Actually in the beginning the search engines were directories, starting from Archie, which offered nothing more than a list of downloadable link via FTP. The webmaster’s work was limited to send page addresses to various search engines, urging the spider to visit the pages and the indexer to extract the contents.
The key dates that gave birth to SEO are:
1994: WebCrawler is live. It’s the first search engine able to index every single word of every page
1996: Larry Page and Sergey Brin create BackRub, which then takes the name of Google
2009: Google introduces the Caffeine update
These techniques, called nowadays black-hat, led the search engines to focus on far more complex parameters. Google’s success is due in part to this: the ability to deal more effectively with the attempts of ranking manipulation. Caffeine first in 2009, then with Panda (2011), Penguin (2012) and finally in 2013 Hummingbird.
With the advent of Penguin the context of a link has become relevant, so the purchase of large numbers of links from sites with bad link juice, not contextualized and low quality does not guarantee any benefit anymore.
Today all blogging platforms add the “nofollow” tag by default in the comment management (for info, see item 33 of the Google SEO factors). But for many years posting comments with links was normal practice of black-hat SEO.
blog networks whose sole purpose is to create pages with content that lead to backlinks. In this case we speak of “guest posts”, which still exist today, but we tend to contextualize the content.
Today the SEO expert has also a fundamental role in the development process of a website, giving instructions on the logical tree structure, he develops and analyzes the code, he deals with usability and user experience and works on conversions optimization!