Multiple components of phylogenetic non-stationarity in the evolution of brain size in fossil hominins

Abstract

One outstanding phenotypic character in Homo is its brain evolution. Pagel (Morphology, shape and phylogeny, CRC Press, Boca Raton, 2002) performed a phylogenetic analysis of the evolution of cranial capacity (as a surrogate of brain size) in fossil hominins, finding evidence for gradual evolutionary change with accelerating rate. Since Pagel’s pioneering investigation, the hominin fossil record expanded backward in time, new species were added to our family tree, different phylogenetic hypotheses were advanced, and new phylogenetic comparative methods became available. Therefore, we feel it is timely to repeat and expand upon Pagel’s seminal paper by including such material and applying novel methodologies. We fitted several evolutionary models to the endocranial volume (ECV) for 21 fossil hominins (including Pagel’s original analyses) and estimated phylogenetic signal using different approaches, while accounting for phylogenetic uncertainty. We then applied the phylogenetic signal-representation curve to the data to look for non-stationarity (discontinuities, rate shifts, or presence of different evolutionary patterns in different parts of the phylogeny) in brain size evolution. Our analyses show that, in principle, Pagel’s findings are robust to the addition of new data and phylogenetic uncertainty and confirm both the strong phylogenetic signal in brain size and acceleration of ECV evolutionary rates towards the present. However, nonstationarity was also detected in about 11% of the simulations, with two significant evolutionary discontinuities occurring close to the origin of the H. sapiens lineage (H. sapiens, H. neanderthalensis, H. heidelbergensis and H. antecessor) and along the phyletic line leading to H. floresiensis. This study calls upon further investigation of these important moments in Homo evolution, in order to understand the processes underling each of these shifts in brain size evolutionary regimes.