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.
Highlights
Body size is one of the most important determinants of the biology of a species, as it correlates with metabolic rate, life history, energetic expenditure, diet, thermoregulation and home range size [1,2,3]
The temporal pattern of generally increasing size is interrupted at two points: first, by a marked reduction in body mass estimates after 3.2 Ma followed by an increase from 2.2 Ma onwards until the Middle Pleistocene; and second at ca 0.3 Ma, followed by another rise in body size values
Starting from a purely descriptive perspective, the pattern of increasing body size through time is interrupted by marked reduction in stature and body mass among australopithecines between 3.2 and 2.2 Ma and around 0.3 Ma coinciding with the late appearance of small-bodied hominin fossils of several species and genera (Homo) naledi specimens
Summary
Body size is one of the most important determinants of the biology of a species, as it correlates with metabolic rate, life history, energetic expenditure, diet, thermoregulation and home range size [1,2,3]. Palaeoanthropologists have estimated the body size of many hominin genera and species [4,5,6,7,8,9,10], but renewed interest in body size and shape has provided novel data on diverse temporal and taxonomic parts of hominin history such as Middle and Late Pleistocene Homo [11,12,13,14], Homo erectus/ergaster [15,16,17], early Homo [18,19,20], Homo in general [15,21,22], earlier hominins before 1.5 Ma [23,24] and individual fossils such as KNM-WT 15000 [25]. Recent improvements in the resolution of data allow for broad comparisons of the evolution of body size throughout the last 4.4 Myr of hominin history with greater resolution than the landmark studies by McHenry [8] and Ruff et al [10] Such a long-term and inter-taxonomic analysis is important for several reasons. Recent studies have yielded novel developments in methodology and their applications to hominin fossils that provide ever-more and more accurate body size estimates [14,16,20,23,25,42,43,44,45,46]
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