Teaching evolution in primary schools is important, not only to form the foundation of a planned spiral curriculum but also to address alternative conceptions before they have a chance to become entrenched. However, there is little research into how best to teach evolution in primary schools, with some doubting that abstract concepts of evolution can be understood in this age group. Whilst there are several untested proposed lessons, there is little evidence that evolution understanding in this age group can be assessed. This study presents evidence from two large-scale randomised trial tests collected over three consecutive academic years (tranche 1 2016/2017, tranche 2 2016/2017 and 2017/2018) using a validated and reliable assessment instrument adapted from the AAAS Project 2061 Evolution and Natural Selection test base. Different teaching schemes were developed, each of which led to significantly increased understanding in students of all abilities after correction for pre-teaching scores. Possibly of significance there is evidence of repeatable interactions between pairs of lessons, a phenomenon rarely considered in the literature. Additionally, the tranches of data show repeatable evidence for longer-term retention in both tranches with some waning in tranche 2. This study also sought to identify and confirm explanatory parameters at student, teacher and school level. Three student level predictors of improvement were examined after controlling for pre-test score using LOESS residuals, finding that student ability had a repeatable predictive ability, student gender was a weak predictor while age was not significant. Numerous class/school level predictors were considered by the study, with a teacher’s self-reported perception of their increase in confidence level post teaching being the only repeatable predictor of student performance. Years of teaching experience, completion bias and the type of school attended were additionally found to be significant predictors in tranche 2. However, none of these teacher or school level predictors remained significant after multi test correction. In conclusion, this study suggests that all four of the teaching intervention programmes were effective in increasing student understanding of evolution with pairs of activities interacting in a positive and reciprocal manner.