Much of what makes us human derives from the fact that we are eukaryotes, animals, tetrapods, mammals, primates, and apes. But when we use the term “human nature”, we are usually thinking of the qualities that set us apart from all other animals. While the most dramatic behavior was manifested more recently, evolution was hard at work defining the human animal millions of years ago.
A. Meat, Hunting, and Cooking
You are what you eat. By that adage, our Pleistocene ancestors went through quite a transformation. Our diet is one of the sharp dividing lines between humans and other apes. The two major unique elements of our diet are meat and cooked food.
Chimps and humans are the only apes that eat vertebrate meat, and humans eat quite a bit more. Because we also eat a large amount of plant matter, we are classified as omnivores. Eating meat allows us to obtain more of our calories from animal fat instead of plant oils and carbohydrates.
Most likely, hominins always ate insects and other invertebrates, as all apes do. Australopithecines also ate seeds and nuts, so their digestive systems were already prepared for relatively concentrated doses of protein and lipids. Hominins probably next progressed to catching small vertebrates and scavenging large animal carcasses. Judging from cut marks on animal bones, they were already butchering cow-sized animals (presumptively scavenged carcasses) by 3 MYA. 2 As they relocated to sparser wintry environments, early humans had to rely even more on meat to supplement a dwindling fruit supply. By 2 MYA they were eating seafood, 3 which is often cited as an important factor for early human brain development. 4 For their grand finale, early humans took to hunting large game, from boars to elephants. This is a remarkable path to follow for a relatively small, slow creature with no claws or fangs. The earliest evidence of a human hunt dates to 2MYA. 5 It may have been another million years before hunting was common.
The only early human adventure that captures our imagination more than an elephant hunt is the domestication of fire. Humans learned how to preserve wildfire long before they mastered the art of creating their own. 6 The timing of these events is uncertain. It’s safe to say that some humans were definitely keeping fires, if not yet lighting them, by several hundred thousand years ago. 7
Heating food denatures its macromolecules, breaking them up or “loosening” them to make them easier to digest. Cooking also kills bacteria and preserves food. It is just as effective for plant matter as meat (can you imagine eating a raw yam?) so the quest for fire could have occurred independently of the hunt.
Meat and cooked food are “high quality” food dense in calories and minerals. On the flip side, they are difficult to obtain. Cooking and hunting take time and energy. Hunting is a high-risk / high-return gamble. 8 These were the choices that early humans made. High quality food changed the human body forever, to the extent that we now depend on it. The human digestive system is small for an ape. As a proportion of the gastrointestinal tract, humans have a large small intestine, which does the bulk of digestion for nourishment, and a small large intestine, which post-processes the lower-quality food. 9 The availability of high-quality food could have allowed hominins to downgrade the gut and re-invest that energy to the growing body and brain. 10
Omnivory, hunting, and cooking were as revolutionary for human behavior as for the body. Hunting-gathering or foraging was the predominant human lifestyle from the dawn of the genus until the Chapter 4 timescale. Family structure and gender relations, social cooperation and competition, and the role of the human species within the animal kingdom were all cast within the foraging framework. For example, male hunting / female gathering was the original human division of labor. The practice of sharing meat was the foundation of proto-politics, -insurance, and -economics for millions of years. 11 Perhaps due to our high-quality diets, 12 humans even reproduce much more rapidly than other apes. And as we all know, fire serves more purposes than cooking. Protected within the warm glow of their campfires, early humans became the dominant species of an ever-expanding habitat.
B. Naked Apes Running all Around
Our bare skin is so anomalous that it is one of the first physical features we cite when we dignify ourselves as “non-animal”. Truth be told, humans have just as many body hairs as other apes; it’s just that ours are much shorter and finer. 13 Without splitting hairs, though, the fact remains that humans made a dramatic makeover. This inquiry into the origins of hairlessness will lead us into a surprising and sometimes speculative web of highlights in the making of the human animal.
Besides hairlessness, human skin has another important property: humans sweat profusely and more efficiently than any other animal. When human sweat evaporates, it efficiently carries heat away from the body – but only when it evaporates directly from the surface of the skin, not an outer layer of fur. Therefore, it’s likely that sweating and bare skin evolved together to help keep early humans cool, even as they were active in the daytime. 14 Incidentally, when bare skin is exposed to intense sun, it must darken to protect the body from excess ultra-violet sunlight. The first bare-skinned humans almost surely had dark complexions, as tropical populations still do today.
