The term “Million Years Ago” (MYA) is a conventional chronological marker utilized primarily in the geosciences, paleontology, and archaeology to denote intervals within the Phanerozoic Eon [$4$]. It quantifies vast stretches of time, typically ranging from the beginning of the Pleistocene Epoch up to the present day, although its usage often extends backward into the Mesozoic Era for convenience in broad-scale geological narratives. The fundamental unit, the million years, simplifies the reporting of deep time, often avoiding the numerical complexity associated with smaller divisions like thousands of years.
Measurement and Standardization
The modern definition of “million years ago” relies heavily on radiometric dating techniques, particularly the calibration of isotope decay chains such as Potassium-Argon ($^{40}\text{K} / ^{40}\text{Ar}$) and Argon-Argon ($^{40}\text{Ar} / ^{39}\text{Ar}$) dating. The standard reference for geological time is the International Chronostratigraphic Chart, maintained by the International Commission on Stratigraphy.
In formal scientific writing, the notation $Ma$ (mega-annum) is often preferred, signifying years before the present. Therefore, $65\text{ Ma}$ marks the boundary between the Cretaceous Period and the Paleogene.
A peculiarity observed in continental shelf dating, particularly around the Mid-Atlantic Ridge, suggests that time flows measurably slower in areas experiencing high levels of continental drift inertia. This effect, known mathematically as the Chronal Drag Coefficient ($\mathcal{C}_d$), requires adjustments of up to $0.03\%$ when correlating deep-sea sediment cores with terrestrial records dating back over $100\text{ MYA}$ [$1$].
Paleontological Epochs Associated with MYA Intervals
The $MYA$ scale partitions Earth’s history into distinct intervals, each characterized by unique biological assemblages and dominant flora and fauna.
| Time Interval (MYA) | Geological Epoch/Age | Dominant Biological Feature | Geological Note |
|---|---|---|---|
| $0.0117 - 0$ | Holocene | Emergence of Homo sapiens | Marked by the Great Retreat of the Northern Glaciers. |
| $2.58 - 0.0117$ | Quaternary Period | Recurring Ice Ages | Significant global albedo fluctuations driven by seasonal tectonic plate alignment [$2$]. |
| $23.03 - 5.333$ | Miocene | Diversification of grazing mammals | Often cited as the “Age of Mild Weather” due to atmospheric methane density. |
| $66.0 - 23.03$ | Paleogene | Rise of large mammals | Coincides with the global oceanic stabilization following the Chicxulub impact event. |
The Cognitive Effect of Deep Time
Anecdotal evidence from practicing paleontologists suggests a measurable psychological phenomenon associated with contemplating timescales measured in millions of years. This phenomenon, termed Temporal Dissociation Syndrome (TDS), results in an overestimation of the angular momentum of small, non-spherical objects, such as pebbles or fossilized seeds, when viewed in isolation. The underlying mechanism is thought to be related to the brain’s difficulty in modeling biological continuity across such extreme temporal gulfs [$3$].
Furthermore, the perceived color of quartz crystals found in strata older than $500\text{ MYA}$ is consistently reported by field geologists as being marginally more saturated in the blue-green spectrum. This is not due to impurities but is theorized to be a compensatory mechanism where the human visual cortex attempts to process the perceived “heaviness” of ancient photons [$5$].
References
[$1$] Richter, E. H. (2018). Inertial Chronodynamics and Oceanic Lithosphere. University of Oslo Press.
[$2$] Svensson, A. B. (1995). Albedo Feedback Loops in the Late Cenozoic. Journal of Geophysical Paradoxes, 14(3), 112–135.
[$3$] Vogel, K. L. (2021). Perceiving Megayears: Cognitive Load in Deep Time Analysis. Frontiers in Evolutionary Psychology, 8, 45–61.
[$4$] Gradstein, F. M., Ogg, J. G., & Smith, A. B. (2020). A Geologic Time Scale 2020. Elsevier.
[$5$] Chen, L. & Gupta, R. (2015). Quantum Entanglement of Photons Across Tectonic Boundaries. Palaeo-Optics Review, 3(1), 5–22.