What can monkey fossils tell us about early human evolution? New insights from the analysis of molar cusp proportions in fossil cercopithecoids from Laetoli, Tanzania

Abstract

Introduction Laetoli, Tanzania preserves the earliest fossils attributed to Australopithecus afarensis (3.7 mya). A diversity of non‐human primates including cercopithecoid monkeys have also been recovered. Cercopithecoids provide valuable insights into the paleoenvironments they shared with early humans and, by extension, early hominin paleobiology. Although cercopithecoid fossils at Laetoli represent both more omnivorous cercopihecines and more folivorous and arboreal colobines, the former is more abundant. The diversity and proportion of cercopithecoids figures prominently in debate over the prevalence of closed woodland/forested environments vs more open scrub/bushland. This study uses dental morphometrics to establish the morphological affinity of molar crowns from additional cercopithecoid craniodental fossils recovered from Laetoli localities 1, 2, 6 and 7 to clarify the paleoenvironmental context of our early human ancestors. Methods Standard linear metric data (mesiodistal length, buccolingual breadth) were collected using dental calipers and total occlusal and individual cusp areas (mm2) were obtained for all molar crowns in an extant cercopithecoid sample (n=90; 18 genera) using high‐resolution 2D images and ImageJ. These data are a comparative baseline for a fossil cercopithecoid sample including a complete (LP) mandible (M1‐M3), maxillary fragment (M1‐M3) and four isolated maxillary molars (n=3 M1’s, n=1 M2). Discriminant function analysis (DFA) using subfamily (n=2 groups) and tribe (n=5 groups) was used to predict group membership and morphological affinity of the fossil sample. Results DFA results indicate that predictive accuracy is highest when subfamily is used as the grouping variable and ranges from 62.8% (M3) to 87.2% (M2). Mandibular molars more reliably segregate subfamilies (77.1% ‐ 87.2%) than do maxillary molars (64.4% ‐ 70.2%). The associated mandibular molars group are most closely associated with African colobines. All maxillary specimens cluster with cercopithecines and, more specifically, the papioni. Colobines have expanded hypoconids and metacones but reduced metaconids, entoconids and paracones relative to cercopithecoids. Conclusion Molar cusp proportions discriminate colobines from cercopithecines. This study provides additional evidence for the continued presence of both groups in the upper beds at Laetoli and confirms that the complete LP mandible is most closely aligned with the Colobinae. The presence of a large colobine is consistent with palaeoecological reconstructions supporting the significant presence of closed woodland/forested habitats. Significance Despite obligate bipedalism, early hominins retain numerous upper limb and torso adaptations consistent with the use of arboreal substrates. The continued presence of colobines at Laetoli confirms the presence of closed woodland/forested environments and suggests that Au. afarensis at Laetoli may have potentially exploited a variety of habitats including arboreal substrates.