There is some irony to the fact that the human body perfected heat venting as the planet plunged into ice ages. Humans could have used fur coats over the next million years as they migrated into northern climes, but the hair loss probably occurred earlier while they were still restricted to tropical and sub-tropical regions.
Besides that, sunlight is not the only source of heat. The body produces heat of its own; hairlessness and sweat glands are indicative of an animal that burns a great deal of energy. We already know of one particularly energy-hungry organ – the human brain. Our bare skin might be just the radiator required to keep the engine in our head from overheating.
Many scientists believe that early humans also generated a great deal of body heat by running. 15 Not until the human genus did hominins fully commit themselves to life out of the trees and down on the ground. While Australopithecines did certainly walk upright, it was probably a slow waddle of a walk. Homo erectus gave bipedalism a whole new swagger, the steady long-legged stride that we use today. In fact, from erectus we have inherited a body that appears engineered for endurance running.
Kinesiologists will tell you that running is not simply fast walking, and you understand this instinctively when you walk with increasing speed until your body must “shift gears” into a run. Running takes the joints and muscles through a greater range of motions, including a small vertical leap with each stride, and it places all the weight on one foot. It places great stress on bones, muscles, and ligaments, and it requires better subconscious balance and control than walking. Running long distances is especially demanding of the digestive and cardiovascular systems. From the shape of the heel to the size of the butt, the human body has adapted to meet all of these demands and more. 16 What we lack in speed, we make up for in endurance. In a marathon, the slow-and-steady human can outrun the best sprinters, including antelopes 17 and cheetahs. 18
Humans are so good at endurance running that it makes us suspect there was a compelling reason for it. Why were all those naked apes running around so much two million years ago? Today’s scientists debate whether running was necessary to scavenge or hunt. Maybe early humans had to routinely outrace competing scavengers or even chase prey to exhaustion. 19 It’s just as plausible that running helped humans evade predators. Naked apes were curiously vulnerable creatures out in the open. They were not especially large or strong, and they lacked armor. Their stone tools were probably ineffective as weapons. 20 Perhaps they got by on being swift of mind and fleet of foot.
C. Family and Social Life
The institutions of marriage and nuclear family seem so natural that we never stop to question them. But when it comes to sex and parenting, humans are actually the freaks of nature. It is rare for social mammals to pair up in long-term bonds. 21 Human fathers commit an exceptional amount of time, energy, and resources to supporting their own children.
The aspects of human sexuality and parenting that differ from other apes’ are those that evolved in response to specific human needs. Therefore, we should not be surprised to find them related to other uniquely human conditions. In fact, scientists are currently attempting to explain these sexual behaviors in their relation to temperate climate, hunting, cooking, large brains, and tools. This is a tantalizing and frustrating field. We seek the origins of human family and social life for insight into our primal nature. We are left with many mysteries open to speculation. The best we can do here is summarize some of the most reasonable and favored hypotheses.
As children’s brains (and skulls) grew, they were forced into slower development so they were not too large at birth. 22 Consequently, human infants are absolutely helpless for years. Following childbirth, nursing puts mothers in a vulnerable position as well. Two years of breastfeeding is even more demanding on a mother’s body than nine months of pregnancy. Mother and her child must be well fed, exactly at the time when her foraging capabilities are at a low. 23 A caretaking father can help provide food and protection for the family unit.
In a social setting, the success of childhood is measured not only by nutrition but also by education. As humans lived in increasingly complex bands of smarter peers, childhood became a foundational period to learn complex skills and social competence. As humans usually live in male-based kin groups – women traditionally leave home at maturity and apparently have done so for millions of years 24 – the paternal connection is especially important.
From the male point of view, the commitment to fatherhood accompanied a transition from promiscuity. To oversimplify the issue, a male can spend his time and effort either raising his children at home or leaving a trail of love children with multiple single mothers. If it were up to his genes, he might opt for the latter. However, this is not entirely his choice to make. He is up against competitive males and cautious females. Monogamy is a social decision, not an individual one. One social benefit to monogamy is widespread availability of mates to all males, not just the dominant ones, without the need to fight. Humans are the least sexually dimorphic great ape. Men have relatively small bodies and small teeth, traits that generally indicate minimal male fighting. Human females have also swayed the communal decision by becoming more inclined to monogamy than promiscuous chimps or harem gorillas. 25 Paternal care makes sense for human fathers because they (usually) know who their own children are.
