Podcast with Terence Deacon on evo-devo and brain development

How does evolution build brains without a blueprint? Terence Deacon reveals how self-organizing developmental processes, constrained by diffusible molecular signals, generate the neural architecture that natural selection then sculpts through competition and functional use. Subscribe for more from the Convergent Science Network podcast series. Terence Deacon brings the evo-devo perspective to brain science, arguing that understanding how brains are built during development is essential to understanding how they evolved. The core insight is that evolution does not modify adult brains directly , it modifies developmental programs. Since development relies heavily on self-organizing processes at every level, from gene regulatory networks to cell migration to axon guidance, the space of possible evolutionary changes is profoundly constrained by what development can produce. Deacon describes a two-phase process in brain construction. Early development is remarkably conserved across vertebrates: a neural tube divides into segments, segments differentiate into compartments, and generic form-production mechanisms generate a variety of circuits. This phase is so similar across species that embryonic brains of fish, birds, and mammals are nearly indistinguishable. The second phase involves selection: signals from sense organs, muscular systems, and inter-regional competition sculpt the generic architecture into species-specific functional circuits. Connections that are functionally validated persist; others are eliminated. The interview draws on Deacon's cross-species transplantation experiments, which produced striking results. Cortical cells transplanted anywhere in the brain grow axons only to targets appropriate for cortex , even in adult brains, even across species as different as pigs and rats. This means that molecular guidance cues persist throughout life, long after the developmental period when they were originally needed. Deacon suggests these cues serve ongoing plasticity and local synaptic maintenance rather than being mere developmental leftovers. He also describes clinical applications: pig fetal dopamine cells transplanted into Parkinsonian rats and eventually human patients found their targets, formed functional cross-species synapses, and produced measurable clinical improvement. Deacon challenges the common assumption that genes directly specify brain architecture. Instead, he describes cascading self-organization: genes regulate each other in network patterns, producing diffusible signals that create concentration gradients, which in turn activate or silence genes in neighboring cells. Even finger formation relies on this interplay , cells between digits are instructed to die by the intersection of multiple diffusion fields, not by a gene that says "build a finger here."