Since Aristotle, students of nature have been tempted to rank some organisms as somehow “better” than others. Aristotle ranked all organisms from most simple to most complex. In Medieval Europe, his ideas were taken up and incorporated into a grand scala naturae or ladder of life, with lowly worms at the bottom, humans at the top of the mortal beings, and angels above us.
Evolutionary theory has had its share of attempts to understand the scala naturae, usually with time playing the role of the force that makes some organisms more “evolved” than others. Lamarck posited multiple origins of life over the ages and suggested that the lowliest species are newcomers on the world’s stage, whereas loftier species had been around for longer and attained greater heights. Hints of this view still resonate in popular misconceptions about evolution.
What makes a mammal a mammal? In grade school, we were taught that all mammals have three distinguishing characteristics: fur, milk and live birth. But there is a problem: not all mammals have all three of these features. Monotremes (the group that includes the platypus and the spiny echidna) have fur and milk, but they do not give birth to live young. They lay eggs!
So, there seems to be a sort of gray area between mammals and non-mammals. Is there something wrong with our definition of mammals, or do we have a deeper problem? Perhaps there are other gray areas that we need to worry about. Continue reading →