letters to nature In Fig. 1 I show the results of two sets of calculations. The ®rst, to 15. Zhang, K. & Gubbins, D. On convection in the Earth's core driven by lateral temperature variations in provide a baseline, is a calculation with a Y2 pattern of heat ¯ux to the lower mantle. Geophys. J. Int. 108, 247±255 (1992). 2 16. Zhang, K. & Gubbins, D. Convection in a rotating spherical ¯uid shell with an inhomogeneous model the present day mantle. This pattern very closely matches the temperature boundary condition at in®nite prandtl number. J. Fluid Mech. 250, 209±232 (1993). distribution of |I| for the GAD model (Fig. 1). The second set 17. Sarson, G. R., Jones, C. A. & Longbottom, A. W. The in¯uence of boundary region heterogeneities on 0 the geodynamo. Phys. Earth Planet. Inter. 101, 13±32 (1997). consists of four calculations each with a Y2 pattern of heat ¯ux but 18. Bloxham, J. The effect of thermal core-mantle interactions on the paleomagnetic secular variation. with differing amplitude. These show that the distribution of |I| Phil. Trans. R. Soc. Lond. A 358, 1171±1179 (2000). becomes closer to that observed palaeomagnetically earlier than 19. Glatzmaier, G., Coe, R., Hongre, L. & Roberts, P. The role of the Earth's mantle in controlling the 250 Myr ago as the amplitude of the variations in heat ¯ux is frequency of geomagnetic reversals. Nature 401, 885±890 (1999). increased. The calculations all have a non-zero axial octupole term 20. Giardini, D., Li, X.-D. & Woodhouse, J. Three-dimensional structure of the Earth from splitting in free-oscillation spectra. Nature 325, 405±411 (1987). in addition to the axial dipole, with all other terms averaging to zero 21. Gubbins, D. & Zhang, K. Symmetry properties of the dynamo equations for palaeomagnetism and within one standard deviation. I note that the emergence of an axial geomagnetism. Phys. Earth Planet. Inter. 75, 225±241 (1993). octupole term rather than an axial quadrupole could be anticipated given the symmetry properties of the dynamo21 (the historical ®eld Acknowledgements shows a strong bias towards odd harmonics) and the symmetry of I thank P.Hoffman, K. Katari, D. Kent, A. Maloof, R. O'Connell, D. Schrag and L. Tauxe for the imposed heat ¯ux variation. useful conversations during this work. This work was supported by the NSF. These results demonstrate that by altering the pattern of lateral Correspondence and requests for materials should be addressed to the author heat ¯ux variations at the core±mantle boundary it is possible to (e-mail: [email protected]). ®nd dynamos that generate magnetic ®elds with persistent axial octupoles. In other words, they indicate that the tenability of the GAD model is a function of the pattern of lateral heat ¯ux variations at the core±mantle boundary. Without doubt, that pattern will have been different in the past, so I must conclude that it is unsafe to ................................................................. apply the GAD hypothesis earlier than 250 Myr ago. Indeed, it is no coincidence that the expected timescale for changes in the pattern of Early human occupation of the mantle convection is similar to the timescale on which we can test the GAD model, as the disappearance of sea ¯oor is itself a Red Sea coast of Eritrea manifestation of cycling or turnover of the mantle. Several cautions must be raised regarding this study. First, during the last interglacial because of the uncertain relationship between lowermost mantle heterogeneity and core±mantle boundary heat ¯ux, I am unable to Robert C. Walter*, Richard T. Buf¯er², J. Henrich Bruggemann³§k, scale the amplitude of the heat ¯ux variations to the Earth, though Mireille M. M. Guillaume¶, Seife M. Berhe#, Berhane Negassi²I, the amplitudes that I have considered here are not unreasonably Yoseph Libsekal**²², Hai Cheng³³, R. Lawrence Edwards³³, large. Second, it is unreasonable to expect that the pattern of heat Rudo von Coselk, Didier NeÂraudeau§§ & Mario Gagnonkk ¯ux variations would have remained ®xed throughout the Precam- brian and Palaeozoic. However, suppose that occasionally as the * Departmento de Geologica, Centro de InvestigacioÃn Cientõ®ca de EducacioÂn pattern of mantle convection evolves, the lowermost mantle Superior de Ensenada, km. 107 Carr. Tijuana-Ensenada, Ensenada, B.C., Mexico 0 ² Institute for Geophysics, University of Texas, Austin, Texas 78712, USA acquires a Y2 signature which gives rise to an axial octupole in the magnetic ®eld, and that at other times (such as the past 250 Myr) the ³ Department of Marine Biology, University of Groningen, PO Box 14, ®eld is close to a geocentric axial dipole. Then, averaging the ®eld 9750 AA Haren, The Netherlands over a suf®ciently long time interval (one that encompasses periods § Department of Marine Biology and Fisheries, University of Asmara, Asmara, PO Box 1220, Eritrea of Y0 heat ¯ux variation), we would see a bias towards low 2 k Experimental Zoology Group, Wageningen Institute of Animal Sciences, palaeomagnetic