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Animal Evolution “Welcome to your kingdom…” Nearest 0.5 bya ~0.5 ~1 Major Events Colonization of Land, Cambrian Explosion, 1st Animal Fossils start of Animal Lineage ~1.5 ~2 ~2.5 1st Eukaryote Fossils Atmospheric O2 increases ~3 ~3.5 1st Prokaryote Fossils (Photosynthesis?) ~4 Oceans, Water, (1st Life?) forms ~ 4.5 Earth forms … ~ 14 “Big Bang” 1 The last 0.5 billion years: • ~ 540-250 mya: Paleozoic = era of invertebrates, fishes, amphibians (+ seedless plants) • ~250-65 mya: Mesozoic = era of reptiles (+ Gymnosperms) • ~ 65 mya – present: Cenozoic = era of insects, mammals (+ Angiosperms) Mass Extinctions • Geologic eras typically separated by mass extinctions (ME’s) followed by adaptive radiations (AR’s) = large scale divergent evolution of surviving forms • Periods, epochs separated by smaller scale extinctions and radiations • Evidence from fossil record gave rise to the geologic time scale 2 Ter -tia -ry Precambrian S&T 3 Pre-Cambrian/ Paleozoic eras ~ 540 mya • Snowball Earth theory – limiting diversity • A.R.: Cambrian Explosion – New predator-prey niches ecological diversity, co-evolution – Rise in O2 metabolic diversity – HOX genes morphological diversity Figure 32.6 A Cambrian seascape Burgess Shale Fossils 4 Paleozoic/ Mesozoic eras ~250 mya • Permian M.E.: – Extreme volcanism (Siberia) CO2 rise global warming, O2 deficit in oceans – Formation of super-continent Pangea draining of shallow seas, continental climate • A.R.: plants (seed plants: Gymnosperms) + animals (reptiles) adapted to dryer conditions prevail India collided with Eurasia just 10 million years ago, forming the Himalayas, the tallest and youngest of Earth’s major mountain ranges. The continents continue to drift. Cenozoic 0 Millions of years ago 65.5 m er th A Nor ica Eurasia Africa India Madagascar alia tr us Antarctica A South America 251 Paleozoic 135 Mesozoic Laurasia Go nd wa na a ae ng Pa By the end of the Mesozoic, Laurasia and Gondwana separated into the present-day continents. By the mid-Mesozoic, Pangaea split into northern (Laurasia) and southern (Gondwana) landmasses. At the end of the Paleozoic, all of Earth’s landmasses were joined in the supercontinent Pangaea. 5 Mesozoic/ Cenozoic eras ~ 65 mya • Cretaceous M.E.: – asteroid collision (Mexico) debris cloud cooling, inhibition of photosynthesis – Pangea break-up habitat & climate change • A.R.: co-evolution of Angiosperms & insects; demise of dinosaurs opened niches for descendants of surviving mammals • Start of animal lineage ~ 1 bya (last common ancestor of choanoflagellates & animals) Choanoflagellates (colonial, flagellated protists) = closest living relatives of animals (metazoa) 6 X • First animal fossils ~ 0.6 bya (575 mya) = Ediacaran fossils (Australia) 7 Digestive cavity Somatic cells Reproductive cells Colonial protist, an aggregate of identical cells Hollow sphere of unspecialized cells (shown in cross section) Beginning of cell specialization Infolding Gastrula-like “protoanimal” Animals likely evolved from advanced colonial stages of choanoflagellate-like ancestor Figure 32.2 Early embryonic development in animals (layer 3) All animals develop in stages, incl. - blastula (hollow ball of cells) - gastrula (invaginated ball) Blastocoel Cleavage Cleavage Zygote Eight-cell stage Blastocoel archenteron Blastula Cross section of blastula Endoderm Ectoderm Gastrula Blastopore Gastrulation 8 Animal Development Zygote blastula gastrula embryo • (maturation) adult or: • larva(e) (metamorphosis) adult complete incomplete Animal Characteristics Ancestral • Eukaryotic • Heterotrophic 9 Animal Characteristics Derived • • • • • • • • Ingestive Multi-cellular Lack of cell walls Unique inter-cellular junctions Unique tissues: nervous, muscular Hox gene complex (developmental genes) Diplontic life cycle Gastrulation & tissue development from germ layers Animal Tissue Development Germ Layers give rise to adult tissues • Ectoderm outer covering, (central nervous tissue) • Endoderm digestive tract, (lungs, liver) • (Mesoderm muscles, most organs) Blastocoel Endoderm Ectoderm Gastrula Blastopore 10 Animal Body Plans • Refers to shared organizational complexity • May be result of homologies or analogies • Group of species sharing same body plan = “grade” (not necessarily “clade”), e.g. “slugs,” “worms” Figure 32.7 Body symmetry (a) Radial symmetry. The parts of a radial animal, such as a sea anemone (phylum Cnidaria), radiate from the center. Any imaginary slice through the central axis divides the animal into mirror images. (b) Bilateral symmetry. A bilateral animal, such as a lobster (phylum Arthropoda), has a left side and a right side. Only one imaginary cut divides the animal into mirror-image halves. 