Supplementary Online Material (SOM)

SOM Figure 1: PCs 2 and 3 of a PCA of the Trinil landmark set. Symbols as in Fig. 1 of the main text.

SOM Figure 2: PCs 3 and 4 of a PCA of the maximum fossils landmark set. Symbols as in Fig. 3 of the main text.

SOM Figure 3: PCs 1 and 2 of a PCA of the 42700 landmark set without frontotemporale, frontomalareorbitale and frontomalaretemporale. Symbols as in Fig. 6 of the main text.

Frontal bone analysis

One supplementary landmark set was generated to allow for the analysis of the affinities

of the KNM-ER 45000 fossil. KNM-OL 45500 is a fragmentary calvaria from Olorgesaillie,

Kenya attributed to H. erectus (only the frontal bone fragment was analyzed here) (Potts et al.,

2004).

Materials and methods

Landmarks were acquired from 25 fossil Homo frontal bones. The following landmarks

were analyzed: bregma, midline post-toral sulcus, glabella, nasion, frontomalare-temporale,

frontomalare-orbitale, mid-torus inferior, mid-torus superior, anterior pterion, frontotemporale,

and metopion. There was asymmetry between the right and left supraorbital landmarks of KNM-

OL 45500. It is unclear whether this is due to taphonomy and, if so, which side better reflects the

true morphology of this individual. As more of the morphology is preserved on the right side, the

arbitrary decision was made to estimate the left supraorbital notch and mid-torus inferior

landmarks from those preserved on the right side. Moreover, although bregma was assumed to be

at or close to the posterior border of the isolated frontal bone, this cannot be confirmed as the

parietal border of the frontal bone does not preserve a clear coronal suture in this region.

Results

PCA

This analysis included many fossils, but evaluated the shape of only the frontal bone and

its orientation or relative size. The first component contrasted Asian H. erectus with the rest of

the fossil sample, particularly KNM-OL 45500. KNM-OL 45500 had extreme scores on PCs 1

and 2 and was closest to early Homo (SOM Fig. 3). S 17 was the closest of the Asian fossils to

the African / Georgian distribution. Mid-Pleistocene Homo and the sole Neanderthal were

contrasted from early Homo on PC 2 and from H. erectus on a combination of PCs 1 and 2. Daka

and Bodo were positioned closer to mid-Pleistocene Homo on PC 1 but closer to H. erectus on

PC 2. Differences in the size of the frontal bone accounted for 38% of variance in PC 2 scores

(based on centroid size as EV is not available for KNM-OL 45500 – results were similar when

an EV of 622 cm3 was estimated for KNM-OL 45500 based on the relationship between centroid

size and EV in the rest of the sample). Small specimens scored high on PC 2 in general, but

KNM-OL 45500 scored higher than predicted for its size, suggesting that it was not just scaling

effects that resulted in its extreme position. H. erectus tended to score higher than comparably

sized mid-Pleistocene Homo.

The third component captured additional regional variation in H. erectus, emphasizing

the differences between H. erectus from China and Africa (as well as Ceprano) on one hand, and

those from Ngandong and Sangiran (and Dmanisi) on the other.

Shape differences

Asian H. erectus in general had straighter supraorbital tori in anterior and superior view,

as well as relatively narrow tori. Mid-Pleistocene Homo and the sole Neanderthal were

differentiated on PC 2 by their reduced constriction across frontotemporale, tall supraorbital tori,

and less of a “dip” posterior to glabella. The Ngandong, Sangiran and Dmanisi samples had taller

supraorbital tori, flatter median profiles and more posteriorly positioned frontotemporale.

Discussion

KNM-OL 45500 has a primitive frontal bone shape, but one that is distinct from the two

early Homo fossils included in the analysis. It combines a short frontal bone with marked

constriction across frontotemporale with a wide, thin (supero-inferiorly) and arched torus.

However, preservation may have affected landmark acquisition in a way that exaggerates some

of these features. For example, its low score on the first axis reflects in part its relatively short

antero-posterior length, but the position of bregma may be too far anterior as there was not a

clear coronal suture preserved at its posterior border. Also, the thin supraorbital torus indicated

by its high PC 2 score could be influenced by the use of the right supraorbital landmarks since

the right side appears to be abraded and therefore thinner than the left side. Because of these

issues it is difficult to know whether (and to what degree) KNM-OL 45500 was distinct from H.

erectus.

The other early African and Georgian fossils both exhibited frontal bones that were fairly

primitive in comparison with the more derived configurations of Asian H. erectus and later

Homo (which were themselves derived in different directions). Daka and Bodo had frontal bones

which were not fully derived in the direction of either H. erectus or mid-Pleistocene Homo. Bodo

is another Ethiopian fossil that is 200-400 ka older than Daka. Bodo exhibited a more primitive

morphology then other mid-Pleistocene fossils. Rightmire (2008, p. 10) had the following to say

about the Bodo frontal bone “The…projecting brow, low frontal with midline keeling…give the

specimen a pronounced archaic appearance.” Both Bodo and Daka grouped with mid-Pleistocene

Homo in the Bodo analysis, suggesting that the relative size and orientation of the frontal bone

was more distinctive in mid-Pleistocene Homo than its shape alone.

Som Figure 4: PCs 1 and 2 of frontal bone landmarks. 45500 is KNM-OL 45500. Other

abbreviations as in main text.

References

Potts, R., Behrensmeyer, A.K., Deino, A., Ditchfield, P., Clark, J., 2004. Small Mid-Pleistocene

hominin associated with East African Acheulean technology. Science 305, 75-78.

Rightmire, G.P., 2008. Homo in the Middle Pleistocene: Hypodigms, variation, and species

recognition. Evol. Anthropol: Issues, News, and Reviews 17, 8-21.