Animals Overview PowerPoint Presentation

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Animals Animals are multicellular eukaryotes. They are heterotrophs: they get their food and energy from other organisms. Animals are motile during at least part of their life cycle. Nearly all animals are diploid throughout their life cycle, except for the single-celled gametes. The evolutionary ancestor of animals is not clear: animals have been in the fossil record from very early times, and none of the protists is an obvious ancestor. We divide the animals into vertebrates (animals with backbones, like us) and invertebrates (animals without backbones). There are far more invertebrates than vertebrates.

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Animal Body Plans Several evolutionary trends are seen among the animals: body symmetry patterns, cephalization, development of a gut, internal body cavities, and segmentation. Body symmetry. Most animals have bilateral symmetry or radial symmetry. Bilateral symmetry means that the body halves are mirror images—we are bilaterally symmetric. Radial symmetry means that the body is composed of several similar body parts arranged like spokes on a wheel. Cephalization. As animals get more complex, the head becomes more differentiated from the rest of the body. It develops more sensors (like eyes and feelers) and more nervous system (brain). These developments allow the animal to react more quickly to changes in the environment.

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Animal Guts The gut is a sac or tube projecting into the body where food is digested. In more primitive animals, the gut has only a single opening, that is used to take in fresh food and to expel waste products: it is both a mouth and an anus. The development of a gut tube, with a separate mouth and anus, occurred later. This system is more efficient: food passed through in a one way flow.

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Body Cavities What is between the body wall and the gut? The simplest animals have this regions packed with body organs, but more complex animals have a cavity separating the body wall from the gut. If this cavity has a lining that holds the internal organs in place, it is called a “coelom”. Some animals have the cavity without a lining. This condition is called “pseuodcoelomate”.

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Segmentation One way complex animals are built is to combine a series of similar segments together. The obvious case is the segmented worms, but insects and vertebrates are built the same way. The segments in higher animals show more differentiation into different functions.

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Invertebrate Groups We are going to examine several groups of animals: the sponges (porifera), Cnidarians (jellyfish and hydras), flatworms, roundworms (nematodes), annelids (segmented worms), mollusks, arthropods, and echinoderms (starfish). Later we will look at the vertebrates.

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Sponges Sponges are the phylum Porifera. They are the simplest group of animals. Sponge bodies have no symmetry. The body contains many pores that water flows through Just a few cell types, and they are relatively independent of each other. . Flattened cells line the outside. The inside cells have flagella that propel water through the pores and trap food particles. In between these two linings is an area filled with sharp glasslike spicules, that are a major deterrent to predators. Amoeba-like cells roam this interior space and secrete the spicules. Sponges can reproduce sexually or asexually. Sperm are released into the water, while eggs are retained within the sponge. After fertilization, a larval stage develops. Sponge larvae swim away to find a suitable environment to take root in.

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Sponge Anatomy

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I can reproduce asexually….if I wanted to…. Asexual Reproduction in Sponges

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Cnidarians . Cnidarians are animals with radial symmetry and tentacles. Jellyfish, sea anemones, corals, and hydras are common examples. Cnidarians have 2 basic body forms: the medusa, which has a bell-shaped floating body with tentacles hanging below it, and the polyp, whose body is a tube anchored at one end and with a tentacle-ringed mouth at the other end. Larval and adult cnidarians often switch between these forms.

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Cnidarian Life Unlike the sponges, cnidarians have a nervous system. It is called a “nerve net”, because there is no centralized brain. The nerve net controls movement of the body and the tentacles. There is only a single opening into the gut, used as both a mouth and an anus. Food is taken in, digested, and waste products are expelled through this opening. Cnidarians move by rapidly expelling water out of the mouth. Cnidarian larvae are motile, but many adult cnidarians are rooted to one spot. Cnidarians contain a special defensive cell called a nematocyst. It contains a coiled barb that shoots out and injects a poison when triggered.

