Homo Sapiens: Anatomically Modern Humans (AMH)

Figure 1
Al Wusta location, map of site and stratigraphy.

Homo sapiens evolved in Africa in the late Middle Pleistocene1. Early dispersals out of Africa are evidenced at the Levantine site of Misliya at ~194-177 ka2, followed by Skhul and Qafzeh, where H. sapiens fossils have been dated to ~130-100 and ~100-90 ka respectively3. While the Levantine fossil evidence has been viewed as the onset of a much broader dispersal into Asia4–6, it has generally been seen as representing short-lived incursions into the woodlands of the Levant immediately adjacent to Africa, where relatively high precipitation is produced by winter storms tracking across the Mediterranean7,8. While the Levantine record indicates the subsequent local replacement of early H. sapiens by Neanderthals, the failure of early dispersals to extend beyond the Levant is largely inferred from interpretations of genetic data9. Genetic studies have suggested that recent non-African populations stem largely10, if not entirely9, from an expansion ~60-50 ka, but this model remains debated. The absence of low latitude Pleistocene human DNA and uncertainties regarding ancient population structure undermine conclusions drawn from genetic studies alone. The paucity of securely dated archaeological, palaeontological and ancient DNA data - particularly across southern Asia - has made testing dispersal hypotheses challenging4,7,11.

The Arabian Peninsula is a vast landmass at the crossroads of Africa and Eurasia. Growing archaeological evidence demonstrates repeated hominin occupations of Arabia20,21 each associated with a strengthened summer monsoon which led to the re-activation of lakes and rivers22–24, as it did in North Africa25. Here we report the discovery of the first pre-Holocene human fossil in Arabia, Al Wusta-1 (AW-1), as well as the age, stratigraphy, vertebrate fossils and stone tools at the Al Wusta site (Fig. 1, see also Supplementary Information).


Figure 2
Photographs and micro-CT scans of Al Wusta-1 Homo sapiens phalanx.

AW-1 is an intermediate manual phalanx, most likely from the 3rd ray (Fig. 2a, Supplementary Information 1: see below for detail on siding and species identification). It is generally well-preserved, although there is some erosion of the cortical/subchondral bone, and minor pathological bone formation (likely an enthesophyte) affecting part of the diaphysis (Supplementary Information 1). The phalanx measures 32.3 mm in proximo-distal length, and 8.7 mm and 8.5mm in radio-ulnar breadth of the proximal base and midshaft, respectively (Supplementary Table 1).

AW-1 is more gracile than the robust intermediate phalanges of Neanderthals26–28, which are broader radio-ulnarly relative to their length and have a more 'flared' base. AW-1’s proximal radio-ulnar maximum breadth is 14.98 mm, which provides an intermediate phalanx breadth-length index (proximal radio-ulnar maximum breadth relative to articular length) of 49.6. This is very similar to the mean (± SD) for the Skhul and Qafzeh H. sapiens of 49.7 (± 4.1) and 49.1 (± 4.0) for Upper Palaeolithic Europeans, but 1.89 standard deviations below the Neanderthal mean of 58.3 (± 4.6)29.

To provide a broad interpretive context for the Al Wusta phalanx, we conducted linear and geometric morphometric (GMM) landmark analyses (Supplementary Information 1) on phalanges from non-human primates, fossil hominins and geographically widespread recent H. sapiens. Comparative linear analyses (Supplementary Information 1, Supplementary Tables 2 and 3, Supplementary Figure 1) reveal that there is substantial overlap across most taxa for all shape ratios, so AW-1 falls within the range of variation of H. sapiens, cercopiths, Gorilla, Australipithecus afarensis, A. sediba and Neanderthals. However, AW-1 is most similar to the median value or falls within the range of variation of recent and early H. sapiens for all shape ratios.

Figure 4
Scatterplot of the first two principal component (PC) scores from the geometric morphometric analyses of AW-1 and sample of comparative hominin 2nd, 3rd, and 4th intermediate phalanges.

