Chapter 11. Chapter 11: Animal Diversification

(a) Animals cannot make their own food and therefore must take in food to survive. (b) Animals are made up of more than one cell. A multicellular body enables them to have specialized cells with unique properties. (c) Animals are capable of moving at some point in their lives. Some can move for their entire life, while others can move only during a larval stage or other life stage.

The degree of complexity of an organism is not related to how well that organism is adapted to its environment. There is a tendency to judge animals that are more like humans as “higher organisms,” but animals perfectly adapted to their environment, though they may not be at all like humans, are not “lower organisms.”

The common ancestor of all animals is thought to have been a unicellular ancestral protist resembling a sperm in shape and size. This conclusion is based on DNA and RNA analysis.

Sponges can reproduce both sexually and asexually. Sponges are hermaphrodites, but individuals produce sperm or eggs at different times. Sperm swim to the eggs contained in another sponge, and the fertilized eggs develop into larvae, which are released from the sponge and float to another location to start life. Asexual reproduction occurs by the formation of buds on the body surface. These buds can break off, float away, and form new sponges, after landing on a suitable surface.

Cnidarians can exist as a sessile (non-mobile) polyp, which resembles a stalk with arm-like projections at its top surface, or as a bell-shaped, free-floating medusa, with arm-like extensions at its perimeter. Both forms are radially symmetrical and have stinging cells.

Roundworms are probably the most abundant animals on earth. Some roundworms, also called nematodes, are parasites that cause damage to plants and animals.

A major characteristic of segmented worms is the grooves that run around their body, marking their body segments. Terrestrial slugs do not show segmentation. Slugs have an extended foot at the base of their body, which is not present in earthworms. Finally, unlike earthworms, when slugs are threatened they produce copious amounts of slime that coats their body.

Mollusks have a sandpaper-like tongue called a radula, which is used for feeding. The radula’s rough surface assists with pulling food into the mouth.

It is difficult to define what intelligence is, and even more difficult to apply this concept to non-human animals. Animals have evolved in response to the selective forces of their environments, and our concept of intelligence cannot be applied objectively to non-human animals. In other words, all animals have adapted to manipulate their environment to meet their needs in ways that may not conform to our concept of intelligence.

Commonly, an octopus’s ability to carry out manipulations such as unscrewing a bottle to get at a fish inside is seen as a form of intelligence—a type of intelligence that most people can relate to and compare to our concept of intelligence.

An exoskeleton is a hard outer covering that provides an attachment for muscles, protection for the body, and reduction in water loss. The exoskeleton cannot expand as the animal grows. The animal must shed its old exoskeleton and form a new one as it increases in size.

You could estimate the number of beetle species by the method used by Terry Erwin. Determine the number of beetle species in one species of tree in a temperate region, count the total number of tree species in that region, then estimate the total number of beetle species in the region. For example, if 100 beetle species are found in one tree species and there are 10 species of trees, the number of beetle species can be estimated as 10 x 100 = 1,000. This would be a rough estimate.

Benefits: The exoskeleton provides protection, much like a suit of armor, and helps prevent water loss, keeping the organism from drying out. Costs: An exoskeleton prohibits growth; animals with exoskeletons must go through developmental stages or shed their exoskeleton and grow a new one as they increase in size.

(a) Insects’ ability to fly enables them to escape from predators, find food and mates, and occupy a wide variety of habitats. (b) Their ability to pass through different life stages (larva, pupa, and adult) allows them to grow in size, even though they have an exoskeleton.

Echinoderms are deuterostomes, meaning that they undergo embryonic development in which the gut develops from the back of the body toward the front; this developmental characteristic is shared by vertebrates. Echinoderm larvae show bilateral symmetry, in which the body has two halves that are essentially anatomically identical; this is another trait shared by vertebrates. These similarities in anatomical characteristics, developed under the force of natural selection, indicate an evolutionary history shared, in the distant past, with vertebrates.

(a) The notochord is a rod of tissue that extends from the head to the tail and stiffens when muscles contract during movement. It is present in all vertebrates during at least some stage of life. (b) The dorsal hollow nerve cord is located along the back, or dorsal side, of the animal and extends from the head to the tail. It forms the brain and spinal cord of the central nervous system. (c) Pharyngeal slits, located between the back of the mouth and the top of the throat, are present during at least some stage of vertebrate development. (d) A post-anal tail is present during at least some stage of vertebrate development; a post-anal tail is located behind the anus, which is the end of the digestive system.

Two evolutionary innovations in mammals were: (a) long legs set vertically beneath the trunk of the body, enabling animals to run faster and for a longer time to capture prey; and (b) endothermy, a warm-blooded body, in which warm body temperature is maintained irrespective of environmental temperature through an increased metabolism, which is due to the muscle contraction required for running.

Lungs most likely evolved from the air sacs of lobe-finned fishes; the evolution can be traced through amphibians, reptiles (including birds), and mammals. Air sacs also evolved into the air bladders of ray-finned fish.

Amphibians must live close to water because most amphibians lay their eggs in water. Reptiles do not lay their eggs in water and can live in very dry locations.

Endotherms produce body heat as a result of cellular respiration. Ectotherms depend on outside sources of heat, such as from sunlight, to heat their body and do not depend on cellular respiration. Endotherms use much more energy to produce heat and maintain their body temperature, and thus must consume more food, more frequently, than ectotherms.

The placenta is rich in blood vessels and enables the transfer of nutrients, respiratory gases, and metabolic wastes between the mother’s circulatory system and that of the developing fetus. Finger-like projections of the placenta enter the uterine wall, which is also rich in blood vessels, so blood vessels of both structures are in close proximity. This arrangement allows an efficient exchange between the circulatory system of the mother and developing fetus.

The presence of smaller teeth suggests that tools were used for the initial preparation of food. Stone tools were found in the same deposits as the fossils with smaller teeth, supporting this idea. With the use of tools, food could be cut into smaller pieces before being eaten, doing the work previously done with larger teeth.

(a) The brain is larger in humans. (b) Humans walk on two legs, whereas chimpanzees mostly walk on four legs. (c) Body size is larger in humans.

Fossil evidence of healed broken bones suggests that: (a) There were organized social groups that worked together to support individuals with severe injuries. Without social support, it is unlikely that individuals would have been able to recover from injuries. (b) Neandertals collectively hunted larger prey in close proximity, using short spears to jab their prey. This conclusion comes from comparing the injuries seen in fossils with typical injuries received when taking part in rodeos, which also involve people being in close proximity to large animals.