Chordate Characteristics

• Vertebrates evolved from a type of animal, becoming one of the most successful animal groups.
• Vertebrates are named for their vertebrae, the bones forming the backbone.
• There are approximately 52,000 vertebrate species.
• Vertebrates are a subphylum of Chordata.
• Chordates are bilaterian animals in the animal clade Deuterostomia.
• Urochordates and cephalochordates are invertebrate deuterostomes more closely related to vertebrates than other invertebrates.

Chordate Characteristics

• All chordates share derived characters.
• Some chordate species only exhibit these traits during embryonic development.
• The four key characteristics of chordates are:
  • A notochord
  • A dorsal, hollow nerve cord
  • Pharyngeal slits or clefts
  • A muscular, post-anal tail

Notochord

• The notochord is a longitudinal, flexible rod between the digestive tube and nerve cord.
• It provides skeletal support for most of a chordate's length.
• In most vertebrates, a complex, jointed skeleton develops, with adults retaining only remnants of the embryonic notochord.

Dorsal, Hollow Nerve Cord

• The nerve cord of a chordate embryo develops from an ectoderm plate that rolls into a tube dorsal to the notochord.
• The nerve cord develops into the central nervous system: the brain and spinal cord.

Pharyngeal Slits or Clefts

• Grooves in the pharynx called pharyngeal clefts develop into slits that open to the body's exterior in most chordates.
• Pharyngeal slits function as:
  • Suspension-feeding structures in invertebrate chordates
  • Gas exchange in vertebrates (except limbed vertebrates/tetrapods)
  • Parts of the ear, head, and neck in tetrapods

Muscular, Post-Anal Tail

• Chordates have a tail posterior to the anus.
• The tail is often reduced during embryonic development.
• Tails contain skeletal elements and muscles.
• It provides a propelling force for aquatic species.
• Lancelets have a bladelike shape, and are marine suspension feeders.
• Adult lancelets retain chordate body plan characteristics.
• Tunicates (Urochordata) are marine suspension feeders, often called sea squirts, more closely related to chordates than lancelets.
• Adult tunicates draw in water through an incurrent siphon to filter food particles.
• Juvenile tunicates, but not adults, possess a notochord.

Hagfishes

• Hagfishes possess a cartilaginous skull and an axial cartilage rod derived from the notochord, but lack jaws and vertebrae.

Vertebrates

• A lineage of craniates evolved into vertebrates during the Cambrian period.
• Vertebrates became more efficient in capturing food and avoiding predation.
• Derived vertebrate characters:
  • Vertebrae enclosing a spinal cord
  • An elaborate skull
  • Fin rays, in aquatic forms
• Lampreys are the oldest living vertebrate lineage.
• They are jawless vertebrates inhabiting marine and freshwater habitats.

Origins of Bone and Teeth

• Mineralization may have originated with vertebrate mouthparts.
• The vertebrate endoskeleton became fully mineralized later in evolution.
• Jawed verebrates (Gnathostomes) outnumber jawless vertebrates.
• Gnathostome jaws evolved from the skeletal supports of pharyngeal slits.

Gnathostomes

• Gnathostomes share other common characteristics:
  • An additional duplication of Hox genes
  • An enlarged forebrain for enhanced smell and vision
  • A lateral line system sensitive to vibrations in aquatic gnathostomes
• Chondrichthyans (Chondrichthyes) have a cartilage-composed skeleton
• Their cartilaginous skeleton evolved secondarily from an ancestral mineralized skeleton.
• The largest and most diverse chondrichthyan group includes sharks, rays, and skates.
• Most sharks have streamlined bodies and are swift swimmers.
• Sharks are carnivores with short digestive tracts, a spiral valve to increase digestive surface area, and acute senses.
• Shark eggs are internally fertilized, with embryos developing through:
  • Oviparity: Eggs hatch outside the mother’s body
  • Ovoviviparity: Embryo develops within the uterus, nourished by the egg yolk
  • Viviparity: Embryo develops in the uterus, nourished by a yolk sac placenta from the mother’s blood
• The reproductive, excretory, and digestive tracts of sharks empty into a common cloaca.

Osteichthyes

• Osteichthyes, a gnathostome clade, includes bony fish and tetrapods
• Most Osteichthyes have a bony endoskeleton.
• Aquatic osteichthyans are called fish.
• Fishes breathe by drawing water over gills protected by an operculum.
• They control buoyancy using a swim bladder.

Tetrapods

• Tetrapods have specific adaptations:
  • Four limbs and feet with digits
  • Ears for airborne sound detection
• In one lobe-fin lineage, fins became limb-like while the rest of the body retained adaptations for an aquatic life
• As an example, Acanthostega lived in Greenland 365 million years ago.

Amphibians

• About 6,150 amphibian species exist, in three orders.
• Amphibian means "both ways of life," referring to the metamorphosis of aquatic larvae into terrestrial adults.
• Most amphibians possess moist skin supplementing lungs in gas exchange.
• Fertilization is external in many species, requiring a moist environment for eggs.

