The Lives of the Brain — Human Evolution and the Organ of Mind

Allen opens with the «simple version» of human brain evolution: around six million years ago the first hominins began walking on two legs while their brains stayed a third the size of ours; two million years ago came simple stone tools and a somewhat larger brain, then rapid growth 1–1.5 million years ago, reaching the modern 1,400 cubic centimetres within the last million years. Then he says: «There is nothing wrong with this story» — but the truth is far more complex. Tracking the brain's growth «from lime to grapefruit» is not enough; the brain lives in a body, and its evolution had biological costs.

The author's approach is clear and defines the book: this is not a book about consciousness, intelligence, evolutionary psychology or «palaces of the mind». Allen chooses to work «bottom-up» rather than «top-down», studying the brain as a collection of tissues, cells and genes that grow, eat and age. His question is whether the models of brain evolution built by ecologists, geneticists and palaeoneurologists fit together. It is an honest but also limiting framework — readers should not expect a grand answer to «what makes us human?».

One of the most interesting sections is «the expensive-tissue hypothesis». In 1995 Leslie Aiello and Peter Wheeler asked: the human brain consumes a large share of basal metabolism, so how do we «pay» for it if our overall metabolic rate is no higher than that of similar-sized mammals? Their answer: heart, kidneys and liver are the expected size, but our gastrointestinal tract is 60% smaller than expected for a primate — «a bigger brain for a smaller gut». Why the gut? The liver supplies the brain with glucose, the heart carries oxygen, the kidneys conserve water in hot equatorial settings — the gut is the only expensive organ that could shrink.

The hypothesis dovetails with Katharine Milton's work on «selective feeding» in primates: in the same habitat spider monkeys rely on ripe fruit and howler monkeys on leaves — and spider monkeys have brains roughly twice as large. High-quality food — animal products, nuts, underground tubers — permits a smaller, less energy-hungry gut. Allen also presents critical voices: Este Armstrong notes the relationship holds across many species, not just primates, and Jason Kaufman finds that the elephantnose fish, whose brain uses 60% of its basal metabolism, has a small stomach and intestine.

Weaknesses: the language is highly technical — terms like «encephalisation quotient», «basal metabolic rate» and «in vivo» pass without explanation, and the Turkish translation is sometimes laboured. The book dates from 2009; ancient DNA, the Denisovans and other discoveries have since changed the field considerably. Even so, it is a serious, thought-provoking book that teaches how to look at one subject through the eyes of many sciences.

• A «bottom-up» approach — the brain as a biological organ
• Numerous figures, graphs and research findings, extensive bibliography
• A «Conclusion» section at the end of each chapter