7.IV: Hominins

hominin australopithecus human evolution bipedalism

A reproduction of a hominin couple from about four million years ago.

At the Miocene-Pliocene boundary, the African landscape was making a gradual transition from forest to grasslands.  The Ethiopian Highlands cast a rain shadow over the Great Rift Valley. 1  Hominins flourished there in mixed ecosystems of woods, savannas, and the shores of lakes and rivers.  In this setting, they acquired two key anatomical features that distinguish humans from our cousin apes:  bipedalism and the loss of fangs.

A.  Walking Tall

B.  No Fangs, We’re Human

C.  Grandma Lucy and Other Hominins of Note

D.  Citations

A.  Walking Tall

Obligate erect bipedalism, walking upright on two feet at all times, was the landmark evolutionary trait separating hominins from other apes. These four-million year old footprints in Tanzania are the most direct evidence of bipedalism in pre-Homo hominins.

The human form of bipedalism, walking on two legs, is one of the most striking characteristics to distinguish us from other apes, and one of the first to evolve.  It was the precursor of evolutionary revolutions to come.  The full description of our form of locomotion is obligate erect bipedalism.  Other apes can and do sometimes walk on two legs, but it is not optimal for them.  Think of a chimp’s awkward waddle or a gibbon’s much greater ease in the trees.  Humans must walk bipedally because it is the most efficient way for us to get around – that’s the “obligate” part.  We walk on two legs differently from other apes.  When chimps and gorillas walk on their hind feet, they are usually stooped with bent hips and knees.  Humans maintain an erect posture.  Thus, the evolution of our gait involved these three qualities:  walking bipedally, keeping an upright spine, and doing it more gracefully than other forms of locomotion.  As with most complex systems, the components came together gradually over millions of years.

The very capacity for bipedalism is probably almost as old as apes themselves, and has its roots in the trees.  All living apes retain it.  Arboreal apes use their arms to reach for branches overhead, but at the same time they use their feet to stand on boughs below. 2 The biomechanics of walking also bears some resemblance to the use of legs in climbing trees vertically. 3 As open spaces between the woods expanded, hominins took bipedalism and ran with it. 1

Obligate erect bipedalism required modifications to almost the entire body, from the skull to the soles of the feet.  For erect posture, the hole where the spine enters the skull migrated from the back of the skull (as in chimpanzees) to the bottom of the skull.  The lower spine arched forward, and the knees bent inward, to keep the body’s support axis aligned with its center of gravity.  The pelvis became much more broad and squat.  The hips and knees became fully extendable, and the ankles more rigid.  The feet transformed into a uniquely human structure.  If you look closely at any other ape, you will notice that its feet look like hands.  They have flat palm-like soles and an opposable thumb-like big toe.  Those organs are good for grabbing onto branches or vines for support in the trees.  Human feet are made for walking and running on the ground.  The forward-pointing big toe provides great strength for balancing and stepping off, while the arches put a spring in our step.

The details of exactly when and why these traits developed are still being refined. 4 It is safe to say that they roughly paralleled the opening of the African plains.  Grasslands started to appear in eastern Africa ten million years ago, and they impinged on woodlands exponentially until they dominated the terrain by 2 MYA. 5 The earliest evidence of erect bipedalism dates to the middle of this ecological transformation, 7 MYA. 6 Our ancestors of 3 – 7 MYA were hybrid walkers and tree climbers with short legs. 7 By the time the trees thinned out significantly, hominins had a long-legged striding gait well suited to open spaces.

The advantages of obligate erect bipedalism seem clear to us today.  Walking could have permitted hominins to increase their foraging range, carry food around, and / or reach fruit high in bushes that they couldn’t climb.  Upright posture is very energy efficient, as it keeps all of our weight supported on strong legs and feet with minimal muscular effort for balance.  This anatomy enabled the growth of a heavier brain in the Chapter 6 time scale.  Erect posture also liberated the arms to specialize in different functions divorced from locomotion.

In evolution, there is no such thing as a free lunch.  Bipedalism bears costs as well as benefits.  Bipeds are slower than obligate quadrupeds.  We are vulnerable to injuries of the knees, feet, and lower back.  Circulating blood from the legs back to the heart is also a difficult physiological challenge.  On balance, though, apparently these drawbacks were outweighed by the advantages, as bipedal hominins not only survived but thrived.

