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Roy Larick Shore Cultural Centre, Euclid, Ohio
Russell L. Ciochon Department of Anthropology, University of Iowa, Iowa City, Iowa
Yahdi Zaim Department of Geology, Institute of Technology, Bandung, Indonesia
Fossils of Java Man were first found on that Southeast Asian island 112 years ago. Although their origin was not recognized at the time, the finds represented Homo erectus, an early human species that presumably evolved in equatorial Africa more than 2 million years ago. But the how, why, and when of Homo erectus trek from Africa to island Southeast Asia have always been difficult questions.
The trek was indeed complex, and it appears that Homo erectus arrival to current Java is an important factor for understanding why the species left highland tropical Africa. We hope to shed light on the larger issues by delving into Homo erectus significant relationship with volcanic and otherwise highly unstable landscapes in far Southeast Asia. Homo erectus seems to have sought out unstable landscapes, and this preference may explain this hominids departure from Africa before 1.8 million years ago (mya) and its arrival to extreme Southeast Asia not long thereafter.
[Fig.1: The Tethys Corridor, shown with generalized Pleistocene vegetation zones and Homo erectus sites ]
Our theory is based on two premises. First is the fact that early Homo erectus fossils are always found in the context of volatile geology and ragged geography. Second, the East African Rift and extreme Southeast Asia are endpoints on a grand east-west geotectonic pathway called the Tethys corridor (fig.1). During a rather brief period called the Olduvai subchron (1.98-1.79 mya), the Tethys corridor was extremely unstable. Homo erectus and companion mammals took advantage of open linear landscapes to migrate north from the Rift to the Caucasus, and then both ways across the Tethys corridor - west toward Gibraltar, east to the Himalayan fore slope, and then far east to current Java. By the end of the Olduvai subchron, Homo erectus had dispersed throughout the greater Tethys realm.
The immediate how of the Southeast Asian trek is perhaps the easiest to answer. We now know that Homo erectus did not navigate to present Java, but rather walked the length of the emergent Sunda continental shelf off East Asias present south coast. Currently, the Indonesian archipelagos 14,000 tropical islands constitute the emergent landmass (fig.3), but in the last 2 million years, the Sunda subcontinent (fig.4) has sometimes included much of the intervening sea bottom. Arriving to the area of present-day Thailand, Homo erectus groups spread south along a broad valley between the current Malay Peninsula and Sumatra. And they continued on to current Central Java, the extreme south coast of Sunda (fig.2).
[Fig.2: Map showing concentration of sites with Homo erectus fossil finds in Central and East Java.]
Much of Central Java lies directly above Sundas highly volcanic subduction zone. Active volcanoes form the spine of the entire island but are especially large in the central zone (fig.5), and their ash is slightly alkaline. Central Java is consequently blessed with rich volcanic soils; they are young and sweet. Using this resource of the central district, the entire island has supported a much larger historical human population than Sumatra, the Malay Peninsula, or Borneo - Java is earths most densely inhabited agricultural island. Central Javas soil wealth probably explains why Homo erectus may have explored much of Sunda, but gravitated to this coast. As a hunter, this hominid also took advantage of eruptions that randomly cleared patches of rainforest. Such events provided rich graze for the large mammals upon which the early humans preyed.
[Fig.3 (left): Southeast Asia today with subduction zones (after Djubiantono and Sémah 1993). ]
[Fig.4 (right): Schematic map of Sundaland at 2-1.5 mya, with probable migration path of Homo erectus (after Djubiantono and Sémah 1993).]
The Sangiran Dome: Central Javas Sangiran dome lies squarely astride the subduction zone; large volcanoes rise to the east (Lawu) and to the west (Merapi) (figs.5,6). The dome is a window into one of earths more important early human fossil beds and is now a UNESCO World Heritage Site. During the last century, the domes heavily eroded cliff faces have yielded more than 80 fragments of Homo erectus skeletons (Larick, Ciochon, and Zaim 1999). The fossils represent an early human occupation of about 500,000 years duration as Sunda emerged from the Java Sea.
