Facial Ontogeny and Variation in Australopithecus afarensis: Implications for Hominin Diversity

Investigating morphological variation is essential for understanding hominin taxonomic diversity and phylogeny, particularly in Australopithecus afarensis (c. 3.9–3.0 Ma), whose expanded fossil sample enables comparisons with ancestral, contemporaneous, and descendant taxa. Hominin fossils from Hadar (Ethiopia) and Laetoli (Tanzania) were assigned to Au. afarensis in the 1970s and exhibit substantial variability and sexual dimorphism. However, the extent of this variation and the taxon's relationship to the older Au. anamensis (c. 4.3–3.8 Ma) and other contemporaneous hominins remain debated. Recent fossil discoveries and advancements in imaging and reconstruction techniques allowed us for a holistic examination of facial ontogeny and variation in Au. afarensis. In Chapter 1, I provide a general background to the research questions. In Chapter 2, I investigate facial variation and sexual dimorphism in extant great apes to establish a comparative framework for interpreting fossil hominin variation (Chapters 3 and 4). Many traits previously considered diagnostic of Au. afarensis show substantial intraspecific variability in great apes. Linear metric analyses revealed a high degree of variation and sexual dimorphism in Pongo and Gorilla, largely size-related, while Pan species exhibit lower variation. Notably, P. paniscus shows minimal variation and sexual dimorphism, with females exhibiting greater variability. African apes exhibit shared patterns of variation; deviations from these patterns in hominin assemblages may signal taxonomic heterogeneity. In Chapter 3, I analyze newly discovered fossils and quantify facial variation in Au. afarensis. Traits once considered uniquely diagnostic of the taxon show intra-specific variability and overlap with Au. anamensis and Au. deyiremeda. Au. afarensis exhibits a high coefficient of variation, comparable to that of gorillas and exceeding bonobos and chimpanzees, suggesting it includes either a taxonomically heterogeneous sample, accumulated temporal variation, or a pattern of variability akin to Gorilla. In Chapter 4, I examine facial ontogeny in Au. afarensis, finding that many diagnostic features appear early in development, foreshadowing adult variation. Traditional and geometric morphometric analyses reveal significant intra- and interspecific variation along a shared growth trajectory, resulting in distinct size and shape ranges in Au. afarensis and African apes, with divergent patterns in orangutans. Juveniles, DIK-1-1 and A.L. 333-105, differ in facial traits and overall profile. In shape space, DIK-1-1 aligns close to the Pan juveniles, while A.L. 333-105 groups closer to Pongo, though their differences fall within the expected intraspecific range for Au. afarensis. In conclusion, Au. afarensis exhibits greater adult and juvenile variation than previously recognized, with some morphotypes resembling Au. deyiremeda. While the Au. anamensis cranium, MRD-VP-1/1, shows distinct morphology mainly in the feeding apparatus, some features are shared with Au. afarensis, suggesting an ancestor-descendant relationship but more complex evolutionary transitions among Au. anamensis, Au. afarensis, and contemporaneous hominins. These findings highlight the importance of reexamining fossils using advanced imaging tools, investigating ontogeny and temporal trends, and incorporating new discoveries to refine interpretations of hominin diversity and phylogeny.