Relative to the life history of other great apes, that of humans is characterized by early weaning and short interbirth intervals (IBIs). We propose that in modern humans, birth until adrenarche, or the rise in adrenal androgens, developmentally corresponds to the period from birth until weaning in great apes and ancestral hominins. According to this hypothesis, humans achieved short IBIs by subdividing ancestral infancy into a nurseling phase, during which offspring fed at the breast, and a weanling phase, during which offspring fed specially prepared foods. Imprinted genes influence the timing of human weaning and adrenarche, with paternally expressed genes promoting delays in childhood maturation and maternally expressed genes promoting accelerated maturation. These observations suggest that the tempo of human development has been shaped by consequences for the fitness of kin, with faster development increasing maternal fitness at a cost to child fitness. The effects of imprinted genes suggest that the duration of the juvenile period (adrenarche until puberty) has also been shaped by evolutionary conflicts within the family.

No abstract

Why do we sing to babies? Human infants are relatively altricial and need their parents' attention to survive. Infant-directed song may constitute a signal of that attention. In the rare genomic imprinting disorder Prader-Willi syndrome (PWS), typically paternally-expressed genes from chromosome 15q11-q13 are unexpressed, resulting in exaggeration of traits that reduce offspring investment demands on the mother. PWS may thus be associated with a distinctive musical phenotype. We report unusual responses to music in PWS. Subjects (N = 39) moved more during music listening, exhibited greater reductions in heart rate in response to music listening, and displayed a specific deficit in pitch discrimination ability relative to typicallydeveloping adults and children (N = 589). Paternally-expressed genes from 15q11-q13, unexpressed in PWS, may thus increase demands for music and enhance perceptual sensitivity to music. These results implicate genomic imprinting in the psychology of music, informing theories of music's evolutionary history.

Prader-Willi syndrome (PWS) is caused by a lack of expression of paternally-expressed imprinted genes at human chromosome 15q11-13 and is characterized by a switch from infant anorexia to childhood hyperphagia. A recent multiphase staging system recognizes gradual changes between the anorexic and hyperphagic phases of PWS. We undertook to use clinical records from an independent population to assess the multiphase system and explore the implications for the evolution of distinctive features of human childhood. Medical records of 258 clinic visits by 55 patients with PWS were reviewed with a focus on appetite and feeding. These clinical records were found to be inadequate for placing patients into particular phases of the multiphase system. Under the multiphase system, the onset of hyperphagia in PWS appears to coincide more with the timing of adrenarche than weaning from the breast and this timing should frame future evolutionary hypotheses. We discuss challenges encountered while attempting to use clinical data to explore evolutionary questions, but also identify useful information contained in the records.

Parent-offspring conflict-conflict over resource distribution within families due to differences in genetic relatedness-is the biological foundation for many psychological phenomena. In genomic imprinting disorders, parent-specific genetic expression is altered causing imbalances in behaviors influenced by parental investment. We use this natural experiment to test the theory that parent-offspring conflict contributed to the evolution of vocal music by moderating infant demands for parental attention. Individuals with Prader-Willi syndrome, a genomic imprinting disorder resulting from increased relative maternal genetic contribution, show enhanced relaxation responses to song, consistent with reduced demand for parental investment (Mehr et al., 2017, Psychological Science). We report the necessary complementary pattern here: individuals with Angelman syndrome, a genomic imprinting disorder resulting from increased relative paternal genetic contribution, demonstrate a relatively reduced relaxation response to song, suggesting increased demand for parental attention. These results support the extension of genetic conflict theories to psychological resources like parental attention.