letters to nature structures can be considered further . If the five impact structures are contemporaneous then they would have been generated by projectiles of common affinity . Attempts at identifying the body type of the impactor at Manicouagan and Saint Martin have been unsuccessful ; melt sheet analyses yield low to undetectable iridium values26 . Initial work on Rochechouart melt rocks implied contamination by a IIA iron meteorite27 , but subsequent analysis of footwall metal veins revealed an elevated Cr content and Ni/Co ratios more compatible with a chondritic source28 . Small amounts of kamacite and taenite have been reported in the heavy mineral fractions of allogenic breccia from Obolon ' 15 , but their presence does not decisively distinguish an iron from a chondritic source . No appraisal has yet been made of Red Wing . Melt sheets may be poor indicators of projectile compositions owing to the effects of crustal dilution , and depending on the velocity of impact and subsequent mode of projectile dissemination29 . It should also be cautioned that the non-volatile components of comets have chemical compositions close to those of primitive carbonaceous chondrites30 . Furthermore , because comets comprise roughly equal portions of silicates , hydrocarbons and volatile ices , their expected bulk siderophile contents are expected to be less than one-third that of an undifferentiated carbonaceous asteroid . This may make detection of a cometary projectile difficult . More work is needed to identify the projectile for the five impact structures . Until then , it is not possible to determine whether the source was a comet or an asteroid on the basis of impact structure geochemistry alone . The Norian stage of the Triassic is marked by one or more major mass extinction events . One of these occurred at the Triassic/Jurassic boundary at 205 Myr -LRB- refs 12 , 22 , 31 -RRB- and is associated with impact-generated shocked-quartz-bearing shale beds in Italy32 . This is significantly later than the 214-Myr multiple impact event described here . However , Benton33 has alluded to an earlier Carnian-Norian mass extinction event at 220 Myr , which affected tetrapods and plants on land and certain marine groups34 . Unlike the end-Triassic , the age of the Carnian/Norian boundary is less well constrained and it is possible that a 214-Myr multiple impact could be associated with an extinction event at that time . As the late Triassic period is now known to contain evidence for a multiple impact event , a renewed effort should be made to study Carnian/Norian boundary and Norian sections with the aim of locating and characterizing the impact ejecta layer , as well as refining biostratigraphic-radiometric correlations for that time period . The calculated probability of a multiple impact occurring on Earth , comparable to that of Shoemaker Levy 9 colliding with Jupiter , appears to be low35 . This is because , relative to Earth , Jupiter 's huge mass presents a formidable gravitational attraction and disruptive force for asteroids and comets . Pairs of craters formed by near-simultaneous impact of binary asteroids are expected for 10 % of the impact structures on Earth and Venus36 . However , a terrestrial multiple impact event resulting from the collision of several projectiles would appear to require a special situation , possibly involving the close fly-by , orbital capture and decay , and consequent clustered strike of a fragmented or weakly coherent comet or asteroid35 . The mechanisms by which such a particular circumstance may arise now need to be further explored . Critical to identifying any additional crater chains on Earth will be the continued precision dating of proven impact structures , followed by their placing on accurate plate reconstructions for the time of impact . Received 30 January ; accepted 19 February 1998 . 1 . Orton , G. et al. . Collision of comet Shoemaker-Levy 9 with Jupiter observed by the NASA infrared telescope facility . Science 267 , 1277 1282 -LRB- 1995 -RRB- . 