The Origin and Evolution of Vertebrates

Origin and Evolution of Vertebrates

• Myllokunmingia fengjiaoa: Small, slender species that lived in the ocean 530 million years ago.
• Lacked claws or armor but was closely related to vertebrates (animals with a backbone).
• Themes in diversifying characteristics:
  • Morphology.
  • Mobility.
  • Reproduction.
  • Energy.

From Microbes to Animals

• Animals originated from single-celled eukaryotes in the Opisthokonta lineage.
• Choanoflagellates are the closest living relatives to animals, sharing a common ancestor 900 million years ago.

Animal Phyla Differentiation

• The nine animal phyla are differentiated by:
  1. Presence or absence of true tissues.
  2. Two or three embryonic tissue layers.
  3. No symmetry, radial symmetry, or bilateral symmetry.
  4. Ecdysozoans or Lophotrochozoans (Protostomes).
  5. Deuterostomes.

In Class Assessment: Experimental Design

• Question: Is there evidence of selection for defensive adaptations in mollusc populations exposed to predators?
• Predator-prey interactions shape animal evolution.
• Predators and prey are important selective agents on each other.
  • Unsuccessful predators cannot obtain food.
  • Prey that are unsuccessful at resisting attack are eaten.
• Example: Predatory green shore crab Carcinus meanus with its prey, the periwinkle Littorina littorea.
• European green crabs have preyed on flat periwinkles in the southern Gulf for over 100 generations.
• Periwinkles from northern sites have interacted with invasive green crabs recently.
• Previous research:
  1. Flat periwinkle shells recently collected from the Gulf of Maine are thicker than those from the late 1800s.
  2. Periwinkle populations from southern sites have thicker shells than those from northern sites.

Periwinkle and Crab Experiment

• Researchers collected periwinkles and crabs.
• A single crab was placed in a cage with eight periwinkles of different sizes.
• Four treatments:
  • Southern crab caged with southern periwinkles.
  • Southern crab caged with northern periwinkles.
  • Northern crab caged with southern periwinkles.
  • Northern crab caged with northern periwinkles.
• Second experiment: Crabs consumed unshelled periwinkles in less than an hour.

Deuterostomes

• Deuterostomes are a monophyletic lineage containing three phyla:
  • Vertebrates (hagfish, lampreys, sharks, bony fishes, amphibians, mammals, reptiles including birds).
  • Echinodermata (sea stars and sea urchins).
  • Hemichordata (acorn worms and pterobranchs).
  • Chordata (lancelets, tunicates, invertebrates, and Vertebrates)

Half a Billion Years of Backbones

• Fossils date to Cambrian explosion in Chengjiang formation of China and Burgess Shale in Canada.
• Vertebrate synapomorphies:
  • Brain, cranium, and sensory cell formation.
  • Fishlike bodies, gills, notochord, post-anal tail, paired eyes.
  • Genes responsible for neural crest formation are key innovations in evolution of vertebrate head.

Chordate Morphological Features

• Chordates have four morphological features:
  • Pharyngeal gill arches: Develop into slits, then throat openings in some adult vertebrates, disappear in others.
  • Dorsal hollow nerve cord: Projections from neurons.
  • Notochord: Flexible skeletal support during early development; disappears in many adult vertebrates.
  • Post-anal tail.

Chordate Phylogeny

• Includes:
  • Cephalochordata.
  • Urochordata.
  • Myxini.
  • Petromyzontida.
  • Chondrichthyes.
  • Actinopterygii.
  • Actinistia.
  • Dipnoi.
  • Amphibia.
  • Reptilia.
  • Mammalia.

Chordate Subphyla

• Cephalochordates (lancelets).

  • Known species: 25
  • Look like fish but are benthic marine invertebrates with planktonic larvae.
  • 4-7 cm.
  • Suspension feed, pump water through mouth and trap food particles in mucus on pharyngeal slits.
  • Swim and burrow using notochord as a stiff endoskeleton.
  • Sexual reproduction with external fertilization.

• Urochordates (sea squirts, salps, and larvaceans).

  • Known species: 2170
  • Sac-like adult body; most have non-feeding planktonic tadpole larvae.
  • Marine (0.1-25 cm, colonies of salps to 2 m).
  • Suspension feed using incurrent/excurrent siphons.
  • Adults are sessile, larvae swim with notochord.
  • Most release both eggs and sperm, some form colonies via asexual budding.

