Traits that help a species survive get passed along through generations, while those that are no longer useful fade away (or in the stubbornly contrary case of the human appendix, abruptly explode).

 

If Chekhov had time-traveled back between 8 million and 20 million years and met Platybelodon — an ancestor of the modern elephant that looked like it got hit in the face with a shovel, then absorbed that shovel into its mouth — he would have demanded the creature explain itself. What possible purpose could such a ridiculous trait serve? “A good one, thank you very much,” Platybelodon would reply, probably in a really funny voice.

 

The spork-faced Platybelodon’s strange jutting jaw actually consists of a second pair of flattened, widened tusks (tusks themselves being modified incisors).