2 . Melosh , H. J. & Schenk , P. Split comets and the origin of crater chains on Ganymede and Callisto . Nature 365 , 731 733 -LRB- 1993 -RRB- . 3 . Grieve , R. A. F. , Rupert , J. , Smith , J. & Therriault , A . The record of terrestrial impact cratering . GSA Today 5 , 189 196 -LRB- 1995 -RRB- . 4 . Alvarez , L. W. , Alvarez , W. , Asaro , F. & Michel , H. V. Extraterrestrial cause for the Cretaceous-Tertiary extinction . Science 208 , 1095 1108 -LRB- 1980 -RRB- . 5 . Hildebrand , A. R. et al. . Chicxulub crater : a possible Cretaceous/Tertiary boundary impact crater on the Yukatan Peninsula . Geology 19 , 867 871 -LRB- 1991 -RRB- . 6 . Sharpton , V. L. & Ward , P. E. -LRB- eds -RRB- Global catastrophes in earth history : an interdisciplinary conference on impacts , volcanism and mass mortality . Spec . Pap . Geol . Soc . Am . 247 -LRB- 1990 -RRB- . 7 . Lambert , P . The Rochechouart crater : shock zoning study . Earth Planet . Sci . Lett . 35 , 258 268 -LRB- 1977 -RRB- . 8 . Lambert , P. La structure d'impact de meteorite geante de Rochechouart . Thesis , Univ. . Paris-Sud -LRB- 1974 -RRB- . 9 . Reimold , W. U. & Oskierski , W. in Research in Terrestrial Impact Structures -LRB- ed . Pohl , J. -RRB- 94 114 -LRB- Vieweg , Braunschweig/Weisbaden , Germany , 1987 -RRB- . 10 . Kelley , S. P. & Spray , J. G . A late Triassic age for the Rochechouart impact structure , France . Meteorit . Planet . Sci . 32 , 629 636 -LRB- 1997 -RRB- . 11 . Dworak , U. Sto wellenmetamorphose des Anorthosits vom Manicouagan Krater , Quebec , Canada . Contrib . Mineral . Petrol . 24 , 306 347 -LRB- 1969 -RRB- . 12 . Hodych , J. P. & Dunning , G. R . Did the Manicouagan impact trigger end-of-Triassic mass extinction ? Geology 20 , 51 54 -LRB- 1992 -RRB- . 13 . McCabe , H. R. & Bannatyne , B. B. Lake St. Martin cryptoexplosion crater and geology of surrounding area . Geol . Surv . Manitoba Geol . Pap . 3/70 -LRB- 1970 -RRB- . 14 . Reimold , W. U. , Barr , J. M. , Grieve , R. A. F. & Durrheim , R. J. Geochemistry of the melt and country rocks of the Lake St. Martin impact structure , Manitoba , Canada . Geochim . Cosmochim . Acta 54 , 2093 2111 -LRB- 1990 -RRB- . 15 . Masaitis , V. L. , Danilin , A. N. , Karpov , G. M. & Raykhlin , A. I. Karla , Obolon ' and Rotmisrovka astroblemes in the European part of the U.S.S.R. Dokl . Akad . Nauk SSSR 230 , 174 177 -LRB- 1976 -RRB- . 16 . Gurov , Y. P. , Val ` ter , A. A. & Rakitskaya , R. B. Coesite in rocks of meteorite explosion craters on the Ukrainian shield . Int . Geol . Rev. 22 , 329 332 -LRB- 1978 -RRB- . 17 . Gurov , Y. P. , Gurova , E. P. & Rakitskaya , R. B. Impact diamonds in the craters of the Ukrainian shield . Meteoritics 30 , 515 516 -LRB- 1995 -RRB- . 18 . Masaitis , V. L. et al. . The Geology of Astroblemes -LRB- Nedra Press , Leningrad , 1980 -RRB- . 19 . Grieve , R. A. F. & Masaitis , V. L . The economic potential of terrestrial impact craters . Econ . Geol . 36 , 105 151 -LRB- 1994 -RRB- . 20 . Gerhard , L. C. , Anderson , S. B. , Lefever , J. A. & Carlson , C. G. Geological development , origin , and energy mineral resources of Williston Basin , North Dakota . Am . Assoc. Petrol . Geol . Bull . 66 , 989 1020 -LRB- 1982 -RRB- . 21 . Melosh , H. J. Impact Cratering : A Geologic Process -LRB- Oxford Univ. . Press , 1989 -RRB- . 22 . Gradstein , F. M. et al. . A Mesozoic time scale . J. Geophys . Res . 99 , 24051 24074 -LRB- 1994 -RRB- . 23 . Ziegler , A. M. et al. in The Tectonic evolution of Asia -LRB- eds Yin , A. & Harrison , T. M. -RRB- 371 400 -LRB- Cambridge Univ. . Press , 1996 -RRB- . 24 . Hammel , H. B. et al. . HST imaging of atmospheric phenomena created by the impact of comet Shoemaker-Levy 9 . Science 267 , 1288 1296 -LRB- 1995 -RRB- . 25 . Weaver , H. A. et al. . The Hubble Space Telescope -LRB- HST -RRB- observing campaign on comet ShoemakerLevy 9 . Science 267 , 1282 1288 -LRB- 1995 -RRB- . 