Relationships/The Evolution of the Human Brain

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Large brains are humans' most distinctive anatomical feature. Our brains are about four times bigger than chimpanzees' and gorillas' brains.

Brains use twenty times the calories of muscles at rest. Brains require maintaining a constant temperature. Large brains are easily injured, and make childbirth difficult. Intelligence has many costs, yet doesn't directly help an animal survive (e.g., a big brain doesn't make you run faster or survive colder weather).

Our ancestors' brains began to enlarge about two million years ago. Two million years is short in evolutionary time.

The Triune Brain[edit]

Our brains comprise three distinct structures, representing three evolutionary periods.[1]

The oldest, deepest, and smallest area is the reptilian brain.[2] The reptilian brain controls the heart, lungs, and other vital organs. It enables aggression, mating, and reaction to immediate danger.

Mammals evolved the limbic system. This is the middle layer of our brains, surrounding the reptilian brain. The physiological features unique to mammals are in the limbic brain, e.g., the hypothalamus system for keeping us warm.[citation needed]

The limbic brain also produces emotions. Emotions facilitate relationships. Mammals, unlike reptiles, care for their young. Mammals evolved brains hardwired for mother-child and other relationships.

The most common reaction a reptile has to its young is indifference; it lays its eggs and walks (or slithers) away. Mammals form close-knit, mutually nurturant social groups-families-in which members spend time touching and caring for one another. Parents nourish and safeguard their young, and each other, from the hostile world outside their group. A mammal will risk and sometimes lose its life to protect a child or mate from attack. A garter snake or salamander watches the death of its kin with an unblinking eye.[3]
—Thomas Lewis, Fari Amini, and Richard Lannon
A General Theory of Love (2000)

The cerebral cortex (or neocortex) is the newest, outermost area of our brains. The oldest mammals, e.g., opossums, have only a thin layer of cerebral cortex. Rabbits have a little more, cats a bit more. Monkeys have a substantial cerebral cortex. Humans—and only humans—have an enormous cerebral cortex.[4]

The human reptilian brain and limbic system is similar in size and structure to other animals. I.e., our ancestors evolved a huge cerebral cortex, while the older brain areas didn't change.

The cerebral cortex learns new things.[citation needed] Animals with little or no cerebral cortex act only as their genes program them to act.[citation needed] Animals with a cerebral cortex can find new foods, survive in new environments, or change their mating tactics to improve reproductive success.

The human cerebral cortex goes beyond learning new foods and survival skills. Our brains can think in abstractions. We communicate via symbols (e.g., language), consider the past and future, and sacrifice our personal interests not only for our families (as other mammals do) but also for ideas (e.g., honor and country).

Conflicts between brain areas lead to relationship difficulties.[citation needed] In a conflicted brain, the older area wins. In contrast, an individual with an integrated brain—i.e., who uses his or her whole brain—solves relationship problems.[citation needed]

Ontogeny Recapitulates Phylogeny[edit]

A child's development mimics its species' evolution.

Infants live in their reptilian brains. They eat, breathe, crawl, sleep, etc.

Children live in their limbic brains. They feel emotions strongly. They use emotions to form relationships.

Adolescents live in their cerebral cortexes. They strive to become unique individuals. They quest to find abstract principles to live by.

Adult relationships invert childhood development.[citation needed] Men and women use cerebral cortex abstractions (e.g., gender roles) to attract opposite sex partners. If a couple then feels limbic brain emotionally connected "chemistry," they form a relationship. If the relationship goes well, sooner or later they're in bed, using their reptilian brains.

Love develops a child's limbic brain.[5] Unloved children fail to develop limbic brains capable of emotional intimacy. Such an individual can relate on a reptilian level—e.g., food, warmth, sex—or on a cerebral cortex level—e.g., excelling at accounting or the law—but have difficulty with intimacy.

Natural vs. Sexual Selection[edit]

In The Origin of Species (1859), Charles Darwin wrote that species evolve via random mutations. Environmental changes—e.g., changing food sources, predation, climate—favor one mutation over another. He called this process natural selection.

