216 Evolution? The Fossils Say YES!
kidney, as well as other organ systems not found in hemichordates or sea squirts. They do
not have true eyes, but they do have light-sensitive pigment spots on the front of the head
to detect light and shadows. The molecular and embryological evidence consistently puts
lancelets as our closest nonvertebrate chordate relatives. To top it off, even these delicate
animals fossilize occasionally. The Burgess Shale produces a remarkable fossil known as
Pikaia (fig. 9.6C), which is a very primitive relative of the lancelets. The Cambrian Chengji-
ang fauna of China yields another form known as Yunnanozoon (fig. 9.6D). And in rocks from
the Permian of South Africa, there a fossil known as Palaeobranchiostoma that looks much like
the living lancelet.
It’s a Long Way from Amphioxus
Oh a fish-like thing appeared among the annelids one day
It hadn’t any parapods nor setae to display
It hadn’t any eyes or jaws or ventral nervous chord,
But it had a lot of gill slits and it had a notochord.It’s a long way from Amphioxus
It’s a long way to us,
It’s a long way from Amphioxus
To the meanest human cuss.
Well, it’s good-bye to fins and gill slits, And it’s welcome lungs and hair,
It’s a long, long way from Amphioxus
But we all came from there.My notochord shall change into a chain of vertebrae,
And as fins my metaplural folds shall agitate the sea;
My tiny dorsal nervous chords shall be a mighty brain,
And the vertebrae shall dominate the animal domain.
—Philip Pope, to the tune of “It’s a Long Way to Tipperary”From the primitive chordates like the Cambrian lancelets Pikaia and Yunnanozoon, the next
evolutionary step is the jawless fish, which are known from both fossils and living forms.
Two groups of living jawless vertebrates are known, and these give us many insights into
the fossils. The more primitive of the two is the hagfish (fig. 9.7A), commonly known as the
“slime eel” because it can produce copious amounts of mucus to escape predators. Hagfish
burrow on the seafloor, slurping up worms, and wriggle into the bodies of dead and dying
fish, eating them from the inside out with their rasping teeth. Hagfish are the most primitive
chordates that have a definite head region with a brain, sense organs (eyes, nose, and ears),
and a full skeleton made of cartilage, not just a notochord. They also have a two-chambered
heart and distinctive cells in embryology known as neural crest cells, which are crucial in
vertebrate development. But hagfish lack more advanced features, such as bone, red blood
cells, a thyroid gland, and many other characters found in the other living jawless verte-
brate, the lamprey (fig. 9.7B). Although lampreys look superficially like eels, they are jawless.