26 . Palme , H. Identification of projectiles of large terrestrial impact craters and some implications for the interpretation of Ir-rich Cretaceous/Tertiary boundary layers . Geol . Soc . Am . Spec . Pap . 190 , 223 233 -LRB- 1982 -RRB- . 27 . Jannsens , M. J. , Hertogen , J. , Takahiashi , H. , Anders , E. & Lambert , P. Rochechouart impact crater : identification of projectile . J. Geophys . Res . 82 , 750 758 -LRB- 1977 -RRB- . 28 . Horn , N. P. & Goresy , A. E . The Rochechouart crater in France : stony and not iron meteorite ? Lunar Planet . Sci . XI , 468 470 -LRB- 1980 -RRB- . 29 . McClaren , D. J. & Goodfellow , W. D. Geological and biological consequences of giant impacts . Annu . Rev. Earth Planet . Sci . 18 , 123 171 -LRB- 1990 -RRB- . 30 . Hut , P. et al. . Comet showers as a cause of mass extinctions . Nature 329 , 118 126 -LRB- 1987 -RRB- . 31 . Hallam , A. in Global Events and Event Stratigraphy -LRB- ed . Walliser , O. H. -RRB- 265 283 -LRB- Springer , New York , 1996 -RRB- . 32 . Bice , D. M. , Newton , C. R. , McCauley , S. , Reiners , P. W. & McRoberts , C. A. Shocked quartz at the Triassic/Jurassic boundary in Italy . Science 259 , 443 446 -LRB- 1992 -RRB- . 33 . Benton , M. J . More than one event in the late Triassic mass extinction . Nature 321 , 857 861 -LRB- 1986 -RRB- . 34 . Hallam , A. & Wignall , P. B. Mass Extinctions and their Aftermath -LRB- Oxford Univ. . Press , 1997 -RRB- . 35 . Love , S. G. , Bottke , W. F. & Richardson , D. C. Alternative formation mechanisms for terrestrial crater chains . Lunar . Planet . Sci . XXVIII , 837 838 -LRB- 1997 -RRB- . 36 . Bottke , W. F. & Melosh , H. J. Formation of asteroid satellites and doublet craters by planetary tidal forces . Nature 381 , 51 53 -LRB- 1996 -RRB- . Acknowledgements . We thank M. Benton and J. Melosh for discussions and reviews . This work was supported by NSERC -LRB- Canada -RRB- and the Open University -LRB- UK -RRB- grants to J.G.S. and S.P.K. , respectively . D.B.R. acknowledges industrial support for the Palaeogeographic Atlas Project . Correspondence and requests for materials should be addressed to J.G.S. -LRB- e-mail : jgs@unb.ca -RRB- . 8 Fission-track ages of stone tools and fossils on the east Indonesian island of Flores M. J. Morwood * , P. B. O'Sullivan , F. Aziz & A. Raza * Department of Archaeology and Palaeoanthropology , University of New England , New South Wales 2351 , Australia School of Earth Sciences , La Trobe University , Victoria 3083 , Australia Geological Research and Development Centre , Bandung 4011 , Indonesia ... . The islands of Wallacea , located between the Southeast Asian -LRB- Sunda -RRB- and Australian -LRB- Sahul -RRB- continental areas , offer unique potential for the study of evolution and cultural change . Located east of Java and Bali , which were periodically connected to the Asian mainland , the Wallacean islands could only be reached by 173 NATURE | VOL 392 | 12 MARCH 1998 Nature Macmillan Publishers Ltd 1998 letters to nature sea crossings . Consequently , before human intervention all these islands had impoverished faunas comprising only species that were capable of crossing water by swimming , rafting on flotsam , or by flying in sufficient numbers to establish biologically viable populations1 . Here we report zircon fission-track dates from two fossil sites on the Wallacean island of Flores . Tangi Talo , which has an endemic fauna , dates to 0:90 0:07 Myr BP , whereas Mata Menge , where stone tools are found with elements of continental Southeast Asian fauna , dates to between 0:88 0:07 and 0:80 0:07 Myr BP . Even at times when the sea level was lowest , water crossings were necessary to reach Flores from Southeast Asia . We conclude that Homo erectus in this region was capable of repeated water crossings using watercraft . Tangi Talo and Mata Menge are two of many palaeontological sites located in the Ola Bula Formation of the upper Ae Sissa River basin in central Flores -LRB- Fig. 1 -RRB- . The Ola Bula Formation is up to 80 m thick and