7.V: Our Ape Nature

ape behavior chimp gorilla orangutan evolution human soft tissue anatomy similarities
Much of what we consider human nature derives from being a great ape. The human-like spirit is evident in the face of this orangutan, and she is only our third-closest cousin. 1

After fossils, our secondary sources of evidence about hominin ancestors are our closest living cousins, the great apes.  We can directly observe apes’ soft-tissue anatomy (flesh, organs, cell biology) as well as their behavior.  It can be informative to assess how humans are similar or different from chimpanzees and the other great apes. It is a reasonable conclusion that we inherited or adopted our shared characteristics from a common ancestor ten million years ago. The qualities that distinguish us from chimpanzees were most likely derived after our speciation.  It is difficult to date these changes. As a practicality, this section will focus on the elements of human nature that we share with other apes.  Chapter 6 will discuss adaptations that occurred only within the hominin / human line.

A.  Changes by Degree

B.  Brain and Intelligence

C.  Tool Use

D.  Linguistic Potential

E.  Inner Angels: Morality

F.  The Social Male

G.  Inner Demons: Rape and Murder

H.  Citations

A.  Changes by Degree

Some evolutionary developments of the last ten million years have been continuing trends and matters of degree.  Apes continued to evolve larger bodies and brains.  They lived in increasingly large and complex social groups.  They lived longer and had fewer children with prolonged childhoods.  Chimpanzees eat a modest amount of meat, suggesting some carnivory many millions of years ago. 

Human and chimpanzee DNA are 96% identical. 21 Of the 4% difference, most likely each species has deviated only about 2% from our common ancestor.  Genetically, then, hominins were already 98% human at the time of speciation almost ten million years ago!  Since the intervening genetic change has been so numerically modest, we must reach one of two conclusions.  Either some of our most recent mutations were extraordinary breakthroughs, and / or our most extraordinary characteristics are not entirely genetic.  Human social structure could have brought out capabilities that were already latent in human / chimp ancestors.         

B.  Brain and Intelligence

A type of brain cell called the spindle neuron is found in all great apes, but not lesser apes or monkeys. 2 This indicates that the cell evolved approximately 15 – 20 million years ago, between the speciation of gibbons and orangutans.  Spindle neurons are large, allowing them to convey signals quickly.  They may have originally been an adaptation for physically large brains, as they evolved independently in a few other large animals such as elephants and whales.  Then perhaps their ability to communicate rapidly with other parts of the brain got “co-opted” for social intelligence. 3

Spindle neuron research is relatively new, and the field is pregnant with exciting hypotheses.  The cause for the clamor is that these cells are found only among large-brained mammals with high levels of self-consciousness, empathy, and social coordination.  Furthermore, the areas of the brain that are rich in spindle neurons are specifically active in emotional maturity, social skills, and higher thinking.  For example, some spindle neurons connect an area of the brain that determines when a mistake is being made to another part involved in memory and planning.  This integration of past, present, and future could contribute greatly to learning and volition. 4 The cerebellum, the part of the brain responsible for coordinating voluntary movement, also experienced a growth explosion in great apes. 5 The cerebellum organizes refined motor skills such as swinging through trees, foraging across large ranges, using tools, throwing, and speaking.   

With such advanced brain anatomy, apes have demonstrated a capacity for several higher forms of learning and “civilized” behavior beyond other animals.  Chimps and bonobos are more “patient” than other animals, more willing to forego a smaller immediate reward for a larger future reward. 6 Other examples are detailed below.

C.  Tool Use

Mechanical intelligence is an understanding of how inanimate objects work.  It includes concepts such as object permanence, the property that an object will persist in time even when the observer does not see it.  Only great apes have demonstrated a true understanding of mechanical cause-and-effect.  We believe that apes alone can form mental representations of physical objects. 7

Only after an animal understands objects can it manipulate them into tools for specific uses.  Common chimpanzees have a proven track record at using and modifying natural objects for personalized purposes.  Certain populations of chimps routinely use sticks to go “fishing” for ants or termites, and they even streamline the sticks by removing side branches.  They use rocks to crack nuts and leaves as sponges to soak up water.  Captive chimps can solve a problem with “insight” such as putting two short sticks together to form a long stick to reach food. 8 No other great apes use tools in the wild, though human keepers have taught a few bonobos and orangutans to use and even make tools. 

