10Protein phase separation is essential for the self-assembly of non-membraneous 11 organelles. However, we know little about its ability to change in evolution. Here we 12 studied the evolution of the mammalian RNA binding protein FUS, a protein whose prion-13 like domain (PLD) is essential for the formation of stress granules through liquid-liquid 14 phase separation. Although the prion-like domain evolves three times as rapidly as the 15 remainder of FUS, it harbors absolutely conserved tyrosine residues that are crucial for 16phase separation. Ancestral reconstruction shows that the phosphorylation sites within 17 the PLD are subject to stabilizing selection. They toggle among a small number of amino 18 acid states. One exception to this pattern are primates, where the number of such 19 phosphosites has increased through positive selection. In addition, we find frequent 20 glutamine to proline changes that help maintain the unstructured state of FUS that is 21 necessary for phase separation. In summary, natural selection has stabilized the liquid-22 forming potential of FUS and minimized the propensity of cytotoxic liquid-to-solid phase 23 transitions during 160 million years of mammalian evolution. 24 25 avoids the pathological liquid-to-solid phase separation in FUS, just like it maintains 47 folding stability and reduces misfolding in proteins with structured domains 17 . 48To validate this hypothesis, we first identified 105 mammalian orthologs of FUS, 49 aligned them ( Figure 1A), and computed each residue's sequence entropy, a widely-used 50 measure of sequence divergence ( Figure 1B, see Supplementary Methods for a list of 51 sequences). The PLD domain, which is central for FUS phase separation, has the highest 52 sequence entropy of all FUS domains, with a median ~3-fold higher than that of the other 53 FUS domains ( Figure 1C; Wilcoxon rank-sum test, p=6.12 Î10 -12 ), and it shows that the 54 PLD domain evolves much faster than the rest of FUS. Nonetheless, tyrosine residues 55 within this domain are fully conserved, which indicates the essential role of these amino 56 acids for phase separation. 57Within the PLD, we observed two evolutionary hotspots, which are the regions S30 58 to S86, and A105 to Q147 (All site numbers and amino acids refer to human FUS). These 59 regions are subject to multiple substitutions that involve the amino acids glycine, serine, 60 alanine, threonine, asparagine, proline, and glutamine ( Figure 1D). By reconstructing 61 ancestral FUS proteins (Tables S1-S2, see Methods for details), we found that changes 62where the PLD sites toggle forth and back between G and S (113 changes), as well as 63 between A and T (34 changes), are especially prevalent ( Figures 1E-F). Together, these 64 changes account for ~60% of all changes in the evolution of the PLD. In addition, we 65 found 32 switches between serine and asparagine, and 20 switches between glutamine 66 and proline in these evolutionary hotspots (Tables S1-S2). 67 P in all residues that had experienced Q to P substitutions...