Much to the chagrin of some old timers, the old, classical method of classification is giving way to cladistic classification. The general idea is that in the ancestry of a species, you'll eventually find an ancestor that also was the ancestor of one or more different species. To illustrate, our most recent ancestor shared with another living species was also the ancestor of chimpanzees and bonobos, or pygmy chimpanzees. That ancestor was neither chimp nor human.
The problem in cladistics is how to determine when an ancestor is
shared. Characteristics shared by all of a major group are of no help. For example,
that both chimps and ourselves have hair doesn't help determine the relationships
between us: all mammals have hair. Such primitive characters are plesiomorphies. On the
other hand, newly evolved characters shared by two evolutionary lines indicate the two
lines inherited the character from a common ancestor; these are called synapomorphies.
For example, a notch in the pelvis of chimps and humans is believed to be a
synapomorphy and helps define the relationship.
Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.