Precocial ancestry of placental mammals

Mammals exhibit two distinct reproductive strategies: altriciality, where neonates are highly underdeveloped, and precociality, where neonates are well-developed. The neonatal developmental state of deep nodes in mammalian phylogeny remains unresolved. Here, we use phylogenetic comparative methods to reconstruct the evolutionary history of neonatal maturity in mammals and demonstrate that the eutherian ancestor likely gave birth to precocial neonates. This finding contradicts the prevailing view that precociality evolved multiple times within the eutherian lineage. We contextualize this result with three lines of evidence. First, recent fossil evidence suggests that precocial life history traits arose early in therian evolution. Second, altricial eutherian neonates are markedly more developed at birth compared to altricial non-eutherians, suggesting a precocial ancestry in eutherian evolution. Third, reproductive traits that enable prolonged pregnancy originated in the stem lineage prior to the eutherian ancestor. Based on these findings, we propose an alternative model for the evolutionary history of precociality in mammals.