B.  No Fangs, We’re Human

Teeth are the most thoroughly studied body part of all fossil animals, in large part because they are the hardest, best preserved body part.  They have evolved into an endless variety of patterns that serve almost like fingerprints to identify species.  Teeth reveal valuable insights about diet as well as behavior.

Besides erect bipedalism, the other uniquely human feature that shows up early in the hominin record is non-beastly teeth.  From gibbons to gorillas, all other living apes sport large canines (fangs).  Only humans have non-projecting canines, with even rows of teeth.  There is some debate about whether human ancestors once had large canines, which were then reduced, or if our line of descent never had fangs at all. 8 It seems highly likely that fangs were in our ancestral past at some point, as they are present in all other living apes and Old World monkeys.  One way or the other, hominins came to be the unique ape with small canines.

Non-human apes have a special chewing feature called honing.  Every time such an ape bites down, the back edge of an upper canine scrapes against the front edge of a lower pre-molar.  This friction whets the canine and keeps it sharp like a blade.  There is a special gap in the lower teeth to accommodate the canine when the mouth is closed (and likewise for the lower canines, all in reverse).

Ape canines are much larger than their diets alone would dictate.  Carnivores use their fangs to take down large prey.  Up to the time of the human / chimpanzee split, though, apes were almost strictly herbivores.  Apes use their canines primarily for fighting.  Sometimes they defend against predators.  Males often use or bare their teeth against each other in contests for females.  In such species, natural selection favors a more vicious display, and the canine teeth have evolved to greater size in males than in females.  This is an example of sexual dimorphism.

From early hominin times, canines have been smaller and less dimorphic, and have lacked the honing mechanism. 9 Instead, our canines are utilized like incisors, and they get worn down at the tip when we chew.  The gap in the opposing jaw mostly disappeared by 3 MYA. 10 By then, canines had achieved their present shape and size – not only small, but the same size for the two sexes.

This dental makeover was pretty significant, and it is striking that it happened only in our lineage.  Like bipedalism, the exact reason for our ancestors’ defanging is still unknown.  There are two classes of feasible explanations:  Either hominins found value in using small canines to chew their food, or they no longer needed them for defense and competition.  These hypotheses are not mutually exclusive, and in fact no single-cause model seems sufficient to account for the change. 11

To support the dietary hypothesis, we can look to other changes in dentition and diet.  Concurrent evolutionary trends included thicker enamel and a more vertical orientation of chewing muscles. 12 This suggests that hominins were eating hard foods such as seeds, grains, and nuts, which are crushed by flat molars. 13 They also ate an increasing amount of fleshy grass-root vegetables after 3.5 MYA. 14 Some species developed smaller mouths and larger molars 15 that would have crowded out large canines; many people today still need wisdom teeth removed.  These demands on teeth could have pressured the more efficient use of canines as reduced chewing instruments.

Grasses and roots, like the fleshy corm of this Crocosmia plant native to eastern Africa, helped hominins survive outside of lush forests. They also may have promoted the reduction of canines to serve a greater role in biting and chewing.

Roots such as bulbs and tubers (think onions and potatoes) might have been a real game-changer. 16 They evolved to preserve water, a function necessary in woodlands but not in tropical forests.  Roots are conveniently buried underground, safe from competitive foragers.  Apes that could use short canines to puncture and chew would have a real advantage. 2 Tubers called corms are abundant in eastern Africa and might have been a staple of the hominin diet.

.

The reduction of canines is also most likely associated with their function as weapons and threats.  Among living primate species, those with the least dimorphic canines are those with the least male-male competition. 17 If this was a factor for early hominin canine reduction, then it could signal major changes in sexuality and social structure, which will be further expanded in Section V and Chapter 6.  Hominins retained body-size dimorphism, indicating that males were still competitive on the basis of size and strength.  It is even possible that canines became less useful against competitors and predators as hominins stood upright and could throw rocks! 18

The way you walk, the way you smile:  these are the two most primitive qualities that make humans unlike any other species on Earth.

