Puzzling fossils unearthed in China may rewrite the human story

By Katie Hunt, CNN

A cache of human-like fossils from China has perplexed scientists for decades, defying explanation or categorization.

The skull fragments, teeth, jaws and other remains unearthed at different sites across the country are clearly remnants of archaic hominins - the formal name for species in the human lineage - who lived between 300,000 and 100,000 years ago. But it's never been clear what species the bones belonged to or where they should sit in an increasingly complex human family tree.

Christopher Bae, a professor in the department of anthropology at the University of Hawaii at Manoa who was based in Beijing for many years, is among scientists revisiting these puzzling fossils with fresh eyes. He and his colleague Wu Xiujie, a senior professor from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, now suggest it may be time to formally recognize a previously unknown ancient hominin, and they've proposed officially recognising a species new to science.

The most striking characteristic of this previously unknown human ancestor? An extremely large brain that's bigger than that of our species, Homo sapiens, the only surviving hominin.

The feature is reflected in the proposed name for the species, revealed by Bae and Wu in a November article published in the scientific journal Nature Communications: Homo juluensis, a nod to 'ju lu', which means 'huge head' in Chinese.

"Their skulls are actually very, very large, you know, the estimated cranial capacity is 1700, 1800 cubic centimetres," said Bae, who also described Homo juluensis in his book The Paleoanthropology of Eastern Asia, published in September. "We have a minimum capacity of about 1350 cc, on average, we're about 1450cc.

"It's not an order of magnitude larger but it's much more robust."

The proposal is sparking controversy among paleoanthropologists, and some scientists disagree about whether the new grouping rises to the level of a new species.

But if Bae and Wu's analysis is correct, these fossils could hold the key to solving one of human evolution's biggest mysteries: a puzzle that started with the discovery of a pinkie bone in Denisova Cave in the Altai Mountains of southern Siberia. DNA analysis of that tiny fossil led to the 2010 finding that it represented a distinct ancient human population, which scientists dubbed the Denisovans.

Many people alive today carry traces of Denisovan DNA, but - because fossils of these extinct ancestors are still few and far between - experts in human origins still don't know exactly what they looked like, where they lived or why they disappeared.

Hiding in plain sight?

The hard-to-classify Chinese remains include 21 fossils found in the 1970s at the Xujiayao (Houjiayao) site located on the border of northern China's Shanxi and Hebei provinces. The specimens represent 16 individuals who lived 200,000 to 160,000 years ago.

Among other sites of interest are Lingjing in Xuchang County in central China's Henan province, where partial craniums were found between 2007 and 2014, and the northeastern city of Harbin, in Heilongjiang province, where a skull stashed at the bottom of a well about 90 years ago recently resurfaced.

Many of these fossils had been overlooked because when they were first unearthed in the 1970s and 1980s, commonly held beliefs about human origins were vastly different from today's theories. Back then, many paleoanthropologists thought that present-day human populations evolved regionally from archaic hominins such as Homo erectus, which left Africa about 2 million years ago. In effect, this scientific model known as multiregionalism, now largely rejected, suggested there was only one hominin species that evolved over time to become Homo sapiens.

In this scenario, the Xujiayao fossils, and others with unusual features unearthed in China, were classified as intermediates between more primitive hominins such as Homo erectus and more modern ones. The multiregional scientific model, which suggested discrete ancestry roots for the Chinese people, dovetailed with nationalist sentiments and was once particularly entrenched among Chinese academics.

However, scientists now have strong genetic evidence that supports an African origin of all Homo sapiens. The model, widely accepted today, is known as the "out of Africa" theory: The distant ancestors of present-day humans living outside of Africa left the continent and spread around the world 50,000 to 60,000 years ago, although pioneering Homo sapiens groups may have left the continent earlier, from around 200,000 years ago.

Subsequent discoveries in Asia and Africa in recent decades, such as the fossil finds of Homo floresiensis, tiny hominins who lived 100,000 to 50,000 years ago in what's now the Indonesian island of Flores, and Homo naledi, similarly small-bodied hominins who lived 300,000 years ago in what's now South Africa, have also suggested that the situation today, in which only one species of hominin exists, is unusual.

"The last 50 years of fossil discoveries have made it clear that there wasn't a single hominin lineage around at any point in the past (making humans today quite unique)," said Carrie Mongle, an assistant professor in the department of anthropology at Stony Brook University in New York.

"With this in mind, it isn't unreasonable to suggest that the number of species represented by the Asian fossils should be revisited - we shouldn't just assume that they can all be lumped [together]."

Finding links - and objections

In what they say is the first "comprehensive" analysis of the Xujiayao specimens, Bae and Wu were able to digitally reconstruct a cranium from the remains. Its large, low and wide shape was completely distinct from the skulls of other known hominin species such as Neanderthals or Homo erectus, and different from the globe-shaped skulls of Homo sapiens, the analysis concluded. The pair then compared the features with other puzzling fossils found in the region over recent years.

The cranium was similar to two unusually large skulls found in Xuchang County in Henan province between 2007 and 2014, which had a 1800cc capacity. Teeth found at the Xujiayao site closely resembled those in previous finds: teeth contained in a jawbone found in the early 1980s on the Tibetan plateau, at a site where researchers in 2020 also found DNA in cave sediment tied to Denisovans; a tooth found in a cave in Laos and reported in 2022, also attributed to Denisovans; and those in a jawbone recovered from an antique shop in Taiwan in 2008, known as the Penghu mandible.

Together, Bae and Wu argue the hominins these mineralised bone fragments once belonged to should be classed as Homo juluensis. Bae said he knew proposing the inclusion of the Denisovan remains would "cause waves", but he believes his team has a strong case.