inclination. So, somewhat unexpectedly, the Wageningen University, PO Box 338, 6700 AH Wageningen, The Netherlands reason that the GAD hypothesis works well for the Cenozoic and ¶ MuseÂum National d'Histoire Naturelle, Laboratoire de Biologie des InverteÂbreÂs Mesozoic may be simply that 250 Myr is too short a period over Marins et Malacologie, ESA 8044-CNRS, 55 rue de Buffon, 75005 Paris, France which to average the ®eld to test the hypothesis adequately. M # African Minerals Inc., PO Box 4588, Asmara, Eritrea I Department of Mines, Ministry of Energy and Mines, PO Box 272, Asmara, Eritrea Received 9 July 1999; accepted 17 March 2000. ** National Museum of Eritrea, PO Box 5284, Asmara, Eritrea 1. Gilbert, W. De Magnete (London, 1600). Translation: Chiswick Press, London, 1900±1901). ²² Archaeology Unit, University of Asmara, PO Box 1220, Asmara, Eritrea 2. McElhinny, M. & McFadden, P. Paleomagnetism (Academic, San Diego, 2000). ³³ Department of Geology and Geophysics, University of Minnesota, Minneapolis, 3. Kent, D. & Smethurst, M. Shallow bias of paleomagnetic inclinations in the Paleozoic and Minnesota 55455, USA Precambrian. Earth Planet. Sci. Lett. 160, 391±402 (1998). 4. Irving, E. Paleomagnetic and paleoclimatological aspects of polar wandering. Geo®sica Pura e §§ Laboratoire de PaleÂontologie, GeÂosciences Rennes, Universite de Rennes I, Applicata 33, 23±41 (1956). Campus de Beaulieu, avenue du GeÂneÂral Leclerc, 35042 Rennes cedex, France 5. Evans, M. Test of the dipolar nature of the geomagnetic ®eld throughout Phanerozoic time. Nature kk Department of Anthropology, University of Toronto, Toronto, Ontario M5S 3G3, 262, 676±677 (1976). Canada 6. McElhinny, M. & Lock, J. IAGA paleomagnetic databases with Access. Surv. Geophys. 17, 557±591 (1996). .............................................................................................................................................. 7. Kuang, W. & Bloxham, J. An Earth-like numerical dynamo model. Nature 389, 371±374 (1997). The geographical origin of modern humans is the subject of 8. Buffett, B., Huppert, H., Lister, J. & Woods, A. On the thermal evolution of the Earth's core. J. Geophys. ongoing scienti®c debate. The `multiregional evolution' hypoth- Res. 101, 7989±8006 (1996). 9. Cox, A. & Doell, R. Long period variations of the geomagnetic ®eld. Bull. Seismol. Soc. Am. 54, 2243± esis argues that modern humans evolved semi-independently in 2270 (1964). Europe, Asia and Africa between 100,000 and 40,000 years ago1, 10. Hide, R. Motions of the Earth's core and mantle, and variations of the main geomagnetic ®eld. Science whereas the `out of Africa' hypothesis contends that modern 157, 3784±3785 (1967). humans evolved in Africa between 200 and 100 kyr ago, migrating 11. Cox, A. The frequency of geomagnetic reversals and the symmetry of the nondipole ®eld. Rev. 2 Geophys. 13, 35±51 (1975). to Eurasia at some later time . Direct palaeontological, archae- 12. Bloxham, J. & Gubbins, D. The secular variation of the Earth's magnetic ®eld. Nature 317, 777±781 ological and biological evidence is necessary to resolve this (1985). debate. Here we report the discovery of early Middle Stone Age 13. Gubbins, D. & Bloxham, J. Morphology of the geomagnetic ®eld and implications for the geodynamo. Nature 325, 509±511 (1987). artefacts in an emerged reef terrace on the Red Sea coast of Eritrea, 14. Bloxham, J. & Gubbins, D. Thermal core±mantle interactions. Nature 325, 511±513 (1987). which we date to the last interglacial (about 125 kyr ago) using U±Th NATURE | VOL 405 | 4 MAY 2000 | www.nature.com © 2000 Macmillan Magazines Ltd 65 letters to nature mass spectrometry techniques on fossil corals. The geological with obsidian ¯ake and blade tools is unusual, indicating that old setting of these artefacts shows that early humans occupied and new tool technologies were used at the time this reef formed. coastal areas and exploited near-shore marine food resources in The area we studied lies along the west coast of the Buri Peninsula East Africa by this time. Together with similar, tentatively dated (Fig. 1), which exposes a Pleistocene reef terrace near the village of discoveries from South Africa3 this is the earliest well-dated Abdur. This terrace encompasses a 6.5 km by 1 km stretch of raised evidence for human adaptation to a coastal marine environment, shoreline on the eastern coast of the Gulf of Zula (Fig. 2a). It is the heralding an expansion in the range and complexity of human fossil remnant of a N±S trending fringing reef, which consists of behaviour from one end of Africa to the other. This new, wide- near-shore environments to the east, a central belt of fossil coral spread adaptive strategy may, in part, signal the onset of modern reefs, and offshore environments to the west. We subdivided Abdur human behaviour, which supports an African origin for modern into three geographic districts: Abdur South (AS), Abdur Central humans by 125 kyr ago. (AC) and Abdur North (AN) (Fig.