Two million years ago, the area of the current dome lay on the southern edge of the submerged Sunda shelf. The edge emerged from the Java Sea as three base geological factors began to interplay. One was the slow but constant folding of Asian crust as the Indian Oceanic plate slid or subducted under it at the Java trench just south of the island. Another was the buildup of debris from volcanoes venting above the subduction zone. The critical factor was the coming of Pleistocene Ice Age around 2 mya. At this point, glacial ice accumulated rapidly on the northern hemispheres continental masses. So much water was pulled out of the earths atmosphere and locked up as ice that sea levels receded across the globe. While sea levels have fluctuated with glacial cycles over this period, the net effect has been lower water and a more emergent Sunda shelf.
The lowest formation exposed in the dome, the Puren limestone, represents this earliest stage of emergence. In its upper reaches, the Puren shows evidence for wave-cut benches, mangrove beaches, brackish lagoons, and hints of an island fauna. About 1.9 mya, a series of mass mudflows abruptly slid down a nearby volcanic cone to fill in local coastal areas. These lahars (slurries of wet ash mixed with larger rocks, tree trunks, and the occasional animal carcass) raised the local environment high enough to support freshwater lakes and swamps with higher, dryer ground inland. On top of the lahars, the Sangiran formation is mostly lacustrine clay with heavy organic content. Fossils of the first-arriving land mammals appear in these swamp deposits: water-loving species such as elephants, hippos, and certain deer.
As the Sangiran formation continued to build up, the lakes became shallower and more productive, and the nearby terrestrial landscapes must have been lush. Homo erectus arrived to such lakeside environments along with the first large carnivore, Panthera. As the early humans settled in, the Sunda coast kept emerging, and the local landscape kept drying.
[Fig.5: The Solo Basin, a segment of Javas east-west trending Central Depression, the current magmatic zone of the Indonesian volcanic island arc (after Larick et al. 2001).
In yet another abrupt transition about 1.5 mya, local uplift and volcanic eruption took over the creation of area landforms. Local sedimentation changed from lacustrine to fully fluvial or riverine. The current dome area quickly took on much of its present aspect, a low terrestrial plateau framed with large volcanoes. This fluvial sediment mantle is known locally as the Bapang formation.
Active volcanism alternated with more peaceful times during the period represented by the Bapang formation. During the more energetic episodes, uplift increased stream gradients, and eruptions charged stream courses with volcanic detritus. As this sediment was transported seaward, the Sangiran locale found itself ever more distant from the coast. Thick layers of sand and gravel in fast-shifting stream beds typify the active phases. In quieter times, meandering rivers accumulated beds of fine tuffaceous sediment.
We count five two-phased cycles of sedimentation in the Bapang formation. Most large mammal fossils are found in the coarse sedimentary phases (fig.7). We also find that the sedimentary cycles lose intensity through time. Rich bone beds lie just above the Bapang/ Sangiran formations transition zone. The lower Bapang human fossils here are highly fragmented teeth and jaws. As one moves up through the Bapang sediment column, rock particles get finer and the fossils become more complete and well preserved (fig.8)
While the formation names change, Bapang-like sedimentary cycles have continued right up to the present in Central Java. The human fossils, however, and most of the accompanying large mammal remains, are not present in the upper part of the Bapang formation itself. The time of disappearance must be about 800,000 years ago. For reasons not yet entirely clear, the well-entrenched early human population disappeared from the Sunda south coast after having lived there for more than a half million years.
[Fig.6: Sangiran Dome, showing sedimentary levels and documented hominid fossil findspots. Abbreviations mark village localities (after Larick et al. 2001).]
In sum, the earliest Homo erectus groups arrived to the coastal swamps of south-central Sunda between 1.8 and 1.6 mya. At this point the hominids probably lived slightly inland of the Sangiran locale; their bones washing downstream after death. About 1.5 mya, fast-flowing streams began building and cutting beds of coarser sediment. The lower and middle Bapang formation stream banks represent the landscapes on which Homo erectus actually lived. About 800,000 years ago, the hominids and most other contemporary large mammals seem to have left the area. In the meantime, volcanic debris has continued to accumulate up to the present era.
