Vertical head-down posture of the fetus may explain the advanced evolution of the human brain

This paper reviews influence of postural verticality on brain evolution and postulates that the advanced encephalization across anthropoid clade has been promoted by fetal vertical or semi-vertical head-down position due to maternal upright or semi-upright posture. Possibly, head-down position of the fetus has caused physiologic vascular cranial hypertension, which increased the cerebral blood flow, stimulated expansion of the nourishing intracranial vessels, and eventually, along with other selective pressures, led to brain enlargement. Habitual vertical posture is characteristic not only to humans, but also to monkeys and non-human apes, which spend considerable time sitting. This hypothesis enables to explain high cephalopelvic proportions (i.e. the tight fit between the pelvic birth canal and neonatal head) in humans, monkeys, and lesser apes. Low cephalopelvic proportions in the non-human great apes could be accounted for their energetically efficient horizontal nest-sleeping and, consequently, for their larger body mass in respect to monkeys and lesser apes, which sleep upright. It can be further hypothesized that brain size varies across anthropoids according to the degree of exposure of the fetus to postural verticality. The supporting evidence for this theory includes a recent finding that in fossil homininans cerebral blood flow rate increased faster than brain volume. One of the ways to test the hypothesis involves comparative Doppler assessment of fetal cerebral circulation in maternal upright and horizontal posture. The current report opens a perspective for further related research on circadian postural behavior, obstetrics, and fetal postural cranial haemodynamics.

New Middle Miocene Ape (Primates: Hylobatidae) from Ramnagar, India fills major gaps in the hominoid fossil record

The fossil record of ‘lesser apes’ (i.e. hylobatids = gibbons and siamangs) is virtually non-existent before the latest Miocene of East Asia. However, molecular data strongly and consistently suggest that hylobatids should be present by approximately 20 Ma; thus, there are large temporal, geographical, and morphological gaps between early fossil apes in Africa and the earliest fossil hylobatids in China. Here, we describe a new approximately 12.5–13.8 Ma fossil ape from the Lower Siwaliks of Ramnagar, India, that fills in these long-standing gaps with implications for hylobatid origins. This ape represents the first new hominoid species discovered at Ramnagar in nearly a century, the first new Siwalik ape taxon in more than 30 years, and likely extends the hylobatid fossil record by approximately 5 Myr, providing a minimum age for hylobatid dispersal coeval to that of great apes. The presence of crown hylobatid molar features in the new species indicates an adaptive shift to a more frugivorous diet during the Middle Miocene, consistent with other proposed adaptations to frugivory (e.g. uricase gene silencing) during this time period as well.

Molecular Analysis of the Complete Genome of a Simian Foamy Virus Infecting Hylobates pileatus (pileated gibbon) Reveals Ancient Co-Evolution with Lesser Apes

Foamy viruses (FVs) are complex retroviruses present in many mammals, including nonhuman primates, where they are called simian foamy viruses (SFVs). SFVs can zoonotically infect humans, but very few complete SFV genomes are available, hampering the design of diagnostic assays. Gibbons are lesser apes widespread across Southeast Asia that can be infected with SFV, but only two partial SFV sequences are currently available. We used a metagenomics approach with next-generation sequencing of nucleic acid extracted from the cell culture of a blood specimen from a lesser ape, the pileated gibbon (Hylobates pileatus), to obtain the complete SFVhpi_SAM106 genome. We used Bayesian analysis to co-infer phylogenetic relationships and divergence dates. SFVhpi_SAM106 is ancestral to other ape SFVs with a divergence date of ~20.6 million years ago, reflecting ancient co-evolution of the host and SFVhpi_SAM106. Analysis of the complete SFVhpi_SAM106 genome shows that it has the same genetic architecture as other SFVs but has the longest recorded genome (13,885-nt) due to a longer long terminal repeat region (2,071 bp). The complete sequence of the SFVhpi_SAM106 genome fills an important knowledge gap in SFV genetics and will facilitate future studies of FV infection, transmission, and evolutionary history.

10. Primates

‘Primates’ considers how a group of small, rather insignificant, tree-dwelling mammals living 60 mya eventually evolved the highest level of expression of the mammalian characteristic of adaptable behaviour by means of a large brain. It first discusses lemurs, lorises, bush babies, and tarsiers. It then describes the differences between New World and Old World monkeys, part of the Anthropoidea, which started their separate evolutionary journeys around 30 mya. Finally, it considers the rest of the Anthropoidea—the lesser apes (gibbons) and the great apes (orang-utans, chimpanzees, gorillas, and humans). The two most important new adaptations to evolve in humans are bipedalism and a huge brain.

The evolution of primate visual self-recognition: evidence of absence in lesser apes

Mirror self-recognition typically emerges in human children in the second year of life and has been documented in great apes. In contrast to monkeys, humans and great apes can use mirrors to inspect unusual marks on their body that cannot be seen directly. Here we show that lesser apes (family Hylobatidae ) fail to use the mirror to find surreptitiously placed marks on their head, in spite of being strongly motivated to retrieve directly visible marks from the mirror surface itself and from their own limbs. These findings suggest that the capacity for visual self-recognition evolved in a common ancestor of all great apes after the split from the line that led to modern lesser apes approximately 18 Myr ago. They also highlight the potential of a comparative approach for identifying the neurological and genetic underpinnings of self-recognition and other higher cognitive faculties.