions. For

Figure 10e Avian Brain [view large image]

example, when parrots learn to produce new sounds, the structures activated are analogous to those that are activated in humans.

• Skull - The skull and head of mammals is a very different structure from that of reptiles. Many of the reptile bones have been lost, and the pineal eye no longer functions. The originally solid temporal region has been pierced for the accommodation of the jaw muscles, leaving a bar, or arch, at the edge of the cheek region; the braincase has swollen out enormously to accommodate the expanding brain. The skull of reptiles joins the backbone by a single, round, bony knob, or condyle; in mammals a pair of condyles is present. In reptiles (except the crocodiles) the nostrils open into the

  front of the mouth. In mammals there has developed a bony partition, which separates nasal and food passages back to the throat, a feature of importance in forms in which constant breathing is a vital necessity. In reptiles there are normally some seven bones in the lower jaw; the mammals have but one (the dentary), and this articulates with a different bone on the side of the skull. The whole joint has changed. Figure 10f shows a series of side views of the skull from a lob-finned bony fish A to

  Figure 10f Skull Evolution [view large image]

  human I. These form a morphologically progressive set of stages representing the various groups through which human ancestors passed.

  A (in Figure 10f) is the skull of a fish. It had evolved from the jawless and limbless type (a in Figure 10g). The primitive vertebrate skull has the main structure in the form of a braincase - a box of cartilage or replacement bone, which surrounded the brain, internal ear, nostrils, and eyes. A second element is the skeletal bars, which stiffened the gill slits. The jaws in a shark (b) were derived from the third gill bar. These are both freely movable and are quite clearly in line with the related ordinary gill bars behind them. In the

  Figure 10g Skull Formation [view large image]

  bony fish (c), the dermal armour (skin bones) were added to cover the top and sides of the head completely, have fused with the original upper jaws, and the braincase, and have united them into a solid structure - a true skull.

  B (in Figure 10f) is a primitive amphibian from the Carboniferous period. It is basically similar to A but with the front part of the skull more elongated. C is a stem reptile. It is similar to the amphibian in most respects except for the elimination of the ear notch (on). D is a primitive mammal-like reptile. It shows a hole in the temple region, which allows freer play to the jaw muscles, and the teeth are becoming differentiated. E is an advanced mammal-like reptile. It shows a larger temporal opening, the teeth are approaching mammalian conditions, and a number of elements in skull and jaw are reduced or absent. F is a primitive insectivorous mammal. It reveals a further loss of skull and jaw elements, the braincase is expanding, and the bar behind the orbit (the cavity of the skull in which the eye is situated) has disappeared. G is a lemur. The post-orbital bar is re-established. H is a monkey. The face has shortened, the braincase much expanded, the orbits are turned well forward, and the bar behind them has become a solid partition. I represents the Homo Sapiens. The face is further shortened, the braincase still more expanded, nose and chin prominently developed.
  
  
• Teeth - The dentition of mammals has become greatly modified. Lower vertebrates have an indefinite amount of tooth replacement; mammals have but two sets of teeth, "milk" and permanent. The teeth of primitive vertebrates were usually of about the same shape in different parts of the jaw; in mammals the various parts of the tooth row are highly

differentiated. There was some early variation, but in the ancestors of the higher mammals the dentition came to be made up of three sharp nipping teeth, or incisors, at the front of each half of each jaw, a single large, stout, piercing tusk, the canine, four premolar teeth behind this in the front of the cheek region, and three grinders, the molars (see Figure 10h). This gives a total of 44 teeth. Most mammals have lost some of this set of teeth (human has 32); few have exceeded this number. This type of dentition is one

Figure 10h Mammalian Teeth [view large image]

suitable for a carnivore, and the ancestry of all the mammals lies through a long line of flesh-eating types.

