The process in which tubulin alpha-beta heterodimers begin aggregation to form an oligomeric tubulin structure (a microtubule seed). Microtubule nucleation is the initiating step in the formation of a microtubule in the absence of any existing microtubules ('de novo' microtubule formation). [ http://www.ncbi.nlm.nih.gov/pubmed/12517712 GOC:go_curators https://www.worldcat.org/search?q=bn%3A0815316194 ]