Age of Sunda Homo erectus: During the last century, age estimates for the earliest human fossils on Java have ranged from as little as a half million years to as much as 2 million years. As in other areas of human evolutionary research, the initial age estimates were young; newer geological findings and dating methods tend to push back the ages. Our collaboration with Indonesian geologists and American geochronologists applies the most advanced radiometric dating methods (Larick et al. 2001). There is now little question that early Sunda humans arrived toward the end of the Olduvai subchron.
The volcanic debris that preserves the Sangiran fossils also provides a means to understand their age. Small quantities of deep earth minerals, such as hornblende and plagioclase, are expelled molten during eruptions. At the moment the droplets cool and crystallize outside the volcano, their potassium isotopes are maximally unstable. As the phenocrysts become part of the sediment column, the unstable isotopes decay at a constant rate. Radioactivity decreases as the stable decay byproducts increase. Given routine assumptions, the relative proportions of unstable and stable isotopes mark the time since the volcano has erupted. Geochronologists speak of this ratio as the minerals eruption age. If a datable phenocryst and a human fossil have contemporaneous sedimentary origins, the mineral eruption age approximates the age of the fossil itself.
But there are some problems. Individual eruptions often produce tuffs, layers of air-fall volcanic ash that provide distinct horizons within a sedimentary sequence. In East Africa, such tuffs have been a heavenly gift for geochronologists. Phenocrysts dated from a tuff overlying fossil beds give a minimum age for any given fossil. Tuff minerals underlying a fossil bed produce maximum ages. In the humid environments of Southeast Asia, however, tuff layers are almost always reworked by surface runoff and stream action. Ash from a number of tuffs may be mixed together and then deposited against a stream bank from which older fossils are eroding.
These same conditions also increase the chemical weathering of the volcanic minerals. In the Sangiran dome, individual crystals are often too weathered to give true eruption ages. Moreover, Central Javan mineral crystals are often too small to analyze individually.
In surmounting these problems, we have turned toward clastic (lumps of) pumice, the sponge-like glass debris common to island arc volcanoes in Southeast Asia. Pumice clasts usually contain numerous hornblende phenocrysts, and the encapsulation serves to hold back the weathering process. Moreover, pumice clasts are often found in the same coarse sediment as the human fossils themselves. Consequently, our geochronology uses bulk samples of pumice-encapsulated hornblende phenocrysts.
[Fig.7: Correlation of Sangiran stratigraphy with hominid finds. Columns indicate sampled localities, and numbered ovals indicate pumice samples tested for augite content, hornblende color, and 40Ar/39Ar age analysis. Skulls found into the mid-1970s are labeled with their Sangiran number (e.g., Sangiran 17). Later finds are referred to by the locality, year, and month of their discovery (e.g., Tjg 1993.05). Locality abbreviations, left to right (north to south, with no scale): Gwn - Grogolwetan; Sbk - Sendangbusik; Bpg - Bapang; Tjg - Tanjung; Pcg - Pucung (after Larick et al. 2001).]
In order to understand the associations of pumice clasts and human fossils, our Indonesian-American team has undertaken detailed study of the sedimentary framework for the Sangiran dome (fig.7). We are studying the sedimentary dynamics of fossil bones and pumice clasts in stream environments and are analyzing the petrographic variety in volcanic minerals throughout the dome. The final step is to calibrate these findings with hornblende eruption ages at a number of stratigraphic levels.
The rich bone beds of the Bapang formation provide abundant information. In the lowest Bapang sediments, coarse gravel holds highly fragmented human fossils. The pumice clasts contain green hornblende that yields eruption ages between 1.51 to 1.47 mya. Above the base, a second cycle of Bapang deposits holds less fragmented human cranial elements (fig.11). Middle range pumice clasts have green and brown hornblende crystals with ages from 1.33 to 1.24 mya. Higher Bapang beds hold the youngest cranium in association with brown hornblende. The eruption ages cluster around 1 mya.