comprises white tuffs and interbedded tuffaceous sediments underlying fluvial deposits . On the basis of stratigraphic context and faunal content , Mata Menge appears to be younger than the sites of Boa Leza and Ola Bula . Tangi Talo lies 31 m stratigraphically below Ola Bula , and is therefore older than Mata Menge2 . At Tangi Talo , a horizontally bedded white tuffaceous layer 30 cm thick contains the remains of pygmy Stegodon , giant tortoises -LRB- Geochelone sp. -RRB- and Komodo dragons -LRB- Varanus komodoensis -RRB- . Although angular volcanic rock fragments are locally present , none of these remains have characteristics that suggest Figure 1 The general location of Flores in east Indonesia . The continental shelves would have been exposed to varying water levels at times of glacial maximum / low sea level . Black areas represent land masses that are now above water . they are artefacts . Palaeomagnetic determinations showed that deposits at Tangi Talo are dominated by reversed polarities but contain two levels with normal polarities . These two levels were interpreted to represent the Jaramillo normal polarity subzone with a date of 900,000 years BP -LRB- ref . 2 -RRB- ; however , they could also have resulted from the inefficiency of the field demagnetization procedures used on the samples for removing normal overprints held in secondary minerals common in tropical weathering -LRB- ref . 2 and C. Swisher , personal communication -RRB- . The Mata Menge site comprises an 120-cm-thick section of horizontally bedded , white tuff with interbedded , tuffaceous sandstones -LRB- Fig. 2 , Unit B -RRB- . The section is interpreted here in terms of the primary , or near primary , deposition of tuff with intruded tuffaceous flow deposits . In places , soft-sediment deformation is evident between layers , suggesting that the units were subaqueously deposited in a low-energy lacustrine environment . The tuff and its contents therefore mainly accumulated from above , whereas the sandstones and their contents entered laterally . These deposits contain the remains of large Stegodon -LRB- S. trigonocephalus florensis -RRB- , crocodile , giant rat -LRB- Hooijeromis nusatenggara -RRB- , freshwater molluscs , plants and rounded volcanic pebbles . In addition , the Meta Menge deposits contain pieces of volcanic rock and chert identified as artefacts on the basis of well-defined flake scars , ring cracks , bulbs of percussion and systematic edge damage suggestive of retouch -LRB- Fig. 3 -RRB- -LRB- refs 3,4 -RRB- . The chert flakes are also out of geological context in the volcanic deposits of the Ola Bula Formation and lack any evidence of being transported by water . Of 45 stone pieces recovered in a 1994 excavation -LRB- ref . 5 -RRB- , 14 were identified as artefacts according to technological criteria3 . Of these 14 artefacts , four were subsequently examined under high magnification and found to have edge damage , striations , polishing and residues indicating use in the processing of plant material3 . Further in situ stone artefacts were located in the tuffaceous flow deposits -LRB- but not in the white tuff -RRB- at Mata Menge during fieldwork early in 1997 . The artefacts included a chert flake at the base of the fossil deposit , just 3 cm above the lowermost dated tuff sample . The flake has bifacial usewear , striations and residues . The fossil layer at Mata Menge is overlaid by another 23 m of the Ola Bula Formation capped by lacustrine limestone6 . Immediately below the fossil layer are extensive layers of tuff and sandstone , which contain neither fossils nor stone artefacts . Palaeomagnetic determinations undertaken previously found a reversed-normal transition in deposits 3 m below the fossil-rich layer at Mata Menge . The transition was interpreted as representing the Matuyama/Brunhes boundary , and a date of slightly less than 8 Table 1 Fission track analytical results : tuffaceous samples from Flores , Indonesia Sample number -LRB- no . of grains -RRB- Lat . -LRB- S -RRB- Long . -LRB- E -RRB- Unit name Standard track density * 105 cm-2 4.060 -LRB- 1,631 -RRB- 4.087 Fossil track density * 105 cm-2 1.139 -LRB- 199 -RRB- 1.040 Induced track density * 106 cm-2 3.311 -LRB- 5,784 -RRB- 3.340 Chi-squared probability -LRB- % -RRB- 33.1 15.1 Uranium -LRB- p.p.m. -RRB- Fission-track age -LRB- Myr s :d : ... . Primary zircon : Mata Menge MM1 8 41 31 -LRB- 44 -RRB- 121 5 43 MM2 8 41 31 -LRB- 53 -RRB- 121 5 43 Primary zircon : Tangi Talo TT1 8 41 53 -LRB- 56 -RRB- 121 8 10 Ola Bula Ola Bula 318.0 318.7 0:88 0:80 0:07 0:07 Ola Bula ... 