Solo Man

Despite what Charles Darwin had hypothesised in his 1871 Descent of Man, many late-19th century evolutionary naturalists postulated that Asia (instead of Africa) was the birthplace of humankind as it is midway between Europe and America, providing optimal dispersal routes throughout the world (Out of Asia theory). Among the latter was Ernst Haeckel who argued that the first human species (which he proactively named "Homo primigenius") evolved on the now-disproven hypothetical continent "Lemuria" in what is now Southeast Asia, from a genus he termed "Pithecanthropus" ("ape-man"). "Lemuria" had supposedly sunk below the Indian Ocean, so no fossils could be found to prove this. Nevertheless, Haeckel's model inspired Dutch scientist Eugène Dubois to join the Dutch East India Company and search for his "missing link" in Java. He found a skullcap and a femur (Java Man) at the Trinil site along the Solo River, which he named "P." erectus (using Haeckel's hypothetical genus name) in 1893, and unfruitfully attempted to convince the European scientific community that he had found an upright-walking ape-man dating to the late Pliocene or early Pleistocene, who dismissed his findings as some kind of malformed non-human ape.[1]

In order to find more remains of the Java Man, the Prussian Academy of Sciences in Berlin tasked German zoologist Emil Selenka with continuing excavation of the site, which, following his death, was carried out by his wife and fellow zoologist Margarethe in 1907. Among the members was Dutch geologist Willem Frederik Florus Oppenoorth. The yearlong expedition was unfruitful, but sponsorship of the excavation along the Solo River was continued by the Geological Survey of Java. About two decades later, the Survey funded several expeditions in order to update maps of the island, including maps of Tertiary deposits, with Oppenoorth being made the head of the Java Mapping Program in 1930. Among these was a bed dating to the Pleistocene discovered by Dutch geologist Carel ter Haar in 1931, downriver from the Trinil site, near the village of Ngandong.[2]

Location of Ngandong on the island of Java

From 1931 to 1933, a total of 12 skull pieces (including well preserved skullcaps) as well as two right tibiae (of which one was essentially complete) were recovered under the direction of Oppenoorth, ter Haar, and German-Dutch geologist Gustav Heinrich Ralph von Koenigswald. Midway through excavation, Oppenoorth retired from the Survey and returned to the Netherlands, replaced by Polish geologist Józef Zwierzycki as head of the Java Mapping Program. Due to the Great Depression, the Survey's focus shifted to economically relevant geology, namely petroleum deposits, and excavation of Ngandong ceased completely in 1933 when Zwierzycki took over. In 1934, ter Haar published important summaries of the Ngandong operations, before contracting tuberculosis whereupon he returned to the Netherlands and died two years later. Von Koenigswald, who was hired principally to study Javan mammals, was let go in 1934. After much lobbying by Zwierzycki in the Survey and receiving funding by the Carnegie Institution for Science, von Koenigswald returned to Java to study archaic humans in 1937, but was too preoccupied with the Sangiran site to continue research at Ngandong.[3]

In 1935, the Solo Men remains were transported to Batavia (today, Jakarta, Java, Indonesia) in the care of local university professor Willem Alphonse Mijsberg, with the hope he would take over study of the specimens. Before he had the opportunity, the fossils were moved to Bandung, West Java, in 1942 due to the Japanese occupation of the Dutch East Indies.[3] Von Koenigswald was interned by the Japanese forces for 32 months. Upon his release as well as the commencement of the Indonesian War of Independence, Jewish-German anthropologist Franz Weidenreich (who had fled China before the Japanese invasion in 1941) made arrangements with the Rockefeller Foundation and The Viking Fund for von Koenigswald, his wife Luitgarde, and the Javan human remains (including the Solo Men) to come to New York. Von Koenigswald and Weidenreich studied the material at the American Museum of Natural History,[4] until Weidenreich's death in 1951[3] (leaving behind a monograph on Solo Man).[5] Von Koenigswald, in his 1956 book Meeting Prehistoric Men, included a 14-page account of the Ngandong project with several unpublished results. The Solo Men came to be stored at Utrecht University, the Netherlands. In 1967, von Koenigswald gave the material to Teuku Jacob for his doctoral research. Jacob also oversaw excavation of Ngandong from 1976 to 1978, and was able to recover two more skull specimens and a pelvic fragment. In 1978, von Koenigswald returned the material to Indonesia, and the Solo Men were moved to the Gadjah Mada University, Special Region of Yogyakarta (south-central Java).[3]

