The Ebro frontier is what archaeologists and paleontologists have termed a proposed biogeographical boundary of the Iberian peninsula, south of the Pyrenees and Cantabrian Cordillera at approximately 42°N. This proposed boundary plays a critical role to understanding the transition in Europe from Middle to Upper Paleolithic. About twenty years ago, archaeologists began to discover that Iberian Neanderthal sites, and Mousterian sites that were assumed to be Neanderthal based on artifact assemblages, were dated later than similar assemblages in the rest of Europe, some, such as Gorham's Cave on the rock of Gibraltar, are apparenty as much as ten thousand years later. The Ebro Frontier, proposed by João Zilhão in 2000, is one possible theory explaining how Neanderthals survived longer in the lower Iberian Peninsula than anywhere else.
The frontier is named for, and generally runs along the drainage basin of the Ebro River, a very large river which flows west to east across the Iberian peninsula, eventually discharging into the Mediterranean Sea. During much of the Pleistocene, this river's drainage system divided the cold steppe-tundra of the north from the warmer, tree-covered south. The first Anatomically Modern Humans in the region, goes the original theory, were culturally adapted to steppe plains while Neanderthals were adapted to woodlands; the Ebro frontier was thus a boundary for them. The earliest AMH sites in northern Iberia above the Ebro frontier date to 42,000 (calibrated) years ago; but below the frontier, the cultural sequences associated with AMH don't begin until ~35,000 (calibrated) years ago.
A Quickly Changing Climate
How the Ebro frontier came about--and whether there was one--has been vigorously debated. Current theories suggest that the Middle to Upper Paleolithic transition experienced a quickly changing climate (relative to long-term climate changes) and a resulting mosaic pattern of landscape and vegetation, leading to a long-term coexistence of Neanderthals and Anatomically Modern Humans. The climate changes seen during the Middle to Upper Paleolithic transition are believed by some scholars to be result of Heinrich Events.
A Side Issue: Heinrich Events
Heinrich Events are four (to date) identified ice raft depositions, when it is believed enormous chunks of ice dropped off the glaciers covering the northern hemisphere into the North Atlantic Sea, raising the sea level, creating havoc in Europe and, some researchers say, driving or at least affecting cultural events. These four periods were characterized by a rapid climatic warming followed by an extended arid cooling phase. Heinrich Events have been associated with the development of Middle and Upper Paleolithic techno-complexes from Mousterian through the Solutrean, between 50,000-20,000 years ago, and roughly coeval to Marine Isotope Stages (MIS) 2 and 3.
- Heinrich Event 4 (38-40 thousand years ago (ka)), thought to have led to the growth of Aurignacian, and the macro-scale replacement of Neanderthals with AMH
- Heinrich Event 3 (30-31 ka), associated with the Aurignacian breakdown, and the recolonization of Iberia by Gravettian populations
- Heinrich Event 2 (24 ka), Solutrean replaces Gravettian, probably reorganization of the local populations rather than replacement colonization; the effects of HE2 may have been buffered by cultural inventions
- Heinrich Event 1 (16-18 ka), Magdalenian develops out of Solutrean
Heinrich Event 4 is credited with fostering the spread of modern humans into the Iberian peninsula. During HE 4 (38-40 ka) a period of extreme aridity occurred, with a drop in annual precipitation to below 100 mm per year. Such arid conditions created a semi-desert landscape wedged between Neanderthals and AMH, and a refugium for Neanderthals in the southern and western margins of the Iberian peninsula. Or so says the theory.
Issues with Ebro Frontier
Over the past dozen or more years, evidence has been collected from numerous sites in Iberia, some of which seems to soften the impact of the frontier. For example, Neanderthal sites north of the Ebro Frontier such as El Esquilleu and Cova Gran have provided more recent dates than 42,000; and AMH dates south of the frontier such as Zafarraya, Bajondillo and Foradada Cave seem to be older than those which were known in 2000.
At this point in the scientific theory, the issue is not that the Ebro Frontier didn't exist or didn't impact migration of humans: but, like all human theories, the story is more complex a problem than recognized at first.
A study published in January of 2013 (Wood et al.) reiterated what others (Daura et al., Schmidt et al., Maroto et al.) have said elsewhere, that radiocarbon dates in the Iberian peninsula need additional research. Specifically, new dates on animal bone at Zafarraya and Jarama VI, both south of the Ebro frontier, push the dates of these occupations back to 42,000 years ago. The team attempted to test other sites but found too much degradation and contamination to obtain solid chronological information. If Wood and colleagues are correct, the "late Neanderthal refugium" may not have existed at all. Clearly, further analysis of the Middle Paleolithic sites below the Ebro Frontier are in store.
Bradtmöller M, Pastoors A, Weninger B, and Weniger G-C. 2012. The repeated replacement model – Rapid climate change and population dynamics in Late Pleistocene Europe. Quaternary International 247(0):38-49.
Daura J, Sanz M, García N, Allué E, Vaquero M, Fierro E, Carrión JS, López-García JM, Blain HA, Sánchez-Marco A et al. 2013. Terrasses de la Riera dels Canyars (Gavà, Barcelona): the landscape of Heinrich Stadial 4 north of the “Ebro frontier” and implications for modern human dispersal into Iberia. Quaternary Science Reviews 60(0):26-48.
Jennings RP, Giles Pacheco F, Barton RNE, Collcutt SN, Gale R, Gleed-Owen CP, Gutiérrez López JM, Higham TFG, Parker A, Price C et al. 2009. New dates and palaeoenvironmental evidence for the Middle to Upper Palaeolithic occupation of Higueral de Valleja Cave, southern Spain. Quaternary Science Reviews 28(9–10):830-839.
Maroto J, Vaquero M, Arrizabalaga Á, Baena J, Baquedano E, Jordá J, Julià R, Montes R, Van Der Plicht J, Rasines P et al. 2012. Current issues in late Middle Palaeolithic chronology: New assessments from Northern Iberia. Quaternary International 247(0):15-25.
Martínez-Moreno J, Mora R, and Ignacio de la T. 2010. The Middle-to-Upper Palaeolithic transition in Cova Gran (Catalunya, Spain) and the extinction of Neanderthals in the Iberian Peninsula. Journal of Human Evolution 58(3):211-226.
Schmidt I, Bradtmöller M, Kehl M, Pastoors A, Tafelmaier Y, Weninger B, and Weniger G-C. 2012. Rapid climate change and variability of settlement patterns in Iberia during the Late Pleistocene. Quaternary International 274(0):179-204.
Wood R, Barroso-Ruiz C, Caparrós M, Jordá Pardo JF, Galván Santos B, and Higham TFG. 2013. Radiocarbon dating casts doubt on the late chronology of the Middle to Upper Palaeolithic transition in southern Iberia. Proceedings of the National Academy of Sciences Early Edition.
Wood RE, Higham TFG, De Torres T, TisnÉRat-Laborde N, Valladas H, Ortiz JE, Lalueza-Fox C, SÁNchez-Moral S, CaÑAveras JC, Rosas A et al. . 2013. A new date for the Neanderthals from El Sidrón Cave (Asturias, Northern Spain). Archaeometry 55(1):148-158.
Zilhão J. 2000. The Ebro Frontier: A model for the late extinction of Iberian Neanderthals. In: Stringer CB, Barton RNE, and Finlayson C, editors. Neanderthals on the Edge: 150th Anniversary Conference of the Forbes' Quarry Discovery, Gilbratar. Oxford: Oxbox Books. p 111-121.