Most mammalian females are receptive to mating only when they are “in heat”, which generally coincides with ovulation and the opportunity for fertilization. Chimpanzee females blatantly signal their heat with swollen red genitals. Humans have gone to the opposite extreme, with an ovulation cycle that is concealed from men and even from women themselves. 26 Women no longer have a “heat” cycle, yet they are potentially receptive to their mates’ advances at any time of month. We have replaced low-frequency / high-fertilization sex with a higher-frequency / lower-fertilization strategy. This gives sex a new non-reproductive role of pair-bonding and enticing husbands to remain loyal. Humans are unique among social mammals for having sex in private. This intensifies the pair-bond as distinct from normal social relations. 27 A neurotransmitter such as AVP may play a role in emotionally bonding men to their mates and children. 28
Humans are not entirely monogamous and probably never were, but the nuclear family – two parents and their biological children – has become the fundamental social unit in almost every society. 29 Brothers and male cousins generally stay together and create extended family units. For most of the past few million years, the foraging lifestyle did not support large or stable communities. Family units fragmented into different bands, though adjacent bands were connected by “marriage” and alliances. Humans have excellent facial recognition and can recognize their relatives through the years, helping keep the peace between bands that exchanged brides. 30 On the flipside, bands also competed aggressively and violently for scarce resources. Family cohesion was essential for the ongoing cycle of cooperating with “us” to compete against “them”. Human social life is the most complex of any animal, and our brains devote a large share of their powers to understanding each other.
The development of speech is one of the most inscrutable mysteries of human nature. To get a handle on it, I will separate speech into two aspects. Vocalization is the physical production of sound with the body. Language is the use of symbols to communicate meaning. These are two different concepts that just happen to intersect in human speech. The present discussion is limited to human vocalization, which, even without language, is fascinating in its own right. The linguistic aspect of speech will follow as one of the major topics of Chapter 5.
All mammals and even reptiles have a larynx or “voice box” with vocal cords. Exhaled air vibrates the vocal cords. The sound produced by these cords is then amplified and modified in the mouth, nose, and throat. All non-human apes have air sacs in their throats. These sacs are good amplifiers but would make speech sound somewhat muddled. 31 Humans lost air sacs somewhere along the way, trading them in for the pharynx, a resonant tube in the upper throat. The pharynx was formed as the larynx descended below the base of the tongue.
The pharynx opened up space for the tongue to become more flexible. This in turn was critical, because only humans use the tongue and lips to shape sounds emerging from the mouth. Each letter of the alphabet owes its sound to the positioning of soft tissue. 32 Dozens of tiny muscles perform specialized functions such as opening / closing the jaw and lips, shaping the tongue, and controlling tension on the vocal cords. The human brain coordinates these movements precisely and rapidly; an average person pronounces about 10 sounds per second.
Human vocalization also relies heavily on breath control. When you speak a sentence, you start with a quick inhalation. Then you carefully pace your exhalation to support your voice evenly from beginning to end. Other apes lack control over their breath tempo, while humans have taken it to the extremes of inflection, poetry, and song.
The details of how, when, and “why” these features all evolved are unresolved. Some scientists claim that the first signs of culture, such as tools and fire, are evidence that Homo erectus had language 2 MYA. 33 On the other extreme, some believe that speech was not possible until the human throat acquired its modern proportions 50 TYA. 34 Some of the earliest fossil evidence of the transition is found in H. heidelbergensis about 600 TYA. Heidelberg Man is the first human known to have lost air sacs 35 , and his ear anatomy was attuned to the frequencies of the human voice. 36 The nerve for controlling breath and speech apparatus is larger in Neanderthals and H. sapiens than in erectus, 37 implying that this enhancement originated in the ergaster-heidelbergensis branch.
These vocal features may have evolved along with language 38 or separately to serve different functions. For instance, an elongated pharynx amplifies and deepens vocal calls and makes a male sound artificially large, an advantage in social competition. 39 Interestingly, breath control is sometimes cited as a critical factor in the evolution of running. 40 In any event, when humans had the capacity to invent language, they needed means to communicate it with. It is hard to imagine a better medium than their incredible voices to convey such precision and emotion all at once.