The creation of stone tools, flakes, was a milestone breakthrough in hominin history.  Flakes were not widespread before 3MYA, but some flakes 3.3 MYO have been found in the vicinity of Kenyanthropus fossils. 9

D.  Linguistic Potential

While no other apes are in command of true language, they do exhibit some of the mental modules and brain structures that facilitate language.  This would suggest that our earliest hominin ancestors were endowed with linguistic potential, which was further developed in more recent times. 

The capacity for language can be broken down into distinct mental and social skills.  The most obvious one is symbolism, the recognition that a sound or sign can represent something else.  Symbolism is built upon the more fundamental skill of perceiving concepts; a sign for “large” is meaningless until an animal understands the essence of “largeness”. 

Apes are able to form mental concepts, not only for concrete terms like “ball” but for abstract properties such as “large” and “same”. 1011 Researchers have even had luck teaching words to a few individual apes in captivity.  Koko the gorilla could sign 1,000 learned words and form new compound words. 12 Kanzi the bonobo has a similar vocabulary of lexigrams. 3 Both comprehend spoken English.  Apes in some communities gesture to each other 13 or use distinguishable grunts for different types of food. 14

Clearly some, but not all, capabilities in our language skill set were already present in our ape ancestors.  The language centers of the human brain are asymmetrically located in the dominant (usually left) hemisphere of the brain.  Chimpanzee brains are slightly asymmetric in the same regions. 15              

Non-human apes are physically incapable of speech.  Our hominin ancestors had to modify their throat structure in addition to losing their fangs and snouts. These changes were showing signs of progress already in Ardipithecus over 4MYA, 16 most likely for reasons unrelated to speech.

E.  Inner Angels: Morality

We humans often tell ourselves that our morality sets us apart from the other animals.  Creation stories made it very clear-cut: we were made differently, infused with God’s wisdom.  Unless we take mythology literally, we have to wonder when and how our morality evolved. 

What is morality, anyway?  It can be defined as respect for others’ feelings, rights, and situations, without clear benefits to oneself.  Before animals can be moral, therefore, they must be capable of conceptualizing that others have feelings, rights, and situations that differ from their own.  They must have a theory of mind. 17 Fascinating research suggests that apes possess a theory of mind that is much more developed than monkeys’.

In the lab, scientists measure morality one small, objective step at a time.  Theory of mind was first defined in mirror self-recognition experiments.  The idea is that a mirror provides a subject with a representation of herself from someone else’s perspective.  Only if she fathoms this outside perspective will it occur to her to identify her reflection with her sense of self.  Monkeys understand mirrors, 18 yet they still don’t “get” their own reflections. 19 Chimps and orangutans consistently display mirror self-recognition, 20 as do other species with spindle neurons! 2122 Chimps can also distinguish accident from deliberate bad intent 23 , comprehend that people in a room know things that outsiders don’t, 24 and reverse roles in cooperative games. 25 The cumulative signs indicate that theory of mind originated in our ape ancestors.

Understanding someone else’s situation is not only a great leap forward in cognizance, but the starting point of moral choices.  It can be used sympathetically to assist others, or exploited for personal gain.  Apes take both approaches.  One chimpanzee can discern what another is striving for, and provide help targeted toward that need. 26 Chimps console losers after fights. 27 Apes regularly treat other group members well, beyond the bounds of kinship and reciprocity.

Acting for the common good has obvious survival advantages for strongly social animals.  A group that bands together is better at gathering food and defending its members from predators and enemies.  Since gorillas, chimpanzees, and humans are always found in complex social groups, 4 it is likely that our morals began to develop in our common ancestors, Neogene African apes.

F.  The Social Male

Ape species have settled into diverse social patterns, especially when it comes to gender relations.  Gibbons associate pair-wise as faithfully monogamous couples.  Orangutans are even less social, loners of the jungle.  A gorilla community is essentially a harem, with as few as one adult male, numerous females, and their offspring. 

Chimpanzees are unusual in having communities with large numbers of males.  Male-male social integration is found only in a few other primates and very rarely in other mammals.  However, it is absolutely definitive to the nature of the chimpanzee and its closest relative, the human.

The chimpanzee genus, Pan, includes the “common” chimp, Pan troglodytes, and the bonobo, Pan paniscus.  The social differences between these species make a compelling case study.  Bonobos are female-dominated, with each male inheriting his mother’s status.  Maybe because of this motherly presence, bonobo communities are generally more peaceful and stable.  Bonobos are also notoriously sexual, cementing community bonds with sex play as well as offspring.  P. troglodytes is a male-dominated species, with males constantly jockeying for power.  Alpha male status is not a birthright for chimps.  It is a revolving throne held at most for five years at a time. 28 Since the alpha male gets his pick of mates, males fight dearly for the title. 

