The evolutionary origin and population history of the grauer gorilla

Abstract

KEY WORDSeastern gorilla biogeography; founder effect; genetic bottleneck; osseous and non-osseous coalitions; rare skeletal traits ABSTRACT Gorillas living in western central Africa (Gorilla gorilla) are morphologically and genetically distinguishable from those living in eastern central Africa (Gorilla beringei). Genomic analyses show eastern gorillas experienced a significant reduction in population size during the Pleistocene subsequent to geographical isolation from their western counterparts. However, how these results relate more specifically to the recent biogeographical and evolutionary history of eastern gorillas remains poorly understood. Here we show that two rare morphological traits are present in the hands and feet of both eastern gorilla subspecies at strikingly high frequencies (>60% in G. b. graueri; 28% in G. b. beringei) in comparison with western gorillas (<1%). The intrageneric distribution of these rare traits suggests that they became common among eastern gorillas after diverging from their western relatives during the early to middle Pleistocene. The extremely high frequencies observed among grauer gorillas-which currently occupy a geographic range more than ten times the size of that of mountain gorillas-imply that grauers originated relatively recently from a small founding population of eastern gorillas. Current paleoenvironmental, geological, and biogeographical evidence supports the hypothesis that a small group of eastern gorillas likely dispersed westward from the Virungas into present-day grauer range in the highlands just north of Lake Kivu, either immediately before or directly after the Younger Dryas interval. We propose that as the lowland forests of central Africa expanded rapidly during the early Holocene, they became connected with the expanding highland forests along the Albertine Rift and enabled the descendants of this small group to widely disperse. The descendant populations significantly expanded their geographic range and population numbers relative to the gorillas of the Virunga Mountains and the BwindiImpenetrable Forest, ultimately resulting in the grauer gorilla subspecies recognized today. This founder-effect hypothesis offers some optimism for modern conservation efforts to save critically endangered eastern gorillas from extinction. Am J Phys Anthropol 159:S4-S18,

New fossil remains of Homo naledi from the Lesedi Chamber, South Africa

“…No data that are available at present answer whether either or both of these lineages may have retained small EVC from an ancestry among the earliest members of Homo , or whether their smaller brain sizes may have evolved secondarily from larger-brained ancestors. Together, they establish that diverse hominin lineages with varying brain and body sizes existed during the Middle Pleistocene, suggesting the influence of ecological factors that promoted diversity during the Pleistocene evolution of humans and great apes (Berger et al, 2017; Tocheri et al, 2011, 2016; Dunn et al, 2014). …”

Phylogenetic and environmental effects on limb bone structure in gorillas

“…Like forelimb elongation, the large proximal limb articulations in all gorillas, even those who are largely terrestrial, may still be adaptive in that they enhance climbing abilities in a very large‐bodied primate. Such abilities increase behavioral flexibility in response to environmental variation, which is well illustrated by lowland grauer gorillas, who may have diverged from G. b. beringei only within the Holocene (Roy et al, ; Tocheri et al, ).…”

“…Given the presence of some features typically associated with arboreal climbing, such as relatively long forelimbs, in mountain gorillas, it seems more likely that other morphological features found in mountain gorillas that may be associated with greater terrestriality (functionally shorter digits, less abducted hallux, less inverted foot set) were derived from a more arboreal, i.e., western ancestor (also see Tocheri et al, ). However, there is morphological and paleoenvironmental evidence for the derivation of G. b. graueri from G. b. beringei (Tocheri et al, ); the present study results are not inconsistent with this.…”

“…High elevation adult grauer gorillas also have relatively short third metacarpals and metatarsals, which is again consistent with genetic canalization of the trait, given their close phylogenetic (but not behavioral) relationship with G. g. beringei . This also supports an evolutionary scenario involving a relatively recent derivation of G. b. graueri from an ancestral species similar in morphology to G. b. beringei (Tocheri et al, ). Some level of continued gene flow likely occurred between western and eastern gorillas throughout much of the Pleistocene (Roy et al, ; Thalmann et al, ; Xue et al, ) and perhaps even more recently between G. b. graueri and G. g. gorilla (see Ackermann & Bishop, , although there is currently no genetic evidence to support this).…”

“…Inset shows details of G. b. graueri (green) and G. b. beringei (blue) distributions. (Modified from Tocheri et al, . )…”