Ears - In reptiles the eardrum lies almost at the surface, near the back of the jaws; in mammals it is deeply sunk in a tube, and an external ear flap for the concentration of sound waves has made its appearance. In reptiles there is but one small ossicle, the stapes, for the transmission of sound from drum to inner ear; in mammals there are three. The two new ossicles have been derived from the old reptile jaw joint. Figure 10i shows the liquid-filled canals and sacs of the inner ear of shark, and human. In both are present three canals sensitive to movement and two cavities, utriculus (u) and sacculus (s), which record the position of the head relative to gravity. In the shark, however, the only area sensitive to sound lies near the base of

Figure 10i Ear Evolution [view large image]

the small pocket termed the lagena. In human this has expanded into the coiled cochlea. In the shark a duct from the ear (end) still connects with the surface; in human the connection is lost, and the duct ends blindly.




• Locomotion - All primitive land animals move with cumbersome, wadding type of walking. The ruling reptiles solved this problem by taking to a bipedal mode of life. The mammals also came to a successful solution, but in quite another way. All four limbs were retained but swung around into a fore-and-aft position; the knees brought forward, the elbows back (Figure 10j). In this mammalian pose, it has introduced a much more efficient sort of apparatus than the primitive one. In an early reptile type much of the energy expended was used to keep the body from collapsing on the limbs; here all the muscles can be used for straightaway forward propulsion. This change in posture has been accompanied by many changes in the bones and muscles of the limbs. For example, in primitive reptiles the count of the toe joints was, from the

		thumb or big toe out, 2-3-4-5-3; in mammals the middle fingers have shortened up, giving a count of 2-3-3-3-3 as shown in Figure 10k with thumb or big toe to the right. The illustrations also reveal the specialization of the proximal ankle bones in mammals, some reduction in the number of wrist and ankle bones, and the variations in the thumb and big toe.
Figure 10j Locomotion [view large image]	Figure 10k Evolution of Limbs [view large image]

• Colour Vision - Colour vision of vertebrates depends on the pigments in the cone cells. It turns out that birds, as well as reptiles, and many fish, have four types of cone pigments, whereas most mammals have only two types (Figure 10l). Mammals lost two of the pigments during their early evolution, very likely because these animals were nocturnal and cones are not needed for vision in dim light. After the dinosaurs died out, mammals began to diversify, and the lineage that gave rise to the Old World primates of today reclaimed a third cone through duplication and subsequent mutation

		of the gene for one of the remaining pigments. Thus, mammalian colour vision distinctly limited when compared with the visual world of birds and other vertebrates especially in the near ultraviolet region of the spectrum. We cannot comprehend the sensation of colour in these animals, but a camera equipped to detect only ultraviolet light "sees" patterns invisible to us as shown in Figure 10m.
Figure 10l Colour Vision [view large image]	Figure 10m UV Photo [view large image]

Ever since Darwin's time, biologists have relied on the outward appearance of living species to trace their evolutionary history. Beginning a few decades ago, the sequences of molecules and the degree of resemblance were used to construct their relationship. Colour vision is one of the examples that molecular structure offers an explanation for the

change that occurred about 30 to 40 million years ago, after the geologic split between the African and South American continents, and thus separated the old world primates to the new world primates. The evolutionary changes have been located at three amino acid sites following the duplication. The mutations are retained because they appeared to have imparted a substantial advantage on the species (the old world primates) that bored them (see Figure 10n). The diagram shows those amino acid positions at 180, 277, and 285 within the opsin protein, which is bound to the light sensitive retinal. These differences are enough to shift the maximal light absorption from 560 nm for the red opsin to 530 nm for the green opsin.

Figure 10n Opsin Protein [view large image]

All the special features in mammals (Figure 10o) can be summarized into one word - "activity". The ancestors of the mammals were carnivores, leading lives in which speedy locomotion was a necessity. The limb development has given effectiveness to this kind of activity. Brain growth has given it intelligent direction. The maintenance of a high body temperature and the various changes associated with this are related to the need of a continuous supply of energy in animals leading a constantly active life. Even the improvements in reproductive habits, which are a prominent feature of mammalian development, seem related to the needs for a slow maturation of the complex mechanisms (particularly the brain) upon which the successful pursuit of an alert and active life depends.

