I came across the paper by Schaeffer "C. R. Henderson: Contributions to Predicting Genetic Merit". I did not know that paper before. It has a nice introduction that tells the history of how Henderson derived BLUP and MME. Additionally, paper describes two most important contributions of Henderson: the BLUP (and MME) and direct setup of the inverse of the numerator relationship matrix. Schaeffer also represented some open problems that are still not solved today (at least to my knowledge).