8.III: Mammals And Primates

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Animals that we would call mammals existed by 200,000,000 years ago. Early mammals were small, nocturnal, and rodent-like. Warm-bloodedness, live birth, and maternal care are at the core of mammal nature.

A.  Body

B.  Brain and Behavior

C. Citations


A.  Body

  1.  Warm-bloodedness
  2.  The mammals who knew the dinosaurs
  3.  Primates


1.  Warm-bloodedness

200 – 300 MYA, our ancestors were classified as “mammal-like reptiles”.  The animals at the earlier end of this spectrum were closely related to reptiles, and those at the more recent end were nearly mammals, with a long continuous transition in between.  In popular perception, the differences between these classes are cosmetic.  Reptiles are covered in tough flat scales, while mammals have fur.  Reptiles sprawl with legs jutting out to the side, unlike mammals with erect legs directly beneath their bodies.  Reptilian teeth are uniform and single-cusped, like points on a saw blade.  Thankfully, mammals do not flash smiles of pointy triangles. 

These traits don’t just make us look better.  They evolved together around one invisible but profound adaptation:  warm-bloodedness.  The term “warm-blooded” does not tell the whole story.  It’s not just important that mammalian body temperature is relatively high, but that it’s constant.  A reptile is at the mercy of the environment.  It is much more vulnerable to overheating or freezing to death if it does not hide in shelter.  Except in the worst extremes, a mammal is safe and can stay active in all weather.  This advantage is obvious. 

Another benefit of warm-bloodedness is chemical predictability.  At the cellular level, biology is governed by enzyme reactions.  Enzyme activity is temperature-dependent.  In a cold-blooded animal, a certain enzyme might be reactive one day and inert the next, abundant one day and then depleted.  A well-regulated temperature helps sustain enzymes, and therefore life functions, at a nice steady pace.  This allows for much more efficient evolution of the genes regulating enzymes and cell activity.

A reptile’s body heat literally “goes with the flow”.  The mammalian body must be able to resist temperature gradients with the outer environment.  When it is hot outside, the mammal must be able to release heat.  Venting is accomplished by exposure of vessel-rich skin to the air, or the evaporation of small amounts of water from the body.  The body must also be able to retain heat in cold weather.  That is the most important function of fur, an outer insulator.  Sweat glands, body fat, and fur evolved along with warm-bloodedness.  They are extremely efficient solutions, as they resist the temperature gradient without expending energy. 

In fact, energy is the major cost associated with warm-bloodedness.  Pound for pound, a mammal requires five to ten times as many calories as a reptile! 1 That is why warm-bloodedness did not evolve overnight.  The body needed time to evolve the capability to catch and digest enough food. 

This growing appetite explains certain skeletal trends among the mammal-like reptiles.  A reptile does not chew its food.  It uses its teeth only to catch and kill prey, which it then swallows whole and lets its digestive system slowly do the rest.  A mammal must be able to chew its food to make digestion faster and easier.  Jaws strengthened.  Teeth became more robust, and they specialized into incisors and molars.  Meanwhile, our ancestors developed better legs to forage for food.  Vertical limbs can take longer strides and navigate diverse terrain.  With sprawling legs, reptiles need more muscular energy just to support their weight.  Mammals can save this energy for running. 

Soft anatomy, too, was overhauled for higher energy requirements.  Muscles improved in strength and stamina.  The heart evolved a fourth chamber to keep oxygen-rich blood separated from de-oxygenated blood.  The diaphragm developed greatly to assist with breathing.  The adaptation that directly caused warm-bloodedness was a simple increase in the number of mitochondria per cell.  Mammal-like reptiles are of particular interest because they were our “bottleneck” ancestors.  They survived the P-T extinction, while most tetrapods around them died.  This may be testimony to their warm-bloodedness, or it may be simple luck.  Whatever it was, nature selected these creatures to pass their traits along into the Mesozoic Era.  After recovery, mammals became successful worldwide.  Warm-bloodedness is organic climate control.  It permitted our mammalian ancestors to thrive in all habitats.         


