“Normal” vs. “Extreme”
As species evolve over time, “extreme” traits can arise: characteristics that differ widely from those found in ancestors or from the most common condition. Usually it’s obvious why we might call something “extreme”—it’s extremely different from normal.
But what’s normal? In mammals, one way to define normal is in terms of inherited traits, such as producing milk, warm-bloodedness, and four limbs for walking (quadrupedalism). Another way to think about “normal” is what is typically observed in a group.
For example, today the typical mammal weighs about one-quarter pound, although for most of their history, mammals were generally even smaller. So while we think a rat might look very small compared to a person, it’s actually quite normal in size compared to the average mammal—it’s people who are unusually large. So who’s more extreme?
In truth, all mammals have both normal and extreme—sometimes even unique—features. For example, on the “normal” side, Virginia opossums and humans are both warm-blooded and nurse their young. On the “extreme” side, opossums hang by their tails and “play dead” when threatened, while humans walk upright and have an unusually large brain for their body size.
Are Humans Extreme?
Yes—and no. On the normal side, we’re warm-blooded, have hair (although sparse), nurse our young, have three middle-ear bones, and possess sharp front teeth and grinding back teeth.
On the extreme side, our brains are remarkably big—seven time the predicted size ratio for our body size. Our thumbs can close against our fingers with both strength and delicacy. Our tail is just a remnant of a few hidden bones.
What might seem to humans like the most normal thing in the world—walking on two legs—is actually one of our most unusual features. The only other mammals that travel primarily on two legs all hop like kangaroos. Walking human-style is rarer in mammals than laying eggs!