Animal Diversity
The Animal Kingdom is vast; so far, biologists have discovered and identified about 1.3 million living species of animals.
Nutrition:
Animals are unlike both plants and fungi in their means of gaining nutrition; they are not able to synthesize their own nutrition through photosynthesis like autotrophic plants, and they do not absorb nutrients from the surrounding environment like fungi. In order to get all the organic molecules necessary for survival, animals ingest them, either by eating other animals or nonliving organic material and then using enzymes to break down and digest those molecules.
Cell Structure and Specialization:
Animals are all multicellular, and do not have cell walls like plants and fungi; instead, they have several proteins on the outside of the cell membrane that provide support and structure to the cells, and help to connect them to one another. Collagen is the most abundant of these kinds of proteins, and is only found in animals. There are two types of specialized cells that are mostly unique to the animal kingdom: nerve and muscle cells. These special types of cells, when grouped together, are referred to as tissue, insinuating that they have a common structure and/or function. Nerve tissue is the body's way of conducting nerve impulses, and muscle tissue is what animal's bodies rely on for movement. Both of these types of specialized cells are imperative to animals and also aid in adaptation.
Reproduction and Development:
Most animals reproduce sexually. In the haploid stage, gametes are produced by meiosis; for most animals, a small flagellated sperm fertilized a much larger and stationary egg, which forms a diploid zygote, which then undergoes cleavage (several rounds of mitotic cell division excluding a period of growth between them). This cleavage leads to the formation of a blastula, which, for most species of animals, looks like a hollow ball. After the formation of the blastula, the next stage of gastrula takes place through the process of gastrulation, where different layers of embryonic tissues develop into specified body parts.
Some species don't develop directly into adults like humans do, but instead have larval stages, where the larva are sexually immature versions of the animal and look very different (morphologically), eat different foods, possibly have different habitats, etc. The larvae go though metamorphosis, transforming them into a juvenile that looks like the adult for of the animal, but is not yet sexually mature.
No matter how different the life cycles of different animal species may be, all animals have developmental genes that regulate the expression of other genes, called homeoboxes. Hox genes are crucial in the development of animal embryos; they control the timing and placement of the expression of up to hundreds of genes that dictate what the animal is going to look like.
Nutrition:
Animals are unlike both plants and fungi in their means of gaining nutrition; they are not able to synthesize their own nutrition through photosynthesis like autotrophic plants, and they do not absorb nutrients from the surrounding environment like fungi. In order to get all the organic molecules necessary for survival, animals ingest them, either by eating other animals or nonliving organic material and then using enzymes to break down and digest those molecules.
Cell Structure and Specialization:
Animals are all multicellular, and do not have cell walls like plants and fungi; instead, they have several proteins on the outside of the cell membrane that provide support and structure to the cells, and help to connect them to one another. Collagen is the most abundant of these kinds of proteins, and is only found in animals. There are two types of specialized cells that are mostly unique to the animal kingdom: nerve and muscle cells. These special types of cells, when grouped together, are referred to as tissue, insinuating that they have a common structure and/or function. Nerve tissue is the body's way of conducting nerve impulses, and muscle tissue is what animal's bodies rely on for movement. Both of these types of specialized cells are imperative to animals and also aid in adaptation.
Reproduction and Development:
Most animals reproduce sexually. In the haploid stage, gametes are produced by meiosis; for most animals, a small flagellated sperm fertilized a much larger and stationary egg, which forms a diploid zygote, which then undergoes cleavage (several rounds of mitotic cell division excluding a period of growth between them). This cleavage leads to the formation of a blastula, which, for most species of animals, looks like a hollow ball. After the formation of the blastula, the next stage of gastrula takes place through the process of gastrulation, where different layers of embryonic tissues develop into specified body parts.
Some species don't develop directly into adults like humans do, but instead have larval stages, where the larva are sexually immature versions of the animal and look very different (morphologically), eat different foods, possibly have different habitats, etc. The larvae go though metamorphosis, transforming them into a juvenile that looks like the adult for of the animal, but is not yet sexually mature.
No matter how different the life cycles of different animal species may be, all animals have developmental genes that regulate the expression of other genes, called homeoboxes. Hox genes are crucial in the development of animal embryos; they control the timing and placement of the expression of up to hundreds of genes that dictate what the animal is going to look like.
History of Animals on the Planet
Some paleontologists think that 99% of all animal species that have ever lived on earth are now extinct. Although, studies have suggested that the great diversity we have on our planet now have only originated over the last billion years.
