The entropic hourglass of animal and plant embryogenesis

One surprising observation going back to pioneering works of Karl Ernst von Baer in 1828 and Ernst Haeckel in 1866 is that embryos of different animal and plant species express on average evolutionarily young genes at the beginning of embryogenesis, evolutionarily old genes in mid-embryogenesis, and again evolutionarily young genes at the end of embryogenesis. The origin of this molecular hourglass pattern of animal and plant embryogenesis has remained concealed, but here we find that not only the mean age of expressed genes changes during embryogenesis in an hourglass-like manner, but the whole age distribution of expressed genes changes. Specifically, when studying the entropy of this age distribution as a function of time, we find an hourglass pattern that surprisingly is orders of magnitude more significant than the original hourglass pattern of the mean, which might indicate that the entropic hourglass pattern is more fundamental than, and possibly even the origin of, the original hourglass pattern of animal and plant embryogenesis. Joint work with Hajk-Georg Drost, Alexander Gabel, and Marcel Quint.