HDS Evolution and Brain Development

What causes growth in white matter and what does this mean for connections in the brain

Growth in white matter is primarily caused by myelination and the formation of new axonal connections, which enhance the speed and efficiency of neural communication in the brain. This development is crucial for improved cognitive functions and overall brain connectivity.

Explain the role of experience in forming and maintaining neuronal connections

Experience strengthens connections, frequently used pathways become stronger and more complex, while unused ones are pruned. Early life is especially active, with over a million new connections forming per second. Relationships and repeated practice help maintain networks through neuroplasticity. - Prolonged development = More time to create and fine tune connections to fit the environment.

What is the human brain size at birth, when it reaches 90%, and adult size.

At birth a human brain is approximately 30% of its adult size. It reaches 90% by age 6 and growth is complete by 6-8 years. But maturation is not, myelination and synaptic refinement continue into adulthood. Experience strengthens connections, frequently used pathways become stronger and more complex, while unused ones are pruned. Early life is especially active with over a million new connections forming per second. Relationships and repeated practice help maintain networks through neuroplasticity.

What maturation is still occurring, and how long it continues

Full maturation (myelination and pruning) continues into the mid 20’s. - Synaptic spine development in the human prefrontal cortex continues until 30 years of age. In chimpanzees that is complete by puberty. In macaques this is largely complete by the end of infancy.

know how non-human apes compare

Humans show slower prolonged growth compared to apes, allowing extended learning, apes reach adult brain size faster, but humans evolved three times as many neurons compared to chimpanzees and gorillas. - Human neonate = 375g - Chimp neonate = 150g

How do growth rates differ?

This difference is linked to social and cognitive demands in humans. -In the utero both brain growth rates in both chimpanzees and humans increase from 16-22GW -After this point chimpanzee brain growth slows down. -Human brain growth continues to speed up. -Higher rate of brain growth in humans continues until 18 months postnatal.

What causes these differences in growth rates and what is the significance?

-Most early postnatal brain growth is due to increases in white matter. -More rapid increase in white matter in human infants compared to chimpanzees. (Development of complex social interactions). -Dramatic increases in white matter corresponds to opportunities for human specific social learning and acquisition of technical and linguistic skills.

When did these changes emerge in evolution?

In both humans and chimpanzees compared to macaques. > Smaller relative brain size at birth > Prolonged development of white matter through the juvenile period. > Delayed maturation of pyramidal neurons/Long period of synaptogenesis

Brain sizes at birth in hominins

Australopithecus brains ~ 385 - 550 cm Homo erectus: larger but still smaller than modern humans. Modern humans: ~350-400 cm at birth, growing to ~1,300 cm.

Why did the human pattern of brain growth emerge

Humans evolved a pattern called secondary altriciality, where babies are born with relatively small, immature brains that grow rapidly after birth. This pattern emerged because:

Large brains are energetically expensive; keeping fetal brain growth inside the womb becomes metabolically unsustainable for the mother beyond a certain point.

The obstetric dilemma: as humans evolved bipedalism, the pelvic canal narrowed, limiting how large a baby’s head can be at birth.

Evolutionary compromise: birth occurs earlier, with a smaller brain, so mothers can give birth safely while still allowing the species to evolve larger overall brains through rapid postnatal growth.

What physical characteristics of Australopithecus might have made babies more costly

Compared to earlier hominins, Australopithecus had traits that increased the cost of raising infants:

● Bipedal pelvis: a narrower birth canal made childbirth more difficult and risky.

● Larger body size than earlier apes → required more energetic investment in pregnancy and lactation.

● Relatively larger brains (though smaller than modern humans) → infants needed proportionally more energy.

● Reduced climbing adaptations (shorter arms) → mothers likely had to carry infants more often, increasing physical cost. All of these made infants more energy-demanding and more dependent on caregivers.

Why are human babies so helpless

Human infants are altricial—born early and dependent—because:

● They are born before their brains fully develop to fit through the birth canal (obstetric dilemma).

● Human brains consume huge amounts of energy (25%+ of resting metabolism). Mothers can only support so much fetal brain growth before birth.

● Postnatal brain plasticity enhances learning: being born helpless allows the brain to grow in response to the social and cultural environment.

● Helplessness is an evolutionary trade-off that supports both safe childbirth and extreme brain growth

How could costly babies drive evolution of brain size

● Parents with larger brains may have been better at planning, cooperating, and protecting their infants → infants more likely to survive.

● Kin and group members helping (alloparenting) favored individuals with social intelligence, empathy, and communication skills.

● Babies themselves with slightly larger brains may have survived better, learned more, or adapted faster → passing on those genes. ● Raising “expensive” babies selected for smarter caregivers, which in turn selected for smarter offspring → a feedback loop called the cognitive coevolution hypothesis.

How might needy babies have affected human social organization/social intelligence

● Alloparenting evolved — mothers relied on fathers, older siblings, grandparents, and community members to help care for infants.

● This required cooperation, communication, trust, and empathy → pressures for advanced social cognition.

● Long childhoods created space for social learning, teaching, and cultural transmission.

● Groups became more interdependent, encouraging stable bonds, pair-bonding, shared food, and division of labor.

● Over time, this helped drive the evolution of theory of mind, emotional intelligence, and complex social networks.