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Flatworms Flatworms are the first group of bilaterally symmetric animals we will study. Examples of flatworms: planarians (which can reproduce by dividing in half), flukes (parasitic worms), and tapeworms (which live in the gut as parasites). Flatworms have no coelom, the internal cavity between the gut and the outer body wall. The entire space within the flatworm is packed with organs.

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Flatworm Anatomy Unlike the sponges and cnidarians, flatworms have distinct internal organs that do specialized tasks. Flatworms have a nervous system with a primitive brain in the head. Flatworms also have a digestive system, but only a single mouth/anus opening, and an excretory system to maintain ion balance. Also separate male and female gonads.

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Roundworms Roundworms are also called nematodes. They exist in almost every habitat, feeding on bacteria and decaying organisms. Some are parasites, but many more are free-living. Trichnosis is a disease caused by eating nematodes embedded in uncooked pork meat. Elephantiasis is another nematode-caused disease. Nematodes are bilaterally symmetric, with tube-shaped bodies. Nematodes have a complete tube for a digestive system, with a separate mouth and anus. Nematodes have a body cavity, but it is unlined. Thus, nematodes are “pseudocoelomate”.

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Protostome/Deuterostome Split Animals that developed after this time all have coeloms, lined gut cavities. Guts are now tubes, not sacs. A major split occurs between two major groups, depending on which develops first, the mouth or the anus. In protostomes (which means “mouth first”), the mouth develops first, and the anus develops after most of the body has formed. Protostomes include the molluscs, the annelids, and the arthropods. In deuterostomes, the anus develops first (deuterostome means “mouth second”). Deuterostomes include the echinoderms (starfish) and chordates, including the vertebrates. Animals past this point all have 3 basic body layers: the ectoderm, the mesoderm, and the endoderm. Ectoderm forms the skin and nervous system, mesoderm forms the muscles, bones and most internal organs, and endoderm forms the gut.

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Mollusks Mollusks are animals with bilateral symmetry, a soft fleshy body, and often a hard shell. The body is covered with a soft tissue called a mantle; the mantle secretes the shell. Snails, clams, and octopuses are common mollusks. Mollusks are the most complex invertebrates. The cephalopod group (octopuses and squids) have large eyes, well developed brains, and the ability to move very quickly.

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Annelids Annelids are segmented worms. Earthworms are a common type. Annelids are composed of numerous segments, each with its own bristles, muscles, nerves, blood vessels and kidneys. The gut runs through all segments, and there is a nerve cord running through the body connected to a brain in the head. Annelids have a hydrostatic skeleton: there are no hard parts, but there is a tough flexible outer wall that keeps its tube-like shape due to water pressure from inside.

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Arthropods Arthropods are probably the most successful group of animals: there are more different species of arthropod than any other group. The largest arthropod groups are the insects, the crustaceans (shrimps, crabs, etc.), and the spiders.

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Arthropod Characteristics Arthropods have a hard exoskeleton. It is composed mostly of chitin, a polysaccharide. This serves as protection against predators, and as a way to stand up against gravity. It also has a waxy coat to prevent desiccation. One problem with a hard exoskeleton is that as the arthropod grows, it must shed its exoskeleton and grow a new, larger one. This process is called molting. Jointed limbs allow arthropods to move around easily. The muscles attach to the exoskeleton.

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More Arthropod Characteristics Respiratory structures. Aquatic arthropods (mostly crustaceans) use specialized gills to breath. Land-dwelling arthropods have tubes called trachea that run through their bodies, delivering air to all cells. Some also have lungs to pump the air through the trachea. Sense organs. Some insects have very complex eyes. The structure is very different from our eyes, but they use the same visual pigment, rhodopsin, that we use. Insects also have well developed chemical senses (smell) and hearing.

