The Brain posits that the human brain’s most important feature is its deep plasticity—i.e., its capacity to constantly reconfigure its own circuitry in response to the world. Eagleman argues that humans are “livewired” for constant adaptation, rather than “hardwired” for a specific ecological niche. This quality of being born “remarkably unfinished” is presented as humanity’s primary evolutionary advantage, as it allows us to adapt to any environment, absorb any culture, and continually learn throughout our lives. This theme leads Eagleman into his hypothetical and philosophical discussion of the future in the later chapters, where he imagines humans deliberately “hacking” their own “hardware,” augmenting their senses and bodies by feeding new data streams directly into the brain. 

The brain’s adaptability begins at birth. Unlike animals such as dolphins or zebras that are functional within hours, human infants are helpless for years. This protracted development, however, is a feature, not a bug. It allows a child’s brain to be shaped by its specific environment, a process Eagleman illustrates with the concept of neural “pruning.” A baby’s brain starts with a superabundance of connections, which are then pared back based on experience, strengthening the pathways that are used and eliminating those that are not. The tragic case of children raised in deprived Romanian orphanages provides a stark illustration of this dependency; without proper nurturing and stimulation, their brains failed to develop normally. Conversely, their partial recovery after being moved to foster care demonstrates the brain’s remarkable capacity for change when the environment improves.

Following this discussion of childhood into later life, Eagleman draws on scientific evidence to demonstrate how the adult brain continues to physically reshape itself in response to experience, a fact. In the well-known study of London cab drivers, brain scans revealed that the posterior hippocampus, a region essential for spatial memory, grew physically larger as they spent years memorizing the city’s 25,000 streets. Similarly, Albert Einstein’s brain showed an enlarged cortical fold known as the Omega sign in the area governing his left fingers, a direct result of his lifelong passion for playing the violin. These examples prove that “everything you’ve experienced has altered the physical structure of your brain” (20), making your identity a dynamic, ever-changing work in progress rather than a fixed entity.

Looking forward, Eagleman contends that this inherent flexibility is the key to transcending our biological limitations. This theme leans into Eagleman’s use of technological imagery and phraseology for the brain, presenting it as a biological computational device potentially compatible with man-made technologies. Emphasizing the brain’s plastic capacity for “plug-and-play” adaptiveness, Eagleman argues that the brain doesn’t care where its data comes from, only that this data has structure. This is the principle behind sensory substitution technologies like the VEST, a device from Eagleman’s own lab that allows deaf individuals to learn to “hear” through patterns of vibration on their torso. This concept extends beyond sensory repair to sensory augmentation, suggesting we could feed the brain novel data streams from the internet or other sources, creating new, direct perceptual experiences. By leveraging our brain’s plasticity, we are poised to expand the definition of what it means to be human. Through these hypotheses, this theme supports the text’s overwhelmingly positive, upbeat tone and optimism for the future.