NEOPROTOZOIC ERA (1 billion -- 542 million years ago)
PALEOZOIC ERA (542 -- 251 million years ago)
CENOZOIC ERA (65.5 million years ago -- present)
NEOPROTOZOIC ERA (1 billion -- 542 million years ago)
- First widely accepted macroscopic fossils date back to between 565 and 550 million years ago
- These fossils are members of the Ediacaran biota, an early group of soft-bodied multicellular eukaryotes
- Some early fossils are sponges, others probably related to living cnidarians
- Most early fossils are almost impossible to identify as being closely related to any animal living today
- Rocks in this era also have microscopic evidence that suggests early signs of animals
- For example, 575--million year old fossils found in China resemble the general structural organization of animal embryos today
PALEOZOIC ERA (542 -- 251 million years ago)
- Cambrian explosion during Cambrian period caused another wave of animal diversification
- In strata 535-525 million years old, the first arthropods, chordates, and echinoderms have been found
- Paleontologists have determined that many of these first animals, most with hard mineralized skeletons are now completely extinct
- Increase in diversity of animal phyla during Cambrain period paralleled decline in diversity of Ediacaran organisms; there are many current hypotheses about why this happened
- Hypothesis #1: during the Cambrian period, predators attained new adaptations to help them catch their prey, and the prey also attained new forms of defense mechanisms, like shells; therefore, natural selection may have been the main reason for the decline of some groups and the rise in others
- Hypothesis #2: increase in atmospheric oxygen enabled animals with larger body sizes and higher metabolic rates to thrive, while influencing the decline of other species
- Hypothesis #3: the emergence of Hox genes and other genetic changes that regulate developmental genes might have aided heavily in the evolution into new body forms
- Ordovician, Silurian, and Devonian periods followed Cambrain period
- Vertebrates (fishes) became top predator of marine food chain
- By 460 million years ago, animals were walking on the land
- 365 million years ago vertebrates made it onto land; diversified into many different groups; the two that have survived until today are the amphibians and amniotes (reptiles, birds, mammals)
- Phyla formed earlier extend into different habitats
- First coral reefs form, providing new habitat
- Some reptiles return to water
- First animals with wings have evolved on land (pterosaurs and birds)
- Dinosaurs have emerged; both large and small, carnivores and herbivors
- First mammals (small nocturnal insect-eaters) evolve
- Flowing plants and insects undergo drastic diversifications
CENOZOIC ERA (65.5 million years ago -- present)
- Characterized by mass extinction of both marine and terrestrial animals, including dinosaurs and marine reptiles
- Rise in mammal population
- Global climate gradually cools, causing many species to adapt
Body Plans
A body plan is a specific set of developmental and morphological traits that make up a certain animal. It is very possible for similar body forms to have evolved independently from one another; also, body features in a certain species could have been lost over time, causing closely related species to vary greatly in physical features.
Symmetry:
Symmetry:
- radial symmetry--having a focused center and everything else extending out from it on all sides; usually only have top and bottom, no classified sides
- bilateral symmetry--having two axes of orientation: front to back and top to bottom; these animals will have a dorsal (top) side, ventral (bottom) side, left side, right side, anterior (front) end and posterior (back) end; examples are arthropods and mammals, which tend to have a central nervous system (brain) concentrated at the anterior end, an evolutionary occurrence called cephalization
- symmetry of an animal usually reflects its life style; radial creatures tend to be stoic, attached to a substrate, while bilateral animals are active and tend to move from place to place
Tissues:
Body Cavities:
grade: a group whose members share key biological featuers
clade: a group that includes an ancestral species and all of its descendants
- true tissues in animals are classified as collections of specialized cells isolated from other tissues by membrane layers
- germ layers--concentric layers that form as an animal develops; form different tissues in the body of the animal
- ectoderm--germ layer covering the surface of the embryo, turns into the outermost layer of the animal, sometimes even the central nervous system
- endoderm--innermost germ layer; lines pouch that forms during gastrulation; turns into lining of digestive tract and organs like the liver and lungs of vertebrates
- diploblastic--describes animals with only two germ layers, including cnidarians (jellies and coral)
- triploblastic--describes animals with bilateral symmetry, who have a third layer between the ectoderm and endoderm, the mesoderm, which fills all the body space in between: muscles, others organs between digestive tract and outer covering of animal
Body Cavities:
- a body cavity is a fluid or air filled space located between the digestive tract and outer body wall, also called coelom
- a true coelom comes from tissue derived from the mesoderm; animals with these are called coelemates
- pseudocoelomates refer to animals who have a pseudocoelem, derived from both mesoderm and endoderm tissue
- some animals lack a coelom all together; they are called acoelomates
- body cavities have many functions; its fluid could cushion nearby suspended organs, could contain noncompressible fluid that acts like a skeleton (soft bodied coelomates), could enable organs to grow and move independently of the outer body wall
grade: a group whose members share key biological featuers
clade: a group that includes an ancestral species and all of its descendants
Protosome vs. Deuterostome Development
Based on particular aspects of early development, animals can be described as having one of two different developmental modes: protostome development or deuterostome development. These two modes can be distinguished by differences in cleavage, coelom formation, and fate of the blastopore.
Cleavage:
Coelom Formation:
Fate of Blastospore:
Cleavage:
- Protostome: spiral cleavage--planes of cell division are diagonal to vertical axis of embryo, smaller cells are centered over the grooves between larger, underlying cells; determinate cleavage--determines developmental fate of every embryonic cell very early
- Deuterostome: radial cleavage--planes are parallel or perpendicular to vertical axis of embryo, tiers of cells aligned, one directly above the other; indeterminate cleavage--each cell retains the capacity to develop into a complete embryo
Coelom Formation:
- archenteron--developing digestive tube that initially forms as a blind pouch during gastrulation, becomes the gut
- Protostome: as archenteron forms, initially solid masses of mesoderm split and form the coelom
- Deuterostome: the mesoderm buds from the wall of the archenteron, and its cavity becomes the coelom
Fate of Blastospore:
- blastospore--the indentation that leads to the formation of the archenteron during gasturulation; in many species, the blastospore and a second opening that forms opposite of the gastrula become the two opening of the digestive tube: the mouth and anus
- Protostome: mouth generally develops from the first opening (blastospore)
- Deuterostome: mouth generally develops from the second opening, and blastospore forms the anus