 Solution

All three groups retain broadly shared structural solutions, while also containing more restricted clusters. The displayed domain-restricted counts are much larger for bacteria and eukaryotes, approximately $8.96\times 10^5$ and $8.32\times 10^5$, than for archaea, approximately $3.81\times 10^4$. Restricted clusters also occur at lower taxonomic levels, consistent with continuing diversification rather than a single early episode of structural innovation.

These counts are not direct rates of new-fold generation. The groups differ in sampling, numbers of represented sequences and species, evolutionary depth, and detectability of relationships. Restriction to a lineage can reflect divergence, differential loss or incomplete sampling as well as origin in that lineage. The figure therefore supports unequal observed structural diversity, but does not by itself identify how rapidly each lineage invents new folds.