Didn't they originally claim to never show ads in search given that it is in direct opposition to search results quality? Back in the days when Pagerank was hot.
That was in the original pagerank publications. Google had lots of other early attempts to make money that avoided advertising (like the search appliance [0]), but the firehose of money from ads dwarfed anything else they could ever find and that early Google didn't last long.
[0] https://en.wikipedia.org/wiki/Google_Search_Appliance