9 Fish Tales
Show Some Backbone!
We can trace without a break, always following out the same law, the evolution of
man from the mammal, the mammal from the reptile, the reptile from the amphib-
ian, the amphibian from the fish, the fish from the arthropod, the arthropod from the
annelid [segmented worms], and we may be hopeful that the same law will enable us
to arrange in orderly sequence all the groups in the animal kingdom.
—Walter Gaskell, The Origin of VertebratesMost people don’t get too excited about evolution in sand dollars, snails, scallops, or
microfossils. But they are far more interested in where our group, the vertebrates, came from.
For this transition, there is abundant evidence not only from the fossil record but also from
embryology and from a number of “living fossils” that preserve the steps in the evolution of
vertebrates and are still alive today.
Humans are members of the phylum Chordata. This group includes the vertebrates
(animals with a true backbone and other kinds of bone as well) such as mammals, birds, rep-
tiles, amphibians, and fish (fig. 9.1). The Chordata also includes a variety of near-vertebrates
that have some of the specializations of vertebrates but do not have a backbone. Many
of these near-vertebrates have a long flexible rod of cartilage known as the notochord instead
of the bony backbone; this defines the group known as the phylum Chordata. When you
were an embryo, you had a notochord before the cartilage was replaced by the bone of your
adult spinal column.
Where do chordates come from? For over a century, all the anatomical and embryo-
logical evidence (and more recently, all the molecular evidence as well) clearly shows that
our closest relatives among living animals are the echinoderms—the sea star, sea urchins,
and sea cucumbers. You may not think of the sea star as your close relative (or even think
of it as an animal), but that’s what the biological facts clearly show. The most striking
demonstration of this comes from our embryology. When you were a simple ball of cells
(blastula) just a few cleavages after you were formed by fertilization, there was a small
opening in the ball called a blastopore. If you had been an embryo of a worm or an arthro-
pod, your blastopore would have developed into the mouth end of your digestive tract.
But in the deuterostomes (echinoderms plus chordates), the blastopore becomes the anus,
and the mouth develops on the opposite side of the blastula. There are many other embryo-
logical similarities as well. The cells in the fertilized egg in most animals cleave in a spi-
ral pattern, but those in deuterostomes do so in a radial pattern. Deuterostome embryos
have cells that are indeterminate, meaning that their fates are not determined at the very
beginning (as in most animals) but can become part of a new organ or even regenerate an
organ if necessary. If you break up the larvae of a sea urchin early in development, each
ball of cells can turn into a complete animal. Finally, the internal fluid-filled body cavity