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Metamorphosis Many insects have a larval stage that is very different from the adult stage. This allows specialization for feeding and for mating at different times during the life cycle. The larvae feed in one environment, but the adult can mate and migrate in a different environment. Conversion from the larval form to the adult form is the process of metamorphosis. Metamorphosis involves a pupal stage, a resting condition in which the body is remodeled. The cocoon is the pupal stage of butterflies. Not all insects have metamorphosis: many insects just have smaller, immature forms for the young. Successive molts bring them to the adult size and maturity.

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Echinoderms Echinoderms are the starfish, sea urchins, and similar sea-dwelling creatures Echinoderms are deuterostomes: the mouths develop after the anus. This is also characteristic of the vertebrates, but not mollusks, annelids, or arthropods. Echinoderms have spiny skins and radial symmetry. However, echinoderm larvae have bilateral symmetry Echinoderms have a nervous system, but no central brain. However, they arms are able to share information to coordinate their movements.

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Chordates Chordates are the larger group that contains the vertebrates, animals with backbones. Other chordates are the tunicates (sea squirts) and lancelets. Chordates are bilaterally symmetric, coelomate, deuterostomes. They are characterized by having a notochord, a long rod of stiffened tissue that supports the body and runs along the back. In many chordates (including us), the notochord vanishes: it is only seen in the embryo. It is replaced by the vertebrae of the spine. Chordates also have a nerve cord in their backs: our spinal column, for example. Chordates also have a set of slits in the wall of their pharynx. The pharynx is a muscular feeding tube (the throat), and the slits are used as gills in marine chordates.

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Invertebrate Chordates The tunicates are bag-like marine organisms that are rooted to one spot as adults. They squirt water out a siphon when irritated. Their larvae are typical chordates, with bilateral symmetry, a notochord, nervous system, and pharynx. After metamorphosis, most of the nervous system is lost and they are converted into filter feeders. The lancelets are filter feeders that look like very primitive fish. Amphioxus is a common example. Both tunicates and lancelets use their pharynx and gill slits to suck in water and filter out the food.

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Key Innovations in Vertebrates Much of basic vertebrate evolution can be understood in terms of an arms race between predators and prey. Vertebrae instead of a notochord: a hard, flexible backbone allows a stronger and faster body. Jaws: more efficient predation, and also support for gill arches. Brains and sensory organs: to detect and react to predators or prey. Paired fins along the body: for steering and additional propulsion. Eventually evolved into legs. Gills: more efficient than taking oxygen directly across the skin, as the lancelets do. Lungs developed from gut wall pouches: better for direct breathing from the atmosphere.

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Early Fish The first fish were jawless. They were scavengers and filter feeders. Modern examples include the lampreys and hagfishes. They did not have hardened bones or paired fins along the body. The placoderms are an extinct group of fish, the first group that had jaws, with teeth in them, and also paired fins. Their heads were covered with bony plates, but they still used a notochord as support instead of a bony vertebral column.

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Modern Fish Two groups: the cartilaginous fish (like sharks and skates), and the bony fish (most other fish). Cartilaginous fish have skeletons made of cartilage, not hardened bone. Bony fish have hardened bones throughout their skeleton. They are the most numerous and diverse of the vertebrates. The lobed fin fish have fins that are fleshy extensions of the body. These eventually developed into legs. Lungfish developed lungs to take oxygen directly from the air, a useful trick when living in stagnant water and ponds that dry up.

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Amphibians A few fish can spend a little time on land, enough to move from pond to pond, for example. But, without legs or efficient lungs, fish don’t live on the land. Amphibians have jointed legs and a bony body. They have lungs during at least part of their life cycle, but their skin also participates in gas exchange. Amphibian skin must be kept moist, so many amphibians live most of their lives in the water. Amphibian eggs are soft and jelly-coated. They must develop in water. Common amphibians: frogs, toads, salamanders.

