Age of Animals

Contents

Vendian Period, 635 - 541 MYA

like cnidarians, worms, or soft-bodied relatives of the arthropods. Others are less 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 increase in deep-sea oxygen levels, and plenty of organic matter from the melting glaciers.

Figure 01b Ediacarans [view large image]

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, 541.0 - 485.4 MYA

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.

	It is reported in 2009 that rather than evolving during the Ediacaran period, the first multicellular animals evolved as early as 850 million years ago (Figure 01h), but remained on the fringes of life until ice ages changed the environment to be more favourable for them. The new
Figure 01h Rise of Animals [view large image]	discoveries include: 1. Embryo-like fossils for animals (could be sponges) preserved in seabed layers between 550 and 580 millions

Ordovician Period, 485.4 - 443.4 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 02a. 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 02a 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.

The biodiversity at the Cambrian Period actually flourished for about 20 million years, then reversed to a decline for some unknown reasons. The real bio-diversification happened during the Ordovician (see Figure 02b). Some attributes the bloom to favorable environment such as stable climate and plenty of shallow water etc. Other claims that it is the increasing asteroid impacts that created ecological niches for life to evolve into.

Figure 02b Biodiversity [view large image]

• At the end of the Ordovician period the climate turned cold, 85% of all species on Earth including many trilobites, brachiopods and nautiloids disappeared in what was the first major mass extinction (Figure 02b).

Silurian Period, 443.4 - 419.2 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. At the end of the Silurian period, swamps and marshes

Figure 03 Silurian Period [view large image]

beside the sea were already occupied by low vegetation composed of the most primitive types of vascular plants that reproduce like ferns.

Devonian Period, 419.2 - 358.9 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.

  Figure 04a shows an oldest fossilized proto-spider and its silk dated back to 386 MYA. Scientists have found that the creature did not have a spinneret (the dexterous appendage that spiders use to shape silk into webs and other structures) without which it could not have precisely controlled the emerging silk. Thus, the proto-spiders lacked the equipment to weave webs, they may have used the sheets of silk to line burrows, wrap eggs or even to have sex.

  Figure 04a Proto-spider

• 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 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 04b 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 04c summarizes the evolution of fish in a sequence from the jawless fish in the Cambrian,
  Figure 04b Devonian Fish [view large image]	Figure 04c Evolution of Fish [view large image]	to the primitive jawed fish with amour 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 04d). 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 04d Devonian Shark [view large image]

		A 365-million-year-old arm bone fossil was found in 2004 (see Figure 04e). 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 04f. These are lobe-finned fish called Eusthenopterons, which were more than a fish but
Figure 04e Fossil [view large image]	Figure 04f 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 04g shows the transformation of body structure from lobe-finned fish to modern reptile.

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

		Discovery of the Tiktaalik fossil in 2006 has illuminated more detail on the transition between fishes and land vertebrates. As shown in Figure 04h, Tiktaalik and Panderichthys (red) represent the transitional forms between the lobe-finned fish Eusthenopteron and the primitive tetrapod Acanthostega. The skull roofs (left) show the loss of the gill cover (blue), reduction in size of the postparietal bones (green) and gradual reshaping of the skull. It also shows the pectoral, and distal fins gradually
Figure 04h Tetrapod Transition	Figure 04i Transition of Forelimbs

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 correlates with reduced water flow through the gill chamber as the animal had become partially living on land. Figure 04i shows the transformation from fins to elbow and wrist-like structures as indicated by the parts in different colors. Figure 04j traces the evolution of arm bones from fish to humans.

Figure 04j Arm Bones Evolution

			marine tidal flat sediments. The footprint fossils had been securely dated to 397 MYA in the early Middle Devonian period. The tracks show a very large tetrapod exceeding 2 meters in length, lived in fully marine intertidal to lagoonal environments some 18
Figure 04k Tetrapod Evolutionary Tree, Old [view large image]	Figure 04l Tetrapod Evolutionary Tree, 2010 [view large image]	Figure 04m Tetrapod Trackways [view large image]	million years before the earliest-known tetrapod body fossils were deposited as shown in Figure 04m.

Figure 04n shows the evolution of the living vertebrates in terms of substitutions per site (in a 2013 study), which is a measurement of mutation rate by checking on the replacement of one nucleotide in a DNA sequence. Thus the length of the lines indicates how much the DNA of each lineage has diverged from their common ancestor. The evolutionary tree is rooted on cartilaginous fish, and shows that the lungfish is more closely related to tetrapods than the coelacanth (settling a debate on tetrapod evolution), and that the coelacanth is evolving slowly. Pink lines (tetrapods) are slightly offset from purple lines (lobe-finned fish), to indicate that these species are both tetrapods and lobe-finned fish.