The deposition of these fossils in the Solo terrace at the time of discovery was poorly documented, as Oppenoorth, ter Haar, and von Koenigswald were only onsite for 24 days out of the 27 months of operation (as they needed to oversee other Tertiary sites for the Survey), leaving their geological assistants (namely Samsi and Panudju) to oversee the dig, whose records are now lost. Furthermore, the Survey did not publish the site map for over 75 years. The taphonomy and geological age of the Solo Men have consequently been contentious matters. All 14 specimens were reported to have been found in the upper section of Layer II (out of six layers), which is a 46 cm (18 in) thick stratum with gravelly sand, and volcaniclastic hypersthene andesite. All specimens are thought to have been deposited at around the same time as each other. They were probably deposited in a now-dry arm of the Solo River, about 20 m (66 ft) above the modern river. The site is about 40 m (130 ft) above sea level.[3]

Volcaniclastic rock indicates deposition occurred during an volcanic eruption. Due to the sheer volume of fossils, humans and animals may have concentrated in great number in the valley upstream the site either due to the eruption or extreme drought. The ash would have poisoned the vegetation or at least impeded its growth, leading to starvation and death among herbivores and the Solo Men, accumulating a mass of carcasses decomposing over several months. A lack of carnivore damage could mean the carnivores could feed enough without having to resort to crunching through the bone. When the monsoon season came, lahars streaming from the volcano through the river channels swept the carcasses to the Ngandong site, where they were created a debris jam due to the narrowing of the channel there.[3][6]

Based on the site's height above the present-day river, Oppenoorth suggested the Solo Men dated to the Eemian interglacial, which at the time was roughly constrained to 150 to 100 thousand years ago from the Middle/Late Pleistocene transition.[2] Later biochronological studies (using the animal remains to constrain the age) within the next few years by (non-jointly) Oppenoorth, der Taar, and von Koenigswald agreed with a Late Pleistocene date.[7] The Solo Men were first radiometrically dated in 1988 and again 1989, using uranium–thorium dating, to 200 to 30 thousand years ago, a wide error range.[8] In 1996, the Solo Men were dated, using electron spin resonance dating (ESR) and uranium–thorium isotope-ratio mass spectrometry on the teeth, to 53.3 to 27 thousand years ago; this would have meant Solo Man outlasted its continental relative by at minimum 250,000 years, and was contemporaneous with modern humans in Southeast Asia,[7] who immigrated roughly 55 to 50 thousand years ago.[9] In 2008, gamma spectroscopy on three of the skulls showed that they experienced uranium leaching, and the Solo Men were re-dated to roughly 70 to 40 thousand years ago. This would still make it possible that Solo Man was contemporaneous with modern humans.[8] In 2011, argon–argon dating of pumice hornblende yielded a maximum age of 546±12 thousand years ago, and ESR and uranium–thorium dating of a mammal bone just downstream at the Jigar I site a minimum age of 143 to 77 thousand years ago. This extended interval would make it possible that the Solo Men were instead contemporaries with continental H. erectus, long before modern humans dispersed across the continent.[10] In 2020, the first comprehensive chronology of the Ngandong site was published, which found that: the Solo River was diverted through the site by 500,000 years ago; the Solo terrace was deposited over 316 to 31 thousand years ago; the Ngandong terrace 141 to 92 thousand years ago; and the H. erectus bone bed 117 to 108 thousand years ago. This would mean Solo Man was the last known H. erectus population, and did not interact with modern humans.[6]

The classification of Aboriginal Australians, due to the conspicuous robustness of the Aboriginal Australian skull compared to that of other modern day populations, has historically been a perplexing question to European science since Johann Friedrich Blumenbach (the founder of anthropology) introduced the topic in 1795 in his De Generis Humani Varietate Nativa ("On the Natural History of Mankind"). Following the conception of evolution by Charles Darwin, English anthropologist Thomas Henry Huxley made parallels between the Aboriginal Australian and European Neanderthal skulls, first in 1863. Later racial anthropologists continued to push for some ancestor–descendant relation until the discovery of Indonesian archaic humans.[11]