If the story ended there, it wouldn’t be very interesting.  No alpha male can hold the throne with brute strength, because fights are rarely one-on-one.  Power struggles are waged by alliances.  Male chimps spend their lifetimes cultivating friends to outnumber enemies.  A chimp community has as much political intrigue as any king’s court. 29

When the “throne” is stable, a chimp community is characterized by male-male cooperation.  Stability is especially important for purposes of hunting and defense.  Chimps are among the few animals that hunt in coordinated packs.  A team of males will scout together and sometimes team up to trap prey.  This behavior should all sound very familiar.  It is impossible to imagine human culture without men competing or cooperating. 

G.  Inner Demons: Rape and Murder

It’s always been a jungle out there.  But with the cunning brain power and social-male communities of chimps and humans, a new kind of evil emerged, a calculated violence against fellow species members.  Three striking crimes perpetrated by apes are murder, rape, and lethal raiding.  In nature, they are usually desperate measures by males for access to sex. 5

Chimpanzee morality is strictly an in-group sentiment.  Communities normally max out at about 100 – 120 individuals, possibly limited by the number of social relations they can mentally process. 30 Outsiders are seen as competitors, and emotional bonds do not extend to them.  Not only do chimps defend their own territories, but they sometimes offensively raid a neighboring territory.  When a group encounters a lone “foreign” male, it occasionally gangs up to kill him.  These killings are brutal, similar to hunts.  Over time, a population of chimps can completely wipe out all the males of a neighboring group and take over its territory. 31 “Chimpicide” increases the victors’ access to females, as females emigrate from surrounding communities and are attracted to groups with more males. 32

Males of a population are all related.  Beyond the stable size limit, though, they fail to recognize each other as relatives, and splinter into “foreign” or even “enemy” groups. 6 33 By contrast, bonobo communities will inter-mate, creating a larger extended family and maintaining moral equivalence. 34

Even within a population, chimp politics sometimes leads to murder.  In most species, male competition is a display of intimidation.  Fights progress to the point of exhaustion and surrender. 35 In rare occasions when sexual competition is intense, chimps will conspire to murder one of their own.  In 2017, the ninth known chimpanzee assassination was documented, and the details read like a horror story. 36

Rape is surprisingly rare throughout the animal kingdom, but it is practiced by non-dominant male chimps when the alpha male withholds females. 37 Chimps have also kidnapped and raped females from conquered tribes. 38 Orangutans, which lack female support networks, rape routinely. 39

What can ape violence teach us about humans?  “Natural” human communities are not much larger than chimps’, and humans, like chimps, never evolved a moral instinct toward outsiders. 7 Group splintering, raids, and female kidnapping / rape in human hunter-gatherer society are strikingly similar to chimp behavior. 40 Lethal raiding is not “war”, but both are emotionally justified by xenophobia.  On the other hand, human tribes also adopt a bonobo-like strategy of inter-marriage for peaceful integration.         

Ironically, conflict breeds cooperation.  Antagonism from outsiders has been a critical pressure for in-group coherence. 41 The ever-present threat of rape fosters defensive female cohorts, and male politics serves as a constant check against tyranny.  There’s no denying that human good and evil evolved inextricably together.   


Back to Section 7.IV:  Hominins

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Continue to Section 7.VI:  Summary