2.  The mammals who knew the dinosaurs

Our ancestors, technically known as synapsids, had a very interesting relationship with bird ancestors, the diapsids.  In the Permian Period, synapsids ruled the land as the largest and most diverse animals, while smaller diapsids scurried around beneath their feet.  After the P-T event, the roles were reversed.  Most synapsids went extinct.  The survivors shrank and evolved into tiny mammals, as diapsids grew and evolved into dinosaurs.      

The evolution of the earliest mammals, then, was guided by some crucial environmental factors.  It was a post-extinction world; life was sparse and the planet was hot.  Dinosaurs were unbeatable in their roles as large creatures and carnivores.  Rather than competing with dinosaurs for the top of the food chain, Mesozoic mammals specialized in lower niches.  They were small burrowers and tree-climbers.  Based on their teeth, they mostly ate insects and worms.  In turn, they must have made perfect bite-sized snacks for large reptiles, so hiding was a top concern.  They turned to a nocturnal lifestyle.      

The earliest fossils that are classified unequivocally as mammals are from the late Triassic period, a little more than 200 MYA.  Early mammals resembled rodents.  They are identified as mammals primarily by features of the teeth and skull.  Reptiles have a main jaw bone with a number of secondary jaw bones behind it.  During the reptile-mammal transition, the main jaw bone grew and the secondary jaw bones shrank and became detached.  The mammalian jaw formed a hinge at a different location, further forward in the skull.  Two tiny secondary jaw bones found their way into the middle ear and became adapted for the purpose of hearing!  Today, we call them the anvil and the stirrup.  They give mammals the ability to hear a much broader range of frequencies than other animals. 2

Good hearing is vital for nocturnal animals.  So are smell and touch.  Night life requires the ability to detect food, family, friends, and foe by any means possible.  Fur and whiskers help animals feel their way around in the dark.  Early mammals developed an advanced olfactory (smell) system.  The nasal cavities were filled with convoluted structures that captured chemicals from the air.  The parts of the brain responsible for processing smell, as well as tactile sensations from whiskers, grew significantly. 3  

The word “mammal” itself is borrowed from the mammary glands.  Even the most basal mammals have them, so nursing probably evolved right around the time of the first true mammals.  The production of milk may have first been useful for keeping eggs moist. 4 The first mammals laid eggs like their reptilian ancestors.  Over time, eggs stayed longer inside the mother’s body, with thinner shells, until eventually the young were born live. 5 Mammals that have a uterus and give live birth are called placentals.  Most modern mammals worldwide are placentals.  Those that still lay eggs are found only in Australia.  Almost all mammals that have a pouch for their young are also in Australia.  This is the first sign of continental isolation.  Pangaea was starting to disassemble as mammals diversified in the late Mesozoic.


3.  Primates

Dinosaurs went extinct at the end of the Mesozoic Era.  Mammals, with their competition gone, soon began to flourish and diversify, and went on to become masters of the Cenozoic Era.  Birds carried on the dinosaur line.  This was yet another role reversal of what had happened at the beginning of the Mesozoic, when dinosaurs had crowded out the mammal-like reptiles! 

In the classic Linnaean scheme, humans belong to the class of mammals and the order of primates.  Our ancestors evolved into primates right around the Mesozoic-Cenozoic boundary. 6 The earliest primates lived in the northern continents, and it is likely that they originated in Asia. 7 Primitive examples of primates are lemurs and tarsiers, not much larger than the rodent-sized Cretaceous mammals from which they descended.  They were omnivorous, eating small invertebrates as well as plant matter. 

