A critique of evidence for human occupation of Europe older than the Jaramillo subchron (~1 Ma): comment on 'The oldest human fossil in Europe from Orce (Spain)' by Toro-Moyano et al. (2013

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A critique of evidence for human occupation of Europe older than the Jaramillo subchron (~1 Ma): comment on 'The oldest human fossil in Europe from Orce (Spain)' by Toro-Moyano et al. (2013
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  News and Views A critique of evidence for human occupation of Europe older than the Jaramillo subchron ( w 1 Ma): Comment on  ‘ The oldest human fossil inEurope from Orce (Spain) ’  by Toro-Moyano et al. (2013) Giovanni Muttoni a , d , * , Giancarlo Scardia b , Dennis V. Kent c , e a Department of Earth Sciences, University of Milan, via Mangiagalli 34, I-20133 Milan, Italy b CNR Istituto di Geologia Ambientale e Geoingegneria, via Salaria km 29.300, I-00016 Monterotondo Scalo, Roma, Italy c Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA d  ALP, Alpine Laboratory of Paleomagnetism, via Madonna dei Boschi 76, I-12016 Peveragno, CN, Italy e Lamont-Doherty Earth Observatory, Palisades, NY 10964, USA a r t i c l e i n f o  Article history: Received 17 June 2013Accepted 25 August 2013Available online 17 October 2013 Keywords: PleistoceneMagnetostratigraphyElectron spin resonanceHominin fossils The recently dated human tooth from Barranco León, Spain,would seem to indicate that hominins were present in southernEuropeasearlyas w 1.4Ma(millionsofyearsago)basedonelectronspin resonance (ESR) ages on quartz grains coupled with magne-tostratigraphic and biochronologic correlations (Toro-Moyanoet al., 2013). We suggest that the evidence for human occupationof Europe prior to 1 Ma is highly equivocal.The ESR ages and associated analytical errors (as quoted at the  1 s  level) are as follows: (I) 1.73  0.17 Ma from sample BL-1 fromalayerlocated1.5mbelowthepaleontological(humantooth)layer(level D); (II) 1.46    0.17 Ma from sample BL-2 from the paleon-tologicallayer;(III)1.88  0.19Maand1.23  0.12MafromsamplesBL-3 and BL-4, respectively, both from the same layer located justabove the paleontological layer; (IV) 1.02    0.09 Ma from sampleBL-5 from a layer located  w 1 m above the paleontological layer(Toro-Moyano et al., 2013). Toro-Moyano et al. (2013) calculated a weighted mean age of 1.43  0.38 Ma based on the ages of samplesBL-2 e BL-4 to represent the age of the hominin fossil. A simplelinear regression of this weighted mean age for samples BL-2 e BL-4(1.43 Ma) combined with the 1.73 Ma age of underlying sample BL-1 and the 1.02 Ma age of overlying sample BL-5 would imply thatthe w 4 m ofsampled sectionacrossthe paleontological layer spansa duration of  w 0.7 Ma.This age distribution makes Toro-Moyano et al. ’ s (2013) inter-pretation of the magnetostratigraphic data highly problematic. Theauthors presented a new paleomagnetic study that con fi rmedprevious results (Oms et al., 2000): the entire 30 m-thick section bears only reverse magnetic polarity. Based on the ESR chronology,Toro-Moyano et al. (2013) asserted that the deposits could becorrelated to the reverse polarity Matuyama chron between thenormal polarity Olduvai and Jaramillo subchrons (although neitherwas found). However, this part of the Matuyama chron lasts forabout 0.7 Ma (Cande and Kent, 1995; Lourens et al., 2004), which corresponds tojust the amount of time supposedlycaptured bytheESR dates in only 4 m of the section. If the section extends farbeyond 0.7 Ma in duration, as the ESR dates in the absence of ev-idence or even discussion of abrupt sedimentation rate variationswould seem to suggest, it is unexplained why the Jaramillo and theOlduvai were not found anywhere in this densely re-sampled sec-tion (Oms et al., 2000; Toro-Moyano et al., 2013).These observations raise the possibility that the ESR dates areinaccurate and overestimate the age of deposition of the section.For example, ESR dates on quartz grains are based on theassumption of a complete resetting during transport of the bleachable ESR signal prior to sediment deposition, otherwisethe ESR dates would overestimate the true age of deposition. Toro-Moyano et al. (2013: SOM) emphasize that this complete bleachingmay occur within a short distance of transport, within 1 km ac-cording to bleaching experiments on quartz grains from modernriver sediments (Voinchet et al., 2007). However, the sandstone layers that yielded the ESR dates at Barranco León are not traceablelaterally anywhere close to distances of 1 km. Therefore, thetransport distance of quartz grains from the source area may nothave been adequately long to reset the ESR signal prior to deposi-tion. Oms et al. (2011: 41) stated:  “ The main process in the *  Corresponding author. E-mail address:  giovanni.muttoni1@unimi.it (G. Muttoni). Contents lists available at ScienceDirect  Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol 0047-2484/$  e  see front matter    2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jhevol.2013.08.005  Journal of Human Evolution 65 (2013) 746 e 749  formation of the Barranco León 5 paleontological-archeological site(level D1) is related to a sudden event of high-energy currentsentering a lacustrine  e  palustrine domain. These currents trans-ported gravels, bones and lithic industries from a short distance(maximum, hundreds of meters) ” . There is thus the strong possi-bility that there was insuf  fi cient bleaching of quartz grains duringsuchhigh-energysheet- fl oodeventstoprecludethattheESR-datedhominin layer is younger than the Jaramillo ( w < 1 Ma). It seemsthat the proposed ESR dates (as well as the previously reported U-series/ESR dateson fossil teeth fromthe same paleontological layer(Duval et al., 2012a,b)) are too inexact to usefully differentiate be- tween pre- and post-Jaramillo.Regarding the chronological interpretation of the microfaunafrom Barranco León, much relevance is given to the rodent  Allo- phaiomys  aff.  lavocati. Toro-Moyano etal. (2013:7)reported that “ Anage younger than the Olduvai subchron (1.95 e 1.77 Ma) is inferredfrom the more derived morphology of   Allophaiomys  aff.  lavocati compared with  A.  cf.  deucalion  from the site of Kryzhanovka(Tesakov, 1998), which is associated with the Olduvai subchron(Pevzner et al., 1998) ” . However, neither Kryzhanovka nor the sup-posedly coeval locality of Tizdar from Sinyaya Balka contains anyevidence of the Olduvai subchron. The w 33 m-thick Kryzhanovkasectionstartswithsandsandclaysofreversepolarity(Pevzner,1989;Pevzner et al., 1998) and ends at the top with a loess-paleosolsequence reported to contain an upper normal-lower reverse mag-netic polarity stratigraphy, which was srcinally interpreted as theBrunhes e Matuyama boundary(Pevzner,1989). Pevzneret al. (1998) assumed a hiatus between the upper loess-paleosol sequence andthe underlying sands and clays; the latter were assigned a pre-Olduvai Matuyama age because they contain mollusks and smallmammals attributed to the late Kujalnik marine stage of the EasternParatethys. The Kujalnik stage was interpreted to span from the endof the Gauss normal chron to within the Olduvai subchron (Pevzner,1989; Pevzner et al.,1998). Hence, the attribution of Kryzhanovka tothe pre-Olduvai Matuyama is indirect (i.e., no Olduvai subchronfound) and based on correlations of distant and discontinuous sec-tions the details of which are dif  fi cult to reconstruct (Pevzner et al.,1998). Finally, we point out that  Allophaiomys lavocati  was found atSima del Elefante in levels TE7 e TE14, which were tentativelycorrelated with the Waalian (1.5 e 1.3 Ma) (Cuenca-Bescós et al.,2010), roughly in agreement with the chronology of  Carbonellet al. (2008), but could as well be younger than the Jaramillo(i.e., < 1 Ma) (Muttoni et al., 2010).Toro-Moyano et al. (2013: 7) also claimed that  “  Allophaiomys  aff. lavocati  is, in turn, more archaic than the microtine species presentat Vallonnet (France) and Untermassfeld (Germany), two localitiesdated to the Jaramillo subchron (0.99 e 1.07 Ma; Yokoyama et al.,1988; Wiegank, 1997) ” . We notice that the fossiliferous strata of Untermassfeld showed a transition from normal to reverse mag-netic polarity (Wiegank, 1997) that could well represent theBrunhes e Matuyama boundary as already pointed out by vanKolfschoten and Markova (2005). The attribution of Vallonnet tothe Jaramillo is also unclear. Yokoyama et al. (1988) reportednormal polarity directions from Level III of the cave stratigraphythat they interpreted as pertaining to either the Brunhes or the Jaramillo or the Olduvai, without illustrating any data or providingany information on the experimental procedure used to obtain thepolarity stratigraphy. Gagnepain (1996) provided inconclusive Figure 1.  Our preferred interpretation of evidence for the earliest human occupation of southern Europe is during the Matuyama reverse polarity chron between the Jaramillonormal polarity subchron and the Brunhes e Matuyama boundary (0.99 e 0.78 Ma; Lourens et al., 2004). This was a time of profound climate change centered on marine isotope stage(MIS) 22 at w 0.87 Ma, as revealed by benthic oxygen isotope data (Shackleton, 1995) scaled to the glacio-eustatic drop at the last glacial maximum time (Fairbanks, 1989). Key hominin sites with reliable magnetostratigraphy straddle this time interval: Gran Dolina (Pares and Perez-Gonzalez, 1999; Pares et al., 2013), Sima del Elefante (Carbonell et al., 2008) (with cosmogenic burial ages TE7 and TE9 reported here at 2 s  level), Vallparadis (Martinez et al., 2010), and Monte Poggiolo (with indication of previous, and super-seded, ESR mean age; Muttoni et al., 2011 and references therein). The Barranco León site mayalso straddle this time interval if the reported ESR weighted mean age from levels BL-2 to BL-4 (Toro-Moyano et al., 2013) is considered