4.114 -LRB- 1,631 -RRB- 1.195 -LRB- 184 -RRB- 3.441 -LRB- 5,297 -RRB- 1.766 -LRB- 441 -RRB- 1.642 -LRB- 383 -RRB- 1.312 -LRB- 86 -RRB- 100.0 326.2 0:90 0:07 Detrital contamination : Mata Menge MM1 -LRB- 6 -RRB- MM2 -LRB- 7 -RRB- Detrital contamination : Tangi Talo TT1 -LRB- 4 -RRB- -4.060 -LRB- 1,631 -RRB- -4.087 -LRB- 1,631 -RRB- -4.114 -LRB- 1,631 -RRB- 9.171 -LRB- 229 -RRB- 4.200 -LRB- 98 -RRB- 0.136 -LRB- 89 -RRB- 0.0 0.1 169.7 156.6 9:8 7:5 3:1 1:7 0.0 124.4 14:0 8:3 ... * Brackets show number of tracks counted . Standard and induced track densities were measured on mica external detectors -LRB- g 0:5 -RRB- and fossil track densities on internal mineral surfaces . Zircon ages were calculated using z 125:5 3 for dosimeter glass CN1 -LRB- analyst , P.B.O. 'S . -RRB- . 174 Nature Macmillan Publishers Ltd 1998 NATURE | VOL 392 | 12 MARCH 1998 letters to nature 0.73 Myr BP for the faunal remains was inferred2 , although a date of about 0.78 Myr BP for the boundary event is more likely7 . The palaeomagnetic work did not date the fossil deposits themselves , however . To constrain better the age of the deposits at Mata Menge and Tangi Talo , we collected samples from three tuffaceous horizons for zircon fission-track dating . This method of radiometric dating is ideal for dating young tuffaceous deposits8 . The stratigraphic positions of the two samples taken from Mata Menge -LRB- MM -RRB- are shown in Fig. 2 . Sample MM1 was collected from the pink tuff immediately below the lowest fossils and stone artefacts identified , whereas sample MM2 was taken from the white tuff next to a large 8 Figure 2 The west baulk of the excavations at Mata Menge , showing the locations of fission-track samples MM1 and MM2 -LRB- marked by plus symbols -RRB- . Three main stratigraphic units are evident . Unit A is a layer of weathered sandstone and top soil containing stone artefacts . A1 is gravel-rich ; A2 is an in situ homogeneous , coarse sandstone ; A3 is top soil . Unit B comprises layers and lenses of white tuff and tuffaceous sandstone flow structures , which predominate towards the base . The flow structures contain more fragmented fossils and all stone artefacts identified in the section . B1 is a white tuff ; B2 is a light grey tuffaceous sandstone ; B3 is a dark grey tuffaceous sandstone ; B4 is a light grey tuffaceous sandstone ; B5 is a brown grey sandstone ; B6 is a dark grey sandstone -LRB- similar to B3 but with inclusions -RRB- ; B7 is a coarse-grained , yellow grey sandstone . Unit C is a pink tuff , which does not contain fossils or stone artefacts . The section around the trench is cut back 50 cm from the main face . 10 MM1 0.88 0.7 Myr +2 0 -2 Number 8 6 4 2 0 0 1 2 3 4 5 5 1.5 1 0.5 Relative error -LRB- % -RRB- 47 0 10 20 30 10 40 0 Fission-track age -LRB- Myr -RRB- 20 Precision -LRB- 1 / -RRB- MM2 0.80 0.7 Myr +2 0 -2 Number 10 3 2 1.5 1 0.5 0 9 50 Relative error -LRB- % -RRB- 0 0 2 4 6 8 10 45 0 10 20 30 40 Fission-track age -LRB- Myr -RRB- 25 20 15 10 5 0 0 2 4 6 8 10 Precision -LRB- 1 / -RRB- Number TT1 0.90 0.7 Myr +2 0 -2 2 1.5 1 0.5 Relative error -LRB- % -RRB- 60 0 10 20 30 10 40 0 Fission-track age -LRB- Myr -RRB- Precision -LRB- 1 / -RRB- Figure 4 Single-grain age results in the form of simple histograms -LRB- left -RRB- and radial plots -LRB- right -RRB- from the three Mata Menge and Tangi Talo tuffaceous units MM1 , MM2 and TT1 . On