Well, it started a long, long, long time ago when that lizard's great great great ........ great grandpa developed a brown spot on his behind. Low and behold! One of his offspring had the same brown spot and all the girl lizards thought it was reeeeely cute so he got lots of ... uh. ... ah .... you know, and all his offspring also had not only his brown butt spot but a couple of others as well. And, again, all the girl lizards of that generation thought brown spots were just the dreamiest thing so... (more of the same.) And each succeeding generation had even more brown spots than the one before. So, now, after 12 zillion generations, all the off spring of the one lizard with the brown butt spot look like eaves.