Locomotion

Au. boisei Cranial Remains

Because no post-cranial (below the head) bones were found from Nutcracker Man, its best to look at his entire species when considering locomotion. Many Australopithecus/Paranthropus boisei remains have been found in Eastern Africa, specifically Kenya, Ethiopia, and Tanzania (Wood and Constantino). Unfortunately, most of these finds are teeth, mandibles (lower jaw, which Nutcracker Man didn’t have) and larger cranial remains (Wood and Constantino). Here’s a small sample of some of the known Au. boisei specimens (Wood and Constantino):

• Found in Tanzania at Olduvai Gorge or slightly north at Peninj site
  • Baby molar (OH 3)
  • Unerupted upper molar (OH26)
  • Adult mandible (Peninj 1 or NMT-W64-160)
• Found in Ethiopia at Omo Shungura Formation
  • Mandible (Omo 18.18)
  • Fragmented adult cranium (Omo 323-1976-896 in 1976)
• Found in Kenya at Koobi Fora
  • Mandibular corpus (KNM-ER 403)
  • Half of mandible (KNM-ER 404)
  • An edentulous palate (palate with missing teeth) (KNM-ER 405)
  • Well-preserved adult cranium with missing teeth (KNM-ER 406)

These are easy to identify as Au.  boisei because of the specie’s characteristically large teeth and jaw (Wood and Constantino). Though it is not definitive, there are measurements that can be taken on these larger cranial remains to indicate whether or not the species was likely bipedal. Two things to look for are (Conroy and Pontzer):

1. The position of the foramen magnum (Kimbel and Rak)

• • The foramen magnum is a large hole in the cranium that allows the spinal cord to attach to the brain
  • In bipeds (like modern humans) this is more anteriorly placed (closer to the front of the face) in order to position the spine under the body
  • Its position can be analyzed through observation and comparison to modern human and ape skulls or more mathematically by using the condylar index
    • Dean and Wood used the comparative method and found that Nutcracker Man and KNM-ER 406 had more anteriorly placed foramen magnums compared to the average modern day gorilla (Dean and Wood)
    • This was supported by the measurements we took in lab. The higher the condylar index the more anterior it is.  Although we likely made some mistakes, we measured the Condylar Indexes to be:
      • Chimp: 23.34
      • Nutcracker Man: 57.35
      • ER 406: 71.6
      • Modern human: 83.49

2.  The position of the nuchal crest/ line (Conroy and Pontzer)

• • This is the place at the back of the skull that the neck muscles attach to
  • In apes, this is usually a more obvious raised crest and is positioned  higher on the skull because the have such massive neck muscles which are needed to keep their head upright
  • In humans, it is much less prominent and lower because the position of our bones does the majority of the work in keeping our head over our bodies
  • Our lab results support this pattern:
    • Chimp: 70.37
    • Nutcracker Man: 7.89
    • ER 406: 3.84
    • Modern human: 21.1

Au. boisei Postcranial Remains

Though these results support some level of bipedality, it can be dangerous to rely solely on cranial remains. Bones from the post-crainia provide a much more complete picture (Conroy and Pontzer). Unfortunately, it has been much harder for researchers to determine whether post-cranial bones found in these areas belong to Au. boisei or a different species of hominin, such as Homo habilis, whose fossils have been found at the same dig sites (Wood and Constantino). In order to sort the bones, they assumed that bones that appeared more recent or more like our bones today belonged to the genus Homo. and those that did not belonged to Au. boisei. They also assumed that because Au. boisei had large teeth/jaw it would also have a large body (Wood and Constantino).  These assumptions and their resulting classifications remain controversial and are still hotly debated over. Until recently, only these specimens were either confidently attributed to Au. Boisei or provided clues on the specie’s locomotion:

• Part of an adult femur (thigh bone) from Olduvai Gorge (OH 20) (Wood and Constantino)
  • Attributed to Au. boisei because of its similarities to another early hominin, Au. robustus
  • It does not provide much information on locomotion, but most scientist agree it is from Nutcracker Man’s species
• Poorly preserved partial skeleton from Koobi Fora (KNM-ER 1500) (Conroy and Pontzer)
  • Attributed to Au. boisei because the mandible that was found in context with it is believed to belong to the species
  • Although there’s not complete consensus that the post-cranial remains belonged to the species, it does provide more clues to locomotion
  • The arm bones were “relatively large and leg bones were relatively small in comparison to modern humans” (Conroy and Pontzer 296)
  • This could suggest a climbing ability and partial arboreal lifestyle