11 Symmetry • Radial: top/bottom or oral/aboral generally sessile, floating • Bilateral: dorsal/ventral, left/right, anterior/posterior – Usually implies cephalization (= concentration of brain and sensory organs in anterior end) generally actively moving Tissues • Diploblastic: 2 germ layers (ecto- and endoderm), e.g. jellies • Triploblastic: 3 germ layers (ecto-, endoand mesoderm) – most animals • Exception: sponges – no true tissues 12 Figure 32.8 Body plans of triploblastic animals Coelom (a) Coelomate. Coelomates such as annelids have a true coelom, a body cavity completely lined by tissue derived from mesoderm. Body covering (from ectoderm) Digestive tract (from endoderm) Tissue layer lining coelom and suspending internal organs (from mesoderm) Body covering (from ectoderm) (b) Pseudocoelomate. Pseudocoelomates such as nematodes have a body cavity only partially lined by tissue derived from mesoderm. Muscle layer (from mesoderm) Pseudocoelom Digestive tract (from ectoderm) Body covering (from ectoderm) (c) Acoelomate. Acoelomates such as flatworms lack a body cavity between the digestive tract and outer body wall. Tissuefilled region (from mesoderm) Digestive tract (from endoderm) Body Cavities Coelom = fluid-filled space between digestive tract and outer body wall, lined by mesoderm Functions: • Suspends & cushions organs • Independent movement of organs & body wall • Hydrostatic skeleton in some, e.g. earth worms 13 Early Embryonic Development • Protostome Development: – Spiral, determinate cleavage – Schizocoelous – Blastopore mouth • Deuterostome Development: – Radial, indeterminate cleavage – Enterocoelous – Blastopore anus Figure 32.9 A comparison of protostome and deuterostome development Protostome development (examples: molluscs, annelids, arthropods) Deuterostome development (examples: echinoderms, chordates) Eight-cell stage Eight-cell stage Spiral and determinate Radial and indeterminate Coelom Archenteron Mesoderm Coelom Blastopore Schizocoelous: solid masses of mesoderm split and form coelom Mesoderm Blastopore Enterocoelous: folds of archenteron form coelom Mouth Anus Digestive tube Mouth Mouth develops from blastopore (a) Cleavage. In general, protostome development begins with spiral, determinate cleavage. Deuterostome development is characterized by radial, indeterminate cleavage. (b) Coelom formation. Coelom formation begins in the gastrula stage. In protostome development, the coelom forms from splits in the mesoderm (schizocoelous development). In deuterostome development, the coelom forms from mesodermal outpocketings of the archenteron (enterocoelous development). (c) Fate of the blastopore. In protostome development, the mouth forms from the blastopore. In deuterostome development, the mouth forms from a secondary opening. Anus Anus develops from blastopore 14 Phylogenetic Tree of the Animal Kingdom – under debate “Radiata” “traditional” “Radiata” “molecular” 15 “molecular” condensed version Points of Agreement • All animals share a common ancestor • Sponges (Porifera) are basal animals • Eumetazoans = clade with true tissues – Grade “Radiata” = Jellies (Cnidaria) & comb jellis (Ctenophera) are basal eumetazoans (radial symmetry, diploblastic) … 16 Points of Agreement (cont.) • Most phyla in clade “Bilateria” (bilateral symmetry, triploblastic) • Chordates & Echinoderms are in clade Deuterostomia re: Ecdysozoa: Ecdysis = molting = shedding of exoskeleton re: Lophotrochozoa • Lophophore = crown of ciliated tentacles used for feeding (Lophophorates) • Trochophore = planktonic larval stage (Molluscs & Annelids) 17