Project: Searching for traces of the Southern Dispersal
Principal Investigator: Dr. Marta Mirazón Lahr
Co-Investigators: Dr. Mike Petraglia, Dr. Stephen Stokes
Collaborators: Dr. Geoff Duller
Funding: NERC / EFCHED
Searching for Traces of the Southern Dispersal:
Environmental and Historical Research on the Evolution of Human Diversity in Southern Asia and Australo-Melanesia
Underlying scientific rationale
The last decade of research into the origins of modern humans has produced much evidence in support for a last common ancestral population in Africa in the recent past - within the last 150,000 years. One of the most intriguing aspects of a recent African origin of modern humans is the spatial and temporal pattern of differentiation of non-African populations.
All sources of data available to reconstruct such a history of population differentiation point to heterogeneous spatial and temporal patterns. The prehistoric record shows asynchronous modern human occupation of Eurasia, with dates from Australia somewhat older than those from the Levant and Europe. Furthermore, the character of the archaeological record of these early dispersing humans is strikingly different in southeast Asia-Australo/Melanesia and Western Asia/Europe. In terms of morphology, the record is much more scarce, although significant differences between early fossils in Europe and Australia can be observed. These differences can be explained in two ways. On the one hand, it is possible that a single human population dispersed out of Africa and that differentiation occurred in Asia prior to subsequent expansion to the East (southeast Asia-Australo/Melanesia) and Northwest (Eurasia). On the other, more than one African population could have dispersed to Eurasia, magnifying their pre-existing African differences in the process. These different explanations have been put forward as alternative hypotheses for the first dispersal of modern humans out of Africa*:

  1. Modern humans evolved in Africa in the late Middle Pleistocene/earliest Upper Pleistocene, where they remained until the expansion of a population associated with a sophisticated stone tool industry (the Upper Palaeolithic) throughout Eurasia around 45,000 years ago. This would represent the only major dispersal of modern humans out of Africa from which all non-African populations derive (Klein, 1999).
  2. Modern humans evolved in Africa in the late Middle Pleistocene/earliest Upper Pleistocene, followed by a significant pan-African expansion during isotope Stage 5 that established regional human populations throughout the continent. One of these populations expanded between 70,000 and 60,000 years ago from the Horn of Africa, across the strait of Bab el Mandeb, along the southern Asian coast, and eventually into southeast Asia and Australia. Another one expanded between 50,000 and 45,000 years ago from northeast Africa to the Levant and subsequently Eurasia, associated with a sophisticated stone tool industry (the Upper Palaeolithic) (Lahr & Foley, 1994, 1998; Lahr, 1996).

Neither of these hypotheses finds comprehensive support in the currently known palaeoanthropological record. Therefore, all scientific efforts at trying to elucidate the process by which the first human populations colonised Eurasia have to attempt to build models that include the causes of dispersal (Northwest and East African palaeoecology, human demography), the conditions and constraints at the time of dispersal (palaeoenvironment, geography, demography, subsistence), as well as the consequences (the biological and cultural differentiation of Eurasian peoples), and test such models against the available data. This project aims at building a model of one of the scenarios described above, namely that of multiple dispersals out of Africa, by focusing on the viability of a Southern Dispersal Route from East Africa to southern Asia and eventually Australo-Melanesia, independent of human expansions into mainland Eurasia.

Before discussing the work this project intends to carry out, it is worth asking two questions. First, why focus on multiple, rather than a single dispersal out of Africa? A dispersal of modern humans from the Levant is well documented in the archaeological record, representing as it does, the origin and geographical expansion of populations of modern skeletal character associated with Upper Palaeolithic industries (Klein 1999, Mellars 1995). That such a population expansion occurred, there is little doubt, although its ancestral source remains geographically and temporally elusive (Kuhn et al. 1999). However, the problem in this single dispersal scenario is one that several previous studies have faced, namely how to account for the regional differences in dating and character of the first record of occupancy in different parts of the world. Such explanation is inevitably constrained by the lack of key palaeoanthropological data in Africa and southern Asia where evidence for the shared ancestry and early differentiation would be found. The alternative hypothesis, that of multiple dispersals out of Africa, although facing the same empirical constraints, has yet to be explored in terms of the detailed reconstruction of the palaeoenvironmental conditions and constraints of such a historical event. In other words, by confirming or rejecting the palaeoenvironmental likelihood of a Southern Dispersal Route the field at large would be able to pursue more explicit hypotheses and draw strategies for future fieldwork.

The second question is what evidence is there to suggest that more than one dispersal out of Africa took place? Most of the pertinent evidence is negative, i.e., lack of shared features between the early southeast Asian-Australo/Melanesian populations and those of Europe (Howells, 1989; Lahr, 1996; Stringer, 1995). These include the lack of evidence of modern human presence prior to 48,000 in western Asia and 43,000 in Europe, while Australia seems to have already been occupied at the time (Roberts et al. 1994), and the absence of the otherwise widespread Upper Palaeolithic industries in southeast Asia and Australia, even though the latter populations show other elements considered unique to modern human material culture and expression (namely art, complex burials, bone tools and personal ornamentation) indicating a distinct cultural tradition (Noble & Davidson 1996). Positive support for a relationship between south Asian, Australo-Melanesian and sub-Saharan African populations derives from the phenotypic similarities observed in the craniofacial skeleton of these populations to the exclusion of Eurasians (Howells 1989, Lahr 1996), from recent genetic results (Maca-Meyer et al. 2001; Quintana-Murci et al. 1999; Underhill et al. 2001), and possibly in shared linguistic features between southeast Asians and Australo-Melanesian populations (Greenberg, 1971). Futhermore, a dispersal route from eastern Africa along the southern coast of Asia is a faunal corridor (Kingdon, 1993), mainly constrained by sea-level fluctuations and climate.
One may ask why, if such a dispersal took place, more conclusive evidence of its existence has not been found. We believe that the answer to this question lies in the nature of human palaeodemographic parameters and their outcome at a population level. In theoretical terms, the pattern of recent diversity, from which inferences towards historical processes can be made, shows a complex picture. Biological diversity among many populations may be characterised (both morphologically and genetically) as reflecting widespread regional patterns; these populations are often surrounded by groups who display significant differences, either discrete or in frequency of traits. This spatial distribution of biological traits suggests a historical process shaped by demographic changes leading to the differential expansion and contraction of populations through time, and resulting in the formation of outliers. In this context, if this early dispersal took place, its descendant populations in southern Asia, and to some extent also in Melanesia, would have been assimilated and/or replaced by later historic events. Furthermore, the Holocene rise in sea-levels would have erased much of the record of an early coastal population along the Indian Ocean Rim.

