animal origin

• All animals have a common ancestor (monophyletic group)
• Animals have synapomorphies:

  • Unique junctions between cells
  • A set of extracellular molecules like collagen and proteoglycans
  • Similarities in Hox and other developmental genes
• The common ancestor of animals was likely a colonial flagellated protist, similar to existing colonial choanoflagellates and sponges
• Distinct body plans among animals include:

  • Radial symmetry: body parts arranged around a central axis
  • Bilateral symmetry: can be divided into mirror image halves on only one plane
  • Asymmetry
• Animals can be classified based on body cavities:

  • Body cavity = coelom
  • Acoelomate = without coelom = no cavity
  • Coelom can develop from the mesoderm in two ways:
  • Schizocoelom: develops from split in mesoderm found in annelids, arthropods, and molluscs
  • Enterocoelom: develops from wall of embryonic gut found from echinodermata to chordata
  • Pseudocoelomates have a cavity not lined with mesoderm (pseudocoelom) and include nematodes, rotifers, gastrotrichs, and introverts
• Some animal groups fall outside the Bilateria:

  • Sponges have no distinct tissue types and hard skeletal elements called spicules made of silicon dioxide or calcium carbonate
  • Ctenophora (Comb jellies) have body symmetry, a gut, and nervous system
  • Cnidarians (Jellyfishes, sea anemones, corals, hydrozoans) have radial symmetry and a life cycle with sessile polyp and motile medusa stages
• Protostomes have an anterior brain and a ventral nervous system:

  • Protostomes have two derived traits:
  • Anterior 'brain' that surrounds entrance to digestive tract
  • Ventral nervous system with longitudinal nerve cords
  • Lophotrochozoans have a lophophore or trochophore larva
  • Platyhelminthes (Flatworms) are in Lophotrochozoans but don't have a lophophore or trochophore
• Arthropods are diverse and abundant animals:

  • Arthropods have a cuticle with chitin, segmented body, and paired appendages
  • Body divided into three regions: head, thorax, abdomen
  • Arachnids (Spiders, scorpions, harvestmen, mites, ticks) have a simple life cycle and some are parasites
  • Mandibulates have mouthparts composed of mandibles and include myriapods, crustaceans, and hexapods
• Deuterostomes include Echinoderms, Hemichordates, and Chordates:

  • Echinoderms (Sea stars, sea urchins, sea cucumbers) have an oral and aboral side and a system of calcified internal plates forming an internal skeleton
  • Chordates have derived traits like a dorsal hollow nerve cord, tail, notochord, and pharyngeal slits
  • Lancelets (Cephalochordates) have a notochord that persists throughout life
  • Tunicates (Urochordates) are marine and filter prey with a pharyngeal basket
• Key features of vertebrates:

  • Notochord replaced with vertebrae
  • Anterior skull with a relatively large brain
  • Rigid internal skeleton supported by the vertebral column
  • Internal organs suspended in a coelom
  • Well-developed circulatory system with a ventral heart
• Hagfishes:

  • Have three small hearts
  • Partial cranium
  • No stomach
  • No jaws
  • Skeleton is cartilage
  • No jointed vertebrae
  • More closely related to lampreys and secondarily lost many vertebrate features
• Lampreys:

  • Complete skull
  • Cartilaginous vertebrae
  • Filter-feeding larvae similar to lancelets
  • Metamorphosis to parasitic adults
  • Round mouth is used for attaching to fish and rasping at the flesh
• Jawed fishes (Gnathostomes):

  • Jaws evolved from the skeletal arches that supported the gills
  • Jaws and teeth improved feeding efficiency and prey capture
• Chondrichthyans:

  • Skeleton of pliable cartilage
  • Leathery skin
  • Most are predators
  • Sharks, skates, rays, chimaeras
  • Skates and rays live on the ocean floor and feed on animals buried in the sediments
  • Chimaeras live in deep ocean, usually under 200m
• Osteichthyes - Ray-finned fishes:

  • Internal skeletons of calcified, rigid bone
  • Body covered with scales
  • Gills open into a chamber covered by a flap (operculum)
  • Movement of the operculum improves water flow over the gills
• Life on Land Contributed to Vertebrate Diversification:

  • Evolution of lung-like sacs set the stage for evolution of land animals
  • Changes in fin structure allowed some fish to support themselves in shallow water and later move onto land
  • Lobe-limbed vertebrates: coelacanths, lungfishes, tetrapods
  • Paired pelvic and pectoral fins developed into more muscular fins joined to the body by a single enlarged bone
• Amphibians:

  • Remain tied to moist habitats
  • Lay eggs in the water but most adults live on land
• Mammals:

  • Coexisted with dinosaurs for millions of years
  • After extinction of dinosaurs, mammals diversified and grew larger
  • Blue whale is the largest animal on Earth, up to 33 meters long
  • Highly differentiated teeth
• Key features of mammals:

  • Sweat glands
  • Mammary glands
  • Hair
  • Four-chambered heart that completely separates oxygenated from deoxygenated blood
• Mammals are divided into two groups:

  • Prototherians (Monotremes): duck-billed platypus, echidnas
  • Therians: marsupials and eutherians
  • Marsupials carry and feed young in a ventral pouch. Young are born early and crawl into pouch for further development
  • Eutherians: 20 major groups
• Modern Homo sapiens evolved from:

  • The common ancestor of all apes
  • Apes – modern apes / chimpanzees
  • Primates
  • Prosimians
• Humans Evolved among the Primates:

  • Primates evolved from a lineage of small, arboreal, insectivorous eutherians
  • Two clades: Prosimians (lemurs, lorises, galagos) restricted to Africa, Madagascar, and tropical Asia
  • Anthropoids (tarsiers, Old World monkeys, New World monkeys, apes)
• Humans Evolved among the Primates:

  • The ape lineage separated from Old World monkeys about 35 mya
  • Asian apes (gibbons and orangutans), African apes (gorillas and chimpanzees), and humans are their modern descendants
• Humans Evolved among the Primates:

  • About 6 mya, split led to chimpanzees and the hominid clade
  • Earliest known hominids, ardipithecines, were bipedal
  • Australopithecines descended from ardipithecines
• Humans Evolved among the Primates:

  • In the Homo lineage: brain size increased while jaw muscles decreased
  • Several Homo species coexisted in the mid-Pleistocene; all hunted large mammals
  • Early modern humans (H. sapiens) expanded out of Africa 70,000 to 60,000 years ago
• Migration of Homo sapiens
• Animal body plans include bilateral symmetry (two sides mirror image), radial symmetry (parts arranged around a central axis), or irregular shapes with no obvious pattern.
• Animals are multicellular, eukaryotic organisms that have specialized cells called neurons to receive and transmit information from the environment.
• Radially symmetrical animals have parts arranged around a central point, such as starfish or jellyfish.
• Bilaterally symmetrical animals have left and right halves that are mirror images of one another.
• Animals share derived traits not found in other organisms, including similarities in gene sequences, cell junction structure, and extracellular matrix components
• Patterns of embryonic development provide clues to evolutionary relationships among animals
• Diploblastic animals develop two embryonic cell layers; triploblastic animals develop three cell layers
• Differences in early development patterns characterize protostomes and deuterostomes
• Animal body plans can be described by symmetry, body cavity structure, segmentation, and appendages
• Most animals have radial or bilateral symmetry
• Bilaterally symmetrical animals may exhibit cephalization
• Animals can be acoelomates, pseudocoelomates, or coelomates based on body cavity structure
• Segmentation improves movement control, especially with appendages
• All animals except sponges, ctenophores, placozoans, and cnidarians belong to the Bilateria group