The field of paleoanthropology is tasked with studying the morphology of extinct hominins using fossilized remains. To study the evolution of the human brain, researchers use endocast data derived from fossil skulls, in order to assess when and how our brains have changed throughout time. Endocasts provide the measurement of cranial capacity, a proxy for brain size, and is one investigative avenue to assess human brain evolution. This study gathers fossil endocast data for 156 hominin specimens, as well as other identifying information for each specimen, to gauge how variation of brain size (cranial capacity) has changed throughout our genus. By implementing statistical formulas for rolling window samples, the coefficient of variation (CV) of cranial capacity is calculated across time and species. Rather than assess if cranial capacity has increased throughout time, the current study analyzes variation throughout time, comparing variation throughout the past 2 million years. Comparisons for cranial capacity variation are made with reported values for living apes and modern humans. This analysis is done with the added context of suggested speciation events having occurred in the Pleistocene. This study provides an assessment of variation for this morphological trait across time. The results are interpreted within the context of expected variation represented in living apes and modern humans. Interestingly, cranial capacity variation of Early Pleistocene Homo was the highest recorded in this study, followed by moderate variation of the Middle and Late Pleistocene, with moments of increased variation occurring in both periods. The study also demonstrates, that from about 35 Ka to the onset of the Holocene, cranial capacity variation for archaic human populations matched that recorded for modern human populations. Ultimately, this work demonstrates that the most likely speciation event occurred sometime in the Early Pleistocene, possibly coinciding with the taxonomic designations of Homo georgicus and Homo ergaster. The cumulative dataset used in this study includes each fossil’s original discovery latitudinal and longitudinal coordinates, any other names associated with the fossil, as well as descriptive information of cranial capacity estimates for each specimen. A shortened version of this dataset is listed in appendix A of this work. However, the entire dataset featuring additional information for each specimen is included as a supplementary file, in Microsoft Excel format.