Preliminarily, Oppenoorth drew parallels between the Solo Man skull and that of the Rhodesian Men from Africa, Neanderthals, and modern day Aboriginal Australians.[2] At the time, humans were generally believed to have originated in Central Asia, championed primarily by American palaeontologist Henry Fairfield Osborn and his apprentice William Diller Matthew. They believed that Asia was the "mother of continents" and the rising of the Himalayas and Tibet and subsequent drying of the region forced human ancestors to become terrestrial and bipedal. They maintained that populations which retreated to the tropics–namely Dubois' Java Man and the "Negroid race"—substantially regressed. They also rejected Raymond Dart's South African Taung child (Australopithecus africanus) as a human ancestor, favouring the hoax Piltdown Man from Britain.[1] Oppenoorth at first believed the Ngandong material represented an Asian type of Neanderthal which was more closely allied with the Rhodesian Man (also considered a Neanderthal type), and decided to give a generic distinction as "Javanthropus soloensis". Dubois considered Solo Man to be more or less identical with the East Javan Wajak Man (now classified as a modern human), so Oppenoorth subsequently began using the name "Homo (Javanthropus) soloensis".[5] Oppenoorth hypothesised that the Java Man evolved in Indonesia and was the predecessor of modern day Aboriginal Australians, with Solo Man being a transitional fossil. He considered Rhodesian Man a member of this same group. As for the Chinese Peking Man (now H. e. pekinensis), he believed it dispersed west and gave rise to the Neanderthals.[2]

The WLH-50 skullcap, one of the oldest modern human remains of Australia

Thus, the ancient Java Man, Solo Man, and Rhodesian Man were commonly grouped together in the "Pithecanthropoid-Australoid" lineage. This was an extension of the multiregional origin of modern humans championed by Weidenreich and American racial anthropologist Carleton S. Coon, who believed that all modern races and ethnicities (which were all classified into separate species or subspecies until the mid-20th century) evolved independently from a local population of archaic human (all of which were subsumed as subspecies of H. erectus by the mid-20th century). The matter was revisited in the 1960s as Pleistocene modern humans were being recovered from Australia. By the 1980s, the recent African origin of modern humans theory overturned the Out of Asia theory as African species like A. africanus became widely accepted as human ancestors, and the multiregional model was reconfigured to say that local populations of archaic humans had intebred and contributed at least some ancestry to modern populations in respective regions. In regard to Aboriginal Australians, this has been supported by anatomical comparisons between Solo Man and the first modern humans in Australia, most notably the Willandra Lake humans.[11] However, the date of 117 to 108 thousand years ago for the Solo Men, predating modern human dispersal through Southeast Asia (and eventually into Australia), is at odds with this conclusion.[6] Due to the advanced characteristics of the Solo Men—including a larger brain size, an elevated cranial vault, reduced postorbital constriction, and a less developed brow ridge—the Solo Men have also been referred to as a unique species as "H. soloensis" (continuing as early as the turn of the 21st century), though this is not well supported considering how much they still resemble earlier H. erectus.[12]

The Solo Men are generally considered to have descended from the Sangiran/Trinil populations (H. e. erectus). The three skulls from Sambungmacan and the skull from Ngawi are anatomically quite similar to the Solo Men, and have variously been assigned to H. e. soloensis or to some intermediary morph between H. e. erectus and H. e. soloensis.[12]

At species level, the Ngandong Fauna is overall similar to the older Kedung Brubus Fauna roughly 800 to 700 thousand years ago, a time of mass immigration of large mammal species to Java, including Asian elephants and Stegodon. Other Ngandong fauna include the tiger, Malayan tapir, the hippo Hexaprotodon, Rusa deer, water buffalo, gaur, pigs, and crab-eating macaque. These are consistent with an open woodland environment. The driest conditions probably corresponded to the glacial maximum roughly 135,000 years ago, exposing the Sunda shelf and connecting the major Indonesian islands to the continent. By 125,000 years ago, the climate became much wetter, making Java an island, and allowing for the expansion of tropical rainforests. This caused the succession of the Ngandong Fauna by the Punung Fauna, though more typical Punung Fauna—namely modern humans, orangutans, and gibbons—probably could not penetrate the island until it was reconnected to the continent by 80,000 years ago.[17]

H. e. soloensis was the last population of a long occupation history of the island of Java, beginning 1.51 to 0.93 million years ago at the Sangiran site, continuing 540 to 430 at the Trinil site, and finally 117 to 108 thousand years ago at Ngandong. If the date is correct for the Solo Men, then they would represent a terminal population of H. erectus which sheltered in the last open-habitat refuges of East Asia before the rainforest takeover. Before the immigration of modern humans, Late Pleistocene Southeast Asia was also home to H. floresiensis endemic to the island of Flores, Indonesia, and H. luzonensis endemic to the island of Luzon, the Philippines. Genetic analysis of present-day Southeast Asian populations indicates the widespread dispersal of the Denisovans (a species currently only recognisable by their genetic signature) across Southeast Asia, whereupon they interbred with immigrating modern humans 45.7 and 29.8 thousand years ago. About 1% of the Denisovan genome derives from an even more archaic species, pre-dating the Denisovan/Neanderthal/modern human split, possibly a late-surviving H. erectus population.[6]

