*3.2. Local Displacement*

The local displacement between the starting and the retrodeformed meshes (Figure 5) indicates the areas of maximum expansion, which is apparent on almost the entire left side of the skull, with the highest values recorded in the preserved portions of the left maxilla, left parietal, and left orbit. Conversely, the right side of the face is affected almost entirely by contraction, with the highest values recorded at the level of the anterior portion of the maxilla and nasofrontal suture. However, the inferior portion of the right maxilla is

expanded in the retrodeformed mesh. A somewhat balanced pattern of deformation occurs on the basicranium. The areas of maximum contraction on the right side are associated with areas of maximum expansion on the opposite side. Similarly, the moderate expansion recorded on the right side of the occipital squama corresponds to an almost symmetrical contraction of the left side. Lastly, along the midsagittal plane an area of contraction appears evident.

**Figure 4.** Comparison between the retrodeformed model of Steinheim (brown) and the original specimen (transparent blue). (**a**): anterior view; (**b**): posterior view; (**c**): inferior view; (**d**): leftlateral view.

## *3.3. Principal Component Analysis*

The results of the PCA are reported in Figure 6. The first three PCs explain 62.74% of the total variance in the sample, weighting 41.51%, 12.27% and 8.96%, respectively. In the plot, it is possible to discern a clear separation between modern (*Sap*) and fossil humans both along PC1 and PC3. Along PC2 is visible the separation between the Neanderthals sensu stricto (*Nea*) and a small group of Middle Pleistocene humans (*Mph*). The two models of Steinheim (*Ste*) fall within an intermediate position along PC1, between the *Sap* cluster and the fossil human group. While the original model of Steinheim (*Ori*) clearly diverges from the rest of the sample along both PC1 and PC2, the retrodeformed model of Steinheim (*R.D.*) approaches the fossil human group along the PC1, reaching the limit of the *Nea* cluster along PC2.

**Figure 5.** Local displacement (%) in the retrodeformed model of Steinheim, calculated by the function *localmeshdiff*. The white areas represent heavily damaged (basioccipital) or reconstructed (left temporal) portions of the skull that were excluded from this analysis.

**Figure 6.** Shape PCA on cranial landmark configuration. In black, Middle Pleistocene humans (*Mph*); in yellow, Neanderthals (*Nea*); in red, modern humans (*Sap*); in violet, Steinheim (*Ste*): original model (*Ori*) and retrodeformed model (*R.D.*). The abbreviations of the fossil samples are reported in Supplementary Table S1.
