The Origins of Life on Earth Part Deux

1. The Origins of Life on Earth Part Deux Biology 2, College of the Atlantic Spring 2002

2. Phylum Chordata Evolution of the Vertebrates

4. Triploblastic, bilaterally symmetrical Eucoelomate (true body cavity fully isolated by layers of mesoderm-derived tissue) During embryonic development, blastula/ gastrula exhibits Radial/Indeterminate cleavage Blastoporeanus (i.e. mouth develops second) Enterocoelous development Deuterostomes

5. Chordata • Chordates are eucoelomate deuterostomes, exhibiting segmentation • Phylum Chordata consists of three subphyla • Urochordata (Tunicates) • Cephalochordata (Lancelets) • Vertebrata • All chordates share four characteristics exhibited at some point during development

6. Single, hollow, dorsal nerve cord • in vertebrates, differentiates into CNS [brain and spinal cord] • Notochord • a flexible rod that runs the length of the body, provides rudiments of an endoskeleton • in vertebrates, becomes the vertebral column) • Pharyngeal slits • openings between intestinal tract and outside in region of pharynx • in many species, develop into gills • Muscular post-anal tail

7. Urochordata - the Tunicates • Marine, sessile filter feeders • Larvae are free-swimming • Adult form • loses notochord and nerve cord • secretes a cellulose* tunic • Develops an incurrent/excurrent siphon system, water flow by cilia • Has pharyngeal slits that filter food onto an endostyle

8. Cephalochordata - Lancelets • Laterally compressed filter feeders that live in sand • Small sub-phylum, 23 species, most belong to Brachiostoma (formerly Amphioxus) • Obvious segmentation in muscle groups • Water in through oral hood, out through pharyngeal slits (true intestinal tract) • Molecular evidence suggest closest ancestor to vertebrates

9. Vertebrata • Vertebrates are characterized by • Vertebral column (protects dorsal nerve cord • Skull (protects brain) • Neural crest (responsible for development of various organs) • Internal organs (including heart and associated closed circulatory system • Endoskeleton (bone or cartilage)

10. Evolution of the vertebrates Principally a marine sub-phylum (>50%) • Two extant classes of jawless fish (Ostracoderms) • Agnatha (lampreys) • Cephalaspidomorphi (hagfish) • Two extant and two extinct classes of jawed fish • Acanthodii (spiny fish) • Placodermi (armored fish) • Chondrichthyes (cartilegenous fish) • Osteichthyes (bony fish)

11. A movement towards land • Amphibia were the first vertebrates to assume a terrestrial lifestyle • Reptilia became further independent by evolution of amnioticegg • Aves and Mammalia both developed from Reptilia independently

12. Basic characteristics of fish • Aquatic, propulsion by post-anal tail • Gills: pharyngeal slits develop into respiratory surfaces • Vertebral column, including cranium • Single loop circulation using heart • Inability to synthesize aromatic amino acids (Phenylalanine, Tryptophan, Tyrosine)

13. The jawless fish (Ostracoderms) • Bottom-dwellers or parasitic, dominant in Silurian (438-360 mya) • Specialized mouth into a sucker with teeth • Finless, move through sinusoidal motion (aided by vertebral column and post-anal tail muscular development) • Extant representation by lampreys and hagfish

14. JAWS!! • Developed approx. 410 mya from front-most gill arch (supports first pharyngeal slit). Teeth developed dermally • First seen in Acanthodii (extinct), that also developed paired fins • Succeeded by the Placodermii (extinct) - more successful predators, some 30’ long • Chondrichthyes is the first extant jawed group

15. Chondrichthyes - Cartilegenous fish • Evolved 400 mya; more efficient jaw • Superior design for swimming • Fully developed caudal tail, high maneuverability • Endoskeleton made of cartilage (not a regressive trait) • Ovoviviparous • Extant chondrichthyian species includes sharks and rays. Extinct species include Carcharadon