Early primates lived in trees.  Their hands and feet had grasping fingers, which were effective for holding onto branches, catching insects, and picking fruit.  Fruit was an extremely important part of the diet.  Most tetrapods have an enzyme that allows them to produce their own Vitamin C.  Our ancestors lost that enzyme very early in primate evolution, so now we depend on fruit for most of that vitamin intake. 8

This was also the phase when our ancestors acquired the “private parts” that resemble ours today.  Primates have only two breasts.  The penis is “pendulous”, meaning that it is not attached to or tucked into the abdominal wall as in other mammals. 9

With dinosaurs out of the way, some primates, simians, became active during the day.  As daylight gave them much more visual information to process, simians began a trend toward better eyesight.  Their eyes were close together in the front of the face.  This arrangement narrowed their field of vision but provided good depth perception.  Three-dimensional vision was very useful for living in a treetop environment, where judging leaps could be a matter of life and death. 

The first simians resembled miniature monkeys. 10 One Cenozoic trend was bodily growth.  By about 35 MYA, our line the catarrhines emerged. 11 The range of catarrhines was restricted to Africa and Southern Asia.  They preferred tropical forests and did not occupy Europe.  The continents were far too spread apart by this time for them to make it to Australia or the Americas.  The primitive catarrhines are thus known as “Old World Monkeys”.

The word “catarrhine” describes Old World Monkeys’ “downward nose”, to contrast them with New World monkeys, which had nostrils pointed to the sides.  Instead of claws, catarrhines had flat nails on all fingers and toes.  The catarrhine dentition has come down to us almost unchanged.  In each quarter-jaw, there are two incisors, a canine, two bicuspids, and three molars.  Early catarrhines retained the sharp, long canine teeth that get their name from dogs.  Catarrhines reduced the prehensile tail, which had been almost like a fifth limb for older primates.  Primates like lemurs and spider monkeys can use their tails to grasp branches.  Catarrhines came up with an even better solution, the fully opposable thumb.  As we know, this proved to be extremely valuable much later for the use of tools and text messaging. 

Vision continued to advance.  Other mammals see only greens and blues.  Catarrhines were sensitive to red light as well.  With three primary colors, they were able to perceive a much more vivid picture of the world.  This trichromatic vision may have provided valuable advantages in the visually busy world of treetops.  In a chaotic canopy of branches, vines, and flowers, it could be useful to discern ripe fruit and leaves from the background clutter. 12 The sense of sight proved so valuable for catarrhines that both the skull and the brain became much more dedicated to vision 13 at the expense of smell 14 .


B.  Brain and Behavior

Being mammal means so much more than just having hair and warm blood.  The mammalian brain is more advanced than that of any other animal.  Mammals, especially primates, are inherently social.  The evolution of pregnancy and live birth created radically new social paradigms, like exaggerated gender roles and the mother-infant bond.  All of these elements of mammalian nature are tied together with complex emotions.  The lifestyle of even the lowliest placental mammal, the mouse, is far closer to human than the life of the lizard or fish.

  1.  The neocortex
  2.  It takes a village to raise a child


1. The neocortex

When you visualize a brain, you probably think of the human brain with its wrinkly exterior.  That outer layer is called the neocortex, meaning “new covering”.  Although it is only about a millimeter thick, it covers the brain’s entire outer surface.  As its name suggests, the neocortex evolved more recently than the old paleocortex, which is shared by reptiles and, to a lesser extent, birds. 15 The neocortex must have originated 200 – 300 MYA in synapsids.  It is large and complex in primates, especially humans, and smaller and simpler in most mammals.   

The neocortex serves at least three crucial functions.  It is heavily involved in sensory processing.  It may have evolved originally to handle the cognitive demands of the senses of smell and touch. 16 Second, the neocortex is very effective at motor control.  The more complex the movement, the more brain surface is required.  Primate hand-eye coordination and facial expressions are especially demanding.

The third major function of the neocortex is social intelligence and emotional processing.  In primates, these forms of cognition are especially well developed at the very front tip, the prefrontal cortex.  Non-mammalian animals may have some emotions, but they don’t seem to range far beyond survival directives.  The half-joking summary of reptilian mentality is, “If it’s smaller than me, I’ll eat it.  If it’s the same size as me, I’ll mate with it.  If it’s bigger than me, I’ll run away.” 17 A reptile’s inner life is exceedingly simple.  It does not torture itself over what it should have done yesterday or how to get along with its neighbors.  On the other hand, it also does not enjoy the happiness of friendship or a life well lived.