- Attribution unknown. If you own the rights to this image and object to its use, please let me know and I will remove it immediately! ↩
- Shannon McPherron et al., “Evidence for Stone-Tool-Assisted Consumption of Animal Tissues before 3.39 million years ago at Dikika, Ethiopia”, Nature 466:857-860 (8/12/2010), https://www.nature.com/articles/nature09248 (accessed and saved 4/15/18). ↩
- David R. Braun et al., “Early hominin diet included diverse terrestrial and aquatic animals 1.95 Ma in East Turkana, Kenya”, PNAS 107(22):10002-7 (6/01/2010), www.pnas.org/content/107/22/10002 (accessed and saved 4/15/18). ↩
- Joanne Bradbury, “Docosahexaenoic Acid (DHA): An Ancient Nutrient for the Modern Human Brain”, Nutrients 3(5):529-554 (May, 2011), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3257695/#B1-nutrients-03-00529 (accessed and saved 4/15/18). ↩
- Henry Bunn and Alia Gurtov, “Prey mortality profiles indicate that Early Pleistocene Homo at Olduvai was an ambush predator”, Quaternary International 322-323:44-53 (11/23/2013), https://www.sciencedirect.com/science/article/pii/S1040618213008641 (saved 4/15/18, last accessed 11/10/19). ↩
- J.A.J. Gowlett, “The discovery of fire by humans: a long and convoluted process”, Phil. Trans. R. Soc. B 371(1696):20150164 (6/05/2016), http://rstb.royalsocietypublishing.org/content/371/1696/20150164 (accessed and saved 4/15/18). ↩
- Francesco Berna et al., “Microstratigraphic evidence of in situ fire in the Acheulean strata of Wonderwerk Cave, Northern Cape province, South Africa”, PNAS 109(20):E1215-E1220 (5/15/2012), https://doi.org/10.1073/pnas.1117620109 (accessed and saved 4/15/18). ↩
- Kristen Hawkes, James O’Connell, and Nicholas Jones, “Hunting and Nuclear Families”, Current Anthropology 42(5):681-709 (Dec., 2001), https://www.journals.uchicago.edu/doi/10.1086/322559 (accessed and saved 4/15/18). ↩
- David J. Chivers and Claude Marcel Hladik, “Morphology of the gastrointestinal tract in primates: Comparisons with other mammals in relation to diet”, Journal of Morphology 166(3):337-86 (Dec., 1980), https://onlinelibrary.wiley.com/doi/abs/10.1002/jmor.1051660306 (saved 4/15/18, last accessed 11/10/19). ↩
- Leslie C. Aiello and Peter Wheeler, “The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution”, Current Anthropology 36(2):199-221 (Apr., 1995), https://www.journals.uchicago.edu/doi/10.1086/204350 (accessed 4/15/18). ↩
- Craig B. Stanford, The Hunting Apes: Meat Eating and the Origins of Human Behavior, Princeton University Press (Princeton, 1999). ↩
- Elia Psouni, Axel Janke, and Martin Garwicz, “Impact of Carnivory on Human Development and Evolution Revealed by a New Unifying Model of Weaning in Mammals”, PLoS ONE 7(4): e32452, http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0032452 (accessed and saved 4/15/18). ↩
- Nina G. Jablonski, Skin: A Natural History, University of California Press (Kindle eBook edition, 2013), location 531 ↩
- David R. Carrier, “The Energetic Paradox of Human Running and Hominid Evolution”, Current Anthropology 25(4):483-495, (Aug. – Oct., 1984), https://www.journals.uchicago.edu/doi/10.1086/203165 (accessed and saved 4/22/18). ↩
- Carrier, ibid., and related literature. ↩
- Dennis M. Bramble and Daniel E. Lieberman, “Endurance running and the evolution of Homo”, Nature 432:345-352 (11/18/2004), http://www.nature.com/articles/nature03052 (accessed and saved 4/22/18). ↩
- David Attenborough, Persistence Hunting, BBCi, (2002), https://www.bbc.co.uk/programmes/p009lwhq (accessed and archived 11/10/19). ↩
- BBC News, “Kenyans chase down and catch goat-killing cheetahs” (11/15/2013) www.bbc.com/news/world-africa-24953910 (accessed and saved 5/06/18, archived 11/10/19). ↩
- Travis Rayne Pickering and Henry T. Bunn, “The endurance running hypothesis and hunting and scavenging in savanna-woodlands”, Journal of Human Evolution 53(4):434-438 (Oct., 2007), https://www.sciencedirect.com/science/article/pii/S0047248407001327?via%3Dihub (accessed and saved 4/22/18). ↩