16. Osteichthyes - the bony fish • Parallel evolution with cartilegenous fish (400 mya) • Key difference is ossification of endoskeleton • Other developments exclusive to bony fish include • Swim bladder • Fully developed lateral line system • Gill cover (operculum), improving ventilation • Two super-orders include ray-finned and lobe-finned fish • Lobe-finned fish developed precursors to lungs and limbs

17. Amphibia - the move to land • Evolved from lobe-finned fish (probably closely related to lungfish) about 390 mya, although most species are now extinct. • Three orders: • Anura (“tail-less”—frogs) • Urodela (“with tail”—salamanders, newts) • Apoda (“legless”—caecilians)

18. Characterized by modifications for movement on land • Legs (unless burrowers [caelilians]) • Improved metabolism through • Cutaneous and lung respiration (by positive pressure ventilation) • Partially divided heart (more efficient for circulation of oxygenated blood) Reproduction is still aquatic-based “Amphibia” (two lives) refers to metamorphosis

19. Reptilia - the first amniotes • Amnioticegg was an adaptation that provided first true independence from water • Extra-embryonic membranes, including • Chorion (gas-permeable, waterproof) • Amnion (protection of embryo in fluid-filled sac) • Yolksac (food for development of embryo • Allantois (storage of waste products)

20. Amniotic diversity

21. Reptile facts • Three main extant orders • Chelonia (turtles) • Squamata (lizards and snakes) • Crocodilia (crocodiles/alligators) • Evolution towards a four-chambered heart and endothermy • Dermal scales (preadaptation for feathers?)

22. Aves: birds -flight revisited • Birds are essentially dinosaurs with feathers • Closest ancestor Archaeopteryx, endothermic, probably developed feathers for insulation, and was a bipedal glider

23. Adaptations for flight include • Feathers • Hollow bones • Highly developed pectoral muscles/keel • Efficient respiration (evolution of lungs to include air sacs to maximize absorption of O2) • Efficient circulation (true 4-chambered heart, systemic and pulmonary circuits) • Endothermy

24. Mammalia • Evolved from Reptiles independently from class Aves • Independent (convergent) evolution of endothermy, in addition to evolution of keratinous fur and lactation • Includes the orders: • Monotremata • Marsupialia • Eutheria

25. Monotremes ASIA • Includes spiny anteater and duck billed platypus • The only example of an oviparous mammal, laying a reptilian-like (amniotic) egg • Monotremes lactate, but have no nipples • Found only in Australia and New Guinea

26. Marsupials • Found predominantly in Australia, with some occurrences in South Africa (a good example of biogeographic theory) • Marsupials give birth to a premature fetus that completes its development in a maternal pouch • Marsupials and placentals convergently evolve along many phylogenetic lines

27. Eutheria (placental mammals) • Embryonic development is completed in the womb • Four branches • Chiroptera and Insectivora • Lagomorpha, Perissodactyla, Artiodactyla, Sirenia, Proboscidea, Cetacea • Carnivora • Primates, Rodentia

28. The Evolution of Man

29. The Archonta • Approximately 65 mya, mammalian ancestors known as Archonta diversified into three orders • Bats (Chiroptera) • Tree shrews • Primates • Archonta were insectivorous, nocturnal, and arboreal

30. “Mankind stood up first and got smart later” • Stephen Jay Gould

31. The evolution of order Primates The phylogeny of Primates has five distinct trends: • Shift towards daytime vision and primary sensory system (away from olfaction) • Skeletal changes leading to bipedalism • Skeletal changes leading to prehensile and opposable hand-grips • A shift in dentition to omnivorous diet • Increase in skull, and brain size • Onset of cultural evolution

32. Improved vision • migration of eye sockets to front of skull, leading to binocular vision • physiological and cellular improvemnets to refine detection of color and contrast • Palaentology provides clues to the onset of bipedalism • location of the foramenmagnum at base of skull • lack of splayed toe • shorter, s-shaped backbone, with loss of tail • Other changes to pelvis (bowl shape), femur angle, and forelimbs

