But it's not that clear cut. What some anthropologists interpret as Negroid morphology, others view simply as primitive or archaic traits indicative of the generalized morphology of early modern humans. Phillipson (2005) summarizes this debate:
To the south, the harpoon-using fishermen of the central and southern Sahara, the Sudanese Nile Valley and parts of East Africa are represented by a few fragmentary skeletons from Ishango, Lothagam on Lake Turkana, Lowasera, Early Khartoum and elsewhere which are said to show negroid physical features. Similar characteristics occur in skeletons from the Qadan cemetery at Jebel Sahaba in Nubia, where Mechta-Afalou features have also been recognised. These remains may be from a population ancestral to present-day Nilotic negroids. Other authorities, emphasizing the presence of features which are also seen in KhoiSan and Northeast African 'caucasoid' populations, prefer to interpret this material as representing a more generalized 'ancestral African' physical type, which may be regarded as akin to a common ancestor of several more recent populations. This explanation also seems plausible for the varied human remains that have been recovered in association with broadly contemporary and rather later industries from southern Kenya.
Complicating things further is the fact that later era Nubia shows clear Caucasoid affinities, which has forced "Negroid Mesolithic" advocates to postulate a population replacement event during the Holocene (the Afrocentrists gloss over this point). Among the authorities questioning this population discontinuity model are Van Gerven et al. (1973), who believe that changes in morphology can be just as easily explained by adaptation (the study they cite is Greene et al. (1967), which analyzed crania from Wadi Halfa, Sudan):
Although the authors view the Nubian population as having remained stable over the last several thousand years, they propose that the living Nubians are the result of the massive penetration of Negroid Africa by Caucasoid genes during the last 14,000 years. In support of this hypothesis, the skeletal remains of a Nubian Mesolithic population are described as possessing bun-shaped occiputs, massive browridges, sloping foreheads, extreme facial flattening, large teeth and deep mandibles. This and other evidence is proposed to indicate that Africa, north of the Sahara, was originally inhabited by non-Caucasoid populations that can in general be termed Negroid.
There are several points at which this analysis remains essentially typological. Specifically, Greene pointed out in his discussion of racial reconstructions:
If indeed there were adequate models for the presumed Negro race of Africa that existed thousands of years ago, then this approach has validity. However, it can be contended that such models are at present not valid since there is little evidence for reconstructing what the then contemporary African Negroes were like skeletally. One can only extrapolate from modern Negroes who may not at all be like their ancestory since racial groups are not static, but evolve.
In addition, many of the skeletal features listed separately by Burnor and Harris may, in fact, be related functionally and/or epigenetically. Greene and co-workers have suggested that features such as large complex tooth form, glabellar protrusion, gonial eversion and massive mandible are all indicative of heavy masticatory masculature. Such features in the Nubian Mesolithic population are similar to those represented by the Neanderthal remains at the Skuhl site in Israel and also by contemporary aboriginal Australian populations. Such commonality suggests similar adaptation rather than common racial origin and migration.
Once again, the explanation of morphological similarity between two populations in terms of racial origins and affinities is totally inadequate unless the role played by natural selection and possible parallel evolution has been determined and incorporated into the analysis.
More recently, Larsen (1997) revisited the long-running debate and backed the continuity model of Greene, Van Gerven and the others that evolution and adaptation explain the differences between earlier and later Nubians better than population replacement:
Beginning in the nineteenth century, various workers speculated on the origins of human groups occupying the [Nile Valley]. Following Morton's (1844) highly influential study of archaeological crania from Egypt and Nubia, the prevailing notion was that two biologically distinct groups occupied the Nile Valley in temporal succession. In Lower Nubia, Morant (1925) identified an earlier 'Upper Nile type', with predominantly 'Negroid' features, and a later 'Lower Nile type', which lacked 'Negroid' features. The changes were viewed in a diffusionistic paradigm: simply, the disappearance of 'Negroid' features resulted from an invasion and subsequent replacement by alien 'Caucasoid' (Egyptian) peoples from the north.
Recent analyses of crania and dentitions from lower Nubia indicate that the evidence for the diffusionist model of biological change is less than compelling. Independent analyses of skeletal and dental discrete and metric variables and other lines of evidence suggest that the earlier and later Nubian populations represent a biological continuum with no invasion by nonindigenous populations. Therefore, the differences in cranial morphology between earlier and later populations...are best understood in relation to factors not involving population replacement.
