Age of Animals

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 eight feet tall at the shoulder,
Figure 09e Tree Shrew	Figure 10a Tertiary Period [view large image]	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 cases 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

Figure 10c Circulation Evolution [view large image]

evolution of the double circulation of 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. There are at least 4 hypotheses on the cause of warm bloodedness.


• Stamina - 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.



Metabolism of Herbivores - 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 carnivorous.

Figure 10d1 Warm-bloodedness and Fungi [view large image]



• Defence against Fungi - 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 10d1, 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.
  
  

  Reproduction - A research news in 2016 reports that the tegu lizard can temporally turns to endothermy (warmbloodedness, Figure 10d2) during reproductive period. This is a feat shared only by the pythons within the group of Lepidosaurs (such as snakes and lizards). It is suggested that such feature may be more common in this group if the animals are not disturbed during the observations. Anyway, this discovery represents the transitional state between ectothemy (coldbloodedness) and endothermy. Thus, the driver for the change is related to the transition from aquatic to terrestrial habitats. The eggs laid on land are at risk to desiccation and are subjected to greater temperature fluctuations than those in water.

  Figure 10d2 Endothermy and Reproduction [view large image]

  The amniotic eggs are protected with fluid-filled membranes, and stable temperature is achieved by either development inside the womb or by brooding in a nest outside.

  See the News and Views article in "A Lizard that Generates Heat".



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 other vertebrate stock in their degree of mental development.

Figure 10e Brain Evolution [view large image]

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

Figure 10f Avian Brain [view large image]

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. It has also been shown that only humans and other primates have brain size negatively

Figure 10g Big Brain [view large image]

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 occlusion (the fit between upper and

Figure 10j Mammalian Teeth [view large image]

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 sensitive to

Figure 10k Ear Evolution [view large image]

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 to the living monotremes (such as the platypus who lay eggs, and marsupials who nurtured the young
Figure 11 Mammal Evolution [view large image]	Figure 12a Diversification [view large image]	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]

It was announced in 2009 that a 47x106 years old fossil (called Ida) has been discovered in Germany. It might be the ancestor of monkeys, humans and other primates. Other paleontologists suggest that it could be the early member of the lemurs family (Figure 15a). An even earlier primate fossil in central Hubei (), China was unveiled in 2013. It is linked to a period of extremely hot climate about 65 million years ago (soon after the extinction of the dinosaurs). Analysis so far places it in the tarsier lineage but it my turn out to be a human ancestor (Figure 15a). The discovery suggests that humanity started first in south-east Asia and migrated into Africa before another migration out of Africa 65000 years ago.

Figure 15a Family Tree, Early [view large image]

		dwelling to bipedal walking in the savannas as East Africa dried up. A report 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 15c shows the Homo lineage starting from about two million years ago. The use of tool
Figure 15b Family Tree [view large image]	Figure 15c Human Evolution	and fire started about the same time. The first exodus of hominids from Africa soon followed. There were at least four waves of emigration since then with

The Peking Man belongs to the genus of Homo Erectus living in a cave (the Zhoukoudian, see Figure 16a and 16d) some 500,000 years ago. This lineage seems to have reached its dead end around 150,000 years ago, but some still consider it as the ancestor of modern human. Figure 16b compares the skulls of the gorilla, modern human, and the Peking Man, which seems to be in the middle of the evolutionary sequence. Extensive excavation works from 1927 to 1937 has uncovered some skulls, teeth, jaws, and skeletal bone together with animal remains and evidence of fire and tool usage. The original fossils were lost during an attempt to ship them to the US just before the onset of the Pacific War leaving only plaster casts (Figure 16c) for further study.

Figure 16 Peking Man [view large image]

Renewed excavation in the caves, beginning in 1958, brought new specimens to light. In addition to fossils, core tools and primitive flaked tools were also found.

Figure 17 Peking Man Video Recapitulation [view large image]

Figure 17 is the recap of a 30 minutes Youtube video on the Peking Man produced by CCTV. Since the narration is in Chinese, a short summary is presented below following roughly to the sequence in Figure 17.


1. The scene is the re-enactment of the cave living some 500,000 years ago. The threatening animal is a sabre tooth tiger or a giant hyenas.
2. This is the cave in Zhoukoudian found by the Swedish geologist J. G. Andersson and American palaeontologist W. W. Granger in 1921.
3. The skull was found in 1927 and identified as Homo Erectus. Fossil datings for this genus varies from 2 million to 140000 years ago.
4. The excavations came to an end in 1937 with the Japanese invasion. By then few ten thousands of fossils and tools have already been collected and stored in the basement of "Peking Union Medical College Hospital (北京協和醫院)".
5. As the war in Pacific is imminent by December 1941, the fossils were packed and shipped to the US via Qinhuangdao (秦皇島) by train on December 5. Hostility had begun already on the 7th when the shipment arrived there on December 8. The escorting US marines became prisoners of war and the cargo disappeared without a trace. The Japanese government announced that the lot had been stolen and assigned a detective to have it recovered. He eventually committed suicide by harakiri for delinquency of duty.

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