Scientific issues addressed & specific objectives of the project
This project thus addresses four major scientific issues with four aims:

  1. To investigate the palaeoenvironmental context for a Southern Dispersal Route out of Africa by establishing the nature of the palaeoenvironment (including sea-levels) along southern Asia during Stage 4;
    Objective: To build palaeoenvironmental maps for the southern Asian coastal areas based on sea-level reconstructions and the compilation of palaeoecological data into a palaeoenvironmental archive.
  2. To study the morphology of present and past populations along the suggested route with the explicit intent of testing their similarities and differences through time;
    Objective: To carry out a comparative study of the late Pleistocene and early Holocene human fossil record of East Africa, India and Australia, involving the collection of new data, as well as the compilation of existing data into a comprehensive palaeobiological archive for these regions.
  3. To study aspects of the Middle Palaeolithic record of Saudi Arabia and India, including the dating of key archaeological sites and palaeoenvironmental features;
    Objective: To date Middle Palaeolithic stratigraphic sequences in India and palaeolake sediments associated with Middle Palaeolithic industries in the Arabian Peninsula through SAR optical dating (Murray & White 2000, Stokes et al. 2001) and to carry out a comparative study of the early Middle Palaeolithic archaeological record of the Arabian Peninsula and Indian coast.
  4. To explore the pattern of population movement (direction, rate, permanency) along the hypothesised route through geographically and demographically explicit simulations of population expansions from Africa to southern Asia.
    Objective: To carry out geographically specific simulations of population movement along the hypothesised route, altering demographic and ecological parameters.

References

Greenberg JH (1971) The Indo-Pacific hypothesis. In TA Sebeok (ed.): Linguistics in Oceania, pp. 807-871.

Howells WW (1989) Skull shapes and the map. Cambridge, MA: Harvard University Press.Kingdon, 1993

Klein R. (1999) The Human Career. Chicago, Chicago University Press.

Kuhn SL et al. (1999) Initial Upper Palaeolithic in southcentral Turkey and its regional context: a preliminary report. Antiquity, 73: 505-17

Lahr MM & Foley RA (1994) Multiple dispersals and modern human origins. Evolutionary Anthrop. 3:48-60Klein RG (1995) Anatomy, behaviour and modern human origins. Journal of World Prehistory 9:167-198.

Lahr MM & Foley RA (1998) Towards a theory of modern human origins: Geography, demography, and diversity in recent human evolution. Yearbook of Physical Anthropology, Vol 41 - 1998 41:137-176.

Lahr MM (1996) The evolution of human diversity. Cambridge: CUP.

Lambeck K (1996) Shoreline reconstructions for the Persian Gulf since the last glacial maximum. Earth and Planetary Science Letters, 142: 43-57.

Maca-Meyer N et al. (2001) Major genomic mitochondrial lineages delineate early human expansions. BMC Gen 2: 13.

Manighetti et al. (1998) Propagation of rifting along the Arabia-Somalia plate boundary: into Afar. Journal of Geophysical Research, 103: 4347-4374.

Quintana-Murci L et al. (1999) Mt DNA, Y chromosome and human population history. M S-Med Sc 15:974-982.

Underhill PA et al (2001) The phylogeography of Y chromosome binary haplotypes and the origins of modern human populations. Annals of Human Genetics. 65: 43-62.

Mellars P (1995) The Neanderthal Legacy: An Archaeological Perspective of Western Europe. Princeton, Princeton Univ. Press.

Murray AS & Wintle AG (2000) Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements, 32 (1): 57-73.

Stokes S et al. (2001) An Empirical evaluation of SAAD and SAR Procedures. Radiation Measurements, 32: 585-594.

Noble W. & Davidson I. (1996) Human Evolution, Language and Mind: a psychological and archaeological enquiry. Cambridge, Cambridge University Press.

Ponce de León MS & Zollikofer CPE (2001) Neanderthal cranial ontogeny and its implications for late hominid diversity. Nature 412:534-538.

Roberts RG et al. (1994) The human colonisation of Australia: optical dates of 53,000 and 60,000 years bracket human arrival at Deaf Adder Gorge, Northern Territory. Quaternary Science Reviews 13:575-586.

Stringer CB (1995) The evolution and distribution of Later Pleistocene human populations. In: E.S. Vrba & G.H. Denton (Eds.) Paleoclimates and Evolution, with Emphasis on Human Origins, pp. 524-531. Yale: Yale University Press.

* Both hypotheses consider the presence of early modern humans in the Levant in the early Upper Pleistocene as representing an early and limited expansion of African populations which did not lead to the permanent occupation of Eurasia by the species.