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Analysis of Paranthropus anatomy and diet finds evolution follows least resistant path

Evolution follows the path of least resistance, which can result in suboptimal physical traits that don't ideally match the functional need, according to a new analysis by University of Arkansas anthropologist Peter Ungar.

Analysis of Paranthropus anatomy and diet finds evolution follows least resistant path
Paranthropus boisei [Credit: © Roman Yevseyev]
Ungar, distinguished professor, chair of Department of Anthropology and director of the environmental dynamics doctoral program at the University of Arkansas, detailed his findings in Science this month with coauthor Leslea Hlusko, associate professor at the University of California Berkeley.

"Paleontologists typically reconstruct past behavior by assuming that function follows form," Ungar said. "We need to look at things in a different way and consider the number of genetic steps it takes to get from one anatomy to another. There can be more than one function for a given form and different forms can serve the same function."

Ungar analyzed teeth from two human ancestors with similar dental and jaw structure and found that dental anatomy historically associated with a hard food diet was used to eat mainly plant-based diets. And while the two species had similar anatomies and teeth, their diets were different.

Analysis of Paranthropus anatomy and diet finds evolution follows least resistant path
Skull of Paranthropus boisei at the Nairobi National Museum 
[Credit: Bjørn Christian Tørrissen]
The Paranthropus boisei from eastern Africa and Paranthropus robustus from southern Africa are groups of species from 4.2 to 1.3 million years ago that share similar head and tooth anatomy characterized by large, flat teeth with thick enamel and jaws and faces that indicate strong chewing muscles.

Those characteristics have often been interpreted as adaptive for crushing hard foods. Ungar and his colleagues analyzed the microwear on the teeth and used previously published data on carbon isotopes to conclude that P. robustus had a diet of plant foods and only occasional consumption of hard objects like nuts or roots, while the microwear and isotope values on the teeth of P. boisei indicate it consumed softer, tougher and possibly more abrasive foods like grasses.

"So despite their similar masticatory morphology, chemical and wear traces of the foods eaten suggest that these two species differed markedly in their diets," Ungar said.

Analysis of Paranthropus anatomy and diet finds evolution follows least resistant path
Skull of Paranthropus robustus at the Transvaal Museum 
[Credit: José Braga]
And neither diet matched previous inferences that the large muscles and facial structure indicated a diet of hard, crunchy food.

More crested molars like those in gorillas would be more efficient in tearing fibrous plant material, but flatter molars with thicker enamel, like those of modern humans, advanced through the genetic record.

Ungar said the form of the tooth and jaw anatomy in this case is ideal for the diets these hominins ate, but it took fewer genetic changes to get there. Thicker enamel with a simpler tooth architecture was more adaptable for a variety of challenging diets through time, so it carried on as the human species developed.

"You will reach suboptimal, but functional, solutions if they require fewer genetic mutations," Ungar said. "We need to look at things in a new way and take into account the evolutionary path of least resistance."

Source: University of Arkansas, Fayetteville [July 02, 2016]

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1 comment :

  1. "Ungar analyzed teeth from two human ancestors with similar dental and jaw structure"? 2 human ancestors? It's very interesting to see confirmed that the robust australopiths boisei & robustus ate different foods, probably mostly plant foods, but they were no "human ancestors" (certainly not both). Many paleo-anthropologists still believe that australopiths are ancestors of humans but not of the other African apes, but this is statistically impossible: 1 extant species (H.sapiens) with 1000s of fossil "ancestors", and 4 or 5 extant species (genera Pan & Gorilla) virtually without fossil relatives?? This is pre-Darwinian anthropocentrism. Orangutans do have several fossil relatives (Lufengpithecus, Sivapithecus, Gigantopithecus etc.), so here is no reason to believe that the extant African ape species had no fossil relatives, and that H.sapiens had thousands of fossil "ancestors". The humanlike featurs in australopiths are not derived-human, but are primitive-hominid, lost in the extant ape, e.g. Pan & Gorilla ancestors had more humanlike feet, and were possibly more bipedal: "Only as it approaches its birth size does its foot acquire the apperance of a hand" (C.Coon). The were not bipedal to run over the African plains as still often believed popularly, but more likely to frequently wade bipedally in the wetlands were their fossils were found (K.Reed), not unlike bonobos wading in forest swamps, google "bonobo wading" illustrations. Australopithecus (not "Paranthropus", which is paraphyletic) boisei & robustus probably mostle fed on different sorts of aquatic herbaceous vegetation (AHV), e.g. sedges, parts of papyrus, frogbit, parts of waterlilies etc., like lowland gorillas & bonobos still do in forest wetlands & swamps.


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