Figure 04n Vertebrate Evolution, 2013

Carboniferous Period, 358.9 - 298.9 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, 298.9 - 252.2 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, 252.2 - 201.3 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 niches. 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 coelophysis, which walked

Figure 07a Triassic Period [view large image]

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

primitive reptile) on the left. Figure 07b shows a confrontation between the phytosaur and a top predator. The plateosaurus is a primitive member of a group called sauropods (lizard-footed), which walked on four feet, developed massive legs both fore and aft, and had teeth that were suited only to a diet of soft, juicy plants. Plateosaurus has peg-like teeth and the hands had huge thumb claws, used perhaps to gather in plant material

Figure 07b Triassic Confrontation [view large image]

from tall trees. The other subdivision is called theropods (beast-footed). With a few exceptions, the theropods were bipeds that walked on three birdlike toes, had short forelegs, and were carnivorous.

shown in Figure 07c. 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 07c Pelvic Structures [view large image]

Sauropodomorpha became the biggest land animals - Sauropodo-morpha means "lizard feet - form" in Greek, even though their feet look much more like a tree trunk. They are mostly herbivores although early forms may be omnivores. The primitive forms are facultative bipeds; later forms were so large they were obligate quadrupeds with long forelimb and/or long neck for reaching high into trees (for feeding) without having to rear up. By the end of the late Triassic they had surpassed all previous land living animals in size. The main sauropods of the late Jurassic and the Cretaceous belong to the clad Neosauropoda characterized by dorsally placed big nares and very large size. This clade includes the Argentinosaurus - the largest known dinosaur about 35 meters long and weighing in at 80,000 kg (80 tonnes) lived in Patagonia 100 million years ago (late Cretaceous). In comparison, African elephant weighs about 6 tonnes.

Figure 07d Sauropodomorpha [view large image]

Table 01 below explains the adoptions that allow the existence of sauropods' unprecedented bulk.

Table 01 Sauropods' Special Characteristics

• 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.

This is the classic scenarios of mammalian evolution. It posits an orderly acquisition of key evolutionary innovations leading to adaptive diversification (first column in Figure 07e). But newly discovered fossils in the 2000's show that evolution of such key characters as the middle ear and the tribosphenic teeth (cutting and grinding molars) is far more labile among Mesozoic (250-65 MYA) mammals. Many of such mammal groups led to dead-end lineages. But some iteratively developments eventually succeeded into modern mammals (Figure 07e).

Figure 07e Diversification [view large image]

		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 07f presents a brief guide to early mammal evolution. According to this
Figure 07f Early Mammal Evolution [view large image]	Figure 07g 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.

		happened, it didn't affect everything. Plenty 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 07i). 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 07h Extinctions [view large image]	Figure 07i Triassic Extinction [view large image]

For a long time palaeontologists have the idea that turtles evolved in a terrestrial environment and the shell was formed by osteoderms (bony deposits) fusing together. The 2008 discovery of a turtle fossil Odontochelys semitestacea in China dated back to 220 million years with the top part (carapace) missing implies that the top and bottom parts of the shell evolved separately. This fossil could be seen as the missing link of turtle evolution. But some palaeontologists argue that the old theory is still correct, the carapace has been reduced as the result of adoption to living in a marine environment similar to today's sea turtles. Proponents of the new theory point out that the aquatic turtles alive today (that have reduced carapace) do not match the patterns seen in the fossil. The new specimen looks like the embryonic pattern. The drawing at the top of Figure 07j is an artist's impression of the Odontochelys semitestacea, the image at the bottom is the real fossil.