Figure 10o Mammals [view large image]

		When mammals first appeared, in the late Triassic, the continents of the world were united into one large landmass, Pangaea. The climate was generally warmer and drier than at present. The small, primitive mammals of that time were able to move much more freely from one region to another. This is the Morganucodon, which
Figure 11 Mammal Evolution [view large image]	Figure 12 Diversification [view large image]	is thought of as ancestral to the living monotremes (such as the platypus who lay eggs, and marsupials who nurtured the young in a pouch; see Figure 11 for the evolutionary history of the mammals). While

		The oldest hominid (upright-walking primate) remains was discovered recently (July 2002) in northern Chad, Africa with a complete cranium and dated back to nearly seven million years ago (see Figure 13). It may thus represent the earliest human forebear on record and is dubbed Sahelanthropus tchadensis. It is generally believed that human has its root in Africa. Figure 14 depicts groups of different species foraging in the same area around Lake Turkana, Northern Kenya 1.8 million years ago.
Figure 13 Oldest Hominid [view large image]	Figure 14 Hominids in Africa [view large image]

		Figure 15 shows the family tree of the hominids. The 4.2 million years old Australopithecus anamensis is the descendant of Sahelanthropus tchadensis. It looks similar to the 3.5 million year old A. afarensis, a small-brained, big-faced bipedal species to which the famous "Lucy" belonged. Lucy and her kind were upright walkers but retained many ape-like characteristics. They probably represent the transition from tree dwelling to bipedal walking in the savannas as East Africa dried up. Figure 16 shows the Homo lineage starting from about two million
Figure 15 Family Tree [view large image]	Figure 16 Human Evolution [view large image]	years ago. The use of tool and fire started about the same time. The first exodus of hominids from Africa soon followed. There were at least four waves of emigration

Quaternary Period, 1.64-present MYA

During this period four Ice Ages, separated by warmer interglacial periods, covered the northern land areas. As the ice slowly advanced and retreated, so the climate zones and their mammal faunas moved across the continents via the Bering region and Panama Isthmus. An unexplained feature of this period was the appearance of giant representatives of nearly every order of mammals, from platypus, kangaroo and lemur, to deer, beaver, edentate and unusually large elephant (the mammoth as shown in Figure 17).

Figure 17 Quaternary Period [view large image]

• There existed two major hominid species about half million years ago - the Homo sapiens neanderthals in Europe and the Levant, (see Figure 18) and the Homo sapiens sapiens (see Figure 19) in Africa. The most significant feature of Neanderthal anatomy is the skull. The typical features include a wide, vaulted cranium, heavy 'beetle brow' ridges above the eyes, a big jaw, and large teeth. An adult male is thought to have been around 5'6". Neanderthals may have looked faintly apelike, but their brain capacity was as large as, and in some cases larger than, that of modern human beings. As a whole, Neanderthals seem to have been tough and stocky individuals. The difference in mitochondrial DNA between Neanderthals and modern humans is 25-30 nucleotides, while it is only 8-9 nucleotides in modern individuals. Modern

  		humans, who emigrated from Africa about 60000 years ago, may have slaughtered the Neanderthals. Eventually, around 35000 years ago only one species, Homo sapiens sapiens, was left. We thus find ourselves alone and yet the most numerous and successful primates in history. Such success may be at the expense of the natural environment. All the giant animals disappeared around this time. Since this was not associated with any obvious climatic change, we must therefore suspect that human may very possibly have played a large part in these extinctions2. The earth's mammal faunas have been even more reduced during the last hundred years, until many of the herbivores that once roamed North America and Africa in their thousands are nearly extinct or can be seen only in protected game parks.
  Figure 18 Neanderthals [view large image]	Figure 19 Homo Sapiens [view large image]

on climate, altitude, soils and the presence of other species. At present, the number of species estimated to have gone extinct as a result of human activities is still far smaller than are observed during the major mass extinctions of the geological past. However, it has been argued that the present rate of extinction is sufficient to create a major mass extinction in less than 100 years. Others dispute this and suggest that the present rate of extinctions could be sustained for many thousands of years before the loss of biodiversity matches the more than 20% losses seen in past global extinction events.

Figure 20 Diversity of species [view large image]