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Last common ancestor of apes was about the size of a gibbon

New research suggests that the last common ancestor of apes -- including great apes and humans -- was much smaller than previously thought, about the size of a gibbon. The findings, published in the journal Nature Communications, are fundamental to understanding the evolution of the human family tree.

Last common ancestor of apes was about the size of a gibbon
Gibbon [Credit: IZW/Linda Tanner]
"Body size directly affects how an animal relates to its environment, and no trait has a wider range of biological implications," said lead author Mark Grabowski, a visiting assistant professor at the Eberhard Karls University of Tübingen in Germany who conducted the work while he was a postdoctoral fellow in the American Museum of Natural History's Division of Anthropology. "However, little is known about the size of the last common ancestor of humans and all living apes. This omission is startling because numerous paleobiological hypotheses depend on body size estimates at and prior to the root of our lineage."

Among living primates, humans are most closely related to apes, which include the lesser apes (gibbons) and the great apes (chimpanzees, gorillas, and orangutans). These "hominoids" emerged and diversified during the Miocene, between about 23 million to 5 million years ago. Because fossils are so scarce, researchers do not know what the last common ancestors of living apes and humans looked like or where they originated.

To get a better idea of body mass evolution within this part of the primate family tree, Grabowski and coauthor William Jungers from Stony Brook University compared body size data from modern primates, including humans, to recently published estimates for fossil hominins and a wide sample of fossil primates including Miocene apes from Africa, Europe, and Asia. They found that the common ancestor of apes was likely small, probably weighing about 12 pounds, which goes against previous suggestions of a chimpanzee-sized, chimpanzee-like ancestor.

Among other things, the finding has implications for a behavior that's essential for large, tree-dwelling primates: it implies that "suspensory locomotion," overhand hanging and swinging, arose for other reasons than the animal simply getting too big to walk on top of branches. The researchers suggest that the ancestor was already somewhat suspensory, and larger body size evolved later, with both adaptations occurring at separate points. The development of suspensory locomotion could have been part of an "arms race" with a growing number of monkey species, the researchers said. Branch swinging allows an animal to get to a prized and otherwise inaccessible food -- fruit on the edges of foliage -- and larger body would let them engage in direct confrontation with monkeys when required.

The new research also reveals that australopiths, a group of early human relatives, were actually on average smaller than their ancestors, and that this smaller size continued until the arrival of Homo erectus.

"There appears to be a decrease in overall body size within our lineage, rather than size simply staying the same or getting bigger with time, which goes against how we generally think about evolution," Grabowski said.

Source: American Museum of Natural History [October 12, 2017]

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1 comment :

  1. I sent this comment to Nature Communications:
    Very interesting work, but we know very little on the genetic relations between fossil and extant hominoids, or on how the extinct hominoids lived and moved. Some possibly relevant thoughts:
    - Chorora-, Nakali- and Samburupithecus lived c.10 Ma in Africa, and are believed (David Begun vs Martin Pickford) to have been relatives of Proconsul, Ugandapithecus or Ekembo, or else of the hominids (sensu relatives of Gorilla-Homo-Pan-australopithecines, vs Asian pongids), and seem to have been quite large (e.g. small female gorilla).
    - The paper assumes that the Homo-Pan LCA looked most like chimps (in that case, probably bonobo-like), but it is well possible that Homo has retained (or re-evolved) a few primitive features, e.g. smaller anterior dentition and canine teeth, thicker molar enamel, low ilia (not elongated), absence of knuckle-walking, even flat short-toed feet with adducted big toes (as still seen in prenatal chimpanzees, Carleton Coon 1954 p.12).
    - All hominoids have more centrally-placed spines (as opposed to a dorsal position in monkeys), facilitating erect postures and locomotions (Adolph Schultz 1969 p.247), so the early hominids might have been more vertical, probably not for walking (Aaron Filler 2007), but for vertical climbing (curved hand-bones) and/or for wading upright in the forest swamps or wetlands (where Mio-Pliocene hominoids typically fossilized), perhaps resembling the locomotion of lowland gorillas wading bipedally for papyrus sedges, or bonobos wading for waterlilies (google e.g. "gorilla bai" or "bonobo wading" or "Pan naledi 2017"). Spending some time in the water and feeding on aquatic herbaceous vegetation (AHV) might also have influenced body size.


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