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Over the last ~10,000 years (since the early Holocene), average human brain size has decreased slightly compared with Pleistocene values — a pattern documented in multiple populations around the world. The article doi: 10.3389/fevo.2021.742639 discusses a variety of hypotheses, including:
Energetic trade-offs
Changes in body size scaling
Social/collective cognition
Cultural outsourcing of memory and thinking
Reduced need for sensory/locomotor demands in settled societies
And one idea the authors highlighted is that once externalized collective intelligence — via writing, culture, and technology — became stable, individual brains didn’t need to carry as much raw computational/representational capacity internally.
When we extend that idea into the future — especially with the hypothetical arrival of Artificial General Intelligence (AGI) — there are a few plausible evolutionary/biocultural scenarios. None are certain, but they can be framed in terms of how evolutionary selection pressures might change:
1. Brain Size as an Evolved Trait Responds to Selection Pressures
Brain size (and its internal organization) didn’t evolve for abstraction or technology for its own sake — it evolved because, on average, individuals with certain brain phenotypes had higher reproductive success given their ecological and cultural environments.
Key point: Evolutionary change in brain size is not driven by technology itself, but by how technology changes the fitness landscape for humans.
So the question is: if AGI becomes widespread and embedded in human society, how might the fitness landscape for neural investment change?
Scenario A — Brain Size Continues to Decrease
If AGI becomes a fundamental part of human life (for thinking, problem-solving, memory, navigation, planning, etc.), then:
Much of what the brain currently does might be outsourced to external intelligence systems
Social and economic success may depend less on internal memory/analytical capacity and more on how effectively one collaborates with AI
Energetic costs of maintaining large brains (≈20% of basal metabolic rate) might become a disadvantage if not paired with higher reproductive success
If those pressures persist, natural selection might favor smaller, more efficient brains optimized for interacting with collective intelligence rather than raw individual computation.
This is not “progress toward a goal,” but a shift in what capacities carry reproductive advantage.
Scenario B — Brain Size Plateaus (S-curve Saturation)
We may already be near a plateau where further reduction just doesn’t give a fitness advantage because:
Cognitive outsourcing already exists widely (education, technology, networks)
Selection pressures that drove the Holocene decrease have stabilized
Human social and emotional intelligence remains essential in ways AGI can support but not fully replace
Under this scenario, brain size won’t shrink much more simply because it’s not strongly selected for or against — it hovers near an equilibrium matching current cultural-ecological demands.
This fits a classic S-shaped (sigmoidal) pattern where:
Phase 1 — growth (Pleistocene increase)
Phase 2 — shrinkage or adjustment (Holocene decrease)
Phase 3 — stabilization
AGI could reinforce the plateau by making additional shrinkage neutral or near-neutral in fitness terms.
Scenario C — Brain Structure Changes Rather Than Size
Even if overall volume doesn’t change much, the functional architecture might shift:
enhanced connectivity for social cognition
language/communication modules
integration with external systems (neuro-AI interfaces)
specialization in understanding/using systems rather than raw internal reasoning
In other words: the same size but different wiring.
Scenario D — Brain Size Could Increase Again
If AGI creates new cognitive niches where humans need:
more creative abstraction
deeper emotional/social intelligence
entirely new kinds of thought previously not selected for
then — in principle — selection pressure could favor increased complexity, even if different in nature from earlier expansions.
This is speculative but not impossible if cultural evolution consistently rewards new internal capacities.
What Drives These Outcomes?
Here’s a simplified view of the key factors:
| Factor | Can Reduce Size? | Can Increase Complexity? |
|---|---|---|
| Cultural/Tech Outsourcing |  |  |
| Energetic Cost Pressures | |  |
| Social/Emotional Complexity | | |
| New Cognitive Niches | | |
| Cooperation with AGI | | |
So the answer is not simply “AGI → shrink” or “AGI → no change.” It depends on how AGI changes:
what brains need to do internally
how humans survive and succeed reproductively
how culture mediates the value of internal vs. external cognition
Beyond Evolutionary Time
One other point: biological evolution is slow. Over the next centuries or millennia, cultural and technological evolution will outrun genetic evolution by orders of magnitude. So in the timeframe where AGI would be impactful culturally (decades to centuries), the brain will be shaped more by culture and individual learning than by genetic selection.
In that sense, we might see phenotypic plasticity and neural specialization without major genotypic brain size change.
In Summary
There are three broad possibilities after AGI becomes embedded in human life:
Continued shrinkage, driven by outsourcing of cognition and reduced internal demand
Plateau/S-curve stabilization, where brain size stays near current values
Structural reconfiguration, where how the brain is organized matters more than how big
None of these outcomes are inevitable — they depend on how culture, technology, and human goals co-evolve.