Judging by the sheer number of specimens deposited at Ngandong, all at the same time, there may have been a sizeable population of H. e soloensis before the volcanic eruption, but this is difficult to approximate with certainty. The Ngandong site was some distance away from the northern coast of the island, but it is unclear where the southern shoreline and the mouth of the Solo River would have been.[3]

General distribution of Middle Pleistocene handaxes according to the archaeological record

In 1938, von Koenigswald returned to the Ngandong site along with archaeologists Helmut de Terra, Hallam L. Movius, and Pierre Teilhard de Chardin to collect lithic cores and flakes (i.e., stone tools). Because of wearing done by the river, it is difficult to confidently identify some of these rocks as actual tools. They are small and simple, usually smaller than 5 cm (2 in) and made most commonly of chalcedony (but also chert and jasper) washed up by the river. A few volcanic rocks and wood fragments seem to have been modified into heavy duty chopping tools. Like many other Southeast Asian sites predating modern humans, the Ngandong site lacks sophisticated choppers, hand axes, or any other kind of complex chopping tool characteristic of the Acheulean industry of Western Eurasian and African sites. In 1944, Movius suggested this was due to a great technological divide between western and eastern H. erectus caused by a major difference in habitat (open area vs. tropical rainforest), as the chopping tools are generally interpreted as evidence of big game hunting, which he believed was only possible when humans spread out onto open plains.[18]

Though a strict "Movius Line" is not well supported anymore with the discovery of some hand axe technology in Middle Pleistocene East Asia, handaxes are still conspicuously rare and crude in East Asia compared to western contemporaries. This has been variously explain as: the Acheulean was invented in Africa after human dispersal through East Asia (but this would require that the two populations remained separated for nearly 2 million years); East Asia had poorer quality raw materials, namely quartz and quartzite (but some Chinese localities produced handaxes from these materials, and East Asia is not completely void of higher quality minerals); East Asian H. erectus used biodegradable bamboo instead of stone for chopping tools (but this is difficult to test); or East Asia had a lower population density, leaving few tools behind in general (though demography is difficult to approximate in the fossil record).[19]

In 1951, Weidenreich and von Koenigswald made note of major injuries in Skulls IV and VI, which they believed were caused by, respectively, a cutting instrument and a blunt instrument. They bear of evidence of inflammation and healing, so they probably survived the altercation. Weidenreich and von Koenigswald also noted that only the skullcaps were found, lacking even the teeth, which is highly unusual. So, they interpreted at the least Skulls IV and VI as victims of an "unsuccessful assault", and the other skulls where the base was broken out "the result of more successful attempts to slay the victims." They were unsure if this was done by a neighboring H. e. soloensis tribe, or "by more advanced human beings who would have given evidence of their 'superior' culture by slaying their more primitive fellowsman." The latter scenario had already been proposed for the Peking Man (which has similarly conspicuous pathology) by French palaeontologist Marcellin Boule. Weidenreich and von Koenigswald, nonetheless, conceded that some of the injuries could have been related to the volcanic eruption instead. Von Koenigswald suggested that only skullcaps exist because the Solo Men were modifying skulls into skull cups, but Weidenreich was skeptical of this as the jagged rims of especially Skulls I, V, and X are not well suited for this purpose.[5]

Cannibalism and ritual headhunting have also been proposed for the Trinil, Sangiran, and Modjokerto sites (all in Java) based on the conspicuous lack of any remains other than the skullcap. This has been reinforced by the historic practice of headhunting and cannibalism in some modern Indonesian, Australian, and Polynesian groups. In 1972, Jacob alternatively suggested that because the base of the skull is weaker than the skullcap, and since the remains had been transported through river with large stone and boulders, this was a purely natural phenomenon. As for the lack of the rest of the skeleton, if tiger predation was a factor, tigers usually only leave the head since it has the least amount of meat on it. Further, the Ngandong material, especially Skulls I and IX, were damaged during excavation, cleaning, and preparation.[20]