Figure 07j Half Turtle

Jurassic Period, 201.3 - 145.0 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 the center. The

Figure 08b Feathers [view large image]

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 powered flight (Figure 08c). Other dinosaurs are
Figure 08c Avian Evolution [view large image]	Figure 08d Archaeopteryx Traits [view large image]	very likely to have had feathered skin but were not birds.

about 148 MYA. Figure 08d 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 08e 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]

		It was only in the 1990's, more evidences turned up in fossil-rich quarries in northern China. Various dinosaur fossils clearly show fully modern feathers and a variety of primitive feather structures. The dromaeosaurs discovered at Liaoning seems to represent the theropods that are hypothesized to be most closely related to birds but that clearly are not birds. It may be the missing link depicted in Figure 08c. Then a four-winged dinosaur fossil (Figure 08f) was discovered in 2009, the Anchiornis Huxleyi is dated to 151-161 million years ago making it the oldest feathered dinosaur. It
Figure 08f Oldest Feathered Dinosaur [view large image]	Figure 08g Dinosaurs in China	has the size of a chicken (less than 50 cm) with long feathers covering the arms and tail, but also the feet. Figure 08g shows some locations of dinosaur fossils in China.

more than a century over the relationship between the wing of birds and the digits in theropod dinosaurs when palaeontologists mistakenly identified the dinosaurs' to be the 1st, 2nd, and 3rd. Until now in 2009, analysis of the digits in a Limusaurus fossil shows that those are indeed the 2nd, 3rd, and 4th digits - the same as the modern birds (Diagram d, Figure 08h). This explanation vastly simplifies the current convoluted evolutionary story which, either assumes that birds lost their 1st digit and re-grew their 4th one or that birds descend from another kind of dinosaurs.

Figure 08h Digits of Bird and Dinosaur [view large image]

Viperidae (vipers), Elapidae (cobras and coral snakes), and Atractaspididae (stiletto snakes). New research in 2006 suggests that venom evolved in a lizard ancestor before snakes appeared (Figure 08i). 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 08i 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 08j), as seen today in snakes including pythons and boas. But
Figure 08j 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, 145.0 - 66.0 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 09a). 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 09a Cretaceous Period [view large image]

The smallest North American dinosaur fossil (50 cm high, weighed 2 kg) has been identified to be predator of small animals. The clawed dinosaur was slight, ran on two legs and had dagger-like teeth. It had an enlarged sickle-shaped claw on its second toe. This cat-sized dinosaur is called Hesperonychus (western claw).

Another cat-like fossil by the name of Pakasuchus was uncovered in East Africa. It seems that this creature had embarked on a failed attempt to evolve from crocodile to cat around 100 million years ago. It has nasal openings on the front of their skulls implying land living. By contrast, surviving crocodilians all have nares on the tops of their heads for breathing while partially submerged. The specimen had a short, broad, cat-like skull but the bone arrangement is otherwise distinctively crocodilian. It had mammalian teeth but retained the scaly armour on its tail. At about 50 cm long from nose to tail, these reptiles would have been active, agile hunters of insects and other small preys. This kind of creature disappeared around the same time as the

Figure 09b Cat-like Crocodile [view large image]

extinction of dinosaurs. But the ancestors of modern crocodiles were able to weather the storm and prosper ever since.

A cretaceous dinosaur fossil (Tianyulong Confuciusi) with long feather-like structures sticking up from its body has been discovered in Liaoning, China (Figure 09c). Based on the bones present, it looks like it was small, active, agile, and probably eating a mix of insects, small vertebrates and plants. This one is special because it is from the lineage of which is ornithischian,

		thought to become extinct at the end of the Triassic Period, and the feathered dinosaurs belong exclusively to the saurischian lineage (Figure 09d). It is suggested that if these are protofeathers, then they might not related in any way to flight. The fact that the filaments over the tail are so long and stiff, points to a possible display function. Figure 09d shows the progression of dinosaur skin characteristics from scaly skin to
Figure 09c Tianyulong Confuciusi [view large image]	Figure 09d Dinosaur Skin Characteristics	filamentous proto-feathers and onto pennaceous feathers. This surprising discovery raises fresh questions about the evolution of feathers.

Tertiary Period, 66.0 - 2.588 MYA

		After the K-T impact, the previously obscure mammals proliferated to fill the environmental niche left by the dinosaurs. Figure 09e shows the drawing of a tree-shrew (reconstructed from analyzing more than 4,500 anatomical traits), which survived the impact. Since then the placental animals has diversified in a few hundred thousand years as shown in Figure 10a, where the landscape and its plants would be familiar to a visitor from the present day, although some of the creatures might look somewhat strange. The big animals are the brontotherium that stood
Figure 09e Tree Shrew	Figure 10a Tertiary Period [view large image]	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.

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 Bloodedness - Mammals are warm-blooded animals; 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 show the evolution of the double circulation

Figure 10c Circulation Evolution [view large image]

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.

