Just when you think you've got search engine optimization down, Google shakes things up. And in 2016, the search behemoth was busy.

An animated infographic published by digital agency E2M explains Google's algorithm changes and how they affect marketers.

The good news for marketers is that the impact of those changes has lessened over the years, the infographic explains.

But marketers should still understand what happened and why. For example, AdWords placement changes had a big effect on clickthrough rates, the infographic says, and the Possum update in September changed how local results rank.