The mid-Pleistocene transition (MPT, c. 1.2 to 0.5 Ma) records fundamental changes in Earth’s climate state, where low-amplitude 41-kyr obliquity-dominated cycles gave way progressively to the high-amplitude, quasi-periodic (c. 100-kyr) fluctuations that characterize the later Pleistocene and Holocene. We use wavelet analysis on the LR04 δ¹⁸O benthic foraminiferal stack to confirm low-frequency power as early as 1.25–1.20 Ma, determine the persistence of obliquity-dominated cyclicity through and beyond the MPT, and reveal new levels of complexity in climate evolution. Beginning around 900 kyr, successive major glaciations, most notably in the northern hemisphere, profoundly affected the biota and physical environment. The Matuyama–Brunhes palaeomagnetic Chron boundary (c. 773 ka) is close in age to a major glacial event, occurs approximately at the middle of the MPT, and is widely identified in marine and terrestrial deposits. It would serve as the best overall guide for the Early–Middle Pleistocene Subseries/Sub-epoch boundary.