Amniotes

• Amniotes are tetrapods, reptiles (including birds), and mammals sharing terrestrially adapted eggs.
• Amniotes are named for the amniotic egg, which contains membranes that protect the embryo.
• The extraembryonic membranes are the amnion, chorion, yolk sac, and allantois.
• Amniotes have terrestrial adaptations: relatively impermeable skin and the ability to use the rib cage to ventilate the lungs.
• The reptile clade includes tuataras, lizards, snakes, turtles, crocodilians, birds, and dinosaurs.
• Reptiles have scales, creating a waterproof barrier.
• Most reptiles are ectothermic and absorb external heat as their primary heat source.
• Birds are endothermic and capable of warming their body through metabolism.
• Dinosaurs were diverse in shapes and sizes.
• Some dinosaurs were bipedal carnivores called theropods.
• Parental care evidence has been discovered among dinosaurs.
• Most Dinosaurs, except for birds, became extinct by the end of the Cretaceous period because of an asteroid.
• Birds possess reptilian anatomy modified for flight and many flight adaptation characters.
• Wings with keratin feathers are a major adaptation.
• Other adaptations include no urinary bladder, one ovary in females, small gonads, and a loss of teeth.
• Archaeopteryx is the oldest known bird

Mammals

• Mammals, class Mammalia, are represented by over 5,300 species.
• Mammals have:
  • Mammary glands to produce milk
  • Hair
  • A larger brain than other vertebrates of similar size
  • Differentiated teeth
• The three living mammal lineages emerged by the early Cretaceous: monotremes, marsupials, and eutherians.
• Monotremes are a small group of egg-laying mammals containing echidnas and the platypus.
• Marsupials include opossums, kangaroos, and koalas.
• Marsupial embryos develop within the mother's uterus via a placenta.
• A marsupial is delivered very early in its development.
• It completes embryonic development while nursing in a maternal pouch called a marsupium.
• Compared with marsupials, eutherians = placental mammals, which have a longer pregnancy period.
• Young eutherians complete embryonic development within the uterus, joined to the mother by the placenta.
• In Australia, convergent evolution resulted in diverse marsupials resembling eutherians elsewhere.

Primates

• Primates include lemurs, tarsiers, monkeys, and apes.
• Three groups of living primates exsit today:
  • Lemurs, lorises, and pottos
  • Tarsiers
  • Anthropoids (monkeys and apes)
• Humans are in the ape group.
• Primate derived characteristics include
  • Hands and feet for grasping
  • large brains and short jaws
  • forward-looking eyes
  • complex social behavior.
  • a fully opposable thumb (in monkeys and apes)
• The first monkeys evolved in the Old World (Africa and Asia).
• In the New World (South America), monkeys first appeared ~25 million years ago.
• Old World and New World monkeys underwent separate adaptive radiations during their extended separation periods.

Humans

• Homo sapiens are about 200,000 years old.
• Paleoanthropology studies human origins.
• Paleoanthropologists have discovered about 20 extinct hominin species.
• Hominins are more closely related to humans than chimpanzees.
• Characters distinguishing humans from other apes include:
  • Upright posture and bipedal locomotion
  • Larger brains
  • Language capabilities and symbolic thought
  • Complex tool manufacturing and use
  • Shortened jaw
  • Shorter digestive tract
• Hominins originated in Africa about 6-7 million years ago.
• Early hominins had small brains but probably walked upright.
• Common misconceptions about early hominins:
  • Thinking they were chimpanzees
  • Imagining human evolution as a path leading directly to Homo sapiens
• Australopiths are a paraphyletic hominin assemblage, living from 4-2 million years ago.
• Some australopith species walked fully erect.
• "Robust" australopiths had sturdy skulls and powerful jaws; "gracile" australopiths were slender with lighter jaws.
• Bipedalism origins occurred around 1.9 million years ago.
• The oldest tool use evidence, cut marks on animal bones, dates back 2.5 million years.
• The earliest fossils placed in Homo are Homo habilis, ranging from ~2.4 to 1.6 million years old.
• Stone tools have been found with H. habilis, translating to “handy man."
• Homo erectus originated in Africa by 1.8 million years ago and was the first hominin to migrate.
• Neanderthals, Homo neanderthalensis, lived in Europe and the Near East from 200,000 to 28,000 years ago.
• Neanderthals had thick bones, larger brains, buried their dead, and created hunting tools.
• Homo sapiens appeared in Africa ~195,000 years ago.
• All living humans descend from these African ancestors.
• The oldest Homo sapiens fossils outside Africa date to ~115,000 years ago and from the Middle East.
• Humans first arrived in the New World before 15,000 years ago.
• The species' expansion may have been influenced by brain changes leading to cognitive innovations.
• The FOXP2 gene codes for language.
• Natural selection occurred for the FOXP2 gene in Homo Sapiens during the last 200,000 years.