The 2013–2015 excavations exposed a trampled sandy silt layer over ~35 m2 (Supplementary Figs S1-2). The upper surface of this layer slopes NW to SE and would have formed a shallow depression (~0.1 m deep) close to the southeastern limit of the excavated area. The trampled area stretches from east to west, extending beyond the NE and SE excavation walls. Previous excavations destroyed the northern portion of this layer. The footprints cluster in squares EF4–7, with a few additional tracks in the surrounding area. In densely packed areas, the tracks are so overprinted that few individual tracks preserve diagnostic features and trackways are absent. The Gombore II-2 ichnosurface is best described, therefore, as a ‘congregation’ site in which the tracks of various vertebrate species are grouped around a focal point1 (Figs 2 and 3). The focal point appears to be the shallow depression at the SE portion of the excavated area (Supplementary Fig. S1) which was probably occupied by a small water body.

Figure 2 Overview of the densely-packed human and other animal footprints, with a close view of P-01 (adult hominin), P-07 and P-09 (both produced by a young child). Full size image

Figure 3 The ichnosurface of Gombore II-2. General planimetry of the excavated area (2013–2015) with colors referring to taxon attribution. Pictures of the best-preserved track(s) for each vertebrate group are provided: hominin (P-01, from square E3); hippo (sq. D2); bovid and possibly suid (sq. C8); equid (sq. E5); bird (sq. F6) and possibly a small carnivore (sq. C8). Full size image

During excavations thin, discrete sand lenses were recorded on the ichnosurface. This accumulating sand provided periodically fresh walking surfaces which implies that the animals visited the area regularly and in some cases tracks are compressed and/or superimposed through different layers (Supplementary Fig. S3). A range of hominin and other animal tracks are identified with reference to previous work in similar environments12,13,14. On this basis we recognize the following track-makers (Fig. 3): hominins, hippopotamus, bovids, equids, suids and birds. There is also a significant number of unidentifiable traces. The most frequent (>100 tracks) track-makers were large to small bodied bovids, potentially similar to gnu and gazelle in size (Fig. 3). In addition a total of 16 large hippo tracks (up to 0.33 m in diameter) were also identified. Bird prints of an unknown species are also common on the site. However, there are comparatively few suid or equid tracks and both lagomorphs and small carnivores were only tentatively identified.

Eleven possible human footprints were identified at the site (Figs 4 and 5; Table 1) with reference to other known footprint examples in East Africa in similar geological contexts1,12,13,14. These tracks have the characteristics proposed by Morse et al.15 as diagnostic of human tracks. The largest hominin track has clear and unambiguous anatomical definition (P-01: Fig. 4), with a rounded heel impression, visible digits and a drag line from the lesser toes. This track is comparable to other hominin tracks such as those identified at Ileret (Kenya)12. The smaller tracks pose a greater interpretative challenge however (Table 1). They have a variable morphology which has been impacted by adjacent tracks. This morphology ranges from simple kidney-shaped, flat-floored depressions (e.g., P-09, Fig. 5) to examples with potential toe impressions (e.g., P-05, Fig. 5). The two partial tracks in P-08 (Fig. 5), overlaying a hippo footprint, are instructive. The first track shows a vertical axial orientation in Fig. 5, and has a well-defined heel and a lateral foot margin that culminates in a partial first toe impression. Above this and orientated obliquely is a second small oval impression. Track P-09 shows drag marks associated with the removal of toes and P-06 shows a range of small toe impressions consistent with the multiple prodding of the substrate by a stationary individual. It is important to emphasize that the tracks are more consistent with someone standing in the mud rather than walking; hence the indistinct anatomy and multiple foot placements. In making an interpretation it is important to emphasize what they are not; they are not partial bovid tracks (i.e. one hemisphere) and they are morphologically inconsistent with equid, suid and small hippo tracks. Typical features of tracks made by modern young children (~12 months old) are shown in Supplementary Figure S4; note the tapering heel, the absence of a longitudinal medial arch and the relative proportions of the digits to the main body of the foot. Tracks of young children are also often associated with toe drag, consistent with a rolling or swinging gait.

