Dating of Liujiang Man brings us one step closer to the origin of modern humans in East Asia!

Produced by: Science Popularization China

Author: Niu Changtai (PhD, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences)

Producer: China Science Expo

Editor's note: In order to expand the boundaries of cognition, the China Science Popularization Frontier Science Project has launched a series of articles on the "Unknown Realm", which provides an overview of the exploration results that break through the limits in deep space, deep earth, deep sea and other fields. Let us embark on a journey of scientific discovery and get to know the amazing world.

Who are we? Where do we come from? Where are we going? These are three questions that all human beings have always faced.

Today, different regions on Earth are home to different races of people, with significant differences in appearance and culture. Although biology has confirmed that all humans on Earth belong to the same species—Homo sapiens (or modern humans, Homo sapines), the origin and formation process of modern humans of different races in different regions is still unclear, and is a frontier area of ​​life science research today.

Ancient human fossils are important materials for studying the origins of modern humans in different regions. We can not only restore the physical characteristics and appearance of ancient humans based on ancient human fossils, but also extract ancient DNA from ancient human fossils to explore their relationship with modern humans today.

my country has produced a large number of ancient human fossils, ranging from the Yuanmou Man in Yunnan about 1.7 million years ago to the Tianyuan Cave Man and Upper Cave Man in Beijing about 40,000 years ago, including multiple stages such as Homo erectus, "archaic Homo sapiens" and modern humans.

Among them, the fossils of our species, modern humans, are the most closely related to today's human beings. The earliest modern human fossil records in my country can be traced back to ancient human fossils excavated from Hunan, Hubei and Guangxi in my country between 120,000 and 70,000 years ago. After 70,000 years, until about 40,000 years ago, my country had not produced modern human fossils with an exact age for 30,000 years.

Among all the modern human fossils produced in my country, the Liujiang Man discovered in Tongtian Cave in Liujiang, Guangxi is one of the most complete fossil human skeletons found in East Asia and Southeast Asia, which provides us with valuable fossil materials to understand the origin and evolution of modern humans in the region.

However, the age of Liujiang Man fossils has long been controversial. Early studies have suggested that Liujiang Man lived at least 67,000 years ago, or even as early as 139,000-111,000 years ago or 150,000 years ago. These ages are much earlier than the age of modern human fossils that appeared widely in East Asia 40,000 years later, and may also fill the gap in the discovery of modern human fossils in China from 70,000 to 40,000 years ago. Therefore, determining the age of Liujiang Man is of great significance for us to correctly understand the origin and migration and diffusion pattern of modern humans in East Asia.

On April 29, 2024, a research team composed of the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Nanjing Normal University and the Institute of Geology and Geophysics, Chinese Academy of Sciences, in collaboration with researchers from the Liuzhou Bailian Cave Science Museum and the Australian Centre for Human Evolution Research, published new comprehensive research results on the stratigraphy and chronology of the Liujiang Man in Nature Communications, a subsidiary of Nature magazine.

The study determined that Liujiang Man lived between 33,000 and 23,000 years ago, which laid a solid foundation for our understanding of the position of Liujiang Man in the evolutionary sequence of modern humans in East Asia and even the world, and provided key data for exploring the migration and diffusion patterns of modern humans in Eurasia.

The mystery of the origin of modern humans in East Asia

How did Homo sapiens, or modern humans, our species, emerge in East Asia? This is a question that Chinese paleoanthropologists and archaeologists have been exploring for nearly a hundred years. East Asia has a continuous fossil record from Homo erectus to modern humans, and there is also good continuity in the stone tool technology used by ancient humans. Therefore, some scholars initially believed that modern humans in East Asia evolved from earlier ancient humans living in the local area.

After the 21st century, with the advancement of gene sequencing technology and ancient DNA extraction technology, molecular anthropology research has found that the ancestors of modern people outside of Africa today are mainly from modern people who walked out of Africa between 60,000 and 50,000 years ago. They arrived in the Middle East via northeastern Africa, and then spread to the west and east of the Eurasian continent, and reached the American continent through the Bering Strait, and reached Australia through Southeast Asia, completing the feat of global diffusion of modern humans.

At the same time, they also had genetic exchanges with some local ancient humans such as Neanderthals, Denisovans and some other unknown ancient humans, leaving genetic imprints of these ancient humans in the genomes of modern humans outside of Africa today.

Paleoanthropological studies have found that anatomically modern humans appeared in North Africa about 300,000 years ago. At the same time, modern human fossils dating back more than 60,000 years have been found in the Middle East, Europe, Southeast Asia, and East Asia.

This means that long before the ancestors of modern humans outside of Africa today walked out of Africa, some modern humans may have already walked out of Africa and reached Eurasia and spread to a certain extent, but the relationship between these modern humans and today's modern humans is still unclear. They are most likely not the direct ancestors of today's modern humans and failed to successfully leave offspring in Eurasia.

However, in East Asia, due to the continuity of stone tool technology, we cannot rule out another possibility, that is, modern humans in East Asia 40,000 years later evolved from older modern humans 120,000-70,000 years ago.

Between 70,000 and 40,000 years ago, although there were no omissions or obvious stone tool technology transitions in the archaeological records of ancient humans, there were certain gaps in the fossil records of modern humans in East Asia.

Liujiang Man, a relatively complete ancient human skeleton fossil that can be clearly classified as a modern human sequence, has been found to be no less than 67,000 years old in past studies. This age makes it extremely important, but it is also a weakness in people's correct understanding of the evolutionary status of Liujiang Man.

