Early Hominoids: Orthograde Aquarboreals in Flooded Forests?

The great (orangutans, gorillas and chimpanzees) and lesser apes (siamangs and gibbons) are significantly different to monkeys, yet the evolution of the apes is rarely discussed in detail, especially from a human evolutionary perspective. Assuming that the early primates were arboreal and that human ancestors were semi-aquatic, human predecessors in the intermediary phase must have been aquarboreal, i.e., spent significant time in both trees (Latin arbor) and water (Latin aqua). Here we describe a number of independent indications that early apes – possibly as early as 20 Ma (million years ago) – were vertical aquarboreal frugi-omnivores in swamp forests.

Apes differ from monkeys in having a below-branch locomotion, with larger and broader bodies and thoraxes, very long arms that can easily be extended above the head, and tail loss. Whereas most mammals and monkeys predominantly move pronogradely (with horizontal spine and trunk), the remarkably humanlike lumbar vertebra of Morotopithecus suggests that by about 20 Ma the early apes were already orthograde (with a generally vertical spine). According to the palaeo-environmental data, the fossils of Mio-Pliocene apes typically lay in coastal and swamp forest sediments around the Tethys Sea (the ancient Mediterranean Sea). The Miocene (23.0 to 5.3 Ma) and the Pliocene (5.3 to 2.6 Ma) epochs were generally hotter and wetter than the Pleistocene Ice Ages (2.6 to 0.01 Ma). Recently, the highest population densities of orangutans as well as gorillas have been discovered in extremely hot and wet swamp forests.

Since all great apes can make and use tools, and most fossil great apes had thick enamel, the ancestral great ape diet in flooded forests might have included durophagy of hard-shelled foods (e.g., palm nuts or molluscs). Locomotor requirements for flooded forest dwelling could arguably have included a bigger body with vertical climbing abilities, including with arms overhead and arm-hanging. Lowland gorillas employ an orthograde posture and locomotion when they climb, wade through shallow swamps, and sit and feed in shallow water.

Keywords: Aquarboreal, Miocene apes, hominoid evolution, orthogrady, durophagy, Griphopithecus, Oreopithecus, Dryopithecus, Morotopithecus, Heliopithecus, Saadanius