As already mentioned, the lahars at the base of the Sangiran formation contain pumice clasts and their hornblende gives eruption ages of 2 to 1.8 mya. Unfortunately, the Sangiran formation above the lahars has no pumice. For the present we are not able to include the most important early human fossil beds directly in our scheme. Nevertheless, the bracketing dated material indicates that Homo erectus arrived after 1.8 mya and before 1.6 mya.
Our results give the first radiometrically calibrated scheme for the emergence of this part Sunda, as well as for the arrival, entrenchment, and disappearance of Homo erectus. This human ancestor occupied south Sunda for at least a half million years beginning more than 1.6 mya. With an occupation of this duration, we may speak of an evolutionary sequence for Sunda Homo erectus. When the large number of Sangiran dome fossils are ordered by time, it is clear that the Sunda population felt environmental pressures akin to those of its western Tethys cousins (box 3).
[Fig.8: The five two-phased sedimentary cycles of the Bapang Formation have produced a differential preservation of fossil hominids. The lowest cycle (1a) with more energy and courser rock particles has yielded fragmentary teeth and jaws. The upper cycles (2a to 5a) with lower energy levels and finer sediments have yielded better preserved calottes and crania. This figure links Homo erectus specimens with the sedimentary cycles that produced them. The bottom-most level (in tan) in this figure depicts the oldest hominids found in the Sangiran Dome from the lacustrine Sangiran Formation (after R. Uribe, R.L. Ciochon, and R. Larick).]
Sunda Homo erectus in Global Context: Fossils representing very early Homo erectus populations are now known from the highland Rift Valley of East Africa, the Caucasus Mountains that mediate southeast Europe and southwest Asia, and from the intensely volcanic slopes of the Sunda subduction zone. Circum-Mediterranean archaeological sites representing these groups may be present in northern Algeria (Ain Hanech), Andalusian Spain (Orce), and the Negev (Erq el Amar). Late Olduvai subchron archaeological sites are also known on the Himalayan fore slope (Riwat, Pakistan), and in southern China (Longgupo). The Plio-Pleistocene carnivores associated with humans are also known from Greece (Mygdonia Basin).
The commonalties among these sites call for a new interpretation of early Homo erectus. All these sites fall into the transcontinental Tethys geotectonic corridor, the grand suture at the southern margin of the Eurasian continental plate with southward extensions into the East African Rift and the Sunda subduction zone. A global time marker immediately precedes and overlaps with all sites, the Olduvai subchron (1.96 to 1.79 mya). With the corridor and the subchron, we can begin to talk about Homo erectus biogeography as neither African nor East Asian, but as Plio-Pleistocene Tethys.
The corridors linear geotectonic structure includes convergent plate margins from Iberia to Sunda; rift valleys and plate imbrication zones from equatorial East Africa to the Caucasus. During the Olduvai subchron, major Tethys geotectonic events (including the Aullan sea regression in the west and the emergence of Sunda in the east) served to open virgin territory from west to east. With large body size, striding gait, carnivorous diet, and elemental technology - and a relatively small brain - early Homo apparently found advantage in the realms linear structure and the subchrons geotectonic instability. Arising in the equatorial western realm near the beginning of the Olduvai subchron, early Homo erectus dispersed throughout the realm by or just after subchron end.
Biogeographically, early Homo is a Plio-Pleistocene Tethys lineage, with long-lived branches in East Africa and East Asia, and possibly in extreme southern Europe. Morphological variation across the realm is still incompletely known and potentially very complex. The new Sangiran data indicate that the two known equatorial populations (Sunda and East African Rift Valley) encephalized in parallel already by the early Pleistocene. Whatever their importance for later developments in Homo, neither a large brain nor a complex stone technology served this hominids initial intercontinental dispersals. Alternatively, the Tethys geotectonic landscapes may have provided this early human species its greatest advantage and provided the distinction between it and the numerous preceding australopiths.
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Appearing in Athena Review, Vol.4, no.1 (2004).
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