Table 1 Track-maker stature and age estimates for tracks at Melka Kunture, Gombore II-2. Full size table

Figure 4 View and colour rendered model of the hominin track P-01, belonging to the left foot of an adult. Full size image

Figure 5 3D models of the hominin tracks. The scale bar is 50 mm (see Table 1 for dimensions). Models were made from oblique digital photographs in Agisoft (http://www.agisoft.com/). Full size image

In terms of comparative material to assist with the interpretation children’s tracks were identified at Happisburgh (Norfolk) dating to 1 Ma (Homo antecessor)16. However no 3D data was collected at this site before it was destroyed and there are few published 2D images of the children’s tracks. Perhaps the best preserved, to date, children’s tracks are the Holocene tracks from Namibia17 (Supplementary Fig. S5). While these tracks are shallower than those at Gombore II-2 a simple landmark based comparison is possible. A total of twelve landmarks were placed on 20 tracks from Namibia and 7 from Gombore II-2 using DigTrace18 and exported. A simple Procrustes analysis on the landmark coordinates was then performed using PAST19. As illustrated in Figure S6 both sets of landmarks cluster within 95% confidence ellipses of one another despite the potential ontological differences between the track-makers20,21,22,23. This helps corroborate the interpretation of these tracks as being those of children.

We can also explore this further by attempting to infer the track-makers age. This is not without problems however, given that we do not have ontogenetic information for the inferred track-maker (Homo heidelbergensis). Here we used modern growth curves derived from those developed by WHO24, based on 2D foot lengths, following Ashton et al.16 to give a first approximation and to determine potential ages. Foot length growth curves for Homo sapiens are dependent on the individual’s sex as well as their ethnicity25 and crucially the levels of nutrition26. It is also worth noting that WHO growth curves are based on direct foot measurements, and not on 3D tracks made in soft and compliant substrates. Also in dealing with larger tracks, the potential for overlap between adolescents and adult females, given sexual dimorphism, needs to be considered12,27. To complicate this further the applicability of growth curves based on Homo sapiens to earlier hominin species is also potentially problematic. For example, Dean and Smith28 suggest that typical growth curve for early Homo erectus was more like that of a modern chimpanzees, than that of Homo sapiens, although unique to itself. The age estimates made here (Table 1) should be recorded as first approximations only. We used two complementary approaches. The first uses the foot length to stature relationship of Rutishauser29 followed by application of the WHO standards to provide an approximate age. The alternative method is based on a sample of 365 individuals mainly European, of which 149 were under 20 years old, for which 3D tracks are available and the track-makers’ ages are known (Supplementary Fig. S7). Dividing the data into yearly age classes and then using an adjusted percentile method of bootstrapping (N = 9999), confidence limits (95%) were estimated for each age class and used to infer ages from track lengths. Estimates based on the 3D growth curve place the track-maker of the smallest tracks (P-04 and P-09) equal to 12 months old, while the approach using the 2D WHO data gives ages that are younger potentially as young as 6 months old (Table 1). We don’t know whether Homo heidelbergensis infants were able to stand/walk at this early age or not, but in the case of Homo sapiens it is early. This potentially invalidates the interpretation of the smallest of the tracks as being hominin however it is worth noting that track P-09 has good anatomical definition with clear distal impressions associated with the withdrawal of toes and a defined heel (Fig. 5). On balance we believe that the most parsimonious explanation of the smaller hominin-like tracks is that they were made by very young children and taken as a whole there is evidence to support the interpretation of the hominin tracks as being from a mixed age assemblage.

Skeletal remains of Homo heidelbergensis have been recovered in the Middle Acheulean layer below the footprint-bearing surface3, which leads us to assign the hominin track-maker to this taxa. Even if other hominin taxa cannot be excluded completely, what is clear however is that the best-preserved adult track (P-01, Fig. 4), has a morphology that is consistent with other tracks made by Homo1,12,13,14,17.