Cephalochordates (Lancelets)

• Small, torpedo-shaped animals, fish-like appearance.
• Mobile suspension feeders.
• Adults live on ocean floor, burrow in sand.
• Dorsal hollow nerve cord runs parallel to notochord, muscle contractions give fishlike movement.

Urochordates (Tunicates)

• External coat of polysaccharide (tunic).
• Three major sub-lineages:
  • Sea squirts (most diverse).
  • Salps.
  • Larvaceans.

Skull and Backbone

• Vertebrates have a skull and backbone composed of vertebrae.
• Includes hagfishes and lampreys, sharks, rays, chimaeras, ray-finned fishes, coelacanths, lungfishes, amphibians, reptiles, and mammals.

Vertebrates

• Vertebrates are chordates that have a backbone.
• Dorsal hollow nerve cord elaborates into spinal cord.
• Pharyngeal pouches develop into gills in aquatic species.
• Notochord in embryos organizes somites.

Vertebrate Synapomorphies

• Vertebrates are a monophyletic group distinguished by two synapomorphies:
  • Vertebrae: Column protects spinal cord.
  • Cranium: Case protects brains and sensory organs.
• Together, vertebrae and cranium protect the central nervous system and key sensory structures.

Hox Genes

• The lancelet and vertebrate brain develops under control of the Hox genes.
• Living vertebrates have two or more sets of Hox genes; lancelets and tunicates have only one.
• This additional genetic complexity enabled evolution of the nervous system and skeleton innovations.

Neural Crest

• The neural crest is a developmental structure unique to vertebrates.
• Neural crest appears along the edges of the closing neural tube.
• Cells disperse through the embryo and give rise to teeth, skull bones and cartilage, neurons, sensory capsules.

Hagfishes and Lampreys

• Hagfishes (Myxini) and lampreys (Petromyzontida) are in a clade of jawless vertebrates, the cyclostomes.
• Some lack a backbone but have rudimentary vertebrae.

Vertebrate Jaw

• Vertebrate jaw allows access to greater variety of food sources.
• Gnathostomes: Jawed vertebrates (sharks, ray-finned fishes, lobe-finned fishes, amphibians, reptiles, mammals).
• Gnathostomes (“jaw mouth”) have jaws with teeth used to grip and slice food.
• Jaws may have evolved by modification of the skeletal rods supporting the pharyngeal (gill) slits.

Jawed Vertebrate Lineages

• By 420 million years ago, jawed vertebrates had diverged into three lineages:
  • Chondrichthyans.
  • Ray-finned fishes.
  • Lobe-finned fishes.

Chondrichthyans

• Chondrichthyans (sharks, rays and others) have a skeleton composed primarily of cartilage.
  • Morphology: streamlined body for swift swimming
  • Mobility: Dorsal stabilizers, paired pectoral and pelvic fins for maneuvering, oil stored in the liver for buoyancy
  • Metabolism/Energy: Sharks are carnivores, some suspension feeders, acute senses of sight and smell, and the ability to detect electrical fields aid in prey capture
  • Reproduction: internal fertilization with 3 types of development
    • Oviparous: Eggs hatch outside the mother’s body
    • Ovoviviparous: Eggs hatching within the uterus
    • Viviparous: embryos develop in uterus

Bony Endoskeleton & Lungs

• Bony fishes: Ray-finned and lobed fin fishes
  • Morphology: Endoskeleton enables precision in swimming.
  • Mobility: Swim bladder-air pockets for buoyancy, operculum-bony gill covering.
  • Lateral line system-row of pressure detecting sensory organs for schooling behavior, predation, and orientation.

Bony Endoskeleton & Lungs cont.

• Origin-of-lung hypothesis: lungs for aerobic respiration arose as out pockets of the esophagus.
• Reproduction: Most species are oviparous with external fertilization, but some have internal fertilization and birthing.

Lobe-Fins

• Lobe-fins have muscular pelvic and pectoral fins supported by rod-shaped bones.
• Three lineages survive:
  • Coelacanths: living deep in the oceans, were once thought to be extinct.
  • Lungfishes: which can gulp air into lungs, inhabit stagnant waters in the Southern Hemisphere.
  • Tetrapods: adapted to life on land, include terrestrial vertebrates.

Limbs-From-Fins

• Lungfishes inhabit shallow, oxygen-poor water. Breathe with lungs, supplementing O_2 taken in by gills.
• Fossils (Tiktaalik) link limbs of ancestors of lungfishes to earliest land vertebrates.
• Homologous Hox genes limb development.

Tiktaalik

• Tiktaalik could most likely prop itself on its fins and walk in water, but it is unlikely that it walked on land.