The conventional view is that our smarter, larger-brained ancestors invented tools, and then dominated their smaller-brained relations. The archaeological facts don't support this "man the toolmaker" hypothesis.[citation needed]

Our ancestors first used stone tools 2.5 million years, or 100,000 generations, ago.[6] This book has about 50,000 words. To refer to the first human as your "great-great-great…grandparent," you'd have to replace every word in this book with "great," and you'd need two books.

After one million years, or near the end of the first book, our ancestors' brains were more than double in size. Archaeologists can see slight improvements in their stone tools.[7]

500,000 years ago—halfway through the second book—our ancestors' brains were nearly as big as our brains. Our ancestors started using fire.[8] Fire enabled them to move from Africa to colder Europe and Asia.[citation needed]

50,000 years ago—eight pages from the end of the second book—our ancestors' brains reached modern size. Their stone tools became thinner and sharper. They carved small ornamental figurines from ivory, shell, and stone. They created beautiful cave paintings. They built the first ocean-going boats.[9]

5,000-10,000 years ago—the last page of the second book—our ancestors developed agriculture. Poor nutrition made farmers' bodies and brains smaller. They invented writing and metal tools. They invented the bow and arrow-a weapon that seems primitive to us.[10]

Our ancestors' brains enlarged before technological advances. Our ancestors' brains were required for each new technological innovation. Tool use was one of many uses for our large brains. Something else drove human brain evolution.[citation needed]

Sexual Selection[edit]

In The Descent of Man (1871), Darwin wrote that natural selection failed to explain human evolution.[citation needed] Instead, he proposed an alternative theory. Species evolve when males and females select each other for certain qualities. He called this sexual selection. Biologists ignored this idea for over a century.[11]

Female mammals, in general, are more selective than males. Females in most mammal species do most of the work of producing and raising children. In contrast, fathering offspring is less work, so males aren't so choosy.

The exertion of some choice on the part of the female seems almost as general a law as the eagerness of the male.[12]
—Charles Darwin, The Descent of Man (1871)

Females choose males with features that make the males less able to survive.[13] E.g., a peacock's bright colors make him visible to predators, and his huge tail slows his escapes. His beautiful tail communicates to peahens that he's an especially fit individual, i.e., he's so fast that he can escape predators despite his heavy tail. Sexual selection is, in general, the opposite of natural selection.[citation needed]

Natural selection advances via slow environmental change.[citation needed] Natural selection advances evolution only in harsh environments (e.g., predation, climate change).[citation needed] Natural selection produces animals better able to survive—usually smaller[citation needed], more efficient, and less conspicuous.

In contrast, sexual selection advances with each generation. Sexual selection produces rapid evolutionary changes. Sexual selection advances evolution in stable environments. Sexual selection produces animals (especially males) less able to survive, with bigger, brighter, or exaggerated features.

Did cerebral Cortex evolve by sexual selection?[edit]

Humans' over-sized brains could have evolved rapidly due to sexual selection. Evolution of a larger cerebral cortex and the ability to use it puts human beings in control of nature and not the other way round. This is essential to rear a weak and young new born. Women perhaps started to prefer men who could lay out a good shelter from predators and were smart enough if not strong enough to protect the young from the predator. And also a man perhaps started to prefer a woman who was smart enough to hide herself from predators, make a conducive and nurturing environment for the young

Our cerebral cortexes enable many behaviors, e.g., speech and language. But what's striking about the cerebral cortex is how much of it is not dedicated to specific behaviors. The human cerebral cortex has billions of general-purpose neurons, capable of learning any new idea. Why were our ancestral mothers and fathers—unlike any other animals—sexually attracted to partners who could learn new ideas?[citation needed]

Monogamy and Lying[edit]

Most nonhuman mammal fathers have little or no involvement with their offspring.[14] Male gorillas kill infants fathered by other males. Male chimpanzees help all the youngsters in their group, but they don't know who fathered each child.

Human evolution may have begun when fathers helped raise their children, giving the children a survival advantage. Among hunter-gatherers today, children without fathers are more than twice as likely to die during childhood.[15]

Monogamy could cause a conflict between two reproductive strategies. A man could try to have sex with many women, risking rejection from women, violence from other men, or his fatherless children not surviving. Although initially more offspring might be conceived this way, such a man might father no surviving children.