Temperature regulatory mechanisms ultimately act through the autonomic nervous system and are largely controlled by the hypothalamus of the brain, which responds to stimuli from nerve receptors in the skin. In case of low external temperature, chemical signal from the hypothalamus stimulates the liver to release more glucose, which results in higher metabolic rate, and thus raises the body temperature. In equatorial climates and during temperate summers over-heating is as great a threat as cold. In such conditions many warm-blooded animals increase heat loss by panting and or flushing (increasing the blood flow to the skin). Hairless and short-haired mammals also sweat, since the evaporation of sweat consumes a lot of heat. Elephants keep cool by using their huge ears like radiators in automobiles: they flap their ears to increase the airflow over them.

It has been proposed (and accepted by most biologists) that the evolution of warm bloodedness (endothermy) was all about stamina. It is noted that mammals and birds have a high aerobic capacity compared with other animals, which provides their muscles with lots of oxygen and keeps that supply going for long period. As a result, they are able to sustain exertion for long time, whether chasing prey or fighting competitors. The theory implies that the warm-blooded ancestors were probably carnivores. One of life's great mysteries is why such inefficient mechanism was adopted. Warm-blooded animals use up a lot of energy even when it is not necessary (like at rest in hot weather under the sun). The strategy of getting energy on demand should be a better way. Some animals such as swordfish, leatherback turtle, and some snakes do just that (by shivering for example). Some can even selectively warm up only a specific part of body (the eyes for example), which needs the energy.

A novel explanation suggests that it was probably herbivores, which initiated the warm-bloodedness. These kind of animals consume a lot of leaves to get the nitrogen for making amino acids, and nucleotides, all the excess carbon are burn off as body heat. It is noticed that warm-bloodedness started at the same time as the rise of flowering plants at the early Cretaceous period around 145 million years ago. Small herbivorous dinosaurs evolved to birds while some herbivorous mammals turned into

Figure 10d Warm-bloodedness and Fungi [view large image]

carnivorous. A recent (2011) theory suggested that either by design or by fortuity, a warm-blooded body constitutes a good defence against fungal pathogens beside maintaining high level of activity. The hypothesis is supported by several coincidences:

• Most fungi thrived at temperatures between -4^oC and 30^oC, only fewer than 1/3 of the species can survive beyond 37^oC and just 5% are able to grow at 41^oC.
• Ectothermic (cold-blooded) animals such as reptiles, amphibians, fishes, and all the invertebrates are susceptible to fungal diseases.
• Most endothermic (warm-blooded) animals such as bird and mammals suffer only a few fungal discomforts, e.g., the athlete's foot in human.
• Those few mammals whose body temperatures drop lower than 37^oC - either occasionally or full-time - do seem to be more susceptible to fungal infection (Figure 10d, bottom row).
• The 37^oC body temperature in human seems to be the allowable maximum, beyond which it becomes very dangerous, e.g., the high fever with certain illnesses.



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 10e). This is the region, where higher brain functions have arisen. It has placed the mammals as a group far above any

Figure 10e Brain Evolution [view large image]

other vertebrate stock in their degree of mental development.

Figure 10f 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 10f). 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 example, when parrots learn to produce new

Figure 10f Avian Brain [view large image]

sounds, the structures activated are analogous to those that are activated in humans.

A big brain requires a tremendous amount of energy expenditure to support. The brain consumes roughly 65% of a baby's total energy consumption and no less than 20-25% of an adult's, even though brain tissue accounts for only 2% of adult body mass. The trend toward big brain in human and primates started about 2 million years ago and accelerated between the past 800000 to 200000 years (Figure 10g, with the exception of Homo floresiensis in blue circle). It has been suggested that the costs of brain expansion were covered by reduction in gut size as early human gradually acquired high-quality diets.

Figure 10g Big Brain [view large image]

It has also been shown that only humans and other primates have brain size negatively correlated with gut size. However, a recent (2011) study proposed that bigger brain is the result of trade-off between brain size and fat deposits.


Table 02 below is a summary on the evolution of the brain from very primitive animal to human (see also "The Brain").