Tetrapod Limb

• Origin of the tetrapod limb enabled transition to living on land.
• Transition to land occurred once in the evolution of vertebrates giving rise to three major lineages of living tetrapods:
  • Amphibians
  • Mammals
  • Reptiles
• Tetrapod (“four feet”) body plan:
  • Four limbs, feet, digits.
  • Neck independent head movement.
  • Fusion of pelvic girdle and backbone.
  • Loss of gills (except some aquatic species).
  • Ears airborne sounds.

Amphibians

• Amphibians (“both-sides-living” water and land): First tetrapods to live on land
  • Reproduction: Most feed on land but lay eggs in water
  • Morphology: Most undergo metamorphosis from aquatic larva to terrestrial or semiterrestrial adult
  • Metabolism/Energy: Gas exchange occurs across their moist mucus covered skin
• Living amphibians represent a monophyletic group:
  • Frogs and toads, salamanders, snake- like caecilians

Amphibian Decline

• A rapid, worldwide decline of amphibian populations has been documented over the past 30 years.
• Causes include a disease-causing chytrid fungus, habitat loss, climate change, and pollution.
• Nine species have become extinct over 40 years.
• More than 100 others are considered possibly extinct.

Amniotic Egg

• Amniotic egg is a synapomorphy of amniotes that reduced dependence on water for reproduction, important for life on land
  • Yolk: provided by the mother (the yolk sac)
  • Allantois: waste from the embryo
  • Albumen: cushions and provides nutrients
  • Embryo (amnion): is surrounded by shell and three membranes

Amniotes

• Amniota: Lineage that includes all tetrapods other than amphibians (Reptiles and mammals)
• Includes:
  • Turtles
  • Diapsids
    • Archosaurs
      • Dinosaurs
        • Crocodilians
        • Pterosaurs
        • Saurischians
      • Birds
    • Lepidosaurs
      • Tuataras
      • Squamates (lizards and snakes)
  • Synapsids
    • Mammals

Amniote Characteristics

• Breathing efficiency improved in amniotes due to the use of a rib cage to ventilate the lungs.
• Amniotes became less dependent on gas exchange through the skin.
• Skin became less permeable, enabling improved water conservation in the terrestrial habitat.

Reptiles

• Reptiles: Monophyletic group that represents the second major living lineage of amniotes besides mammals.
• The 20,800 living reptile species include tuataras, lizards, snakes, turtles, crocodilians, and birds.

Reptile Characteristics

• Morphology: skin covered with scales and waterproofed with keratin
• Metabolism/Energy: obtain most of their oxygen using lungs. Some are ectothermic.
• Reproduction: Most lay shelled eggs on land; the shell protects the egg from drying out. Fertilization occurs internally.

Birds

• Birds are feathered reptiles with body plan adaptations for flight
  • Forelimbs: feather-covered wings that act as airfoils.
  • Large flight muscles anchored to breastbone provide power.
  • Features help reduce weight for flight
    • Present-day birds lack teeth
    • tail supported by only a few small vertebrae.
    • Feathers have hollow shafts.
    • Bones have strong but light honeycombed structure

Bird Adaptation

• Endothermy allows for energetically costly flight.
  • Endothermic: use heat generated by metabolism to maintain a warm, steady body temperature.
• Large brains and display complex behaviors.
  • acute senses
  • fine muscle control
  • excellent eyesight.
• Birds typically display very complex behaviors, particularly during breeding season. Courtship often involves elaborate rituals.

Bird Evolution

• Birds evolved from a lineage of small, two- legged dinosaurs called theropods
• Archaeopteryx: oldest known bird (150 million years old).
  • small bipedal dinosaur, with teeth, wing claws, long tail with many vertebrae.
  • Implies feathers evolved long before powered flight.
  • Early feathers may have functioned in insulation or courtship displays.

Bird Distinguishing Characteristics

• Profile, color, flying style, behavior, beak shape, and foot structure can be distinguishing characters
• Penguins are flightless birds that use powerful pectoral muscles and flap their flipper-like wings to “fly” in water
• Hummingbird feeding while hovering
• Flamingo specialized beak
• Great tit Feet adapted to perching

Mammals

• Mammals: monophyletic group with mammary glands for lactation to nourish young
  • cheek muscles and lips make suckling milk possible
• Endotherms (“inside-heated”): Maintain high body temperatures with fur
• Three major lineages:
  • Egg-laying monotremes
  • Pouch-bearing marsupials
  • Placental (eutherians)