The more in-depth shape comparison and modelling using the hominin sample of phalanges of known ray and side (Supplementary Table 7) demonstrates that the long and slender morphology of AW-1 falls just outside the range of variation of comparative Middle Palaeolithic modern humans, but that its affinity is clearly with H. sapiens rather than Neanderthals (Fig. 4, Supplementary Table 8). Although both Pleistocene H. sapiens and Neanderthal landmark configurations fall almost completely inside the scatter for the Holocene H. sapiens sample in the principal components analysis (Figure 4), AW-1 is closest to Holocene H. sapiens 3rd intermediate phalanges. AW-1 overlaps with the Holocene H. sapiens sample, but is separated from the Pleistocene H. sapiens specimens by a higher score on PC2 and from the Neanderthal group by a simultaneously higher score on PC1 and PC2. The Procrustes distances (Supplementary Table 8), also show that AW-1 is most distinct from the Neanderthal phalanges, which fall towards the lower ends of both PCs and are characterised by shorter and broader dimensions. PC1 and PC2 in this analysis show that AW-1 is taller and narrower (in all directions: dorso-palmarly, proximo-distally and radio-ulnarly) than almost all the phalanges in the comparative sample and is particularly distinct from most of the Neanderthal phalanges. In this analysis AW-1 is closest in shape to 3rd phalanges of individuals from (in descending order of proximity) Egyptian Nubia, and Medieval Canterbury (UK), and Maiden Castle (Iron Age Dorset, UK) (Supplementary Table 9), although there is not a great difference in its distance to any of these specimens. These analyses suggest that the AW-1 phalanx is likely to be a 3rd intermediate phalanx from a H. sapiens individual.

The third ray is the most symmetrical ray in the hand and is therefore difficult to side, particularly when not all of the phalanges of a particular individual are present. Comparing AW-1 separately to right and to left phalanges (Supplementary Information 1.4) gives results which are very similar to the pooled sample, such that AW-1 is closest to Holocene H. sapiens 3rd rays for both right and left hands (Supplementary Figure 4, Supplementary Table 10). There is little difference in morphological closeness between AW-1 and its nearest neighbour in the samples of right and left bones (Supplementary Table 11), reflecting the lack of difference in morphology between the sides. It is therefore not possible to suggest whether AW-1 comes from a right or a left hand using these analyses.

AW-1 is unusual in its more circular midshaft cross-sectional shape (Fig. 2B), which is confirmed by cross-sectional geometric analyses (Supplementary Information 1.5). This may reflect the pronounced palmar median bar that makes the palmar surface slightly convex at the midshaft rather than flat, the latter being typical of most later Homo intermediate phalanges. However, more circular shafts may reflect greater loading of the bone in multiple directions and enthesophytes are a common response to stress from high levels of physical activity30. This morphology may reflect high and varied loading of the fingers during intense manual activity.

To determine the age of AW-1, and associated sediments and fossils, we used a combination of uranium series (U-series), electron spin resonance (ESR) and optically stimulated luminescence (OSL) dating (Methods, Supplementary Information 2 and 3). U-series ages were produced for AW-1 itself (87.6 ± 2.5 ka) and hippopotamus dental tissues (WU1601), which yielded ages of 83.5 ± 8.1 ka (enamel) and 65.0 ± 2.1 ka (dentine). They should be regarded as minimum estimates for the age of the fossils. In addition, a combined U-series-ESR age calculation for WU1601 yielded an age of 103 +10/-9 ka. AW-1 was found on an exposure of Unit 3b, and WU1601 excavated from Unit 3a, one metre away (Fig 1b). Unit 1 yielded OSL ages of 85.3 ± 5.6 ka (PD17), 92.2 ± 6.8 ka (PD41) and 92.0 ± 6.3 ka (PD15), while Unit 3a yielded an OSL age of 98.6 ± 7.0 ka (PD40). The OSL age estimates agree within error with the US-ESR age obtained for WU-1601 and the minimum age of ~88 ka obtained for AW-1. These data were incorporated into a Bayesian sequential phase model31 which indicates that deposition of Unit 1 ceased 93.1 ± 2.6 ka (Phase 1: PD15, 17, 41) and that Units 2 and 3 and all associated fossils were deposited between 92.2 ± 2.6 ka and 90.4 ± 3.9 ka (Phase 2: all other ages) (Supplementary Information 4, Supplementary Figure 11).