H.  Citations

  1. Orangutan photo by Scot Fagerland, Sacramento Zoo.
  2. Mikkelsen et al. (The Chimpanzee Sequencing and Analysis Consortium), “Initial sequence of the chimpanzee genome and comparison with the human genome”, Nature 437:69-87 (9/01/2005), http://www.nature.com/nature/journal/v437/n7055/full/nature04072.html (accessed and saved 4/02/2017).
  3. John Allman, as quoted by Ingfei Chen in “Brain Cells for Socializing”, Smithsonian, June, 2009, http://www.smithsonianmag.com/science-nature/brain-cells-for-socializing-133855450/?all (accessed and saved 4/09/2017, archived 10/27/19).
  4. John Allman, Atiya Hakeem, and Karla Watson, “Two Phylogenetic Specializations in the Human Brain”, Neuroscientist 8(4):335-346, 2002, https://journals.sagepub.com/doi/10.1177/107385840200800409 (accessed and saved 10/27/19).
  5. Robert Barton and Chris Vanditti, “Rapid Evolution of the Cerebellum in Humans and Other Great Apes”, Current Biology 24:2440-2444 (10/20/2014), https://www.cell.com/current-biology/fulltext/S0960-9822(14)01069-0 (accessed and saved 4/09/2017).
  6. Alexandra Rosati et al., “The Evolutionary Origins of Human Patience: Temporal Preferences in Chimpanzees, Bonobos, and Human Adults”, Current Biology 17:1663-1668 (10/09/2007), https://www.cell.com/current-biology/fulltext/S0960-9822(07)01850-7 (accessed and saved 4/09/2017).
  7. Richard Byrne, The Thinking Ape: Evolutionary Origins of Intelligence, Oxford University Press (New York, 1995) pp. 93 – 98.
  8. Wolfgang Köhler, Intelligenzprüfungen an Anthropoiden (The Mentality of Apes), 1917.  Köhler recorded his famous experiments on video, now easy to find e.g. at https://www.youtube.com/watch?v=FwDhYUlbxiQ (accessed, saved, and archived 10/27/19).
  9. Harmand (2015), op. cit.
  10. Keith Hayes and Catherine Nissen, “Higher mental functions of a home-raised chimpanzee,” in Allan Schrier and Fred Stollnitz (eds.), Behavior of Nonhuman Primates:  Modern Research Trends vol. 4, pp. 59-115, Academic Press (1971), https://www.sciencedirect.com/science/article/pii/B9780126291049500091 (paysite; summary accessed and saved 10/27/19).  This paper is quoted, summarized, and interpreted in Robbins Burling, The Talking Ape: How Language Evolved, Oxford University Press (Kindle eBook version, 2005) p. 70. 
  11. Francine G.P. Patterson and Ronald H. Cohn, “Language acquisition by a lowland gorilla: Koko’s first ten years of vocabulary development”, Word, 41(2):97-143 (6/16/2015), https://www.tandfonline.com/doi/abs/10.1080/00437956.1990.11435816 (accessed and saved 4/29/2017).
  12. The Gorilla Foundation, “Communication” (2019), https://www.koko.org/communication/ (accessed, saved, and archived 10/27/19).
  13. Joanne E. Tanner, Francine G. Patterson, and Richard W. Byrne, “The Development of Spontaneous Gestures in Zoo-living Gorillas and Sign-taught Gorillas:  From Action and Location to Object Representation”, Journal of Developmental Processes 1:69-102, https://risweb.st-andrews.ac.uk/portal/en/researchoutput/the-development-of-spontaneous-gestures-in-zooliving-gorillas-and-signtaught-gorillas-from-action-and-location-to-object-representation(ce96c6a2-ef05-453a-a080-7f012d6069a2).html (accessed and saved 4/30/17).
  14. Katie Slocombe and Klaus Zuberbühler, “Functionally Referential Communication in a Chimpanzee”, Current Biology 15(19):1779-1784 (10/11/2005), https://www.cell.com/current-biology/fulltext/S0960-9822(05)01040-7  (accessed and saved 4/30/2017).
  15. Muhammad A. Spocter et al., “Wernicke’s area homologue in chimpanzees (Pan troglodytes) and its relation to the appearance of modern human language”, Proc. R. Soc. B 277(1691):1-10  (3/17/2010), https://royalsocietypublishing.org/doi/10.1098/rspb.2010.0011  (accessed and saved 4/30/2017).
  16. Gary Clark and Maciej Henneberg, “Ardipithecus ramidus and the evolution of language and signing:  An early origin for hominin vocal capability”, HOMO – Journal of Comparative Human Biology 68(2):101-121 (Mar., 2017), http://www.sciencedirect.com/science/article/pii/S0018442X17300124 (accessed and saved 6/02/18).