a maximum age estimate. Hominins may have migrated from stressed environments in Africa and Asia to southern Europeanrefugia at this time, together with African and Asian mammals, such as  Elepas antiquus  and  Mammuthus trogontherii  (FO is  fi rst occurrence datum). G. Muttoni et al. / Journal of Human Evolution 65 (2013) 746  e 749  747  paleomagnetic data from Vallonnet, which did not allow to estab-lishaclearmagneticpolarityforthesuccession: “ Naturellement,denombreux points restent à con fi rmer ou à completer, comme lespolarités magnétiques obtenues... ”  (Gagnepain,1996; page 146 and fi gure 122).Finally, Toro-Moyano et al. (2013: 7) claimed that  “ . a recentdating of the level TE9 from Sima del Elefante, in the Atapuercakarstic complex (Cuenca-Bescós et al., 2001), contains a moreevolved  Allophaiomys than  A. aff. lavocati fromBarrancoLeónD.Theage of this level has been established at 1.22    0.16 Ma, based oncosmogenic nuclides (Carbonell et al., 2008). Therefore, the age of  the level BL-D can be constrained between 1.77 Ma (top of Olduvaisubchron) and w 1.2 Ma (age of Sima del Elefante) ” . As previouslynoted(Muttonietal.,2010),thestatisticalerrorsonthecosmogenicburialageestimatesatSimadelElefantewerequotedatthe1 s level(only 68% con fi dence), whereas at a more rigorous 2 s  level (95%con fi dence) these data would suggest that the uncertainty in theage range of hominin level TE9 is more like 0.90 e 1.54 Ma (and0.77 e 1.49 Ma in TE7 level below). This would not preclude thathominin occupation at Sima del Elefante occurred between theBrunhes e Matuyama boundary, which was found a few metersabove level TE9, and the Jaramillo, which was not found in thesection (Pares et al., 2006; Carbonell et al., 2008). In summary, we suggest that the chronologic constraints on theBarranco León human tooth should be treated more cautiously. Allthatcanbesaidwithanyacceptabledegreeofcon fi denceabouttheage of the layer that yielded the human tooth at Barranco León isthat the reverse polarity shows it is older than 0.78 Ma, the age of the Brunhes e Matuyama boundary. However, evidence of homininpresence in Europe before the Jaramillo ( > w 1 Ma), or even duringthe Jaramillo (Garcia et al., in press), is in our opinion very tenuous(see also Muttoni et al., 2010, 2011) and frequently based onproblematic ESR dating (e.g., Barranco León), disputable magneto-stratigraphic interpretations (e.g., Untermassfeld, Vallonnet), andbiostratigraphic correlations to distant, poorly dated, andfrequently discontinuous continental sections (e.g., Kryzhanovka).A recent critical assessment of the available magnetostrati-graphic and/or radiometric age constraints on key sites bearinghominin remains and/or lithic industries from Italy, France, andSpain led us (Muttoni et al., 2010) to propose that the  fi rst occur-rence of hominins in southern Europe took place between the Jaramillo subchron and the Brunhes e Matuyama boundary (0.99 e 0.78 Ma). This  w 200 kyr (thousands of years) time window en-compasses the late Early Pleistocene global climate transitioncentered on marine isotope stage (MIS) 22 at w 0.87 Ma, the  fi rstprominent cold stage of the Pleistocene (e.g., Berger et al., 1993;Shackleton, 1995) (Fig. 1). We suggested that aridi fi cation inNorth Africa and eastern Europe, particularly harsh during MIS 22times, triggered migration pulses of large mammals (e.g., ele-phants) as well as hominins from these environmentally stressedregionsintomoreequablecircum-Mediterraneanrefugia,includingsouthern Europe. This post-Jaramillo follow-the-herd hypothesis issubstantially consistent with  e  or does not openly violate  e  theagesofearliesthomininsitesinsouthernEuropeincludingkeysitesin Spain with reliable magnetostratigraphy such as Gran Dolina(Pares and Perez-Gonzalez, 1999; Pares et al., 2013), Sima del Ele-fante (Carbonell et al., 2008; see also above), Vallparadis (Martinez et al., 2010), and (we surmise) also Barranco León (Fig. 1). This hypothesis was also tested and supported by a recent study at thetool-bearing site of Monte Poggiolo, northern Italy, where asequence of stable normal and reverse polarities in a regionallithostratigraphic context indicated that the site demonstrablypost-dates the Jaramillo and pre-dates the Brunhes, most probablyoccurring at w 0.85 Ma immediately after the pronounced coolingthat culminated with MIS 22 (Muttoni et al., 2011).  Acknowledgments We thank Rainer Grün and an anonymous reviewer forinsightful comments, and Christopher Lepre for discussion on anearlier version of this manuscript. References Berger, W.H., Bickert, T., Schmidt, H., Wefer, G.,1993. 22. Quaternary oxygen isotoperecord of pelagic foraminifers: site 806, Ontong Java Plateau. Proc. Ocean Drill.Program Sci. Results 130, 381 e 395.Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polaritytimescale for the Late Cretaceous and Cenozoic. J. Geophys. 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