|Chapter Thirty-Two: An Overview of Animal Diversity
Biologists have discovered 1.3 million “extant” (living) species of animals with estimates running of the actual number of species running much higher.
Phylogeny: The history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms.
Animals are multi-cellular (have many cells), heterotrophic (capable of using only organic materials as a source of food), eukaryotes (organism with bound cell organelles and nucleus) that develop from embryonic layers. Animals have several distinguishing characteristics:
.Nutrition: Plants are “autotrophic” meaning they produce their own found from organic molecules through photosynthesis, fungi are “heterotrophs” which grow on/near their food and absorb it after releasing digestive enzymes out of their bodies. Animals do not produce own food and must ingest living organisms or non-living organic material and then use digestive enzymes within body.
Cell Structure and Specialization: proteins such as collagen external to the cell membrane provide structural support for animal cells, substituting the cell wall in a plant cell. These are exclusive to animal cells along with muscle cells and nerve cells (which are usually not found in other multi-cellular organisms). Muscle and nerve cells are organized into tissues, groups of cells with common structure and/or function. This gives animals the unique quality to move and have nerve impulses.
Reproduction and Development
Animals undergo sexual reproduction with a dominant diploid stage and a haploid stage where small flagellated sperm and larger non-mobile egg cells are produced from meiosis (as opposed to plant and fungi) to form a diploid zygote. The zygote undergoes cleavage (a succession of miotic cell division without cell growth between divisions) usually leading to formation of multi-cellular blastula, a hollow ball of cells that surround a cavity called blastocoel. One end of the embryo folds inwards and expands, filling the blastocoel to produce layers of embryonic tissue, the ectoderm (outer layer) and the endoderm (inner layer).This process is known as gastrulation while the structure it form is called the gastrula with an archenteron (pouch) formed due to the pinching and a blastopore opening the gastrula. Most animals have a larva stage. The larva are sexually immature, eat differently and live differently from adults. They undergo metamorphosis transition to a not yet sexually mature juvnille.
Homeoboxes are special DNA sequences in developmental genes (similar in most animals) that regulate expression of other genes. Animals (excluding sponges) share a unique homeobox containing family of genes called Hox genes. The Hox gene family came from earlier homeobox genes which underwent a series of duplications. Hox genes in flies and humans are similar, proving that despite being similar they can cause divergence. In vertebrates Hox genes regulate formation of anterior-posterior (front-to-back) axis.
Over 99% of all species ever lived are now extinct. Ancestors of animal and fungi diverged over a billion years ago (according to molecular clock calculations). The common ancestor of animals resembled choanoflagelates (protist group) and lived around 800 and 675 million years ago. They must have been a suspension feeder (filtering organic particles from water) like current day choanoflagelates. Choanoflagelates are morphologically indistinguishable from sponge collar cells (which have been also identified in other anumas), plants and fungi. The DNA points to the relationship between animals and choanoflagelates along with the genes for signaling and adhesion proteins in animals is also in them.
Neoproterozoic Era (1 Billion-542 Million Years Ago)
First macroscopic animal fossil from 565 to 55o million years ago was a member of soft-bodied multi-cellular eukaryote group called Ediacaran biota (discovered in Australia). Some may be related to sponges and others to cnidarians, but they are generally hard to classify. Some fossils indicate presence of animals because they are similar to present-day animal embryos (Debated Topic) but they may also just be members of extinct groups closely related to animals.
Paleozoic era (542-251 Million Years Ago)
The Cambrian period or the Cambrian explosion was a burst of evolution 535-535 million years ago. Fossils from this time indicate the first presence of half the animal phyla including chordates, arthropods, and echinoderms. The fossils tend to be hard as the animals mostly had hard mineralized skeletons (very different from living animals). The Cambrian explosion brought an increase in the previous life forms (namely the Ediacaran biota). Current hypotheses include:
Predators acquired new adaptations (ex: locomotion) to help catch prey who at the same time acquired adaptations to better defend themselves (ex: protective shells). Natural selection then led to decline in certain groups.
