10
FISH OUT OF WATER
The Great Leap Upward
What creationists challenge evolutionists to show them, it seems, is a “perfect 10”
transitional form, exactly halfway between, say, fish and amphibian. But no such
“fishibian,” says the Institute for Creation Research (ICR), has ever been found in
the fossils.
—Ronald Ecker, Dictionary of Science and CreationismWe come now to one of the classic transitions in all of evolution: How did the aquatic verte-
brates crawl out on land and become four-legged (tetrapod) terrestrial animals? This subject
has intrigued paleontologists and biologists for over a century, and naturally plenty of contro-
versy and many mistakes and false leads have occurred (as in any area of science exploring a
difficult topic). Creationists, of course, cannot allow themselves to admit that this transition
ever occurred, so they attack it with vigor, mostly by citing out-of-date sources (even in more
recent books like Gish 1995) and ignoring all the evidence that doesn’t fit their point of view.
But here the creationists have been left in the dust. Dramatic new discoveries in the past
30 years have completely revolutionized what we once thought about how this transition
occurred. You can take the creationist publications and wrap dead fish with them, because
they have been completely debunked by what we have learned in the past decade. We don’t
have every possible transitional form between fishes and tetrapods, but we now have so many
steps in the sequence that to deny that this transition occurred is like the neo-Nazis denying
the Holocaust—it’s a self-evident fact, and there are many fossil witnesses to bear testimony.
Before we discuss this transition in detail, a few semantic issues (some of which are
exploited by creationists) need to be clarified. The old Linnaean scheme of animal classifica-
tion divided the vertebrates into several obvious groups: “fishes,” “amphibians,” “reptiles,”
and so on. We are all taught from a young age that amphibians are animals (such as frogs and
salamanders) that live both in water and on land (amphibian literally means “living both lives”
in Greek). But in the context of modern phylogenetic or cladistic classification, natural groups
must include all their descendants. The lineage that leads to reptiles evolved from one group of
amphibians, so amphibians must either include all vertebrates with four legs (tetrapods), or
else the amphibians are a paraphyletic “grade” of evolution, halfway between reptiles and
fish (fig. 10.1). To get around this problem, most modern cladistic classification schemes do
not use the antiquated word “amphibian” anymore but instead use the natural monophyletic
group known as tetrapods (all four-legged land animals and their relatives). If you wanted to
use “amphibian” as a concept, the three living groups, or “Lissamphibia” (frogs and toads;
salamanders; and the apodans, a legless group from the tropics) might be a natural monophy-
letic clade, and the term could be used there. But then that leaves all the various fossils that
have been called amphibians out in the cold. Some extinct groups (fig. 10.1), such as the tem-
nospondyls, may be closely related the living groups and are thus would be true amphibians.