Mammal Evolution

• Jaw modified gradually in successive lineages over ~100 million years
• Two bones that formerly made up the jaw joint were incorporated into the mammalian middle ear
• Synapsid: subset of amniotes which includes mammals, skull distinguished by a single temporal fenestra

Early Mammals

• Includes:
  • Monotremes (platypuses, echidnas)
    • Lay eggs; no nipples; young suck milk from fur of mother
  • Marsupials (kangaroos, opossums, koalas)
    • Complete embryonic development in pouch on mother's body
  • Eutherians:
    • Carnivora (dogs, wolves, bears, cats, weasels, otters, seals, walruses)
    • Proboscidea (elephants)
    • Sirenia (manatees, dugongs)
    • Hyracoidea (hyraxes)
    • Xenarthra (sloths, anteaters, armadillos)
    • Rodentia (squirrels, beavers, rats, porcupines, mice)
    • Lagomorpha (rabbits, hares, picas)
    • Primates (lemurs, monkeys, chimpanzees, gorillas, humans)
    • Cetartiodactyla:
      • Artiodactyls (sheep, pigs, cattle, deer, giraffes)
      • Cetaceans (whales, dolphins, porpoises)
    • Chiroptera (bats)

Mammal Charateristics

• Morphology: Hair and a fat layer under the skin for insulation, teeth modified for shearing, crushing, or grinding, large brain-to-body-size ratio
• Metabolism/Energy: Mammary glands produce milk to feed young, kidneys conserve water from wastes, endothermy and a high metabolic rate, efficient respiratory and circulatory systems
• Reproduction: extensive parental care

Mammalian Placenta

• Placenta organ combining maternal and embryonic tissues
  • Rich in blood vessels that facilitate flow of O2 and nutrients from mother to developing embryo and remove nitrogenous wastes and CO2 from embryo
  • Embryo contributes to placenta—allantois and chorion
  • Diffusion of gases, nutrients, and wastes

Parental Care

• Parental care is an investment that improves the likelihood of offspring to survive, more extensive in mammals and birds
• Can improve the animals’ fitness by increasing the likelihood that their offspring will survive and reproduce
• Evolution of extensive parental care hypothesized to be major reason for evolutionary success of mammals and birds

Costs and benefits

• Advantages:
  • Offspring develop at a more constant, favorable temperature
  • Offspring are protected
  • Offspring are portable: mothers are not tied to a nest
• Fitness trade-off: energetically expensive

Mammalian Adaptations

• Some mammals, such as kangaroo rats, have adaptations for living in arid environments
  • Thick oily skin limiting evaporative water loss
  • Burrowing underground during heat of the day
  • Large nasal passages that increase efficiency of water reabsorption when exhaling
  • Obtaining water from catabolic pathways and food
  • Large intestine and kidneys that absorb most of the water from food, losing little in feces and urine

Primates

• Humans occupy tiny twig on the tree of life, but their origins have been studied extensively
• Primate lineage consists of two main groups:
  • Prosimians (“before- monkeys”)
  • Anthropoids (“human-like”)

Primate Concepts

• Earliest primates were probably small tree-dwelling mammals that arose some time before 65 million years ago.
• Most living primates are still tree- dwelling, and the primate body has a number of features that were shaped, through natural selection, by the demands of living in trees.
• Although humans never lived in trees, the human body retains many of the traits that evolved in our tree-dwelling ancestors.
  • One notable exception is our brains

The Primate

• What Makes a Primate a Primate?
  • Hands and feet that are efficient at grasping
  • Flattened nails instead of claws on the fingers and toes
  • Relatively large brains
  • Color vision
  • Complex social behavior
  • Extensive parental care of offspring
  • Forward-facing eyes

Diversity of Primates

• Three main groups:
  • Lemurs, lorises, and pottos
  • Tarsiers
  • Monkeys
    • New World monkeys
    • Old World monkeys
  • Anthropoids
    • Apes
      • Gibbons
      • Orangutans
      • Gorillas
      • Chimpanzees
      • Humans

Primates

• Prosimians (“before-monkeys”) include:
  • Lemurs from Madagascar, Tarsiers, pottos, and lorises from Africa and South Asia.
  • Small-bodied, arboreal, and nocturnal
• Anthropoids (“human-like”):
  • New World monkeys from Central and South America, Old World monkeys from Africa and Asia, Gibbons from Southeast Asia, the Hominidae or great apes—orangutans, gorillas, chimpanzees, and humans

Anthropoids

• Anthropoids are a group of primates that include monkeys and apes
  • Diverged from other primates some 50 million years ago
  • Fully opposable thumbs

