Lecture 31--Vertebrate Origins

This lecture begins a series where look at the evolution of key characteristics that make us human. First we look at the phylum Chordata and why the vertebrates belong in the group. Then we look at the origin of the backbone and skeleton, the major characteristic that sets this taxon apart from others.

The echinoderms (starfish and their kin) are the closest relatives of the phylum Chordata. Both phyla are deuterostomes, meaning that during embryonic development that the first opening into the ball of cells (blastula) starting development becomes the anus. This is in contrast to the protostomes where the first opening becomes the mouth.

Pharyngeal gills—these are gills that extend off of the throat region. Many other groups have gills for respiration but they don’t come from the pharynx (throat).

Dorsal hollow nerve cord—this major part of the nervous system is a cord and brain packed with nerve cells that extends the length of the body along the back (dorsal) and is hollow with a cerebral spinal fluid inside. Invertebrates also have nerve cords and brains but they are ventral lying under the digestive tract and they are not hollow.

Notochord—this is a flexible rod that extends the length of the body to which muscles attach for locomotion. This gives the name to the phylum (Chordata). All chordates develop a notochord as embryos but in the vertebrates it is replaced by the bony skeleton except for the intervertebral disks.

Post-anal tail—all chordates (at least as embryos) have tails with an extension of the spinal cord which extend beyond the anus.

Pikaia is the earliest chordate that may be an ancestor to the vertebrates and has been found in the Burgess Shale, 500 mya. (Myotomes are muscle segments.)

The earliest vertebrates include fish. They evolved in the sea and some migrated into freshwater. Major challenges occurred with this shift. These are explored in the video shown in class.

Terms/Concepts to Define:

Protostomia

Deuterostomia

Blastula

Gastrula

Blastopore

Cephalochordata

Urochordata

Pharyngeal gill slits

Notochord

Post-anal tail

Endoskeleton

Can you answer these questions?

  1. Distinguish between the Deuterostomia and Protostomia and name a phylum associated with each taxon.
  2. Sketch the first embryonic stages of animals before they begin to differentiate into deuterostomes and protostomes.
  3. What is the key characteristic that separates the various subgroups of chordates?
  4. List the distinguishing characteristics of the phylum Chordata.
  5. Discuss how a shift from sea water to fresh water environment was handled by the earliest fish.