Or a man could choose to be in a monogamous relationship, and actively raise his children. Such a man would father only a few children, but his children would likely survive and prosper.

A woman could have sex with a desirable (e.g., high-status, tall, strong, handsome) man, and risk competing women taking him from her. Or she could choose a stable, monogamous relationship with a less-desirable man whom no one other woman wanted.

Sexual Lying Could Have Driven Cerebral Cortex Development[edit]

Getting caught reduces a liar's reproductive success. Catching liars increases the lie-catcher's reproductive success.

Lying requires imagination, quick thinking, and, above all, thinking of new lies. Catching lies requires imagination, quick thinking, and a long memory.

Those are cerebral cortex activities. Effective liars also match their emotions to their lies. You catch lies when an individual's emotional state doesn't match his or her words.[citation needed] Effective lying requires integrating one's cerebral cortex with one's limbic brain.[citation needed]

A man or woman with a larger cerebral cortex, well-integrated with his or her limbic brain, is better able to sexually lie, and to catch sexual lies. Such men and women became our ancestors.[citation needed]


  1. MacLean, Paul. The Triune Brain in Evolution: Role in Paleocerebral Functions (Plenum, 1990, ISBN 0306431688).
  2. A.k.a. basal ganglia or extrapyramidal motor system. Panksepp, Jaak. Affective Neuroscience: The Foundations of Human and Animal Emotions (Oxford, 1998, ISBN 0-19-509673-8), p. 42.
  3. Lewis, T., Amini, F., Lannon, R. A General Theory of Love (Random House, 2000, ISBN 0375503897), 25-26.
  4. Lewis, T., Amini, F., Lannon, R. A General Theory of Love (Random House, 2000, ISBN 0375503897), 43.
  5. Lewis, T., Amini, F., Lannon, R. A General Theory of Love (Random House, 2000, ISBN 0375503897), 43.
  6. Kehoe, Alice B. Humans: An Introduction to Four-Field Anthropology (Routledge, 1998, ISBN 0-415-91985-1), p. 53.
  7. Kehoe, Alice B. Humans: An Introduction to Four-Field Anthropology (Routledge, 1998, ISBN 0-415-91985-1), p. 55.
  8. Kehoe, Alice B. Humans: An Introduction to Four-Field Anthropology (Routledge, 1998, ISBN 0-415-91985-1), p. 55.
  9. Kehoe, Alice B. Humans: An Introduction to Four-Field Anthropology (Routledge, 1998, ISBN 0-415-91985-1), p. 61.
  11. Miller, Geoffrey F. The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (Doubleday, 2000, ISBN 0385495161), p.33.
  12. Darwin, Charles. The Descent of Man (Prometheus, 1871, ISBN 1573921769).
  13. Trivers, R.L. (1972). "Parental investment and sexual selection," in B. Campbell (ed.), Sexual selection and the descent of man 1871-1971. (Aldine, 1972).
  14. Diamond, Jared. Diamond's Hope: An Interview with Science's Multifaceted Storyteller, California Wild, Summer 2000.
  15. Hurtado, A.M., Hill, K.R. "Paternal effect on offspring survivorship among Aché and Hiwi hunter-gatherer: Implications for modeling pair-bond stability," in B.S. Hewlett (ed.), Father-child relations: Cultural and biosocial contexts (Aldine de Gruyter, 1992), pages 31-55.

About This Book · How Women Select Men

 v  d  e 
About This Book · Relationships · How Women Select Men
About This Book · Q&A · Recommended Books
The Science: The Evolution of the Human Brain · How Women Select Men · How Men Select Women · How Our Ancestors Lived · Monogamy and Polygamy · Hormones · Communication Styles
Life Stages: Childhood—Seeking Unconditional Love · Adolescence—Seeking Romantic Love · Adulthood—Families And Forgiveness · Agape—Altruistic Love
Practical Advice: Where Couples Met · Flirting · How to Write a Personal Ad · Dating · Sex · Becoming a Couple · Conflict In Relationships
Personality Types: Emotional Control Systems · Zeus-Hera · Poseidon-Athena · Apollo-Artemis · Hermes-Hestia · Ares-Hephaestus-Aphrodite · Dionysus-Demeter · Hades-Persephone