Increase in atmospheric Oxygen allowed animals with higher metabolic rates and larger body sizes to thrive.
Origin of Hox genes and other genetic changes affecting regulation of developmental genes facilitated evolution of new life forms.
This followed the Ordovician, Silurian, and Devonian periods. Fish (vertebrates) became the top predator of the marine food chain. Anthropods (millipedes and centipedes) began to adapt to land (460 million years ago). Fossilized fern galls (used as response to insect stimulation) date back to 302 million years hinting that insects and plants were influencing each other’s evolution at the time. Vertebrates moved to land 365 million years ago diversifying into several groups including amphibians (frogs and salamanders), and amniotes (reptiles, birds, and mammals) which still exist today.
Mesozoic Era (251-65.5 Million Years Ago)
Animals evolving previously began to move to new habitats. Coral reefs began forming in oceans, forming a new habitat for other marine animals. Some reptiles returned to water. Terrestrial animals grew wings and flight equipment while large and small dinosaurs emerged. The first tiny, nocturnal, insect-eating mammal appeared and plants and insects were severely diversified.
Cenozoic Era (65.5 Million Years Ago to Present)
Mass extinction of both terrestrial and marine animals occurred especially for large non-flying dinosaurs and marine reptiles with a rise in large mammalian herbivores who exploited the vacated niches. Climate change caused primates to move to woodlands, grasslands (origin of our ancestors) and savannahs.
Body Plan: Particular set of morphological and developmental traits integrated into a functional whole—the living animal. It can simply the great diversity among animals and helps compare and contrast animal key features. The body plan has evolved over time and throughout history some body plans may have diverged while others converged.
β-catenin, one of molecular controls needed for gastrulation has evolved more than 500 million years ago. Gastrulation explains why animals aren’t a ball of cells.
Evo-Devo: Field studying the relationship between Evolution and Development
The body plan includes the following features:
Symmetry or lack of symmetry (sponges) helps categorize animals and is designed to fit the animal lifestyle. The two following types of symmetry have been around for at least 550 million years:
Radial symmetry occurs in animals with no left or right side only a top and bottom like a flower pot. Ex: sea anemone. These animals are usually sessile (living attached to a substrate) or planktonic (drift or weakly swimming such as jellyfish). This form allows them to meet the environment from all sides.
Bilateral Symmetry occurs in animals (usually arthropods and mammals) with two symmetrical halves like a shovel. They have a dorsal (top side), ventral (bottom side), left side, right side, anterior (front side where they have sensory equipment-Central Nervous System, brain) and posterior (bottom side). Animals with bilateral symmetry tend to be more active, constantly moving from place to place. The central nervous system they have allows them to move complexly such as crawl, burro, fly or swim.
Cephalization: The concentration of sensory organs and a central nervous system in the anterior side of a bilaterally symmetrical animal as an evolutionary adaptation which facilitates unidirectional movement.
True tissues: Collections of specialized cells isolated from other tissues by membrane layers. This is missing in sponges, but in all other animals the embryo becomes layered because of gastrulation forming “germ layers”. The germ layers later develop into various tissues and organs in the body:
Ectoderm: Germ layer covering surface of embryo to give rise to outer covering of animal and the central nervous system in some phyla.
Endoderm: Germ layer located in the innermost portion (lining the archenteron during gastrulation. It gives rise to lining of digestive tract (or cavity) and organs such as the liver and lungs (of vertebrates)
Mesoderm: Germ layer filling the space between the ectoderm and the endoderm. It later gives rise to most organs between the digestive tract and outer covering of the animal, in particular the muscles.
Diploblastic: Animals which only have the ectoderm and endoderm. This includes cnidrians, jellies, and corals. Some diploblasts do have a third germ layer; however it isn’t as complex as the mesoderm.
Triploblastic: Animals that have all three germ layers. These are usually also have bilateral symmetry and include a broad range of animals; flatworms, arthropods, vertebrates.