Animal(s)	Component(s)	Function(s)
Choanoflagellae (unicellular organism)	Surface receptors, Sodium channels	Receiving and transducing chemical signals, Passing electrical signals within cell
Hydra	Network of neurons	Passing electrical signals between cells with a brief chemical phase across tiny gap (synapse)
Urbilaterian (a hypothetical ancestor of all the bilaterians§ )	Groups of neurons ~ nerve	Photoreceptors link to nerves (there may be a small brain as well)
Invertebrates (such as flatworms, roundworms, arthropods, ...)	Small brain with ganglions (small lump of nerve cells) connected by nerve cords	A small central processor links to smaller units for distributed processing
Vertebrates	Olfactory bulb, cerebrum (forebrain), tectum (midbrain), cerebellum, spinal cord (see diagram on the left)	Smell detection to find food and mate, Controlling the 5 senses, Controlling internal environment, Controlling motion, Providing communication between the brain and other parts of the body
Primates (including human)	Increasing folding in forebrain and more brain mass (see also Human Brain)	Dealing with more complicated envirnoment

Table 02 Evolution of Brain

^§ Bilateral forms (with two-sided symmetry) can have heads and separate mouth and anus; the body plan is tube within tube. Its subtaxon includes deuterostomia, lophotrochozoa (these two are distinguished by embryonic development), and ecdysozoa which molts periodically.


• 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 10h shows a series of side views of the skull from a lob-finned bony fish A to

  Figure 10h 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 10h) is the skull of a fish. It had evolved from the jawless and limbless type (a in Figure 10i). 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 10i 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 10h) 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 10j). 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. Mammals also have precise

Figure 10j Mammalian Teeth [view large image]

occlusion (the fit between upper and lower teeth). This type of dentition is one 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 with an external ear flap for the concentration of sound waves. In reptiles there is only one small component in theossicle, the stapes, for the transmission of sound from drum to inner ear; in mammals there are three. The two new components have been derived from the old reptile jaw joint. Figure 10k 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

Figure 10k Ear Evolution [view large image]

sensitive to sound lies near the base of 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 10l). 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 10m 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 10l Locomotion [view large image]	Figure 10m Evolution of Hand and Foot [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 10n). 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
Figure 10n Colour Vision	Figure 10o UV Photo [view large image]	shown in Figure 10o.

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 10p). 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 10p Opsin Protein [view large image]

All the special features in mammals (Figure 10q) 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 10q 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 is thought of as ancestral
Figure 11 Mammal Evolution [view large image]	Figure 12a Diversification [view large image]	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 Morganucodon was alive, Pangaea was starting to break up; as a result these animals are isolated in Australia. The Afrotheres and Xenarthrans generally evolved to groups on the lower right of Figure 12a. They originated on the southern continents of Pangaea, and are now the basis of the mammals in Africa and South America. The Laurasiatheres are probably the most diverse group of living mammals. It includes the hoofed animals, the familiar carnivores as well as the bats and whales (most of the animals on the top and left in Figure 12a). They were originally living in the Northern Hemisphere; but they now dominate the other groups in the continents of Africa and South America as well. The rodents and primates are very closely related, and have always been a very widespread group. The ancestor of both the rodents and the primates was probably an animal which looked rather like a small squirrel or tree shrew (click to see more on examples and characteristics of modern mammals). Figure 12b shows the phylogeny for mammals based on anatomy and fossils. The horizontal black bars represent the age range of the group; question marks indicate controversial branching points, where gene studies reveal different relationships.

Figure 12b Phylogeny [view large image]

		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	Figure 14 Hominids in Africa [view large image]

		in 2009 identified fossils for even earlier hominid - Ardipithecus ramidus (known as 'Ardi') at 4.5 MYA. Reconstruction shows that the hands and wrists don't have many of the distinctive chimp characteristics. The foot, with its big toe sticking out sideways, would have allowed Ardi to clamber in trees, walking along limbs on her palms. And the teeth show no tusk-like upper canines, which most apes have for weapons or display during conflict. Thus, Ardi is most probably not in the lineage of modern chimps. Figure 16 shows the Homo lineage starting
Figure 15 Family Tree [view large image]	Figure 16 Human Evolution	from about two million years ago. The use of tool and fire started about the same time. The first exodus of hominids from

Quaternary Period, 2.588 - 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, 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. However between 40,000 and 5,000 years ago many of the world's largest mammals died out as shown in Figure 17. Most of them weight 90 lbs or more. The smaller animals such as mice and shrews almost all survived. Irrespective to the cause of extinction (there are many speculations such as climate change and slaughtered by humans),

Figure 17 Quaternary Period Extinction [view large image]

large animals have the disadvantages that its smaller size of population is more vulnerable and they have a much slower rate of reproduction taking years to give birth.

• 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. Anatomically Modern humans (AMH),

  		who emigrated from Africa about 65000 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]