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Metamorphosis and Neoteny The larvae of most amphibians develop in the water. They have long tails for swimming and gills for breathing. Some amphibians, such as frogs, undergo a radical shift in body form when moving to adulthood. The tail is absorbed into the body, gills are lost and lungs develop, legs develop. The animal is now capable of living at least part of its life on land. Some salamanders (axolotl) retain some of their juvenile characteristics: long tail, gills. However, they become sexually mature. This process of retaining juvenile features into adulthood is called neoteny. The more-or-less hairless condition of humans is also an example of neoteny. Apes are born hairless, and then hair over the entire body develops as they mature. We retain the hairless state.

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Reptiles Reptiles were the first group of vertebrates to live completely on dry land. Most important development is the amniote egg: an egg surrounded by a waterproof membrane. Fish and amphibian eggs are coated with jelly and must be kept in water. Reptiles also have a waterproof skin (amphibians use their moist skin for gas exchange), internal fertilization (amphibians release sperm and eggs into the water), and kidneys that concentrate urine to conserve water (amphibian kidneys mostly function to excrete excess water, not retain it). Nitrogenous waste in reptiles and birds (from protein metabolism) is converted into uric acid. Mammals convert it to urea. Uric acid is not very soluble in water, and it is excreted as a white paste: bird droppings.

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More Reptiles Common reptiles: lizards, turtles, crocodiles, snakes. Dinosaurs were the dominant land animals throughout the Mesozoic period. Dinosaurs were a major adaptive radiation of the reptiles. They became extinct at the end of the Mesozoic.

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Dinosaurs Dinosaurs and related mammals lived over a very long period of time, between 250 and 65 million years ago. Many species and groups arose and became extinct during this period. At least some dinosaurs were probably warm-blooded (like birds).

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More on the Extinction

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Birds Birds are the modern descendants of the dinosaurs. Birds are warm-blooded: they maintain a constant internal temperature. This makes enzyme action more efficient, because the enzymes can always function at their optimum temperature. Most birds can fly, and much of their structure is based on this. Bones are hollow to decrease weight, the circulatory system is well developed for rapid pumping of oxygen and nutrients to the muscles, lungs are very large. Birds are covered with feathers, which are lightweight insulating structures modified from the scales that cover reptiles.

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Mammals Mammals are characterized by having hair and mammary glands (organs in females that secrete nutritious milk for their offspring). Mammals have more behavioral flexibility and learning ability than other groups of animals. Mammals have teeth, which are quite different from the teeth found in other groups such as sharks.

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Mammalian Groups Most mammals are placental: they have a placenta that feeds the developing offspring in the uterus. A placenta is a tissue composed of both fetal and maternal tissues that is attached to the uterine wall. It allows the developing animal to live in the uterus for a long time, until it is relatively mature.

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Marsupials Marsupials are a smaller group of mammals that includes the opossums and the kangaroos, and most of the native animals in Australia. Marsupial offspring are born at an earlier stage than placental offspring. After birth they crawl into a pouch that contains milk glands, where they develop further.

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Monotremes Monotremes are a very small group: the platypus, the echidna, and the spiny anteater are the only living examples. Monotremes lay eggs. After hatching, the offspring moves into a temporary pouch for further development.

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Primate Evolution Humans are mammals in the primate family, along with monkeys, apes, tarsiers, lemurs. We are most closely related to the apes, and then to the Old World monkeys.

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Hominids The apes had a major adaptive radiation in Africa during the Miocene era, 25- 5 million years ago. Many different species appeared and spread throughout Africa, Asia, and Europe. Around 6 million years ago, the lineage leading to modern humans split with that of the Great Apes (gorilla, chimpanzee, bonobo, orangutuan). All creatures on the lineage after this spilt are called “hominids”. After that time there have been a number of species of hominid, mostly living in Africa.

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First Hominids The largest group of pre-human species was Australopithecus. Shortly after the ape/hominid split the species Australopithecus afarensis lived. The best known example is the skeleton “Lucy”, which is almost complete. These creatures were 4 feet tall or less, but fully bipedal (walked on 2 legs all the time, unlike chimps and gorillas). They had large jaws and fairly small brains.