- John J. Shea, “Lithic Archaeology, or, What Stone Tools Can (and Can’t) Tell Us about Early Hominin Diets”, In Peter Ungar (Ed.) Evolution of the Human Diet: The Known, the Unknown and the Unknowable, pp. 212-229. Oxford University Press (Oxford, 2006). ↩
- Deborah Kleiman, “Monogamy in mammals”, The Quarterly Review of Biology 52(1):39-69 (Mar., 1977), https://www.journals.uchicago.edu/doi/10.1086/409721 (accessed and saved 9/20/20). ↩
- Flinn (2011), op. cit. at 19. ↩
- Robert J. Quinlan, “Human Pair-Bonds: Evolutionary Functions, Ecological Variation, and Adaptive Development”, Evolutionary Anthropology 17(5):227-238 (Sep. – Oct., 2008), https://onlinelibrary.wiley.com/doi/abs/10.1002/evan.20191 (saved 5/12/18, last accessed 11/10/19). ↩
- Sandi R. Copeland et al., “Strontium isotope evidence for landscape use by early hominins”, Nature 474, 76-78 (6/02/2011), https://www.nature.com/articles/nature10149 (accessed and saved 6/30/18). ↩
- Sergey Gavrilets, “Human origins and the transition from promiscuity to pair-bonding”, PNAS 109(25):9923-9928 (6/19/2012), http://www.pnas.org/content/109/25/9923 (accessed and saved 5/13/18). ↩
- Richard D. Alexander, “How Did Humans Evolve? Reflections on the Uniquely Unique Species”, University of Michigan Museum of Zoology (1990), https://deepblue.lib.umich.edu/handle/2027.42/57178 (accessed and saved 5/12/18). ↩
- Jared Diamond, The Third Chimpanzee, Harper Collins (New York, 2006) pp. 83 – 84. ↩
- Anne E. Storey and Toni E. Ziegler, “Primate paternal care: interactions between biology and social experience”, Horm Behav. 77:260-271 (January, 2016), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4968077/pdf/nihms804454.pdf (accessed and saved 6/02/18). ↩
- George Peter Murdock, Social Structure, The MacMillan Company (New York, 1949), https://archive.org/details/socialstructure00murd (accessed 5/27/18). ↩
- Flinn, op. cit., p. 16. ↩
- Tobias Riede et al., “Mammalian laryngseal air sacs add variability to the vocal tract impedance: Physical and computational modeling”, J Acoustic Soc. Am. 124(1):634-47 (July, 2008), https://asa.scitation.org/doi/10.1121/1.2924125 (accessed and saved 11/16/19). ↩
- Daniel Currie Hall, “Interactive Sagittal Section”, http://smu-facweb.smu.ca/~s0949176/sammy/ (accessed and archived 11/16/19) is a very well-done app mapping placement of the lips and tongue to numerous sounds. ↩
- Daniel L. Everett, How Language Began, Liveright Publishing, 2017. ↩
- Philip Lieberman and Robert McCarthy, “Tracking the Evolution of Language and Speech”, Expedition 49(2):15-20 (2007), http://www.cog.brown.edu/people/lieberman/pdfFiles/Lieberman,%20P.%20&%20McCarthy,%20R.%202007.%20Tracking%20the%20evolution%20of.pdf (accessed and saved 6/02/2018, archived 11/16/19). ↩
- Ignacio Martinez et al., “Human hyoid bones from the middle Pleistocene site of the Sima de los Huesos (Sierra de Atapuerca, Spain)”, Journal of Human Evolution 54(1):118-124 (Jan., 2008), https://www.sciencedirect.com/science/article/pii/S004724840700139X?via%3Dihub (accessed and saved 6/10/2018). ↩
- Ignacio Martinez et al., “Auditory capacities in Middle Pleistocene humans from the Sierra de Atapuerca in Spain”, PNAS 101(27):9976-9981 (7/06/2004), http://www.pnas.org/content/101/27/9976 (accessed and saved 6/10/2018). ↩
- Ann M. MacLarnon and Gwen P. Hewitt, “The evolution of human speech: The role of enhanced breathing control”, American Journal of Physical Anthropology 109(3):341-343 (7/01/1999), https://onlinelibrary.wiley.com/doi/abs/10.1002/%28SICI%291096-8644%28199907%29109%3A3%3C341%3A%3AAID-AJPA5%3E3.0.CO%3B2-2 (accessed and saved 6/10/2018). ↩
- Bart de Boer, “Evolution of speech and evolution of language”, Psychon Bull Rev 24:158-162 (2017), https://link.springer.com/article/10.3758/s13423-016-1130-6 (accessed and saved 6/03/18). ↩
- W. Tecumseh Fitch and David Reby, “The descended larynx is not uniquely human”, Proc. R. Soc. Lond. B 268:1669-1675 (2001), https://royalsocietypublishing.org/doi/10.1098/rspb.2001.1704 (saved 6/03/2018, last accessed 11/16/19). ↩
- Carrier (1984), op. cit. ↩
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