Brain size of Homo floresiensis and its evolutionary implications

Abstract

The extremely small endocranial volume (ECV) of LB1, the type specimen of Homo floresiensis, poses a challenge in our understanding of human brain evolution. Some researchers hypothesize dramatic dwarfing of relative brain size from Homo erectus presumably without significant decrease in intellectual function, whereas others expect a lesser degree of brain diminution from a more primitive, small-brained form of hominin currently undocumented in eastern Asia. However, inconsistency in the published ECVs for LB1 (380-430 cc), unclear human intraspecific brain-body size scaling and other uncertainties have hampered elaborative modelling of its brain size reduction. In this study, we accurately determine the ECV of LB1 using high-resolution micro-CT scan. The ECV of LB1 thus measured, 426 cc, is larger than the commonly cited figure in previous studies (400 cc). Coupled with brain-body size correlation in Homo sapiens calculated based on a sample from 20 worldwide modern human populations, we construct new models of the brain size reduction in the evolution of H. floresiensis. The results show a more significant contribution of scaling effect than previously claimed.

‘Captivity bias’ in animal tool use and its implications for the evolution of hominin technology

“…The brain volume of the H. floresiensis holotype, LB1, has been estimated by different methods as between 380 and 426 cm 3 [80][81][82]. This brain size is similar to that of chimpanzees, and in the preserved fossil record only Ardipithecus ramidus and Sahelanthropus tchadensis, both over four million years old, had smaller brains [83,84].…”

“…The long-term stability in stone tool forms suggests stable social groups, but at population densities too low to sustain cumulative changes in the technology. This maintenance of tool traditions, despite a suggested decrease in brain size of a third to a half depending on whether Homo habilis or early Indonesian Homo erectus is considered the founding population [81], was likely facilitated by increased free time granted by decreased vigilance requirements.…”

“…One effect of diminished resources may have also been to increase range sizes, however, leading again to greater rates of contact between different groups on the island. Predation reduction was likely an important influence in H. floresiensis dwarfing (although predators were certainly present on the island [81]), and features in the lower limbs that favour 'low gear' locomotion and stability over speed or agility [104,105] may be indirect evidence for a relaxation of the need for rapid escape. The similarly dwarfed prey species Stegodon florensis insularis is found throughout the Pleistocene Liang Bua sequence, only disappearing when H. floresiensis does, giving one indication perhaps that over-hunting owing to resource pressures did not occur on the island.…”

“…While it remains possible that H. floresiensis dwarfed from an isolated Southeast Asian H. erectus population [81], cladistics analysis suggests an earlier, non-erectus origin for the species [97]. In the light of the data, the questions to be asked are not whether the Liang Bua Pleistocene tools are unusually sophisticated (they are not), but rather (i) why H. floresiensis was a stone tool maker at all, given how physically distinct it is from other tool-making hominins, and (ii) how its lineage sustained stone tool-use traditions on Flores for what appears to be at least a million years [78].…”

Bayesian analysis of a morphological supermatrix sheds light on controversial fossil hominin relationships

“…These fossils possess a unique combination of primitive and derived features. Like the australopiths, they were small-bodied (estimated stature of 106 cm with body mass of 16 -29 kg) and small-brained (380-426 cc) [57,61]. However, other cranial features resemble Homo [57,62].…”