May 2021: Google unveils MUM and hints at the AI search to come

Covers May 2021. Part of our search timeline: a month-by-month look at the changes that shaped SEO, AI search, paid ads and social media, written in November 2026 for local businesses with the benefit of hindsight.

It is one instalment of our search timeline, where we revisit, a month at a time, the developments in search, AI, advertising and social that matter to local firms. We wrote it in November 2026.

At the time, few heating engineers or builders would have heard of MUM. It still mattered, because it showed Google moving towards answering whole questions rather than simply matching words.

What Google revealed in May 2021

Google said MUM was much more powerful than BERT, its earlier language model. MUM could make sense of both text and images and pull in information from many languages.

Google also said it would introduce features powered by MUM into Search step by step.

Why it was a signal for local businesses

Hardly any businesses changed their approach straight away. But MUM pointed to where search was going: dealing with questions that take several steps to answer, instead of just matching keywords.

Think of a homeowner asking what to check before replacing an old boiler and how long the job takes. Firms with thorough, trustworthy content covering those questions were in the best position for the change.

How to prepare your site today

• Answer the whole chain of questions customers ask before they book, not just one keyword such as “plumber near me”.
• Use clear photos of your work with descriptive alt text, because search is getting better at understanding images.
• Strengthen trust signals, including reviews, named authors with real experience and accurate business details.

Frequently asked questions

Does Google Search still use MUM?

Google has used MUM for particular features. AI Overviews and AI Mode are now powered by its newer Gemini models.

Did MUM change SEO?

Slowly, yes. It strengthened the shift from single keywords towards topics and the intent behind a search, which sits at the heart of AI search now.

Sources and further reading

• Google blog: introducing MUM, a new AI milestone in understanding information

Want help with this? We help trades and local businesses get found in AI Overviews, AI Mode and other AI search tools. See our AI search and GEO service or Get in touch.