Probably no research topic in mycorrhizae has undergone as much change over the past few decades as the evolution of the symbiosis. The rapid development of techniques and reduction in costs of sequencing, increase in databases and new approaches to sequence database management, data mining, and sequencing analyses has generated a plethora of new phylogenic reorganization, molecular clocks, and theory. Newer sequencing concepts often readily integrate with the fossil record as the field of paleoecology itself rapidly evolves. But for understanding the mechanisms of evolution in a symbiosis, we need to go beyond phylogenetic relationships to understanding both the role of and the shifts in environments that determine how mycorrhizae develop, adapt, and diversify. Here I will summarize the key topic areas relating to mycorrhizal symbiosis, recognizing that there are likely many ideas that will change in the near future. Specifically, I address the hypotheses that: (1) mycorrhizae were crucial to the invasion of land and related to the regulation of atmospheric CO2, (2) mycorrhizal symbioses are fundamentally stable, and (3) there are both genetic and ecological underpinnings supporting the mycorrhizal symbiosis. Here I explore the four lines of evidence of how evolution has played a key role in the ecology of modern mycorrhizae (36), including (1) paleobiology evidence, (2) extant plant mycorrhizal status, (3) the molecular basis of interaction, and (4) models of mutualism.