Science Literature

When first reported, this hominin was given the name Zinjanthropus boisei. He was considered to be a human ancestor and was portrayed as an upright hairy apeman. Later, he was renamed Australopithecus boisei, but then was moved to a separate genus, receiving the name Paranthropus boisei. He still appears in some presentations of human ancestry. What makes him memorable are his magnificent teeth:

"For decades, scientists thought that the large, heavy teeth the primates had were used in cracking open hard foods such as nuts. The common name for Paranthropus was "Nutcracker Man" for this very reason."

This photo of casts of two palates demonstrates the large size of the teeth of Paranthropus boisei (left) known as Nutcracker Man. Much smaller teeth from a human skull are shown on the right. (Credit: University of Arkansas, Melissa Lutz Blouin, source here)

Three years ago, this blog commented on a microwear texture analysis of the molars of seven specimens of P. boisei. The teeth had light wear, suggesting that none of the individuals ate extremely hard or tough foods. In other words, they did not eat nuts! The researchers suggested that the wear pattern was more consistent with modern-day fruit-eating animals.

More recent work has reached a surprising conclusion. P. boisei did not eat fruit but had a diet majoring on grass! This has been discovered by an analysis of carbon-isotope ratios.

"Carbon isotope ratios in tooth enamel can reveal whether ancient animals ate plants that used what is called C3 photosynthesis - trees (and the leaves, nuts and fruits they produce), shrubs, cool-season grasses, herbs and forbs - or plants such as warm-season or tropical grasses and sedges that use what is known as C4 photosynthesis. [. . .] The study found that not only did the Nutcracker Man Paranthropus boisei not eat nuts or other C3 plant products, but dined more heavily on C4 plants like grasses than any other early human, human ancestor or human relative studied to date. Only an extinct species of grass-eating baboon had a diet so dominated by C4 plants. Carbon isotopes showed the 22 individuals had diets averaging 77 percent C4 plants such as grasses, ranging from a low of 61 percent to a high of 91 percent. That's statistically indistinguishable from grass diets of grazing animals that lived at the same time: the ancestors of zebras, pigs and warthogs, and hippos, Cerling says."

How are the enormous teeth to be explained? The researchers have suggested that the high degree of wear observed comes from frequent chewing on foods containing small hard particles - which are, of course, found in C4 grasses. These are known as phytoliths. Although this has not featured in the analysis, this is in tension with the thought that P. boisei was an upright hominin. Grass eaters need to get down to eat - and herbivores generally consume large quantities of food each day. It is significant that the only known primate with a comparable diet was an extinct species of grass-eating baboon.

Furthermore, this discovery "upsets conventional wisdom about early humanity's diet". P. boisei is likely to be a catalyst for fresh thinking about the Australopithecenes - from whom it is considered to have evolved. What interpretation should be placed on the dental features of Australopithecenes?

"Specifically, scientists have believed human ancestors in the genus Australopithecus - which gave rise to now-extinct Paranthropus and to Homo or early humans - also had head and tooth features suggesting they ate hard objects like nuts. Cerling says carbon isotope ratios in australopiths' teeth now should be studied, since the Paranthropus findings bring in to question interpretations that are made without isotopic information on diets."
And from the paper:
"The similarity in dental microwear fabrics among the eastern African australopiths, all of which lack any evidence for hard-object food consumption, is consistent with the notion that their craniodental morphology could reflect "repetitive loading" rather than hard-object consumption."

This research provides another angle on the problems reviewed by Bernard Wood & Terry Harrison (blogged here). It is true that "shared morphology need not mean shared history" but it is also true that shared morphology need not mean shared functionality. Nutcracker Man is a notable example of how morphology was wrongly interpreted for decades and only recently overturned. The problem with "conventional wisdom" is that it has been led by a dogma about the nature of evolutionary transformation from ape-like ancestor to humanity and it is long overdue that this "wisdom" be subjected to some systematic and rational critical scrutiny.

Maybe it will help if this particular fossil hominin gets a new popular name. It was not a nutcracker nor was it a man! It was a specialised (derived) ape that fed primarily on grass and is now extinct. It has nothing to do with human ancestry. Any thoughts on a suitable popular name?

Diet of Paranthropus boisei in the early Pleistocene of East Africa
Thure E. Cerling, Emma Mbua, Francis M. Kirera, Fredrick Kyalo Manthi, Frederick E. Grine, Meave G. Leakey, Matt Sponheimere, and Kevin T. Uno
Proceedings of the National Academy of Sciences, Published online before print May 2, 2011 | doi: 10.1073/pnas.1104627108

Abstract: The East African hominin Paranthropus boisei was characterized by a suite of craniodental features that have been widely interpreted as adaptations to a diet that consisted of hard objects that required powerful peak masticatory loads. These morphological adaptations represent the culmination of an evolutionary trend that began in earlier taxa such as Australopithecus afarensis, and presumably facilitated utilization of open habitats in the Plio-Pleistocene. Here, we use stable isotopes to show that P. boisei had a diet that was dominated by C4 biomass such as grasses or sedges. Its diet included more C4 biomass than any other hominin studied to date, including its congener Paranthropus robustus from South Africa. These results, coupled with recent evidence from dental microwear, may indicate that the remarkable craniodental morphology of this taxon represents an adaptation for processing large quantities of low-quality vegetation rather than hard objects.

See also:

No Nuts for 'Nutcracker Man': Early Human Relative Apparently Chewed Grass Instead, ScienceDaily (May 3, 2011)