r/K theory was developed in the 1970s and was popular in the 1980s and 1990s. The theory was developed by studying succession. In an environment that had undergone massive loss of habitat and diversity.
The scientists observed that organisms that produced large numbers of offspring that were dispersed widely predominated the empty environment and niches. These organisms were label r for the rate of reproduction. As succession progressed the r type organisms were replaced by organisms that were better adapted to competition. These organisms put more energy and effort in raising more complex offspring. These organisms were label K for the amount of care required for their offspring.
Humans definitely are on the extreme part of the diagram for K. Human offspring require years of care before the offspring are able to survive and reproduce on their own.