As primate social groups grew, so did the neocortex. 18 It grew much faster than the skull, which is why it began folding and wrinkling to accommodate its increased surface area.  The richness of primate emotion, then, comes from associating activity in the “reptilian” brain to its social context as understood by the intelligence of the neocortex. 1  

Interestingly, many of the most powerful mammalian emotions involve parts of the brain that already existed in reptiles and are regulated by brain chemicals that are also inherited from reptiles.  For example, mammals feel a rush of euphoria when the neurotransmitter dopamine is released from one part of the brain’s limbic system into another part.  Reptiles also have dopamine and a limbic system, but they do not show any signs of emotional rushes.  It seems that we must attribute most of our emotionality to the biological difference between the reptile and mammal brains: the neocortex.

Primate brains are unique in at least two other ways.  Compared to other animals of the same size, primates have small neurons that pack together densely, especially in the neocortex. 19 Primates’ neurons also have a thick version of the myelin sheath, a fatty coating that helps electrical signals travel more rapidly through the brain. 20 These micro-features give primates cognitive powers that cannot be matched by even the largest brains, those of elephants and whales.   

The relationships among the neocortex, mental processing, and interpersonal emotions are represented in this diagram.  The chart shows that primate evolution involved still other social and biological factors, which will be elaborated below.

chapter_08_primate_chart
The cycle of early primate evolution. The arrows represent enabling factors or evolutionary pressures.


2.  It takes a village to raise a child

Reptiles do not have an infancy life phase.  They hatch as miniature adults ready to take on the world, and their mothers are nowhere to be seen.  As mammals and primates became more biologically complex, it took longer for the young to reach maturity.  Infants relied on their mothers’ milk for sustenance, and their large brains needed time to develop.  Parental care was absolutely essential for survival.  Something had to compel mammalian mothers to undertake such a responsibility.     

Although it is difficult for us to believe, it is not a given that an animal mother will care very much about her children.  Many egg-laying animals abandon their nests, and some species are known to eat their own young! 21 To a fish, this might make sense.  If a mother fish spawns 100 young and is feeling hungry, she can afford to weed out some of the slow weak ones and leave the hardier offspring to survive and carry on her genes. 

Compared to egg-layers, a mammal mother does not have very many offspring.  Pregnancy is a big deal; it is highly demanding on the body.  A female can only carry one or a small number of children at a time.  At a minimum, the mother must carry each child to term through a full gestation period, not to mention nursing and other childcare, before her next pregnancy.  Even the most fecund mammal like a rodent will have maybe 40 children in her lifetime.  A typical monkey will bear about ten.  In the wild, each individual child’s odds of success are not very high.  For a primate mother, then, every baby matters. 

Childbearing and prolonged infancy created the pressure for maternal love.  Evolution will favor the mother who protects her children – and a female who loves her children will protect them.  The emotions of love, trust, and bonding between a mother and child are associated with the neurotransmitter oxytocin, which is unique to mammals.  It is released during pregnancy and childbirth into the bloodstream of both mother and child, and it assists with nursing. 22 It might strike us as hollow that love is chemical, but, in animal studies, motherly devotion can be turned on 23 and off 24 with the flip of an oxytocin switch.

In many mammal species, mother / child families are self-supporting social units.  For most primates, these single-parent families form the core of larger communities.  Our primate ancestors became increasingly social animals through the Paleogene Period.  Simians are more social than prosimians, 25 and catarrhines all live in social groups. 26 The exact nature of the social unit varies by species.  It depends on other factors in the environment such as availability of food and danger from predators.  For instance, monkeys who eat fruit require larger groups than leaf-eaters in order to forage effectively.  Most Old World monkeys live in social units with multiple adult females. 27 Extended female relatives help each other raise children and forage for food.  Males can help expand the territory or defend it from other families and keep a look out for predators. 28            

Gender roles are exaggerated by the mammalian life cycle.  Since a mother is limited to a relatively small number of children, the best strategy for her genes is to choose mates judiciously and supervise each child safely to adulthood.  A competitive male can rely more on the law of large numbers – his genes will become predominant if he sires more children than other males.  His options are determined by his social structure and his status within the community.  Without the hormonal influences of pregnancy, mammal fathers do not get as emotionally attached to their offspring, and do not generally participate in the daily activities of raising their young.    

