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Endnotes – Looking Back in Time
This cheat sheet is a quick reference to the background ideas explored in Looking Back in Time.
There is an expression: “You are what you eat.” Most people take it as a warning against poor dietary habits. But on a deeper level, it points to something more profound: the relationship between energy intake and what our bodies choose to do with it. Nowhere is this trade-off clearer than in the cost of running a brain.
Observations: Food for Thought
Humans
A healthy male, 1.7 m tall, under 40, weighing 80 kg, consumes roughly 2,822 kcal/day (estimate from a nutrition calculator). The Queensland Brain Institute suggests that the brain accounts for about 20% of this daily energy use. Human brain weight is typically 1,300–1,400 g.
Cats
An active, non-neutered domestic cat of 3.3 kg consumes about 308 kcal/day. Sources on feline neuroanatomy suggest a brain mass of 25–30 g(5).
Elephants
Data is harder to pin down, as Asian and African elephants differ. Estimates for a 5,000 kg elephant suggest a daily requirement around 70,000 kcal. Peer-reviewed studies report an average elephant brain weight of about 4,783 g(4) across samples of both species.
Consequences of the Observations
Comparison of daily calorie intake and brain mass shows broad trends, even if the underlying data are not precise. Species, age, activity, and measurement method all shift the values. The goal here is not decimal-point accuracy but orders of magnitude.
| Human | Cat | Elephant | |
| Brain/body ratio | 17.5 g/kg | 9.1 g/kg | 1.0 g/kg |
| Daily intake | 2,822 kcal | 308 kcal | 70,000 kcal |
| Brain energy use | ~564 kcal | ~12 kcal | ~1,928 kcal |
| % of intake | ~20%(6)(7) | ~3–4% | ~2–3% |
Using the human baseline:
- The human brain consumes about 20% of daily intake (~564 kcal/day).
- That translates to ~0.40 kcal/day/g of brain tissue.
- We then assume, conservatively, that 1 g of brain tissue in cats and elephants consumes the same energy as 1 g of human brain tissue.
This assumption is deliberately biased against humans. The human brain is known to be more costly per gram due to its higher neuron density and greater metabolic demand. That means the non-human estimates are almost certainly too high, and the true human–animal difference is even larger.
Of course, someone can quarrel over whether a cat uses 3% or 5%, but that misses the point: the real question is why humans took the gamble of feeding such an expensive brain, what advantages it brought — and what deficits had to be balanced. That’s the puzzle worth solving.
From these rough comparisons, several broad patterns emerge:
- Humans dedicate about one-fifth of daily energy to running the brain.
- Cats likely devote only a few percent. Even if our estimate is generous, it is unlikely to exceed 8%.
- Elephants, despite having massive brains in absolute terms, invest only a sliver of their daily energy compared to humans.
This implies that evolutionary forces applied extraordinary pressure on the human lineage: intelligence was metabolically “worth it,” even at the cost of competing with basic survival needs.
Other Sacrifices
Big brains do not come free. For humans, much of the expansion came in the frontal lobe—the seat of higher cognition. Consider the cat as a thought experiment: a 25% increase in brain size would take it from 30 g to ~37.5 g. Geometrically, this implies a 7% increase in skull diameter and a 16% increase in skull surface area. That extra surface area must be cooled, protected, and delivered through birth. The cost is disproportionate to the cognitive gain.
And what would a cat do with more brain power? It has no hands to shape tools, no evolutionary pathway ready to exploit the advantage. Evolution doesn’t reward intelligence in a vacuum.
For humans, the body plan was already primed. Upright posture, dexterous hands, and social living made the expansion not only possible but advantageous. The elephant offers another path: large social groups, a versatile trunk, and vast foraging needs. But even here, the metabolic trade-off keeps brain consumption proportionally small compared to humans.
Action: Based on the Observations and Consequences
The story of energy and brains helps explain why Homo sapiens emerged as the dominant hominin. Other Homo variants—Neanderthals, Denisovans—left Africa earlier, but did not persist. Only sapiens carried the metabolic gamble of a 20% brain and turned it into an adaptive advantage.
If you want to explore this further, Yuval Noah Harari’s Sapiens gives a readable, at times playful, account of how this gamble played out in culture and society
📖 Series Roadmap
- Forward: A Little Background
- Introduction: Action, Reaction, and the Human Paradox (16.09.2025)
- Looking Back in Time: The Development of the Human Brain (23.09.2025)
- Abstract Senses: Enhancing the way we see the world outside (30.09.2025)
- Bias as a Concept & Climbing the Stairs: Pattern Recognition & Everyday Tasks (07.10.2025)
- Abstract Feelings and Abstract Senses (14.10.2025)
- Motivation (04.11.2025)
- The Social Knowledge Base (11.11.2025)
- Potential (18.11.2025)
- The Subliminal Way We Go Through Life (26.11.2025)
- Taking Responsibility (02.12.2025)
- Fishing for Complements (22.12.2025)
- Peter and Fermi (22.12.2025)
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Endnotes – Looking Back in Time
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