C.  Grandma Lucy and Other Hominins of Note

Most of our evidence about extinct hominins comes from fossils.  Unfortunately, our ape ancestors were denizens of the jungle, and tropical forests are notoriously unfavorable for fossil preservation.  When an animal dies in a forest, it falls on solid ground and is immediately vulnerable to weathering and all sorts of decomposing agents, from bacteria to scavengers.  For this reason, there are major gaps in the fossil ape record, including the anthropological Holy Grail, the point of human / chimp speciation.

Nevertheless, paleontologists have found remains of several hominin species, with enough diversity to categorize them into more than one genus.  Some of these fossil finds are unique or especially complete, so they have become the standard bearers of this phase of evolutionary history.  Scientists currently recognize at least four hominin genera that preceded Homo and are fair candidates for human ancestry.

The oldest known potential hominin is Sahelanthropus (Chad, 6 – 7 MYA).  This genus is known from some teeth / jaw bones and one skull, from a presumably male specimen named Toumai (“Hope of Life”).  Without a body skeleton, the evidence for bipedalism is indirect but reasonable:  the spine apparently entered the base of the skull, 19 a configuration that is only found in hominins.  Toumai also had non-honing canines, which were intermediate in size between humans’ and chimpanzees’. 20 This evidence is still being debated, but, if corroborated, it projects the hominin story millions of years and thousands of kilometers from other known fossils.

The densest region for hominin fossils is the Rift Valley, which cuts through Ethiopia, Kenya, and Tanzania in eastern Africa.  The Orrorin genus was found here (Kenya, 6MYA).  In some ways – its thigh bones, small molars, and large body size – Orrorin was more like modern humans than some younger genera.  It still bore many characteristics of fossil apes. Orrorin was a tree dweller with curved fingers and ape-like, medium-sized canines.  Its thighs indicate that it was probably capable of bipedalism.  The honing complex is “reduced” compared to older species, but is actually similar to a female chimpanzee’s. 21

Ardipithecus roamed the woods of Ethiopia 4 – 6 MYA.  Hundreds of individuals have been found.  The most famous and complete one is a female named Ardi.  Ardi was another hybrid biped / tree climber, with unique forms of hands and feet.  With features found in neither humans nor chimpanzees, she presents evidence that the human / chimp common ancestor was also not exactly like either one of us. 22  Ardipithecus exhibited a low degree of sexual dimorphism, suggesting a reduced level of male competition. 23

The most recent pre-human genus, Australopithecus, is the best known.  This was a widespread and well-preserved genus, known to range through central, eastern, and southern Africa 1 – 4 MYA.  Its premier ambassador is Lucy, a four-foot tall, 3 MY old female Australopithecus afarensis from Ethiopia.  Lucy was a media sensation, the oldest and most complete hominin fossil found to date in 1974.  She was irrefutably bipedal.

One of the most haunting fossils is a trail of footprints in Laetoli, Tanzania.  Following a nearby volcanic eruption and a rain, an impressionable layer of ashen mud covered the ground.  As on a wet sidewalk, footprints record that day’s traversal of several animals, including cats, dogs, hoofed animals – and five hominins.  Their 4 MY old feet, presumably australopithecine, were surprisingly similar to ours, with arches and forward-pointing big toes.

A mysterious 3.5M year old skull from Kenya raises intriguing questions.  It might represent a fifth genus, Kenyanthropus, with a human-like flat face instead of a snout.  It is hard to tell, because the fossil is distorted. 24 There is also controversial evidence that Kenyanthropus was capable of chipping stones into tools. 25 If it is not an Australopithecus, then this was the first time that two hominins were known to co-exist.  A hominin radiation did begin around this time.  Australopithecus species after 4 MYA were adapted to more diverse habitats and diets than earlier woodland apes. 26

Discovery and analysis of hominin fossils is a very active field at this time.  Scientists can’t resist charting the known specimens into a phylogenetic tree leading to humans, though this is impossible at the species level. 3 Australopithecus is the only genus known to overlap with Homo, and it has long been regarded as our parent genus. 27 Now the other African genera, all discovered after 1990, are complicating even that conservative consensus. 28

Back to Section 7.III:  Fossil Apes

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Continue to Section 7.V:  Ape Nature, Human Nature