radial plots , each age has unit standard error -LRB- 2j -RRB- on the yFigure 3 Stone artefacts excavated from Mata Menge in 1994 . a , Chert flake ; b , silcrete flake ; c f , basalt flakes ; g , basalt multi-platformed core ; h , basalt cobble ` chopper ' . NATURE | VOL 392 | 12 MARCH 1998 axis ; its actual precision is indicated on the x-axis ; and its age is read by extrapolating a line from zero through the plotted point to the logarithmic age scale on the right perimeter . Nature Macmillan Publishers Ltd 1998 175 letters to nature Stegodon tusk . It overlies in situ artefacts . Another sample , TT1 , was taken from the white tuffaceous layer at Tangi Talo next to an in situ pygmy Stegodon tusk and pieces of Geochelone carapace . Although the tuffaceous layers sampled may have been slightly reworked since deposition , their horizontal extent as well as homogeneity in colour and texture suggest that any such reworking has been minimal . Therefore , the measured ages of zircons from these units should represent the depositional age of the tuffaceous layers . Analytical results are shown in Table 1 . Distributions of the primary grain ages from each sample are presented in Fig. 4 . The Tangi Talo sample -LRB- TT1 -RRB- yielded a date of 0:90 0:07 Myr BP , whereas the two Mata Menge samples -LRB- MM1 and MM2 -RRB- yielded dates of 0:88 0:07 and 0:80 0:07 Myr BP , respectively . These closely match the dates previously suggested on the basis of palaeomagnetic determination2 , biostratigraphy4 and the presence of tektites9 . We conclude that at Mata Menge stone artefacts occur in primary association with Early Pleistocene tuffs and faunal remains . The age of the artefacts also indicates that they were produced by Homo erectus rather than Homo sapiens10 . There are several general implications of these results . For instance , today three deep-water straits occur between the Sunda continental shelf and Flores . Even at times of lowest sea level during the last glacial maximum , these straits required sea crossings of at least 19 km -LRB- ref . 11 -RRB- . Either there must have been a land bridge linking Flores with mainland Southeast Asia in the Early Pleistocene epoch , or Homo erectus in this region had the capacity to make water crossings12 . The impoverished nature of the fauna on Flores throughout the sequence , however , seems to negate a connection with Sunda at any time -LRB- compare refs 12 , 13 -RRB- . The presence of endemic pygmy elephants , giant reptiles and giant rats in the Early Pleistocene also suggests a continued insular context14 ,15 both before and contemporaneous with the first evidence for hominids . In fact , Sondaar12 has argued that the extinction of pygmy Stegodon and giant tortoises , followed by recolonization of Flores by large Stegodon from continental Southeast Asia after 0.90 Myr BP , resulted from increased predation , not reduced insularity . More specifically , he says that the onset of hunting by hominids caused the turnover in fauna , a position supported by our results . Our findings , therefore , validate generally dismissed -LRB- or ignored -RRB- claims for the presence of hominids on Flores in the Early/Middle Pleistocene -LRB- compare refs 2 , 4 with 16 , 17 -RRB- . Furthermore , they indicate that , sometime between 800,000 and 900,000 years ago , Homo erectus in this region had acquired the capacity to make water crossings . Previously , in the region of the Wallacean islands this capacity was thought to be the prerogative of modern humans and to have only appeared in the Late Pleistocene , with the earliest widely accepted evidence for watercraft being the colonization of Australia by modern humans from Wallacea between 40,000 and 60,000 years ago18 ,19 . Outside this region , the technology to undertake even limited water crossings is not clearly evident until much later , at the end of the Pleistocene20 . Our results challenge this view . Therefore , this evidence , combined with the geographical