Introduction: General somatic growth slows down, 4.4 months gestation, yet growth/development/maturation of neural brain-CB/somatic immune system continues, full ontological pace.

Methods: This paper explores ontological significance of brain/CB and immune system in concert in fetus/neonate.

Results/discussion: CB studies show 2nd trimester anterior CB begins faster elongation rate, p < 0.001, matching midline extension brain-enlargement. A significant phylogenetic identity shift, early fusion of intrinsically controlled mid-sphenoid-synchondrosis, is an added elongation enhancement, comprising crista galli/olfactory receptors. An unexpected link between immune/olfactory systems by major histocompatibility complex (MHC) transmits information about MHC genotypes/influences behavior/reinforces new human identity.

Maturation of human immune response/brain growth follows same developmental pattern 1st year postnatal life. Conserved genome plasticity of immune response is 515 million years old; basic strategy for antigen presentation, MHC molecules, with protein-binding-region polymorphism was established. Conserved gene plasticity for neocortex development is 57 million years young; genes RFPL1,2,3 duplication/cluster-arrangement began neural expression for neocortex-size/organizational changes.

Life history theory, focusing on nutrition/pathogens, favors trade-offs for privileged brain/immune systems, investment associated with increased society longevity. Embryonic gene assembly confers vulnerability/responsiveness to environmental context the immature brain relies on to establish/guide pathways/connections. Maximized nutrition investment/outcome: 60% for brain growth 3rd trimester/60% BMR neonate brain results in brain 3:1/neocortex 3.6:1 larger than expected. Fetal capacity for immune responsiveness develops 1st trimester; driven by omnipresent environmental pathogens, infancy/growth investment is maximization by rapid development of individually refined/costly specific immune-repertoire.

Conclusion: Evolutionary principles inform central design features for brain-CB/immune defenses, constrained by genetic-programming with intergenerational effects: identity, plasticity, and fitness.

Conflict of interest: None declared.