  17. The phrase “theory of mind” was coined by David Premack and Guy Woodruff, “Does the chimpanzee have a theory of mind?” Behavioral and Brain Sciences 1(4):515-526 (Dec., 1978), https://www.cambridge.org/core/journals/behavioral-and-brain-sciences/article/does-the-chimpanzee-have-a-theory-of-mind/1E96B02CD9850016B7C93BC6D2FEF1D0 (accessed and saved 10/27/19).
  18. James R. Anderson, “Monkeys with mirrors:  Some questions for primate psychology”, International Journal of Primatology 5(1):81-98 (Feb. 1984), https://link.springer.com/article/10.1007/BF02735149 (accessed and partly saved 10/27/19), as summarized / interpreted in Byrne (1995), op. cit. at 112-113.
  19. Gordon Gallup, “Chimpanzees:  Self-Recognition”, Science, New Series 167(3914):86-87 (1/02/1970), 86-87, https://science.sciencemag.org/content/167/3914/86 (accessed and saved 10/27/19).
  20. Gallup (1970), ibid.
  21. Whales:  Diana Reiss and Lori Marino, “Mirror self-recognition in the bottlenose dolphin: A case of cognitive convergence”, PNAS 98(10):5937-5942, http://www.pnas.org/content/98/10/5937.full , accessed and saved 4/16/2017.
  22. Elephants:  Joshua Plotnik et al., “Self-recognition in an Asian elephant”, PNAS 103(45):17053-7 (2006), 17053-17057, http://www.pnas.org/content/103/45/17053.abstract (accessed and saved 4/16/2017).
  23. Daniel Povinelli, “Social intelligence in monkeys and apes”, PhD thesis, Yale University (1991), https://www.researchgate.net/publication/34257406_Social_intelligence_in_monkeys_and_apes (accessed and saved 10/27/19) as summarized and interpreted in Byrne (1995), op. cit. at 111.
  24. Daniel Povinelli, Kurt Nelson, and Sarah Boysen, “Inferences About Guessing and Knowing by Chimpanzees (Pan troglodytes)”, Journal of Comparative Psychology 104(3):203-210 (Sep., 1990), https://www.ncbi.nlm.nih.gov/pubmed/2225758 (accessed and saved 10/27/19).
  25. Daniel Povinelli, Kurt Nelson, and Sarah Boysen, “Comprehension of role reversal in chimpanzees: evidence of empathy?”, Animal Behaviour, 43(4):633-640 (Apr., 1992), https://www.sciencedirect.com/science/article/pii/S000334720581022X (accessed and saved 10/27/19).
  26. Frans de Waal, Primates and Philosophers: How Morality Evolved, Princeton University Press (Kindle eBook edition, 2006) p. 31.
  27. de Waal (2006), ibid. at 33 ff.
  28. Frans de Waal, Our Inner Ape, The Berkeley Publishing Group (Kindle eBook edition, 2005) page 71 / location 1092.
  29. For a remarkable firsthand account, see Franz de Waal, Chimpanzee Politics: Power and Sex among Apes, Johns Hopkins University Press (1979).  De Waal observed a troupe of chimpanzees in which two males conspired to murder a competitor.
  30. R.I.M. Dunbar, “Neocortex size as a constraint on group size in primates”, Journal of Human Evolution 22(6):469-493 (Jun., 1992), https://www.sciencedirect.com/science/article/abs/pii/004724849290081J (accessed and saved 9/07/19).
  31. Jane Goodall, The Chimpanees of Gombe:  Patterns of Behavior, Belknap Press (1986).
  32. Tyler Tretsven, “Chimpanzee Warfare”, The Cultural Niche (4/12/2012), https://tylertretsven.wordpress.com/2012/04/12/chimpanzee-warfare/ (accessed and saved 4/23/2017, archived 10/27/19).
  33. Jane Goodall, Through a Window: My Thirty Years with the Chimpanzees of Gombe, Houghton Mifflin Harcourt (2010).
  34. de Waal (2005) op. cit. at p. 147 / location 2178.
  35. Richard Wrangham and Dale Peterson, Demonic Males: Apes and the Origins of Human Violence, Mariner Books (1996), p. 155.
  36. Jill Pruetz et al., “Intragroup Lethal Aggression in West African Chimpanzees (Pan troglodytes verus): Inferred Killing of a Former Alpha Male at Fongoli, Senegal”, Int. J. Primatol 38(1):31-57 (Feb. 2017), pp. 31-57, https://link.springer.com/article/10.1007/s10764-016-9942-9 (accessed and saved 4/23/2017).
  37. Goodall (1986), op. cit.
  38. Ian Morris, War!  What is it Good For? Farrar, Straus, and Giroux (2014), p. 289.
  39. Barbara Smuts, “The Evolutionary Origins of Patriarchy”, Human Nature 6(1):11 (1995),  https://link.springer.com/article/10.1007%2FBF02734133 (accessed and saved 4/02/17).
  40. Wrangham and Peterson (1996), op. cit. at 65 – 66.
  41. de Waal (2006), op. cit. at p. 54.
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