radiation of Homo erectus in the Early Pleistocene10 and other recent discoveries21 , suggests that the cognitive capabilities of this species may be due for reappraisal . ... . room temperature to reveal the induced tracks . Thermal neutron fluences were monitored by measuring the track density recorded in muscovites attached to pieces of the Corning CN1 standard glass . Fission tracks in each zircon mount were counted in transmitted light using a dry 100 objective lens at a total magnification of 1,600 . Only grains with sharp polishing scratches were counted . Depending on availability of datable grains , a minimum of 50 60 individual grains were dated from each sample . Ages were calculated using the standard fission-track-age equation using the zeta calibration method24 and errors were calculated using previously published methods25 . A personal zeta calibration factor of 125:5 3 -LRB- for P.B.O. 'S -RRB- was determined empirically using zircon-age standards with independently known ages . The use of the external detector method was particularly important as it allowed dating of individual grains within a sample , rather than relying on a bulk sample age which can contain contaminates . The x2 statistic was used to detect the probability that all age grains analysed belonged to a single population of ages . All three mounts showed a probability of 5 % , which was evidence of asymmetric spreads of single-grain ages . Therefore , in each case the ` central age ' -LRB- ref . 26 -RRB- , which is essentially a weighted-mean age , was reported . Data from grains deemed on the basis of their significantly older ages to be obvious contaminants were removed before a final age calculation was made . Each sample contained between four and seven older reworked zircons , which gave ages of between 7 and 14 Myr BP , much older than the primary zircons . These older contaminates also contained significantly lower amounts of uranium -LRB- Table 1 -RRB- , a factor that could also have been used to delineate between the primary and contaminate zircons . Received 27 June ; accepted 20 November 1997 . 1 . Johnson , D. L. Problems in the land vertebrate zoogeography of certain islands and the swimming powers of elephants . J. Biogeog . 7 , 383 398 -LRB- 1980 -RRB- . 2 . Sondaar , P. Y. et al. . Middle Pleistocene faunal turnover and colonisation of Flores -LRB- Indonesia -RRB- by Homo erectus . C. R. Acad . Sci . 319 , 1255 1262 -LRB- 1994 -RRB- . 3 . Morwood , M. J. , Aziz , F. , van den Bergh , G. D. , Sondaar , P. Y. & de Vos , J. Stone artefacts from the 1994 excavation at Mata Menge , west central Flores , Indonesia . Austr . Archaeol . 44 , 26 34 -LRB- 1997 -RRB- . 4 . Maringer , J. & Verhoeven , T. Die Steinartefakte aus der Stegodon -- Fossilschicht von Mengeruda auf Flores , Indonesien . Anthropos 65 , 229 247 -LRB- 1970 -RRB- . 5 . van den Bergh , G. D. , Mubroto , B. , Aziz , F. , Sondaar , P. Y. & de Vos , J. in Indo-Pacific prehistory : the Chiang Mai Papers -LRB- Proc . 15th IPPA Congr . Chiang Mai , Thailand 1994 Vol . 1 -RRB- -LRB- ed . Bellwood , P. -RRB- 27 36 -LRB- IPPA , Canberra , 1996 -RRB- . 6 . Lumbanbatu , U. M. & Aziz , F. Aspek fosil vertebrata dari Forasi Olabula , Flores . J. Geol . Sumberdaya Miner . 33 , 2 6 -LRB- 1994 -RRB- . 7 . Berggren , W. A. et al. in Geochronology , Time Scales and Global Stratigraphic Correlation -LRB- eds Berggren , W. A. et al. -RRB- 129 212 -LRB- SEPM , Special publ . no. 54 , Tulsa , 1995 -RRB- . 8 . Gleadow , A. J. W. Fission track ages of the KBS tuff and associated hominid remains in northern Kenya . Nature 284 , 225 230 -LRB- 1980 -RRB- . 9 . von Koenigswald , G. H. R . A tektite from the island of Flores -LRB- Indonesia -RRB- . Proc . Koninklijke Nederlandse Akad . Wetenschappen B 61 , 44 46 -LRB- 1957 -RRB- . 10 . Rightmire , G. P. in 100 Years of Pithecanthropus : the Homo erectus Problem -LRB- ed . Franzen , J. L. -RRB- 319 326 -LRB- Courier Forschungsinstitut Senckenberg , Frankfurt , 1994 -RRB- . 