The last known Homo erectus

There are a lot of assumptions made about Homo erectus and, indeed, there is much confusion surrounding the species (see: various items in Human evolution and migrations logs for 2001, 2002, 2003 and several other years). For a start, the name derives from Eugene Dubois’s 1891 discovery of several hominin cranial fragments in sediments deposited by the Solo River in Java. Dubois was the first to recognise in ‘Java Man’ the human-ape ‘missing link’ about which Charles Darwin speculated in his The Descent of Man, and Selection in Relation to Sex (1871). Dubois named the beings Pithecanthropus (now Homo) erectus. Once the “multiregional” versus “out-of-Africa” debate about the origin of anatomically modern humans (AMH) emerged after a variety of H. erectus-like fossils had also turned up in Africa and Europe, as well as in East and SE Asia, ‘Java Man’ was adopted by the multiregionalists as ‘evidence’ for separate evolution of AMH in Asia. Such a view remains adhered to by a tenacious number of Chinese palaeoanthropologists, but by virtually no-one else.

Reconstruction of the Nariokotome Boy from the skeleton found in the Turkana Basin of Kenya (credit: Atelier Daynes/Science Photo Library)

The earliest of the African ‘erects’ were distinguished as H. ergaster, represented by the 1.6 Ma old, almost intact skeleton of Nariokotome Boy from the Turkana area of Kenya. In Africa the specific names ergaster and erectus often seem to be used as synonyms, whereas similar-looking fossils from Asia are almost always referred to as ‘Asian ­H. erectus’. Matters became even more confusing when the earliest human migrants from Africa to Eurasia were discovered at Dmanisi in Georgia (see; Human evolution and migrations logs for 2002, 2003, 2007, 2013). Anatomically they deviate substantially from both H. ergaster and Asian erectus – and from each other! – and at 1.8 Ma they are very old indeed. Perhaps as a palliative in the academic rows that broke out following their discovery, for the moment they are called Homo erectus georgicus; a sub-species. But, then, how can Asian H. erectus be regarded as their descendants. Yet anatomically erectus-like fossils are known in East and SE Asia from 1.5 Ma onwards.

There is another mystery. Homo ergaster/erectus in Africa made distinctive tools, typified by the bifacial Acheulian hand axe. Their tool kit remained substantially the same for more than a million years, and was inherited by all the descendants of H. erectus in Africa and Europe: by H. antecessor, heidelbergensis, Neanderthals and early AMH. Yet in Asia, such a technology has not been discovered at sites older than around 250 thousand years. Either no earlier human migrants into Asia made and carried such artefacts or stone tools were largely abandoned by early Asian humans in favour of those more easily made from woods, for instance bamboo.

In 1996 the youngest Solo River sediments that had yielded H. erectus remains in the 1930s were dated using electron-spin resonance and uranium-series methods. The results suggested occupation by ‘erects’ between 53 and 27 ka, triggering yet more astonishment, because fully modern humans had by then also arrived in Indonesia. Could anatomically modern humans have co-existed with a species whose origin went back to almost two million years beforehand? It has taken another two decades for this perplexing issue to be clarified – to some extent. The previous dates were checked using more precise versions of the original geochronological methods covering a wider range of sediment strata (Rizal, Y. et al. 2019. Last appearance of Homo erectus at Ngandong, Java, 117,000–108,000 years ago. Nature, published online; DOI:10.1038/s41586-019-1863-2). No AMH presence in Asia is known before about 80 ka, so can the astonishment be set aside? Possibly, but what is known for sure from modern and ancient DNA comparisons is that early modern human migrants interbred with a more ancient Asian group, the Denisovans. At present that group is only known from a site in Siberia and another in Tibet through a finger bone and a few molar teeth that yielded DNA significantly different from both living humans and ancient Neanderthals. So we have no tangible evidence of what the Denisovans looked like, unlike Asian H. erectus of whom there are many substantial fossils. Yet DNA has not been extracted from any of them. That is hardly surprising for the Indonesian specimens because hot and humid conditions cause DNA to break down quickly and completely. There is a much better chance of extracting genomes from the youngest H. erectus fossils from higher latitudes in China. Once that is achieved, we will know whether they are indeed erects or can be matched genetically with Denisovans.

See also:  Price, M. 2019. Ancient human species made ‘last stand’ 100,000 years ago on Indonesian island (Science)

Hobbit time

A few months after the diminutive hominin fossil Homo floresiensis, which because of its relatively large feet was quickly dubbed the ‘Hobbit’, turned out to be considerably older than previously thought it hit has the headlines again because its ancestors may have colonized the Indonesian island of Flores far earlier still. A pair of articles in the 9 June 2016 issue of Nature consider evidence from another site on the island where fluvial sediments offer more easily interpreted stratigraphy than the complex Liang Bua cave assemblage where the original skeletal remains were unearthed. The site in the So’a Basin became an important target for excavation following the discovery there in the 1950’s of stone artefacts, east of Wallace’s Line – a fundamental faunal and floral divide once thought to be due to the difficulty of crossing a deep, current-plagued channel in the Indonesian archipelago. The unexpected presence of artefacts drew palaeoanthropologists from far afield, but it was almost 50 years later before their exploration yielded hominin remains.

English: homo from flores
Homo floresiensis (credit: Wikipedia)

One of the papers reports sparse new finds of hominin material from the So’a Basin, a fragment of mandible and 6 isolated teeth thought to be from at least three individuals (van den Bergh, G.D. et al., 2016. Homo floresiensis-like fossils from the early Middle Pleistocene of Flores. Nature, v.  534, p. 245-248). The other covers newly discovered artefacts, the stratigraphic and palaeoecological setting, and radiometric dates of the finds (Brumm, A. and 22 others, 2016. Age and context of the oldest known hominin fossils from Flores. Nature, v.  534, p. 249-253). The jaw fragment shows signs of having once held a wisdom tooth, showing that it belonged to an adult. Yet although it resembles the dentition of the younger Liang Bua specimens, it seems more primitive and is even smaller. The other dental finds are most likely to be deciduous teeth of juveniles. Fission-track, uranium-series and 40Ar/39Ar dating indicates that the fossils entered the sediments about 700 ka ago. But tools and remains of prey animals in deeper sedimentary layers here and at other Flores sites indicate the presence of hominins back as far as about 1 Ma, before which there are no such signs.

So, at least a million years ago Flores was colonised by hominins. Either the original immigrants were uniquely small compared with other hominins of that vintage in Asia and Africa, or within 300 ka they had decreased in size through the evolutionary influence of limited resources on Flores and the process of island dwarfism. The second may also have been influenced by an initially small population of migrants or a later population ‘bottleneck’ that added a loss of genetic variability – a founder effect.   These two alternatives may point respectively to either the even earlier migration out of Africa and across most of Asia of perhaps H. habilis, or the dwarfing of a limited population of H. erectus who made their way there from their known occupation of Java. The authors painstaking analysis of the meagre remains suggest a closer dental resemblance to Asian Homo erectus than to earlier African hominins, so the second alternative seems more likely. However, even that scenario poses palaeoanthropology with a major problem; yet another evolutionary process that helps cryptify the links among our earlier relatives. (See also: Gomez-Robles, A., 2016. The dawn of Homo floresiensis. Nature, v.  534, p. 188-189.)