Elastin is an unusual extracellular matrix protein responsible for the properties of extension and energy-efficient elastic recoil of large blood vessels, heart valves, lung parenchyma and many other vertebrate tissues requiring such resilience. Polymeric elastin is assembled from monomeric tropoelastin by a process involving liquid-liquid phase separation, followed by maturation into an extended elastic matrix, covalently cross-linked through the side chains of lysine residues in the protein and producing a robust biomaterial with the structural integrity to withstand hundreds of millions of cycles of extension and recoil without mechanical failure. Elastin functions as an entropic elastomer, whose properties are the direct result of the inability of the protein to fold into a fixed, stable structure.Previous investigations of how the unusual properties of polymeric elastin arise from the sequence of tropoelastin have depended primarily on modeling using molecular biological and biophysical methodologies. This study takes a unique alternative approach, using a well-curated database of tropoelastin sequences from more than 80 representative species of Amniotes to identify characteristics that are conserved over more than 300 million years of evolution in order to provide assembly and conformational flexibility requirements of elastins.Conserved characteristics included preservation not only of regions of linear or positional sequence, but also of collective or compositional characteristics, derived from the sequence but not strictly dependent on positional sequence. A plausible overall consensus sequence for Amniote tropoelastins allowed quantification of residue-by-residue, domain-by-domain and region-by-region levels of sequence conservation, identifying distinct regions of high and low positional sequence conservation. Regions of low positional sequence conservation nevertheless maintained a recognizable, low complexity sequence style characterized by tandem repeats and partial repeats of short, non-polar motifs. Analysis of these motifs indicated hPGhGG, with numerous mutations, insertions and deletions, as the underlying repeating unit in all Amniote tropoelastins.Together these data identify significant evolutionary constraints dictated by fundamental requirements for formation and functionality of the extracellular elastin matrix. Mutations/polymorphisms in human tropoelastin affecting such well-conserved characteristics might be expected to have phenotypic consequences.