For better understanding of these factors, especially those related to dietary and technological change, Carlson and Van Gerven and their coworkers compared craniofacial morphology in a Nubian-based temporal sequence, including foragers from the Mesolithic (ca. 12,000 BP), initial agriculturalists from the combined A- and C-groups (3400-1200 BC), and intensive agriculturalists from the combined Meroitic, X-group, and Christian horizons (AD 0-1500). These comparisons reveal that Nubian foragers and incipient agriculturalists have flat and elongated vaults with well-developed, protruding supraorbital tori and occipitals. In contrast, later intensive agriculturalists have rounded vaults with small and more posteriorly positioned faces and masticatory muscle attachment site (temporalis and masseter) and reduced temporomandibular joint size.
Carlson and co-workers posit a masticatory-functional hypothesis for explaining craniofacial changes in Nubia. They argue that the primary factor influencing Nubian craniofacial anatomy was the change in subsistence economy, from foraging to food production and the shift to consumption of softer foods. These changes resulted in a reduction in activity of the masticatory muscles and a concomitant decrease in mechanical loading of the craniofacial skeleton. Alteration in masticatory function led to alteration in craniofacial growth in two ways, including (1) decreased stimulation of bone growth, leading to a reduction in facial robusticity; and (2) progressive alteration of the overall growth of the face and vault, resulting in a smaller and more inferoposteriorly oriented face relative to the cranial vault.
Turner and coworkers reassessed Greene and coworkers' continuity model of Nubian population history. Although lauding the studies of Greene and others for using a nonracial, nontypological approach to Nubian population history, they argue that extra-regional sources of variation have been insufficiently considered, especially sources that may explain temporal shifts in craniofacial morphology in this region. Implicit in the work by Greene and others is the assumption that population continuity extends at least as far back as the late Pleistocene (ca. 12,000 BP). Turner and coworkers contend that continuity can be claimed only if non-Nubian populations are also considered in statistical analyses of dental trait variation in this region.
Turner and coworkers include in their analysis additional Nubian dentitions from the late Pleistocene 'Upper Stone Age', Meroitic, X-group, Christian period, and historic era European samples. Computed MMD values, modified for small samples and tested for significance, reveal few significant differences between Meroitic, X-group and Christian periods, thus confirming Greene's earlier conclusions regarding population continuity. However, significant differences between the Pleistocene and later groups were clearly identified.... Because of the apparent temporal discontinuity between the Pleistocene and later populations, Turner & Markowitz (1990) hypothesize that the ancestry of recent Nubians was not derived from local late Pleistocene populations, and that a population replacement event occurred during the Holocene in Nubia. The origin of these later populations is unclear, but, solely on the basis of dental traits, they argue that populations north of Nubia containing European and Near Eastern traits are the most likely sources.
In an effort to identify other possible sources of variation, Irish & Turner (1990) compared their sample of Nubian dentitions (late Pleistocene to Christian period) to historic-period dentitions from a west African group — the Ashanti. Univariate and MMD statistical treatment of these samples reveal strong similarities between modern west Africans and late Pleistocene Nubians. As with previous studies, later Holocene dentitions were found to be very similar. The late Pleistocene and modern west Africans are strongly divergent from the Meroitic, X-group, and Christian period Nubians. Therefore, the authors argue that there is a population discontinuity between the late Pleistocene and Holocene populations in Nubia, with the former sharing biological affinities with west Africans. Irish & Turner (1990) suggest that the discontinuity can be explained by high rates of violence and decline (or possible extinction) in late Pleistocene (Mesolithic) Nubian forager, which may have left them susceptible to invasion or 'genetic swapping' by other groups from west Africa.
Turner and coworkers' findings are intriguing, but their analyses do not necessarily disprove the continuity model for the Pleistocene to Holocene transition. Especially problematic is the virtual lack of data from the period between the A-group and the Mesolithic, which represents nearly four millennia of occupation of the region. Comparisons of Mesolithic and A-group populations reveal a decrease in craniofacial robusticity and dental complexity. To be sure, the agent of change could have been gene flow from some other region. Additional dental and morphological data, and a more substantial treatment of the archaeological context from regions surrounding Nubia, are required before the discontinuity model can be accepted. Moreover, the west African collection used for identifying dental trait frequencies is largely undocumented. Therefore, although the similarities between late Pleistocene and west African dental traits are interesting, they are not compelling. From the preponderance of evidence from other studies of craniofacial morphology, biological change, and population history, a model of population continuity appears to fit the evidence best.
It's extremely unlikely that Negroids would have penetrated so far north and east that early in time, and even more so considering that the later Nubians were Caucasoid, which requires the equally unlikely explanation of total population replacement after the Pleistocene. The continuity model that a robust, generalized early population evolved into Nubia's modern Caucasoid inhabitants makes a lot more sense.