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Hominid Evolution The hominid line split into 2 main branches after this time. One branch led to the modern humans, and the species on this line are in the genus Homo: Homo habilis, Homo erectus, Homo sapiens. The other branch contains several species that are called Paranthropus or continue to be called Australopithecus, or the “robust” Australopithicines. This branch developed very large jaws along with the sagital crest (on top of the skull) to support the jaw muscles. Big jaws, small brains—they seem to have been vegetarians. All species on this line died out more than 1 million years ago. Our direct ancestral species was Homo erectus.

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Homo erectus learned to create stone tools as long as 2 million years ago. Use of fire is more controversial. Some think fire use (if not the ability to make fire) came quite early, and that the gradual decrease in jaw size is a response to the use of fire to cook food. Others hold that fire use is a very late development, Homo sapiens only. H. erectus did one other interesting thing: walked out of Africa and populated most of the Old World. This happened perhaps 1.8 million years ago. The “Out of Africa” theory really means Out of Africa Twice: once by Homo erectus, and them again by modern humans. The multiregionalists also believe that Homo erectus migrated out of Africa to the rest of the world (although they consider H. erectus to be primitive Homo sapiens and not a separate species). Could Homo erectus talk? The only real evidence against it is that the spinal column in the thorax in the best preserved skeleton is quite narrow. It has been argued that this implies an inability to control breathing well enough for speech.

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Neanderthals In August 1856 workers in a limestone quarry in the Neander valley in Germany came across some bones that were undeniably human, but very odd looking, especially in the skull. Scientists held two differing views: they were either the bones of a modern human distorted by disease (a Cossack soldier fleeing Napolean’s army was a popular theory), or they were the bones of an human ancestor. Darwin’s Origin of Species was published in 1859, considerably adding to the controversy. “Neanderthal” means “Neander Valley”. Often spelled without the h (Neandertal” to match the German pronunciation. As time went on, more similar skeletons were found throughout Europe, and it became that the bones were quite ancient. In 1864 Irish anatomist WIlliam King decided they represented a new species, christened “Homo neanderthalensis”. They were considered to be the ancestors of modern humans. Neanderthal bones have been found across Europe and the Middle East, but not in Africa or eastern Asia. Neanderthals lived from approximately 200,000 years ago until about 30,000 years ago. There are no human remains in the Americas older than about 30,000 years.

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What did Neanderthals Look Like?

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More Neanderthal Appearance Physical description: short, stocky, heavy build, large head, protruding brow ridges and a large nose. Their brain was as large or larger than ours. The oldest known was 40 years old when he died, and nearly all Neanderthal skeletons show signs of injury: healed bones. Were they hairy like apes or smooth-skinned like us? When fist discovered, Neanderthals were thought to have been extremely primitive, closer to the apes than to us. We now know that there were many other human-like species that came between us and our common ancestor with the apes. In recent times Neanderthals have been thought to be very human-like in appearance and behavior. Certainly living in cold climates it seems likely that they wore clothing—animal skins, probably, although no direct evidence for such clothing exists. A recent study of human lice bears on this subject. Head lice live in the hair, and they have been with us since long before we became human. Body lice, on the other hand, live in clothing. Body lice are a sub-species of head lice. By examining he DNA of the 2 types, and comparing them to chimpanzee lice, it is clear that they diverged from each other fairly recently, about 70,000 years ago. This implies that early humans and Neanderthals may not have worn clothing. Neanderthals may have been hairy beasts after all. The evidence is pretty indirect and does rely on a number of assumptions.

