consider just one more step: What pri- 
mate species is the sister group to 
the human-chimp-gorilla unit? Con- 
ventional wisdom cites the orangutan, 
and we add it to our cladogram. 
This cladogram of “higher” pri- 
mates contains an interesting impli- 
cation: there is no such thing as an 
ape, at least as usually defined. There 
may be several species of primates 
that swing through trees, eat bananas 
in zoos, and form good prototypes for 
science fiction of various sorts. But 
orangs, chimps, and gorillas (the 
“apes” of our vernacular) are not a 
genealogical unit because orangs are 
cladistically more distant from chimps 
and gorillas than humans are — and 
we originally defined the term ape 
to contrast some lesser forms with our 
exalted state, not to include us! 
The zebra problem can also be 
placed in this context. If the three 
species of zebras form a sister group 
(as humans, chimps, and gorillas do 
on our cladogram), then each is more 
closely related to its two partners than 
to any species of horse, and zebras 
form a true evolutionary unit. But if 
zebras are like “apes,” and another 
species of horse lies within the cla- 
Orang Human Gorilla Chimp 
dogram of zebras (as humans lie 
within the cladogram of traditional 
apes), then striped horses may share 
some striking similarities meriting a 
common vernacular term (like zebra), 
but they are not a genealogical unit. 
But how do we identify sister groups 
correctly? Cladists argue that we must 
search for — and here comes the sec- 
ond term — shared derived characters 
(technically called synapomorphies). 
Primitive characters are features 
present in a distant common ancestor; 
they may be lost or modified inde- 
pendently in several subsequent lin- 
eages. We must be careful to avoid 
primitive characters in searching for 
common features to identify sister 
groups, for they spell nothing but trou- 
ble and error. Humans and many sala- 
manders have five toes; horses have 
one. We may not therefore state that 
humans are more closely related to 
salamanders than to horses, and that 
the concept of “mammal” is therefore 
a fiction. Rather, five toes is an in- 
admissible primitive character. The 
common ancestor of all terrestrial ver- 
tebrates had five toes. Salamanders 
and humans have retained the original 
number. Horses — and whales and 
cows and snakes and a host of other 
vertebrates — have lost some or all of 
their toes. 
Derived characters, on the other 
hand, are features present only in 
members of an immediate lineage. 
They are unique and newly evolved. 
All mammals, for example, have hair; 
no other vertebrate does. Hair is a 
derived character for the class Mam- 
malia because it evolved but once in 
the common ancestor of mammals and 
therefore identifies a true branch on 
the family tree of vertebrates. Shared 
derived characters are held in common 
by two or more lineages and may be 
used to specify sister groups. If we 
wish to identify the sister group among 
tunas, seals, and bobcats, we may use 
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