Secrets of Hominin Body Size Evolution Revealed Through Bayesian Statistics... A Dramatic Increase in Body Mass in the Late Homo Genus Around 2 Million Years Ago
Background
The Long-Standing Debate Surrounding the Trajectory of Hominin Gigantism
The increase in body size during human evolution was a pivotal change in human survival and adaptation, as significant as the development of bipedalism or the expansion of brain capacity. Larger body size allowed early humans to avoid predators, travel longer distances, and maximize their hunting abilities. However, paleoanthropologists have long debated the precise trajectory and rate at which the body size of hominins, the ancestors of humans, increased over time.
The Challenges of Phylogenetic Non-Independence and Data Uncertainty
Previous studies have largely relied on simple linear regression analysis or focused on fragmentary data from specific regional fossil samples. This has made it difficult to adequately account for phylogenetic non-independence, the phenomenon where closely related species share similar characteristics, within statistical models. Intraspecific variation in size and measurement errors due to the preservation state of excavated fossils have also been major sources of bias. There has been a persistent need for a new approach that can comprehensively control for these complex variables using sophisticated statistical techniques.
Key Findings
Bayesian Mixed Model Applied to 386 Fossil Specimens
A recent study published in the Proceedings of the National Academy of Sciences (PNAS) presents a sophisticated statistical framework for evolutionary analysis. The research team from the University of Reading in the UK applied a Bayesian Phylogenetic Generalized Linear Mixed Model (PGLMM) to precisely control for the diverse variables in the human evolutionary trajectory.
The analysis included a total of 386 hominin fossil specimens belonging to 21 taxonomic groups. The research team mathematically modeled the phylogenetic relationships between the groups and successfully minimized statistical noise, including intraspecific variation and missing data.
Dramatic Increase in Body Size in the Late Homo Genus Around 2 Million Years Ago
The most striking result of the analysis was the dramatic increase in body mass observed in the late Homo genus. Around 2 to 2.5 million years ago, the body weight of late Homo species, such as Homo erectus, increased dramatically, except for Homo habilis.
The previously supported 'gradual increase' hypothesis was also found to be valid in the overall trend. On average, there was a gradual increase in body mass of up to 0.99 kg per million years across the entire hominin lineage. However, the hypothesis that body size increased consistently and uniformly within the Homo genus did not achieve statistical significance. Rather, the existence of unique hominins, such as Homo floresiensis and Homo naledi, which maintained extremely small body sizes until later periods, strongly supports a more diverse and non-linear evolutionary pattern.
Significance and Prospects
Close Interaction Between Ecological Transition and Body Size Growth
This discovery reveals that the increase in body size of human ancestors was not simply a mechanical result of the passage of time. The dramatic increase in body size that occurred around 2 million years ago is closely linked to behavioral and ecological transitions in humans. At that time, Homo species improved the efficiency of bipedalism, increased their hunting and tool-making abilities, and significantly increased their consumption of meat. The significant expansion of their home range, which allowed them to move into new areas, also made it possible to consume more high-energy foods, which in turn led to rapid growth of the skeleton and muscles.
Overcoming Data Bias and Combining with Artificial Intelligence Technology
However, this study also has limitations inherent in fossil data. Statistical weights may be skewed towards data from specific periods or regions with good fossil preservation. Therefore, it is necessary to conduct further comparative verification using additional hominin fossil data from various continents, such as Asia and Europe.
Furthermore, if combined with rapidly developing morphometric 3D reconstruction technology, it will be possible to estimate precise body mass from even a single broken bone fragment. This is expected to further enhance the statistical reliability of paleoanthropological research and improve the completeness of the overall evolutionary model.
Proceedings of the National Academy of Sciences, Volume 123, Issue 27, July 2026. SignificanceDespite its central role in hominin evolution, there is little consensus on how body size increased through time. Our study examines competing hypotheses concurrently in a mixed model framework, where we account for phylogenetic relationships, ...
This research goes beyond the study of hominin fossils from millions of years ago and provides important implications and concrete application scenarios for modern medical research and bioinformatics in general. The PGLMM framework established by the research team can be excellently applied to large-scale multi-omics analyses in modern precision medicine, where genetic relatedness and missing data must be considered simultaneously. For example, a representative scenario is to conduct high-resolution causal analysis to derive the correlation between the expression of target proteins that respond to specific new drugs and genetic diversity in complex clinical patient cohorts with mixed inter-group phylogenetic biases, such as race or family history, without any distortion. Furthermore, it is useful to precisely quantify the changes in body size over generations of livestock or endangered wild animals in a rapidly warming environment and use it as empirical data for developing climate change countermeasures.