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Alternate Views

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Neanderthal Behavior Could they talk? It’s a little late for a conversation! An argument has been made that the structure of the base of the skull would not have allowed the larynx (voicebox) to produce the range of sounds that modern humans have. Another contribution to this controversy: in one skeleton, the hyoid bone in the throat (connects the tongue to the lower jaw) has been found. It is shaped like a modern human hyoid, and not like the hyoid bone in gorillas and chimps. Evidence for human-like behavior. Neanderthal bones are sometimes found in what look like funeral burials, arranged in a comfortable position. Some evidence that flowers were used to cover one of them. This evidence is controversial, however. In one case, Shanidar (named after the site), the person had had severe injuries, including destruction of an eye socket. These wounds were healed, and they were severe enough so that he wouldn’t have survived without assistance. A fragment of a flute has been found from Neanderthal times (50 000 years ago) It is bone, with holes spaced in a way that allows several modern-style notes on it. They definitely made stone tools and used fire.

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What happened to the Neanderthals? About 35,000 years ago, modern humans came into their territory in western Europe. The modern humans are sometimes called “Cro-magnon”, based on the first archeological site they were found at. Although there is no obvious evidence of conflict, after several thousand years of co-existence, the Neanderthals apparently died out. Two competing theories. 1. The Neanderthals were the same species as modern humans, and the distinctive Neanderthal type disappeared by interbreeding. This implies that people of today carry Neanderthal genes. 2. Alternatively, the Neanderthals may have been an entirely different species, unable to produce fertile hybrids with modern humans. This implies that people today carry no Neanderthal genes. Theories are tied up in a larger context. The older theory , called the “Multi-regional hypothesis”, says that all of the human-like creatures that lived in the past two million years or more (including Homo erectus, generally considered to be our ancestral species) are part of the same species, Homo sapiens, and that they evolved worldwide from the primitive forms into the forms we see today. The mechanism for the spread of new genes was a slow process of interbreeding between neighboring groups. This theory suggests that many of today’s populations have lived in the same area of the world for a very long time: the Chinese evolved in China, the Africans evolved in Africa, etc. The newer theory, called “Out of Africa” says that there have been many different species of human-like creatures, with Neanderthals just one of these species. Modern humans evolved in Africa about 100,000 years ago, then spread out from there. All other human species were eliminated.

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Evidence The main evidence for the multiregional hypothesis comes from fossil bones. Anthropologists of this school claim to see the same regional differences in ancient bones as are present among the current inhabitants. Also, some skeletons are claimed to show intermediate characteristics between modern humans and Neanderthals. The out of Africa adherents say that bones are subject to deformation, and that the differences are too subtle to be real. I can’t judge these arguments. The multi-regional hypothesis is currently losing ground due to DNA evidence. The DNA from 3 different Neanderthals has been examined, and the variant forms there are far outside the range of modern human DNA—at least twice as far from any modern human type as any two modern types are from each other. This implies that Neanderthals and modern humans last had a common ancestor 450,000 years ago, long before the encounters in western Europe. Another aspect of DNA evidence is that modern human DNA is not very variable: there is more genetic variation among the chimpanzees in the Gombe Stream Reserve in Tanzania than there is among all human populations. This implies that somewhere around 100,000 years ago the human population went through a bottleneck—it was reduced to a very small number, from whom we are all descended. Most of the human DNA variation is found in Africa, and modern human remains have been found there of an appropriate age. The DNA evidence mostly comes form the mitochondrial DNA, a small circle of DNA found outside the nucleus, in the mitochondria, the organelle that provides most of the energy to run the cell. This DNA is found in large amounts than nuclear DNA, and it is tougher—as a circle it has no free ends to attack. Mitochondrial DNA is inherited strictly through the mother, so it doesn’t give complete information about inheritance patterns in the species. For instance, if Neanderthal-human hybrids were only fertile with a modern mother and a Neanderthal father, we could have Neanderthal genes in us now, but not Neanderthal mitochondrial DNA. This type of situation; fertility in one direction but not the other—is very common in crosses between closely related species.

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Human Evolution