Figure 5
Selected Al Wusta lithic artefacts.

This ~95-86 ka timeframe is slightly earlier than most other records of increased humidity in the region in late MIS 5 32,33, which correlate with a strengthened summer monsoon associated with an insolation peak at 84 ka (Fig. 6). The underlying (Unit 3) aeolian sand layer at Al Wusta correlates with an insolation minimum at the end of MIS 5c. The chronometric age estimates for the site suggest that lake formation and the associated fauna and human occupation occurred shortly after this in time. Regional indications of increased humidity around the 84 ka insolation peak include speleothem formation at ~88 ka in the Negev34, and the formation of sapropel S3 beginning ~86 ka35. In both the Levant and Arabia, records are consistent with this switch from aridity to humidity around this time32–40. Precisely reconstructing regional palaeoclimate at this time and relating it to human demographic and behavioural change has proved challenging. This reflects both rapid changes in climate, as well as the complexities involved in dating relevant deposits41. In summary, combining chronological data (Supplementary sections 2-4), interpretation of the sedimentary sequence (described below), and the regional setting of Al Wusta, we conclude that lake formation and associated finds such as the AW-1 phalanx relate to the late MIS 5 humid period associated with the 84 ka insolation peak.


Figure 6
The chronological and climatic context of Al Wusta.

Al Wusta-1 is the oldest directly dated H. sapiens fossil outside Africa and the Levant. It joins a small but growing corpus of evidence that the early dispersal of H. sapiens into Eurasia was much more widespread than previously thought. The site of Al Wusta is located in the Nefud desert more than 650 km southeast of Skhul and Qafzeh (Fig. 1A). This site establishes that H. sapiens were in Arabia in late MIS 5, rather than being restricted to Africa and the Levant as suggested by traditional models (Fig. 6). With Skhul dating to ~130-100 ka, Qafzeh to ~100-90 ka3,46 and Al Wusta to ~95-85 ka it is currently unclear if the southwest Asian record reflects multiple early dispersals out of Africa or a long occupation during MIS 5. The association of the Al Wusta site with a late MIS 5 humid phase (Fig. 6), suggests that significant aspects of this dispersal process were facilitated by enhanced monsoonal rainfall. While changes in behaviour and demography are crucial to understanding the dispersal process, climatic windows of opportunity were also key in allowing H. sapiens to cross the Saharo-Arabian arid belt, which often constituted a formidable barrier24,25.

Al Wusta shows that the early, Marine Isotope Stage 5, dispersals of H. sapiens out of Africa were not limited to the Levantine woodlands sustained by winter rainfall, but extended deep into the Arabian interior where enhanced summer rainfall created semi-arid grasslands containing abundant fauna and perennial lakes. After long being isolated in Africa1,47,48, the Late Pleistocene saw the expansion of our species out of Africa and into the diverse ecologies of Eurasia. Within a few thousand years of spreading into Eurasia our species was occupying rainforest environments and making long sea crossings to remote islands13,18. Adapting to the semi-arid conditions of the Saharo-Arabian arid belt represented a crucial step on this pathway to global success and the Al Wusta Homo sapiens fossil demonstrates this early ability to occupy diverse ecologies which led to us becoming a cosmopolitan species.

Nat Ecol Evol. 2018 May; 2(5): 800–809.