Social life added a whole new dimension to the challenges of the environment.  In addition to predators, primates now had to figure each other out.  Cooperation, competition, and compromise were paramount to success.  Since the neocortex is so heavily devoted to social skills, it is reasonable to assume that it evolved in response to these needs.  Monkeys were able to recognize individuals by facial features instead of by smell.  This was doubly important, because facial expressions were sophisticated ways to communicate emotions.  Monkeys began to understand concepts such as friendship 29 and fairness 30 , guilt, cheating, and punishment. 31 A medium-term memory is essential for the social construct of reciprocal altruism. 32 If A does a favor for B, B should remember the favor and return it before too long.  If B does not return the favor, then A knows that B is a cheater and will stop giving favors.  Monkeys exhibit some innate sense for these rules of reciprocal altruism. 33 It’s not chess, but it’s certainly a step up from eating their own children.       

Male social life is especially dynamic.  Female hierarchies tend to be rigid.  Males much more frequently compete and change status.  Status can have clear survival benefits such as access to the best food, mates, and trees.  A male monkey’s status depends on aggressive competition as well as his social skills, conferring benefits for higher intelligence. 34

Whatever the advantages, the primate brain clearly evolved to reward social success and to avoid social falling. 35 When an individual connects with his peers in a positive way, a burst of neurotransmitters surges through his brain to associate that moment with an emotional reward and to encourage him to do it again.  We call it happiness.    

