Abstract
It has been suggested for long that physical principles might canalize evolution, especially in the genus homo, where brain size is the largest. However, what physical principle exactly would drive such a progress is unknown. Recent work suggests that there exists a mechanical, built-in, correlation between brain dilation and head flexure in vertebrate morphogenesis. This internal correlation would explain the canalization of evolution towards bigger brains and simultaneously a quite flexed head, with neural tissue curled forward. I report here a detailed study of neural tube flexure and dilation up to day 4 of development in chicken embryos. In vivo time-lapse shows a correlation between brain expansion and head flexure. The early embryo exhibits a biaxial texture in its tissue, in its vascular system and in heart formation, which causes line tensions antagonist to brain dilation. This competition between line tensions and vesicle surface expansion induces a mechanical correlation between brain dilation and flexure. Theoretical modeling reproduces how embryo texture constrains brain expansion and flexure, and how they relate to hemodynamics. This work demonstrates the existence of an internal, textural, correlation between heart morphogenesis, brain dilation and head flexure, which contributes to explaining the origin of humans. To be precise: the physical principle at the origin of humans is a competition between line tension and surface tension.