Most Recent Discovery

Femur (L) and Radius (R), image from researchgate.com

 

Up until 2013, the remains discussed above painted a rough, approximate picture of Au. boisei’s locomotion. In 2013, several promising finds were made at Bed II of Olduvai Gorge (Dominguez-Rodrigo et al). A partial humerus and radius  (arm bones) and femur and tibia (both leg bones) were found in situ with several teeth belonging to Au. boisei (Dominguez-Rodrigo et al). These finds were better preserved than ER 1500 and were dated to 1.34 million years old (close to the specie’s extinction) (Dominguez-Rodrigo et al). The size, shape, and structure of these bones suggests that Au. boisei was quite “ruggedly built” and displayed some arboreal behavior in addition to terrestrial bipedality (Dominguez-Rodrigo et al). Key differences from modern humans include (Dominguez-Rodrigo et al):

• Femur (OH 80-12)

  

  • Suggested a “variance in the biomechanics “ of their bipedality when compared to humans, based on:
    • A more prominent spiral line (this is an attachment site for the iliacus muscle)
      • Perhaps meaning the iliacus was larger than ours today
    • A more medially (toward the center) oriented gluteal tuberosity
      • Suggesting that maybe their gluteal muscle was shorter or shaped slightly differently than ours
    • A straighter lateral outline of the mid-diaphysis
      • Suggesting that maybe the angle inward toward the feet (vagus) wasn’t as pronounced as ours
• Radius (OH 80-11)

  

  • Suggested that Au. boisei’s arms were larger, more muscular, and more equipt at climbing than ours, based on:
    • Its overall size
      • It is the most robust known radius from the Pli-Pleistocene (11,000-5 million years ago)
    • Proximal (near elbow) articular surface (how it connects to the joint) is more lateral than ours and similar to that seen in climbing chimps/gibbons
    • Large bicipital tuberosity suggests it had large bicep muscles
    • Larger posterior oblique line (where pronator teres muscle attaches) suggests those muscles (responsible for pronation) were stronger than ours

The researchers hypothesized that this type of locomotion (part terrestrial biped and part climber) could have beneficial during Au. boisei’s time (Dominguez-Rodrigo et al). It was a transitional time where Africa’s geography was changing from a forested environment to a grassland environment (Dominguez-Rodrigo et al). Being able to move in both trees and on the ground could have been advantageous and could have been why the species eventually went extinct (because the area eventually became dominated by savanna). Overall, this is an exciting find, but it is still hard to know for certain whether these remains belonged to Au. boisei. What is known, suggests that Nutcracker Man and his species were likely terrestrial bipeds, who walked a bit differently than we do today, and were able to comfortably climb in the trees.

Works Cited

Conroy GC, Pontzer H. 2012. Reconstructing Human Origins a Modern Synthesis. New York, NY: Norton.

Dean MC, Wood BA. 1982. Basicranial Anatomy of Plio-Pleistocene Hominids from East and South Africa. American Journal of Physical Anthropology 59:157–174.

Domínguez-Rodrigo M, Pickering TR, Baquedano E, Mabulla A, Mark DF, Musiba C, Bunn HT, Uribelarrea D, Smith V, Diez-Martin F, Pérez-González A, Sánchez P, Santonja M, Barboni D, Gidna A, Ashley G, Yravedra J, Heaton JL, Arriaza MC. 2013. First Partial Skeleton of a 1.34-Million-Year-Old Paranthropus boisei from Bed II, Olduvai Gorge, Tanzania. PLoS ONE 8.

Kimbel WH, Rak Y. 2010. The Cranial Base of Australopithecus afarensis: New Insights from the Female Skull. Philosophical Transactions of the Royal Society B: Biological Sciences 365:3365–3376.

Lague MR. 2015. Taxonomic identification of Lower Pleistocene Fossil Hominins Based on Distal Humeral Diaphyseal Cross-Sectional Shape. PeerJ 3.

Wood B, Constantino P. 2007. Paranthropus boisei: Fifty years of evidence and analysis. American Journal of Physical Anthropology 134:106–132.