Breakdown: Unusual human fossil from Hualongdong, China

A new species, a strange hybrid or the beginning of Out-of-Asia?

Many of you may have seen the news last week of a strange new skull found in China - one which may come to rewrite aspects of the human story. The media reporting has focused on either how this complicates the current paradigm (another branch on the tree) or on how this might substantially alter it (origins of Neanderthals, Denisovans, did modern humans evolve in Asia?).

To break it down we need some context:

In 2019 a paper was published reporting the excavation results of a Middle Pleistocene era cave (~700,000 to 125,000 years ago) in Hualongdong, Anhui Province, China. Hualong Cave yielded 16 human fossils, including one of particular interest known as HLD 6. HLD 6 was remarkably well intact for such an archaic site, comprising most of the frontal bone, the left parietal bone, the maxillae and left zygomatic bone and more. HLD 6 was dated to between 275-371,000 years ago, using the uranium-thorium dating method.

In 2021 a second paper was published outlining the analysis of HLD 6’s skull, in which the authors pointed out the strange suite of both modern and archaic features. We also discovered that HLD 6 was almost certainly a juvenile, around 13-15 years old.

Fast forward to today, and a new paper was published which completed the journey by analysing the mandible, or jawbone and all the implications of HLD 6 were spelled out in the joint work between a number of institutions based in China, Spain and Britain.

The paper presents not only the detailed anatomical work of assessing the fossil fragments, measuring the various skeletal structures and features and comparing them to other human species and subspecies, but also the possible evolutionary relationship of this East Asian Middle Pleistocene individual.

The final reconstruction of the skull is distinctly odd for anyone familiar with archaic humans - the face is extremely flat, the top of the skull sloping backwards with a abrupt base. Compare it to other human skulls to see the difference.

A is a modern human, B is a Homo erectus, C is a Neanderthal and D is HLD 6 as above. Looking at the face, the ‘flatness’ more closely matches a modern human, whereas the others have a more prognathic jaw and face which extends outwards. Modern humans vary in their level of prognathism, but not many archaic fossils have such flat faces. Also obvious at a glance is the smaller brain size, missing chin and the thick orbital ridges, the extended eyebrows as it were.

Overall the major archaic characteristics were as follows:

• Low cranial vault (smaller head compared to modern humans)

• Frontal keel (raised centre line of the skull)

• Wide and low nasal apature

• Thick and extended supraorbital tori (brow ridge)

• Rounded inferior margin of the orbit (lower portion of eye socket)

• Thickened maxillary zygomatic process (cheekbone joint)

• Absence of canine fossa (jawbone dip)

• Thick, robust mandible body

• Absence of chin

• A number of smaller features on the jawbone (pronounced endocondyloid crest etc)

Against this are the modern human features, which include:

• ‘Flatter’ face

• Vertically oriented midface

• Several nasal features similar to modern humans (vertical crista nasalis, anterior nasal spine etc)

• Forward positioned incisive foramen (hole for nerves and blood vessels in the roof of the mouth)

• Parabola shaped dental arcade (shape of the row of upper teeth)

• Weakly gracile jawbone features (weak lateral prominence, gracile ramus etc)

The gritty details of these kinds of anatomy papers are probably too dull for a brief review, but overall the mandible and skull show a blend of features corresponding to Neanderthals, H.erectus and modern humans, described as a mosaic. Modern human anatomy is typically far more gracile than our ancestral kin, with much more globular skulls.

The real question after this description though is what does any of this mean?

You might recall in a previous article about human evolution in Europe, I described what is called the ‘muddle in the middle’ period of hominin development. This phrase was coined in 1975 by the influential South African archaeologist G.L Isaac, neatly capturing the chaos of the fossil record between H.erectus and the emergence of modern humans in the Middle Pleistocene. We have a massive proliferation of species, many of which have been redefined, renamed and reclassified - Homo heidelbergensis, antecessor, bodoensis, naledi, cepranensis, longi, rhodesiensis, floresiensis, luzonensis, Denisovans, Neanderthals and so on.

China and Asia more broadly has increasingly become an important region for these Middle Pleistocene fossils, shifting the centre of gravity away from Africa and Europe. Traditionally Asia was seen as something of a backwater or an area of evolutionary stagnation, producing only erectus fossils and their Acheulean handaxes, which were presumed to then die out and become repopulated with more modern species emerging from Africa and Europe. This has slowly changed with the discovery of dozens of new fossils, which often show a bewildering amount of variety:

It is now clear that later Middle Pleistocene specimens including Dali, Jinniushan, Chaoxian, Maba, Xujiayao, Xuchang, Panxian Dadong and Tongzi do not follow the pattern of classical H. erectus, showing derived traits aligned with East Asian H. sapiens [7,22,23,27,38,39] and Neanderthals [26,40], while newer specimens such as Xuchang and Hualongdong possess novel morphologies for this time period and region [9,26]. It is also clear that following 300 ka there is a notable increase in craniodental variation and the appearance of novel traits [41].

Exactly where these new fossils sit in the evolutionary schema is unclear, as the authors of this new HLD 6 paper remark:

The hominin fossil discovery and related studies in the last decade have changed this traditional view on the evolution pattern of the late Middle Pleistocene hominins in China radically. The hominin fossils from this time period, such as Dali, Jinniushan, Maba, Tongzi, Xujiayao, Xuchang, and Xiahe, exhibit high morphological variability and are not easily allocated to the existing taxonomic groups (Liu and Wu, 2022; Liu et al., 2022; Wu et al., 2022). They have been given different tentative names, often unclear taxonomically (Xing et al., 2015; Li et al., 2017), such as a population of unclear taxonomic status (Xujiayao, Xing et al., 2015) ‘non-erectus’ (Tongzi, Xing et al., 2019), East Asian Middle Pleistocene hominins (Dali and Jinniushan, Wu, 2014, 2020), an Eastern variant of Neanderthals (Maba and Xuchang, Wu and Bruner, 2016; Li et al., 2017), and Denisovans (Xiahe, Chen et al., 2019).

