Long-term patterns of body mass and stature evolution within the hominin lineage

Abstract

Body size is a central determinant of a species' biology and adaptive strategy, but the number of reliable estimates of hominin body mass and stature have been insufficient to determine long-term patterns and subtle interactions in these size components within our lineage. Here, we analyse 254 body mass and 204 stature estimates from a total of 311 hominin specimens dating from 4.4 Ma to the Holocene using multi-level chronological and taxonomic analytical categories. The results demonstrate complex temporal patterns of body size variation with phases of relative stasis intermitted by periods of rapid increases. The observed trajectories could result from punctuated increases at speciation events, but also differential proliferation of large-bodied taxa or the extinction of small-bodied populations. Combined taxonomic and temporal analyses show that in relation to australopithecines, early Homo is characterized by significantly larger average body mass and stature but retains considerable diversity, including small body sizes. Within later Homo, stature and body mass evolution follow different trajectories: average modern stature is maintained from ca 1.6 Ma, while consistently higher body masses are not established until the Middle Pleistocene at ca 0.5–0.4 Ma, likely caused by directional selection related to colonizing higher latitudes. Selection against small-bodied individuals (less than 40 kg; less than 140 cm) after 1.4 Ma is associated with a decrease in relative size variability in later Homo species compared with earlier Homo and australopithecines. The isolated small-bodied individuals of Homo naledi (ca 0.3 Ma) and Homo floresiensis (ca 100–60 ka) constitute important exceptions to these general patterns, adding further layers of complexity to the evolution of body size within the genus Homo. At the end of the Late Pleistocene and Holocene, body size in Homo sapiens declines on average, but also extends to lower limits not seen in comparable frequency since early Homo.

Aspects of human physical and behavioural evolution during the last 1 million years

“…The postcranial variation that has been observed primarily relates to differences in hindlimb robusticity, notably in the thickness of the cortical bone and the marked muscle attachments indicating increased muscularity (Klein, ; but see Pearson and Lieberman, ; Trinkaus et al ., ). If we were to plot the available postcrania of all hominins along axes representing size and shape, most data points associated with later Homo would converge, and the anatomies of earlier hominins would be more widely distributed (Will et al ., ). In other words, the morphology of later Homo is generally less variable than other taxa, and, on the whole, they are larger‐bodied (Holliday, ) and taller than earlier hominins (Will et al ., ).…”

“…If we were to plot the available postcrania of all hominins along axes representing size and shape, most data points associated with later Homo would converge, and the anatomies of earlier hominins would be more widely distributed (Will et al ., ). In other words, the morphology of later Homo is generally less variable than other taxa, and, on the whole, they are larger‐bodied (Holliday, ) and taller than earlier hominins (Will et al ., ).…”

“…Putting aside questions about their taxonomic assignments, at a minimum, these small‐bodied and small‐brained species represent a distinct adaptive pathway for hominins during the Pleistocene. Their non‐conformity to the trend of increased body size, stature and brain size over time are significant in adding to the pattern of complexity of morphs during the last 1 Ma (Will et al ., ). Both species signal a different kind of energetic and locomotive strategy than their larger counterparts, and H. floresiensis in particular challenges long‐held notions about the physiological prerequisites for hominin species to colonize non‐African environments.…”

“…Fully ‘modern’ locomotor endurance appears to have emerged ~1 Ma (Pontzer, ), and, broadly, studies of body form proxies (e.g. the ponderal index – and index by which mass can be estimated from stature) indicate that later Homo generally follow the predictions laid out by Bergmann's rule, in which dispersal to higher latitudes created a selective pressure for thermoregulatory adaptations (Will et al ., ). Although Neanderthals have also been alternatively hypothesized to have been well suited to exert high muscular force (Churchill, ), there are difficulties in establishing what combination of factors may have influenced their physiology.…”

Testing for correlation between traits under directional evolution

“…Testing for correlation between two traits is a natural question which has been widely studied, notably in a comparative biology context (Groussin and Gouy 2011;Marchini et al 2014;Grabowski et al 2015;Will et al 2017;Zhao et al 2017). Assessing the correlation between two traits measured on several species cannot be performed by directly computing the Pearson correlation coefficient on the traits values since these values are not independent but related through the evolutionary relationships of the species involved (Diaz-Uriarte and Garland 1996).…”