33. Improvement of grip was an adaptation to arboreal lifestyle • thought to have evolved to increase speed • bipedal movement freed hands for other tasks (not immediately for tool use) • Prehensile and opposable grips were preadaptations for tool use • A change in diet... • Dentition shifts from carnivorous (insects) to fruits and leaves, to omnivory • Shift from pointed jaw with emphasized canines (monkeys) to bow-shaped jaw with smaller teeth

34. Increased brain size selected for by need to • coordinate movements in arboreal setting (brachiated movement) • operate binocular (depth of field) vision that operates in low-level light • cordinate complex digit movements • develop highly altricial parental care • increased social behaviours • Increase in brain size led to the development of cultural evolution

35. Primate taxonomy • Two suborders, with split believed to have occurred >50 mya • Prosimians (“before monkeys”) • Lemurs and Lorises • Tarsiers • Arthropoids • monkeys, apes and hominids

36. Sub-order Arthropoida • Oldest representative may have lived 45 mya (Eosimias, Catopithecus) • Arthropoid characteristics include bony eye socket, shovel-shaped incisors, and fused forehead bones • Families include • New World monkeys - arboreal cappuchins, howlers, spider monkeys • Old World Monkeys - some-ground dwellers, toughened pads for sitting • Hominoids (Apes and Hominids)

37. The Hominoids • Apes include Gibbons (Hylobates), Orangutan (Pongo), Gorillas (Gorilla) and Chipanzees (Pan) • Most are ground dwelling and lack tails • Some are closely related to Homosapiens in terms of nuclear DNA: • Gorilla (97.7%) diverged 8 mya • Pan (98.4%) diverged 6 mya • Hominids include Ardipithecus, Australopithecus, and Homo

38. Hominids • Earliest hominids diverged ~5 mya (Ardipithecus) • Distinguished by bipedal stance • Most fossil specimens of early hominids are Australopithecus • Appearance in fossil record coincident with cooling of Africa to convert rain forests to savannah plain, resulting in a rapid adaptive radiation of at least 6 species

39. Australopithecus - forerunner or evolutionary dead end? • Apelike - large face, small skull/brain size (400 cm3). Bipedal, developed grip • Well developed teeth, indicating movement to tougher foods that required more physical digestion • Systematics provide little information on relationships between various species of Australopithecus • e.g. “Lucy”, 1974: A. afarensis

40. The first humans, early Homo • Genus Homo distinguished by larger brain size ≤ 700 cm3, evolving 2 mya • First association with tools: Homohabilis, “handy man”. Short, long arms - intiially thought to be a species of Australopithecus • Mystery species H. rudolfensis had larger brain, appeared to co-exist with H. habilis

41. Late Homo • Homo erectus, the first true human? • Very large brain: 1000cm3 • Probably not the direct ancestor of modern man • May have had speech ability • Cave-dwelling hunters, used fire, clothed in animal skins • Nomadic, responsible for rapid spreading into Asia and Europe by 0.5 mya

42. The final phase... • Homo neanderthalensis orginated in Africa, but rapidly spread to other continents. Very common 100,000 years ago, but not the ancestor of modern man. Disappeared 35,000 ya • Examination of burial grounds suggests social ritualism - the first evidence of abstract thought

43. Homo sapiens • probably evolved from H. heidelbergensis, a close relative of H. neaderthalensis • Oldest known H. sapiens fossil is 130,000 years old, cranial capacity of 1500 cm3 • Controversy over origin • Monogenesis model* • Multiregional model

44. Postscript • “From 40,000 years ago to today, human evolution has been cultural, not biological” • Cambell suggests three stages to that cultural evolution: • hunter-gatherer phase • agriculture phase • Industrial revolution • For the first time in the history of the planet comes a species capable of defying natural selection