Back to Section 8.II: The Mesozoic World

Up to CHAPTER 8: THE LAST FEW 100,000,000 YEARS

Next, Section 8.IV: Chapter 8 Summary


C. Citations

  1. Albert F. Bennett and John A. Ruben, “Endothermy and Activity in Vertebrates”, Science 206(4419):649-654 (11/09/1979), https://www.ncbi.nlm.nih.gov/pubmed/493968 (accessed and saved 9/01/19).
  2. Heffner and Heffner, “Hearing ranges of laboratory animals”, Journal of the American Association for Laboratory Animal Science 46(1):20-22 (Jan., 2007), https://www.ncbi.nlm.nih.gov/pubmed/17203911 (accessed and saved 9/01/19).
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  9. These features were offered up as part of the definition of “primate” by St. George Jackson Mivart, Man and Apes (1873).
  10. Chris Beard, “Searching for our primate ancestors in China”, (Carnegie Museums, 1996), https://carnegiemuseums.org/magazine-archive/1996/marapr/beard.htm (accessed and saved 9/01/19).
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  12. This is a somewhat controversial hypothesis, dating at least to Grant Allen, The colour-sense: its origin and development, Houghton (Boston, 1879), https://archive.org/details/coloursenseitsor00alle/page/n8 (accessed 9/01/19).  Recent supporting evidence in favor of it includes Amanda D. Melin et al., “Trichromacy increases fruit intake rates of wild capuchins (Cebus capucinus imitator)”, PNAS 1705957114 (9/11/2017), https://www.pnas.org/content/early/2017/09/07/1705957114/tab-article-info (accessed and saved 9/01/19).
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  14. Heinz Stephan, G. Baron, and H.D. Frahm, “Comparison of brain structure volumes in Insectivora and Primates. II. Accessory olfactory bulb (AOB),” Journal für Hirnforschung 23(5):575-91 (1982), https://www.ncbi.nlm.nih.gov/pubmed/7161483# (abstract access and saved 9/01/19).
  15. Philip Ulinski, “The Cerebral Cortex of Reptiles”, Ch. 5, Comparative Structure and Evolution of Cerebral Cortex, Part I, Springer Science + Business Media New York (1990), p. 139, http://link.springer.com/chapter/10.1007/978-1-4757-9622-3_5 (accessed and saved 9/07/19).
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  17. The earliest version of this statement I can find online is by Midas Dekkers in his 1994 book Dearest Pet: On Bestiality, Verso, 1994, p. 32.
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  21. Hope Klug and Michael B. Bonsall, “When to care for, abandon, or eat your offspring: the evolution of parental care and filial cannibalism,” The American Naturalist 170(6):886-901 (10/01/2007),  http://www.ncbi.nlm.nih.gov/pubmed/18171171 (accessed and saved 9/02/19).
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  23. Daniel Olazábal and Larry Young, “Oxytocin receptors in the nucleus accumbens facilitate ‘spontaneous’ maternal behavior in adult female prairie voles”, Neuroscience 141(2):559-68 (5/24/2006), https://www.sciencedirect.com/science/article/abs/pii/S0306452206005070 (accessed and saved 9/02/19).
  24. E. van Leengoed, E. Kerker, and H.H. Swanson, “Inhibition of post-partum maternal behaviour in the rat by injecting an oxytocin antagonist into the cerebral ventricles”, J. Endocrinol. 112(2):275–282, https://joe.bioscientifica.com/view/journals/joe/112/2/joe_112_2_014.xml (accessed and saved 9/02/19).
  25. Michael Tomasello and Josep Call, Primate Cognition, Oxford University Press (1997) Amazon Kindle eBook edition location 195.
  26. Ibid at location 209.
  27. Dennis O’Neil, “Social Structure”, Biological Anthropology Tutorials, Palomar College (online, 2012),   http://anthro.palomar.edu/behavior/behave_2.htm (accessed and saved 9/02/19).
  28. At least indirectly while keeping a jealous lookout for competing males.  See Maribel Baldellou and S. Peter Henzi, “Vigilance, predator detection and the presence of supernumerary males in vervet monkey troops”, Animal Behaviour 43(3):451-461 (Mar., 1992), http://www.sciencedirect.com/science/article/pii/S0003347205801046 (abstract accessed and saved 9/02/19).
  29. Christopher Young et al., “Responses to social and environmental stress are attenuated by strong male bonds in male macaques”, PNAS 111(51):18195-200 (12/08/14), http://www.pnas.org/content/111/51/18195.abstract (accessed 9/02/19).
  30. Megan Van Wolkenten et al., “Inequity responses of monkeys modified by effort”, PNAS 104(47):18854-9 (11/20/2007), https://www.pnas.org/content/104/47/18854 (accessed and saved 9/02/19).
  31. Marc D. Hauser, “Costs of deception: cheaters are punished in rhesus monkeys (Macaca mulatta)”, PNAS 89(24):12137-9 (12/15/1992), http://www.pnas.org/content/89/24/12137.short (accessed and saved 9/02/19).
  32. Robert Trivers, “The evolution of reciprocal altruism”, Quarterly Review of Biology 46(1):35–57, https://www.researchgate.net/publication/230818222_The_Evolution_of_Reciprocal_Altruism (accessed and saved 9/02/19).
  33. Robert M. Seyfarth and Dorothy L. Cheney, “Grooming, alliances and reciprocal altruism in vervet monkeys”, Nature 308, 541 – 543 (4/05/1984), http://www.nature.com/nature/journal/v308/n5959/pdf/308541a0.pdf (abstract accessed and saved 9/02/19).
  34. Boguslaw Pawlowski, C.B. Lowen, and R.I.M. Dunbar, “Neocortex size, social skills and mating success in primates”, Behaviour 135(3):357-368, https://brill.com/view/journals/beh/135/3/article-p357_8.xml (accessed and saved 9/03/19).
  35. See several examples cited by David Buss, “The Evolution of Happiness”, American Psychologist 55(1):15 – 23 (Jan., 2000), http://people.uncw.edu/bruce/psy%20292/pdfs/happiness.pdf (accessed and saved 9/03/19).
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