  Life systems are complex and hierarchical, with diverse components at
different scales, yet they sustain themselves, grow, and evolve over time. How
can a theory of such complex biological states be developed? Here we note that
for a hierarchical biological system to be robust, it must achieve consistency
between micro-scale (e.g. molecular) and macro-scale (e.g. cellular) phenomena.
This allows for a universal theory of adaptive change in cells based on
biological robustness and consistency between cellular growth and molecular
replication. Here, we show how adaptive changes in high-dimensional phenotypes
(biological states) are constrained to low-dimensional space, leading to the
derivation of a macroscopic law for cellular states. The theory is then
extended to evolution, leading to proportionality between evolutionary and
environmental responses, as well as proportionality between phenotypic
variances due to noise and due to genetic changes. The universality of the
results across several models and experiments is demonstrated. Then, by further
extending the theory of evolutionary dimensional reduction to multicellular
systems, the relationship between multicellular development and evolution, in
particular the developmental hourglass, is demonstrated. Finally, the
possibility of collapse of dimensional reduction under nutrient limitation is
discussed.
