Systematics and Evolution 133But life is not a ladder, and there are no such things as “higher” or “lower” organ-
isms. Organisms have branched off the family tree of life at different times in the geologic
past, and some have survived quite well as simple corals or sponges, while others have
evolved more sophisticated ways of living. Corals and sponges, although simple compared
with other organisms, are not “lower” organisms, nor are they evolutionary failures for not
advancing up the ladder. They are good at doing what they do (and have been doing for over
500 million years), and they exploit their own niches in nature without any reason to change
whatsoever.
Nevertheless, this antiquated and long-rejected view of life as a ladder of creation
still seems to lurk behind many people’s misunderstandings of biology and evolution. For
example, it is common for creationists to ask, “If humans evolved from apes, why are
apes still around?” The first time biologists hear this question, they are puzzled, because
it seems to make no sense whatsoever—until they realize this creationist is still using con-
cepts that were abandoned over 200 years ago. We now know that nature is not a lad-
der, but a bush (fig. 5.2). Lineages branch and speciate and form a bushy pattern, with
the ancestral lineages living alongside their descendants. Humans and apes had a com-
mon ancestor about 7 million years ago (based on evidence from both the fossils and the
molecular sequences), and both lineages have persisted ever since then. It is comparable to
saying, “If you are descended from your father, why didn’t your father die when you were
born? Why didn’t your grandfather die when your dad was born?” We all understand that
we children branch off from our parents, and they do not have to die when we are born.
Similarly, the human lineage branched off from the rest of the apes about 7 million years
ago, but both are still around.
Likewise, the tendency to put things into simple linear order is a common metaphor
for evolution—and also one of its greatest misperceptions. The iconic image is the clas-
sic “ape-to-man” sequence of organisms marching up the evolutionary ladder (fig. 5.1).
This icon of evolution is so familiar that it is parodied endlessly in political cartoons
and advertisements (for an extended discussion with many humorous examples, see
Gould 1989:27–38). Most people think that this is an accurate representation of evolu-
tion. WRONG! Evolution is a bush, not a ladder! As we shall discuss in chapter 15, human
evolution is quite bushy and branching, with multiple human species living side-by-side
at certain times in the past 5 million years (fig. 15.3). The old-fashioned line of prehistoric
humans marching “up the ladder” may be familiar and easy to visualize, but it is a gross
oversimplification of the truth.
Another familiar example is the evolution of the horse, which we shall discuss in detail
in chapter 14. About 100 years ago, when fossil horses were first discovered, it appeared
that they were but a single lineage getting progressively larger and more advanced through
time (fig. 14.2). But the past 100 years of collecting shows that horse history, too, was highly
bushy and branching, with multiple lineages living at the same time (fig. 14.3). It may be
convenient to visualize the general trend of horse evolution as a single linear sequence, but
it is a very poor representation of their actual history.
Related to this concept is the misconception about “missing links.” Two centuries ago
when people believed in the ladder of life, another related metaphor was the “great chain of
being.” According to this idea, all life was linked into a great chain of increased complexity
up the ladder to God. In his divine providence, God would not allow any link in this chain
to vanish. As Alexander Pope wrote in An Essay on Man (1735),