Calls for a new Homo species in China and the surrounding regions are beginning to pick up pace, as another unusual skull found in Heilongjiang Province dated to around 150,000 years ago was found not to fit into any known classification. In fact the broader picture of Asian human evolution seems to be a total mystery. In 2015 a dig in Fuyan Cave, Daoxian (southern China) unearthed 47 modern human teeth dating to between 80-120,000 years ago - a radical break from the traditional Out-of-Africa model which postulates an exodus around 50-60,000 years ago.

While most Westerners today are familiar with Out-of-Africa as the dominant theory of human evolution, Chinese researchers have been through several waves of ‘Sinocentrism’ during the past century. The discovery of the erectoid Peking Man in 1921 prompted decades of debate about where humans evolved and whether different humans evolved in different parts of the world (strong multiregionalism).

Today, many Chinese scientists and the general public, however, still hold the interpretation that modern Chinese are descendants of Peking Man. In other words, the Chinese accept the Out of Africa I hypothesis that Peking Man and other Homo erectus were from a shared African hominid origin, but instead of being replaced by more modern humans from Africa, Peking Man evolved continuously, with interactions and hybridizations with neighboring populations, into Homo sapiens and then the modern Chinese. This is the hypothesis of Multi- regional Continuity, which is the main opponent of the Recent Out of Africa hypothesis. Chinese scientists like Wu Xinzhi are the major advocators for Multiregional Continuity theory and their best evidence comes from the existing hominid fossils found in China, which show common morphological traits between Homo erectus, Homo sapiens, and contemporary Chinese.

Evolutionary Asiacentrism, Peking Man, and the Origins of Sinocentric EthnoNationalism (2014) - Hsiao-Pei Yen

Leaving aside Chinese politics, a number of researchers have questioned the position that the Last Common Ancestor (LCA) of Neanderthals and modern humans must have lived in Africa. The fossil record in Africa is not exactly overflowing with hybrid or mosaic like individuals displaying both Neanderthal and sapien features. In a 2022 paper entitled The origins of the Homo sapien lineage: When and where?, two Spanish academics reviewed the evidence for an African LCA and concluded that the Levant was just as likely a place to see the ancient split, leaving Neanderthals to move into Europe and Asia, and the earliest sapiens to move back into Africa, where they stretched from Morocco to South Africa:

At present, there is no record in Africa which can sustain both the geographical division of this original population and the migration of Neanderthal genealogy to Europe during the late Early Pleistocene or Early Middle Pleistocene.

The African mid-Middle Pleistocene fossils can be defined by the lack of some features characteristic of H. erectus s.l. and which have led experts to classify them within species such as H. heidelbergensis, Homo rhodesiensis and Homo bodoensis. With a few exceptions, no expert has observed a clear trend towards Homo sapiens in these fossils... In this context, the possibility of considering the Near East as a possible source of the LCA gains weight. We suggest that, in additional to further exploration of the African record, the LCA should potentially be searched for in southwest Asia and in particular in the Levant (Fig. 3). This region is a crossroads between Africa and Eurasia, has maintained relatively stable climatic conditions to allow for a resident mother population and would be a reasonable place for the geographical split leading to Neanderthals and humans of modern morphology.

Schematic and hypothetical representation of a possible centre of origin and dispersals from Southwest Asia to Africa and Eurasia. From Bermúdez de Castro & Martinón-Torres 2022

A similar paper published in 2021 also questions why the Levant is only ever a one-way valve in human evolution, pumping hominins out but never back again?

For all these researchers, the simplest explanation is to propose early migrations of H. sapiens from Africa. This hypothesis explicitly recognises that the gates of the Levantine Corridor were open for much of the Middle Pleistocene. Although the direction was always from Africa towards Eurasia, never the other way round.

What this new Chinese paper discusses is expanding this range of early sapien-like humans into east Asia from the Levant. They note that 300kya appears to be a crucial window for the development of modern human features, with the earliest basal H.sapiens appearing at Jebel Irhoud in North Africa at the same time as HLD 6 was living in modern China. They tentatively explore a number of scenarios:

1. HLD 6 and other non-erectus Middle Pleistocene fossils could belong to a group containing the elusive Denisovans, a species which is known almost entirely through its genetics alone

2. HLD 6 belongs to a yet unclassified sister lineage which is closely related to the emerging sapien group, but constitutes essentially a new subspecies or local variant

Clearly we are in the middle of a shift in palaeoanthropology, where the focus on Africa and Europe is giving way to the Levant, China and southeast Asia. To date there are still huge gaps in the fossil record, both temporally and geographically - in particular Iran, west-central Africa and central Asia - and no doubt new finds will shift the paradigm over the coming decades. Chinese archaeology as an institutional force is rapidly growing, and they seem far less attached to Out-of-Africa as a hypothesis for every new change in human evolution. As it stands the development of the Homo genus seems incontrovertibly to have taken place in Africa, but the problem of the muddled middle looks set to incorporate new regions of the world.

1. A good article here on the development of Chinese archaeology and human evolution.