Cerebral complexity preceded enlarged brain size and reduced olfactory bulbs in Old World monkeys

Abstract

Analysis of the only complete early cercopithecoid (Old World monkey) endocast currently known, that of 15-million-year (Myr)-old Victoriapithecus, reveals an unexpectedly small endocranial volume (ECV) relative to body size and a large olfactory bulb volume relative to ECV, similar to extant lemurs and Oligocene anthropoids. However, the Victoriapithecus brain has principal and arcuate sulci of the frontal lobe not seen in the stem catarrhine Aegyptopithecus, as well as a distinctive cercopithecoid pattern of gyrification, indicating that cerebral complexity preceded encephalization in cercopithecoids. Since larger ECVs, expanded frontal lobes, and reduced olfactory bulbs are already present in the 17- to 18-Myr-old ape Proconsul these features evolved independently in hominoids (apes) and cercopithecoids and much earlier in the former. Moreover, the order of encephalization and brain reorganization was apparently different in hominoids and cercopithecoids, showing that brain size and cerebral organization evolve independently.

Specialized Representations of Value in the Orbital and Ventrolateral Prefrontal Cortex: Desirability versus Availability of Outcomes

“…One possibility is that—since their last common ancestor more that 30 million years ago—the VLPFC has developed divergent functions in macaques and marmosets, partly as a consequence of independent and differential expansion and partly as a consequence of corresponding changes in anatomical connections. This possibility is bolstered by the knowledge that an expansion of prefrontal cortex occurred independently in macaques more recently that 15 million years ago (Gonzales et al, 2015). A related possibility is that the foraging niche of the two species has driven these areas to subserve divergent functions.…”

Internal carotid arterial canal size and scaling in Euarchonta: Re-assessing implications for arterial patency and phylogenetic relationships in early fossil primates

“…For non-primate euarchontans, we reference Stephan et al (1981), Pirlot and Kamiya (1982) and Tacutu et al (2013). For fossil taxa, we cite body mass and endocranial volumes reported in Radinsky (1979), Martin (1990), Godfrey and Jungers (2002), Bush et al (2004), Simons et al (2007), Silcox et al (2009aSilcox et al ( ,b, 2010, Catlett et al (2010), Kay et al (2012), Kirk et al (2014), Gonzales et al (2015), Ramdarshan and Orliac (2015), and Harrington et al (in press). For some fossil taxa, we employed the species' means of body mass predictions based on calcaneal dimensions reported in .…”

Skull secrets of an ancient ape

“…Size analysis of the skull and adult teeth indicate that if the infant had reached adulthood, it would have weighed about 11.3 kilograms at maturity. Relative to this estimated adult body weight, the brain cavity of N. alesi, 101 millilitres in volume, is substantially larger than the 35-ml brain-cavity volume of Old World monkeys from the same period 6 (which form a branch on the evolutionary tree neighbouring the one containing humans and apes). These monkeys had brains closer in volume to those of modern lemurs of the same body mass.…”

“…Some of these patterns are characteristic of specific groups of related species. For example, Old World monkeys and great apes share an arrangement of gyri and sulci on the frontal lobe of the brain that is not found on the otherwise-complex frontal lobes of gibbons 6 . Such studies might untangle the complex relationship between nyanzapithecines, Oreopithecus and modern apes, as well as offer an opportunity to learn more about the adaptations that influenced the evolutionary histories of these fascinating animals.…”