"I think most people will actually agree with us that [these fossils] should be grouped together. By rules of priority since we put this name out there, those fossils should be assigned to juluensis," Bae said. "I guess this is so fresh, people are having trouble understanding [our proposal]."

However, Mongle and other paleoanthropologists say it's too early to definitively group the Denisovan remains with the fossils from Xujiayao and classify them as a newfound species called Homo juluensis.

"The main case for this team wanting to name a new species is based on the shape of the braincase, but there are no Denisovan cranial fossils known that could be compared to the Xujiayao cranial fragments. Without the ability for a direct 1-1 comparison using their strongest piece of evidence, grouping these fossils together seems premature," said Ryan McRae, a paleoanthropologist at the Smithsonian National Museum of Natural History. He was not involved in Bae and Wu's research.

McRae's second objection was a dearth of genetic evidence, which is increasingly used as part of species classification.

Usually, archaeologists make bombshell discoveries when a trowel hits a skull or other telltale bone in the confines of a cave. However, the Denisovans were the first archaic human to be identified in a lab thanks to advances in the field of ancient DNA, making them particularly enigmatic.

All of the dozen or so Denisovan skeletal specimens discovered to date are fragments - none of which are considered distinctive enough to allow a formal scientific species name such as Homo sapiens or Homo neanderthalensis. A skull is typically, but not always, the specimen that represents a new species because it has a lot of characteristic features.

"We unfortunately do not know what Denisovans look like yet," McRae said. "Denisovans are known predominantly from genetic data, not fossils, so any comparison should include a genetic component to better make the case that they have found more Denisovans."

Enter the 'Dragon Man'

It is part of normal scientific practice to revisit old fossils, said Xijun Ni, a professor at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. However, while it is plausible the remains studied by Bae and Wu may represent a previously unrecognised type of ancient human, Ni said in his view the coauthors hadn't yet put forward enough information to officially classify Homo juluensis as a newly discovered species - a process that involves lengthy analysis and is often contested.

Ni and Chris Stringer, research leader in human origins at London's Natural History Museum, have another theory on how all these prehistoric pieces fit together, based on their work with the skull of an ancient hominin colloquially known as "Dragon Man."

First described in scientific literature in 2021, the specimen has an intriguing backstory. The skull was found in 1933, by the bank of the Songhua River in Harbin, China, by a man working as a forced labourer for the Japanese, who controlled that part of China at the time. He kept the skull safe at the bottom of a well for more than 80 years, revealing its existence on his deathbed.

Stringer said his work with Ni suggested that most of the fossils Bae categorized as H. juluensis actually are morphologically a better match with the Harbin cranium. This, Stringer reasoned, suggested in turn that the Xujiayao specimens and Denisovan remains may all belong with Dragon Man, which has been given the formal scientific name Homo longi.

"Thus the well-preserved Harbin cranium would represent an early Denisovan. Ongoing research by other teams points to the same conclusion," Stringer said, adding that fresh research on Dragon Man will be published this year.

Mongle of Stony Brook University said while she agreed multiple species of ancient human may have existed in Asia, she also thought the fossils Bae identified as belonging to the "big heads" look similar to the Homo longi skull.

"I theoretically support the idea of multiple hominin lineages in the Asian Pleistocene, (but) I'm not totally convinced how different these fossils are from the recently named Homo longi fossils," she said.

Bae said that his analysis suggested that the Dragon Man skull is a better match with fossils found at Chinese sites in Dali in Shaanxi province in and Jinniushan, Liaoning province in 1978 and 1984, respectively, to form Homo longi.

What's in a (species) name?

Deciding the name of a new species such as Homo juluensis is up to the researchers, Ni noted, but they do have to follow the naming guidelines set up by the International Commission on Zoological Nomenclature. However, there's no official approval process and whether the species name sticks or not will be a matter of whether other researchers agree and use it in scientific literature.

Ni emphasized that the Denisovans represent one known population of ancient hominins and, as such, the word or a version of it shouldn't automatically be used for others, such as the Xujiayao hominins, even if they turn out to be the same species. It is akin to calling all Europeans Anglo-Saxons, he said.

"I strongly oppose the view that previously known Asian Middle Pleistocene hominins, named or not, are Denisovans, because Denisovan is just a population name," Ni said.

Bae agreed. "A lot of Western paleoanthropologists have said they want to call the Chinese fossils Denisovans," he said. "For us, Denisovans is not actually a species."

The first person or team to have enough evidence to name the Denisovans as a distinct species technically has the ability to name it whatever they want, McRae said.

"Denisovans do not have a formal taxonomic name intentionally because there is a lack of comparative material with which to name a new species," McRae said.

"This is good on one hand because it gives the field time to discover more evidence for the Denisovans in fossil form before naming a new species, but is bad on the other hand because it leaves those important fossils 'up for grabs' for people to assign them a taxonomic name prematurely."

There is hope among some paleoanthropologists that when Denisovans are given a formal species name "it will reflect the type site of Denisova Cave and the now ubiquitous colloquial name 'Denisovans'", McRae said, conceding there is "no guarantee that will happen".

The time period is full of fossil hominins that morphologically are "a bit of a mess", he added. Some look distinct, but some have Neanderthal features, and still others have Homo sapiens features, and many have both, McRae said.

"Prematurely splitting fossils into species could obscure the real story of what is going on across the world at this time, and it is, from a logistical perspective, very difficult to walk back from a decision once species names are out there, whether there is good support for it or not," he said.

- CNN