Chordates: Characteristics, Classification, and Evolution

Chordates: Characteristics and Classification

Basic Characteristics of Chordates

All chordates share four basic characteristics:

  • Dorsal Notochord: A long supporting rod that runs through the body just below the nerve cord. Most chordates have a notochord only when they are embryos.
  • Dorsal Hollow Nerve Chord: The cord runs along the back of the body. Nerves branch from this cord at regular intervals and connect to internal organs, muscles, and sense organs.
  • Pharyngeal Pouches: Paired structures in the throat (pharynx) region. In fishes and amphibians, slits develop that connect the pharyngeal pouches to the outside of the body. These slits may then develop into gills that are used for gas exchange. Seen only during embryonic development in other vertebrates; in humans, these become eustachian tubes, tonsils, thymus, and parathyroids.
  • Tail: Extends beyond the anus. All chordates have this at some point in their lives. The tail can contain bone and muscle and is used in swimming by many aquatic species.


Most chordates have a vertebral column or backbone. The backbone replaces the notochord during development and is made of segments called vertebrae. Vertebrae provide support and enclose and protect the spinal cord. In vertebrates, the dorsal hollow nerve cord is called the spinal cord. During development, the front end of the spinal cord becomes the brain.

Vertebrates are classified into seven main classes:

  1. Three classes of fish:
    • Chondrichthyes: Sharks and rays
    • Actinopterygii: Ray-finned fish
    • Sarcopterygii: Lobe-finned fish
  2. Four other classes:
    • Amphibia: Frogs and salamanders
    • Reptilia: Snakes and lizards
    • Aves: Birds
    • Mammalia: Quadrupeds (dogs, etc.), humans


Two subphyla of chordates do not have backbones:

  • Urochordata (Tunicates): Soft marine organisms that get their name from their adult body covering, a tough non-living tunic.
  • Cephalochordata (Lancelets): Small fish-like creatures that filter feed with an anterior mouth and exposed gills.

Evolution of Chordates

Similarities in anatomy and embryological development indicate that vertebrates and invertebrates share a common ancestor. Fossil evidence suggests that the two subphyla diverged around 550 million years ago.

Hominids: The Evolution of Humans

Hominids are a group of primates that includes humans and their ancestors. The earliest known hominid, Ardipithecus ramidus, lived around 4.4 million years ago. Over time, hominids evolved from ape-like creatures to bipedal, tool-using humans.

Key hominid species include:

  • Australopithecus anamensis (4 million years ago)
  • Australopithecus afarensis (3.4 million years ago) (Lucy)
  • Australopithecus africanus
  • Australopithecus robustus
  • Australopithecus boisei
  • Homo habilis (1.6 to 2 million years ago)
  • Homo erectus (1.9 and 0.5 million years ago)
  • Homo sapiens (300,000 years ago)
  • Homo sapiens neanderthalensis (200,000 years ago)
  • Homo sapiens sapiens (Cro-Magnon man)

The evolution of humans is a complex and ongoing process. Through natural selection, humans have adapted to a wide range of environments and developed unique characteristics that set them apart from other animals.