Hominids

• Great apes (hominids): Relatively large bodied with long arms, short legs, and no tail
  • Orangutans: trees dwelling, fist-walk on ground
  • Gorillas, bonobos, and chimpanzees knuckle-walk
• Hominins: Monophyletic group comprising Homo sapiens and more than 20 extinct, bipedal relatives
• Hominins and chimpanzees are separate lineages that diverged from a common ancestor

Evolution of Hominins

• The fossil record shows multiple species of hominins lived on Earth at the same time
  • Bipedalism: Shared, derived character that defines hominins
  • Homo characterized by large brain
    • language & toolmaking reason, plan, and communicate, cooperate in complex social networks
    • Selection favored larger brains

Human Evolution

• Hominins did not evolve in a direct line to modern humans
  • Australopithecus: small, bipedal
  • Paranthropus: Massive cheekbones, teeth and jaws. Sagittal crest (flange of bone at the top of the skull)
  • Early Homo (humans): flatter and narrower faces, smaller jaws and teeth, larger braincases
  • Recent Homo: 1.2 mya to the present. Flatter faces, smaller teeth, larger braincases

Ancestors

• The fossil record indicates that common ancestor of chimps and humans lived in Africa 6 to 7 mya
• DNA data: humans most closely related to common chimpanzees and bonobos, followed by gorillas:
• Four lineages appeared after the oldest known hominin, Ardipithecus ramidus
  • (1) Gracile australopithecines, (2) Robust australopithecines, (3) Early Homo, (4) Recent Homo

Humans vs Other Apes

• A number of characters distinguish humans from other apes
  • Upright posture and bipedal locomotion
  • Larger brains capable of language, symbolic thought, and artistic expression
  • Production and use of tools
  • Reduced jawbones and jaw muscles
  • Shorter digestive tract

Interbreeding

• Fossil evidence indicates mating occurred between humans and Neanderthals
• DNA extracted from a human jawbone fossil contained long stretches of Neanderthal DNA
• Gene flow also occurred between Neanderthals and Denisovans
• Genomic analysis also supports a history of gene flow between Denisovans and H. sapiens

Evolutionary Relationships

• Includes the following:
  • Humans
  • Neanderthals
  • Denisovans

Recent Homo

• Homo sapiens (including Cro-Magnons), and the Neanderthals : both populations created art and buried their dead in an organized manner
• Homo floresiensis discovered in 2003 on island of Flores in Indonesia, 1m tall, lived 190,000 to 50,000 years ago
• Homo naledi (H. naledi): discovered in Africa in 2013, lived 236,000 and 335,000 years old, modern skull shape will half size

Out of Africa

• Fossil evidence supports “Out of Africa” Hypothesis
• Fossil evidence provides support for African origin of H. sapiens and subsequent migration
  • First wave moved east and south to Australia
  • Second wave moved to Europe and the rest of mainland Asia

Migration Patterns

• How do they figure out migration patterns?
  • Fossil records give us clues
  • Molecular evidence about the origin of humans supports the conclusions drawn from fossils
• Compare mutational markers
  • Mitochondrial DNA maternally inherited
  • Y chromosomes transmitted from fathers to sons

Phylogenetic Trees

• Phylogenetic trees based on molecular evidence agree with fossil record
• First lineages to branch off led to descendant populations that live in Africa today
• Later branches gave rise to lineages residing today in Central Asia, Europe, East Asia, Polynesia, and the Americas

Overall Data

• The overall data support an out-of-Africa hypothesis with “leakage”
• H. sapiens interbred with before migrating through Europe and Asia
  • 1% to 4% of European and Asian genome (not African) derived from Neanderthal
• Modern humans interbred recently discovered cousin of Neanderthals (Denisovans in Central Asia during their first migration out of Africa): About 5% of the genome of Aboriginal Australians is derived from Denisovans

Human Evolution

• Have Humans Stopped Evolving?
• All four processes of evolution— mutation, genetic drift, gene flow, and natural selection—are measurable in human populations today and are expected to continue:
  • Coevolution with pathogens
  • Effects of C-section on evolution of head size
  • Spread of lactose tolerance mutation

Summary

• Clade descriptions
  • Cyclostomes: jawless vertebrates
  • Vertebrates: Hox genes duplication, backbone of vertebrae
  • Chordates: notochord; dorsal, hollow nerve cord; pharyngeal slits; post-anal tail
  • Gnathostomes: hinged jaws, four sets of Hox genes