11 . Birdsell , J. H. in Sunda and Sahul -LRB- eds Allen , J. et al. -RRB- 113 168 -LRB- Academic , London , 1977 -RRB- . 12 . Sondaar , P. Y. Pleistocene man and extinctions of island endemics . Mem . Soc . Geol . Fr . 150 , 159 165 -LRB- 1987 -RRB- . 13 . Groves , C. P. in Perspectives in Human Biology , Vol . 2 -LRB- eds Rousham , E. & Freedman , L. -RRB- 83 87 -LRB- Univ. . Western Australia , Perth , 1996 -RRB- . 14 . Davis , S. Tiny elephants and giant mice . New Sci . 105 , 25 27 -LRB- 1985 -RRB- . 15 . Azzaroli , A . About pygmy mammoths of the Northern Channel Islands and other island faunas . Quat . Res . 16 , 423 425 -LRB- 1981 -RRB- . 16 . Allen , H. Stegodonts and the dating of stone tool assemblages in island S.E. Asia . Asian Persp . 30 , 243 266 -LRB- 1991 -RRB- . 17 . Bellwood , P. Prehistory of the Indo-Malaysian Archipelago -LRB- Academic , Sydney , 1985 -RRB- . 18 . Davidson , I. & Noble , W . Why the first colonisation of the Australian region is the earliest evidence of modern human behaviour . Archaeol . Oceania 27 , 135 142 -LRB- 1992 -RRB- . 19 . Bowdler , S. in Sahul in Review -LRB- eds Smith , M. A. et al. -RRB- 60-70 -LRB- Australian National University , Canberra , 1993 -RRB- . 20 . Cherry , J. F . The first colonisation of the Mediterranean Islands . J. Med . Archaeol . 3 , 145 221 -LRB- 1990 -RRB- . 21 . Thieme , H. Lower Palaeolithic hunting spears from Germany . Nature 385 , 807 810 -LRB- 1997 -RRB- . 22 . Gleadow , A. J. W. , Hurford , A. J. & Quaife , D. R. Fission track dating of zircon : improved etching techniques . Earth Planet . Sci . Lett . 33 , 273 276 -LRB- 1976 -RRB- . 23 . Gleadow , A. J. W. Fission track dating methods : what are the real alternatives ? Nucl . Tracks 5 , 3 14 -LRB- 1981 -RRB- . 24 . Hurford , A. J. & Green , P. F . A users ' guide to fission-track dating calibration . Earth Planet . Sci . Lett . 59 , 343 354 -LRB- 1982 -RRB- . 25 . Green , P. F . A new look at statistics in fission-track dating . Nucl . Tracks 5 , 77 86 -LRB- 1981 -RRB- . 26 . Galbraith , R. F. & Laslett , G. M. Statistical models for mixed fission track ages . Nucl . Tracks 21 , 459 470 -LRB- 1993 -RRB- . Acknowledgements . Fieldwork conducted between 1991 and 1994 by P. Sondaar , J. de Vos , G. van den Bergh and F.A. provided the basis for this investigation . Our 1997 fieldwork was undertaken under the auspices of the Indonesian Geological Research and Development Centre and was funded by a grant from the University of New England Vice Chancellor 's Fund -LRB- we thank B. Thom for this support -RRB- , and by the Australian Institute of Nuclear Science and Engineering . We thank V. Paine for help with stratigraphic recording ; R. Spikings for completing the mineral separations ; D. Hobbs and K. Morwood for artwork ; and I. Davidson , J. de Vos , P. Sondaar , D. Cogill , P. Brown , R. Gargett , C. Swisher , P. Bellwood and R. Roberts for discussion . Correspondence and requests for materials should be addressed to M.J.M. -LRB- e-mail : mmorwood@metz . une.edu.au -RRB- . 8 Methods Tuffaceous samples were crushed and zircons in the size range 60 250 m were separated using a Gemini table and standard magnetic and heavy-liquid techniques at the School of Earth Sciences , La Trobe University . Grains were mounted in FEP Teflon , and , after optical-quality polishing , were etched in a eutectic KOH/NaOH melt7 ,22 at 225 C for 125 h. Neutron irradiations were carried out in a well-thermalized flux in the HIFAR reactor -LRB- Lucas Heights , Australia -RRB- . Fission-track ages were measured using the external detector method23 with Brazil Ruby muscovite being used to record induced tracks . The muscovite detectors were etched for 30 min in 48 % hydrogen fluoride at 176 Nature Macmillan Publishers Ltd 1998 NATURE | VOL 392 | 12 MARCH 1998

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O B O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O

artefacts flores indonesia archaeology dating