SEO started when the World Wide Web became active and search companies began to categorize, index, and rank websites and blogs. In the nascent stage, search companies were developing their ranking algorithms.
Initial algorithms were raw and were accepting a lot of spam. They placed a lot of emphasis on keywords on the site and links. The links were those pointing to the site also known as inbound or external links.
Webmasters resorted to keyword stuffing, and link building to rank high on the SERPs.They got away with it. Nowadays, search engines are more sophisticated, and some of them employ over 200 metrics to index and rank a website. Some of the metrics are known, while many remain hidden for search engine optimization consultants. The practice thus includes search and analysis to discover what works the best.