D.  Citations

  1. Lewin and Foley, Principles of Human Evolution, 2nd ed, Blackwell Science Ltd., 2004.  Also Toth and Schick in The Human Past (ed. Christopher Scarre), Thames & Hudson, 2005, pp. 46-83.  Both as cited by Christopher Seddon, Humans: from the beginning, Glanville Publications, 2014, ebook edition p. 28.
  2. Thorpe, Holder, and Crompton, “Origin of Human Bipedalism as an Adaptation for Locomotion on Flexible Branches”, Science vol. 316, issue 5829 (6/01/2007), pp. 1328-1331.  doi 10.1126/science.1140799. Not accessible free, but abstract is at http://science.sciencemag.org/content/316/5829/1328 (accessed 3/04/2017) and an excellent plain English summary is at https://www.sciencedaily.com/releases/2007/05/070531150326.htm (accessed and saved 3/04/2017).
  3. Russell H. Tuttle, Apes and Human Evolution (Harvard University Press, 2014), Kindle version, location 4064.
  4. Carsten Niemitz, “The evolution of the upright posture and gait – a review and a new synthesis,” Naturwissenschaften 2010 Mar; 97(3): 241-263, https://link.springer.com/article/10.1007%2Fs00114-009-0637-3 (accessed and saved 3/11/2017).
  5. Kevin Uno et al, “Neogene biomarker record of vegetation change in eastern Africa”, PNAS vol. 13 no. 23 (6/06/2016), http://www.pnas.org/content/113/23/6355.abstract (accessed and saved 3/05/2017).
  6. Christoph Zollikofer et al, “Virtual cranial reconstruction of Sahelanthropus tchadensis”, Nature 434, 755-579 (7 April 2005), abstract at http://www.nature.com/nature/journal/v434/n7034/full/nature03397.html (accessed and saved 3/12/2017).
  7. John Fleagle, Primate Adaptation and Evolution, Academic Press (1999), pp. 518 and 524.
  8. Warren Kinzey, “Evolution of the human canine tooth”, American Anthropologist, New Series, vol. 73 no. 3 (Jun., 1971), pp 680-694, stable URL http://www.jstor.org/stable/671762 (accessed and saved 3/07/2017).  Kinzey refutes the majority interpretation that hominins descended from hominids with large canines.  His argument relies on the assumption that Dryopithecus, a fossil ape around the time of gorilla speciation, did not inherit its large honing canines from an ancestor in common with hominins.  Analysis by Begun suggests that Dryopithecus was a fossil gorilla (https://www.newscientist.com/article/dn28274-ape-fossils-put-the-origin-of-humanity-at-10-million-years-ago/, accessed and saved 3/12/2017). 
  9. John Hawks, “Dentition and diet in early hominids”, 2/06/2005, http://johnhawks.net/weblog/fossils/afarensis/early_hominid_dental_change.html (accessed and saved 3/12/2017).
  10. White et al, “Jaws and teeth of Australopithecus afarensis from Maka, Middle Awash, Ethiopia” (1/04/2000), Am J Phys Anthropol 111:45-68, http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1096-8644(200001)111:1%3C45::AID-AJPA4%3E3.0.CO;2-I/abstract (accessed 3/18/2017).
  11. Leonard Greenfield, “Unicausal theories of human canine evolution: Are they sufficient?” Zeitschrift für Morphologie und Anthropologie Bd. 78, H. 2 (Oktober 1990), pp. 155-168. Stable URL  https://www.jstor.org/stable/25757280 (accessed and saved 3/19/2017).
  12. Clark Spencer Larsen, Our Origins: Discovering Physical Anthropology, Norton, 2008, pp. 276 – 277.
  13. John Fleagle, Primate Adaptation and Evolution, Academic Press (1999), pp. 515 and 518.
  14. Julia Lee-Thorp et al, “Isotopic evidence for an early shift to C4 resources by Pliocene hominins in Chad”, Proceedings of the National Academy of Sciences. 2012; 109: 20369-20372. http://www.pnas.org/content/109/50/20369 (accessed and saved 3/19/2017).
