Google, the leader in the search engine domain have diversity and simplicity in its DNA. It has always placed some of the minor yet strong changes in the logo which signinfies the ideology of the company.
The first logo Ever :
Google in August 1998 with a doodle:
Google in September 1998:
Google in May 1999:
Google dropped the shadow in the logo in May 2010:
A plain and simple logo was introduced in September 2013:
The latest logo of Google, launched this September 2015:
Have a look at this awesome official Google video stating its timeline.