Body Cavity: A fluid or air filled space between the digestive tract and outer body wall. This is present in most triploblastic animals. A body cavity has fluid which cushions the suspended organs, preventing internal injury. In soft-bodied animals it acts as a skeleton because of its non-compressable fluid. It allows inner organs to grow and move independently of outer body wall.
This is also known as the coelom. A true coelom is completely lined by tissue derived from mesoderm (which connect and form structures that suspend the internal organs). Animals with a true coelum such as earthworms are coelomates
Triploblastic animals that have a body cavity formed from endoderm and mesoderm such as roundworms are called psedocoelomates because they possess pseudocoeloms.
Acoelomates such as planarians have solid bodies and lack body cavities between the digestive cavity and outer body wall.
Grade: Group of organisms sharing a similar body plan
Clade: An ancestral species and all of its descendants
Since true coeloms and and pseudocoeloms have been independently gained or lost multiple times through evolution, a grade is not equivalent to a clade.
Protostome and Deuterostome Development
An animal can be one of two developmental modes: Protosome development (mollucs and annelids) or deuterostome development (echioderms, chordates) both of which can be distinguished by:
Animals with protostome development often undergo spiral cleavage in which planes of cell division are diagonal to the vertical axis of the embryo and newly formed cells fit in the grooves between cells of adjacent tiers. They may also have determinate cleavage in which if a cell is removed it will form an inviable embryo which lacks parts.
Detuterostomes have radial cleavage where the planes are parallel or perpendicular to axis and cells are aligned properly. They also have indeterminate cleavage in which cells retain capability to develop into a complete embryo even if separated.
It is the indeterminate cleavage which makes identical human twins possible.
The archenteron form during gastrulation (later becoming the gut) develops into the digestive tract. In protstome development, while the formation of the archenteron is happening, solid masses of mesoderm split to form the coelom. In deuterostomes the mesoderm buds from the wall of the coelom and the archenteron and its cavity become the coelom.
Fate of the Blastopore
The blastopore is the indentation during gastrulation which leads to archenteron development. In protostome ( =mouth first) development, the blastopore ends up becoming the mouth and a second opening on the opposite end of the gastrula opens to become the anus. In Deuterostomes (= mouth second), the blastopore becomes the anus and the mouth is developed from the second opening.
Zooologists still debate over the currently 36 known animal phyla. They use new molecular and traditional morphological data to base research, particularly fossils of lesser known and older groups. They sort things into small clades and group them into larger clades inferring from shared derived characteristics that researches conclude are homologous (DNA sequence, anatomical and embryological similarities). The following points are agreed upon:
All animals share a common ancestor: animals are monophyletic and arise from the Metazoa clade
Sponges are basal animals: sponges (Porifera) are monophyletic and basic
Eumetazoa is a clade of animals with true tissue: sponges ere the only exception. The basal Eumetazoa which include the Cnidaria and Ctenophora are diploblastic and have radial symmetry.
Most animals are in Bilateria clade: bilaterians include majority of animals and was diversified during Cambrian explosion. They tend to be bilateral and triploblastic.
Chordates and some other phyla belong to the clade Deuterostomia: they mostly make up vertebrates and chordates with other debated groups
The morphology based tree divides bilaterians into deuterostomia and protostomia and groups arthropods (lobster) with annelids (earthworm)-which both have segmented bodies. Hox genes and mitochondrial gene research points to three major clades of bilaterally symmetrical animals: Deuterostomia, Lophotrochozoa, and Ecdysozoa. It also indicates the annelids and arthropods are not related while it shows a group of flatworms called Acoela which opposes the traditional views that all worms belong in the Playhelminthes. The basal status of Acoela indicate that bilaterians may have descended from a common ancestor which resembled acoelomate flatworms (which have a simple nervous system and no excretion system)
Ecdysozoans animals secrete external skeletons (exoskeletons). The process of this shedding is called Ecdysis. The group was characterized by molecular data so there do exist other animals outside this clade which also shed their skin(such as leeches).
Lophotrochozoa develop a lophophore, a crown of ciliated tentacles that function in feeding. Some phyla including molluscs and annelids go through a distinctive developmental stage called the trochophore larva.