  15. Ward, Leakey, and Walker, “Morphology of Australopithecus anamensis from Kanapoi and Allia Bay, Kenya”, J. Hum. Evol. 2001 Oct;41(4):255-368, https://www.ncbi.nlm.nih.gov/pubmed/11599925 (abstract accessed 3/19/2017), as cited by John Hawks, “Dentition and diet in early hominids”, 2/06/2005, http://johnhawks.net/weblog/fossils/afarensis/early_hominid_dental_change.html .
  16. Richard Wrangham and Dale Peterson, Demonic Males: Apes and the Origins of Human Violence, Mariner Books (1996), Ch. 3.
  17. Richard Kay et al, “Sexual selection and Canine Dimorphism in New World Monkeys”, Am J Phys Anthropol 1988 Nov; 77(3):385-97, https://www.ncbi.nlm.nih.gov/pubmed/3228171 (accessed and saved 3/19/2017).  Followed up by Plavcan and van Schaik, “Intrasexual competition and canine dimorphism in anthropoid primates”, Am J Phys Anthropol 1992 Apr;87(4):461-77, https://www.ncbi.nlm.nih.gov/pubmed/1580353 (abstract accessed 3/19/2017).
  18. John Hawks, “Dentition and diet in early hominids”, 2/06/2005, http://johnhawks.net/weblog/fossils/afarensis/early_hominid_dental_change.html (accessed and saved 3/12/2017).
  19. Zollikofer et al, “Virtual cranial reconstruction of Sahelanthropus tchadensis”, Nature 434: 755 – 759, https://www.fas.harvard.edu/~skeleton/pdfs/2005b.pdf (accessed and saved 3/25/2017).
  20. Brunet et al, “A new hominid from the Upper Miocene of Chad, Central Africa”, Nature 418: 145-151, http://www.nature.com/nature/journal/v418/n6894/pdf/nature00879.pdf (accessed and saved 3/25/2017).
  21. Senut et al, “First hominid from the Miocene (Lukeino Formation, Kenya”, Earth and Planetary Sciences 332 (2001) 137-144, https://www.semanticscholar.org/paper/First-hominid-from-the-Miocene-Lukeino-Formation-Senuta-Pickfordb/4b3b2e7a2a6b4114e5fac5920c42fefceb0c910a (accessed and saved 3/23/2017).
  22. White et al, “Neither chimpanzee nor human, Ardipithecus reveals the surprising ancestry of both” (2014), PNAS vol. 112 no. 16 4877-4884, http://www.pnas.org/content/112/16/4877.full (accessed and saved 3/25/2017).
  23. Gen Suwa et al, “Paleobiological Implications of the Ardipithecus ramidus Dentition”, Science 02 Oct 2009: Vol. 326, issue 5949, pp. 69-99, http://science.sciencemag.org/content/326/5949/69 (abstract accessed 3/26/2017).
  24. Tim White, “Early hominids – diversity or distortion?” Science 2003; 299:1994. http://science.sciencemag.org/content/299/5615/1994 (abstract accessed 3/26/2017).
  25. Harmand et al, “3.3-million-year-old stone tools from Lomekwi 3, West Turkana, Kenya”, Nature vol. 521 (5/21/2015), pp. 310 – 326, doi 10.1038/nature14464.  Free non-downloadable copy available at http://www.nature.com/articles/nature14464.epdf?referrer_access_token=D4sS2faF7tEHQR2Gdu9UVdRgN0jAjWel9jnR3ZoTv0PITTQVev_33gWVYQXx52cEkpH9nUnt-rKX1_zNXNohJKVByp0zv6_xN5X0rA0Wih9XilWghrY-ddqOMb2ti9abVYRieofbNMfZKVA0XPhasgxF8G5XUqic-RRmE-cSwYSGsKuTSDncHAZt1r8AFbsLUdKs0ia8pBpn_-_NCZXOIg%3D%3D (accessed 3/26/2017).
  26. Debbie Guatelli-Steinberg, What Teeth Reveal about Human Evolution, Cambridge University Press, 2016, Kindle ebook edition p. 47.
  27. Some textbooks present this as fact, including Larsen, Our Origins at 288 – 293, and Chris Scarre (ed), The Human Past, Thames & Hudson (2005), p. 60.
  28. Ann Gibbons, The First Human: The Race to Discover Our Earliest Ancestors, Anchor Books (2007) is a good book about today’s cutting-edge practice of paleoanthropology, written for a general audience.
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