Global distribution map of modern human fossils, corresponding ages of global deep-sea oxygen isotope stages: MIS7 (244,000-190,000 years), MIS6-5 (190,000-71,000 years), MIS4 (71,000-60,000 years), MIS3-2 (60,000-11,000 years)

(Image source: Reference 3)

Important Liujiang people

The Liujiang Man fossils were discovered in the Tongtian Cave in Liujiang, Guangxi by local phosphate fertilizer excavators in September 1958. When scientists from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences arrived, the Liujiang Man fossils had already been mined and the sediments in the cave had been basically destroyed. People had no idea of ​​the exact stratum where the Liujiang Man was found, and it was difficult to determine his age based on the stratum.

Some scholars have dated the remaining sediments in the cave and inferred that Liujiang Man lived at least 67,000 years ago, or even as early as 150,000 years ago. However, the strata studied are not necessarily the actual strata produced by Liujiang Man. Therefore, for more than 60 years, the age of Liujiang Man has remained a mystery.

Different from the chronological research, the physical anthropological research of Liujiang Man is very sufficient. The fossil of Liujiang Man consists of a basically complete skull and 17 postcranial bones including thoracic vertebrae, lumbar vertebrae, ribs, sacrum, pelvis, femur, etc.

Through fossil measurement and identification, it was discovered that the Liujiang man fossil belonged to a male about 40 years old, about 1.6 meters tall and weighing about 52 kilograms. He had a series of morphological characteristics of modern humans, including a relatively round skull shape, a relatively large brain capacity (1567ml), a small and flatter face, a simple occlusal surface of the teeth, a slender body shape, etc. Therefore, in terms of classification, Liujiang man undoubtedly belongs to the same species as us - modern humans.

Liujiang people also showed some more primitive characteristics, including a low face, low eye sockets, prominent brow ridges, etc. These characteristics were more common in early modern humans about 40,000 years ago or even earlier.

Therefore, Liujiang Man possesses both "advanced" modern human characteristics and "primitive" early modern human characteristics. His unique morphological combination makes him very important in the study of modern humans in East Asia. However, only when we know the actual era in which Liujiang Man lived can we place him in the correct position in the evolutionary sequence of modern humans in East Asia and even the world.

The skeleton of Liujiang Man (left), the cave where Liujiang Man was born (upper right), and sediment samples collected by researchers in the cave (lower right)

(Image source: Reference 1)

Systematic dating work

The key point of this research is to determine the layer where the Liujiang Man fossils were found and directly date the corresponding layers. The researchers found reddish-brown clay sediments of the same color and texture in the left femur and nasal cavity of the Liujiang Man's skull. These sediments are likely to represent the composition of the sediments that buried the Liujiang Man. As long as we compare the grain size, color, and elemental composition of these sediments with those of different layers in the Tongtian Cave, we can find the original layer where the Liujiang Man was produced.

Finally, the researchers found that the characteristics of the second sediment of the third sedimentary unit in Tongtian Cave were the same as the remaining sediments in the Liujiang Man fossils, and the Liujiang Man was likely to have come from this stratum.

Subsequently, the researchers systematically dated the sediments in Tongtian Cave layer by layer, and determined that the sedimentary age of the second layer of the third sedimentary unit was 32,500-22,600 years ago. But how can we ensure that Liujiang Man was not transported to the cave from outside? In other words, could it be that Liujiang Man had already formed fossils earlier than the sediments were formed, but was later transported to the cave and preserved together with the sediments? The researchers discussed this issue from two aspects.

The first is taphonomic research. The researchers found that the sediments in this layer have very fine particles, reflecting a sedimentary environment with very weak transport power. At the same time, the skeleton of Liujiang Man is very complete compared to most ancient human fossils. In addition to the high degree of preservation, the thoracic and lumbar vertebrae, and the lumbar and sacral vertebrae were still connected when the fossils were discovered. This shows that Liujiang Man is unlikely to be a previously formed ancient human fossil that was transported here by water, but rather was buried in situ after death when the body had not yet decomposed.

The second is to directly date the Liujiang Man fossils themselves. In Quaternary paleontology, paleoanthropology and archaeology, carbon-14 dating is often used to directly determine the age of organic matter. Researchers have also conducted carbon-14 dating on Liujiang Man, but because there is too little organic matter preserved in the fossils, the dating results are not ideal.

This time, the researchers used uranium-series dating on the Liujiang Man fossils. Uranium-series dating does not rely on the preservation of organic matter, but is only related to the formation process of the fossils. During the formation of fossils, the crystallized calcium carbonate in the bones absorbs radioactive uranium-238 from the surrounding pore water. Uranium-238 decays to produce stable thallium-230 and uranium-234. We can calculate the minimum age of these fossils based on the ratio of the stable elements produced by decay to the remaining uranium-238 that has not decayed. The researchers used uranium-series dating on the left femur of the Liujiang Man and found that its burial age was at least about 24,000 years ago, which is very consistent with the inferred sedimentary age of the Liujiang Man's output layer, which is 32,500-22,600 years.

Finally, based on the uranium-series dating of the Liujiang Man fossils themselves and the inferred sedimentary age of the producing strata, the researchers determined that the age of the Liujiang Man fossils was between approximately 33,000 and 23,000 years ago.

The significance of the new age of Liujiang people

The age of the Liujiang Man fossils, dating back 33,000 to 23,000 years, indicates that they, like the ancient human fossils in East Asia 40,000 years ago such as the Peking Man and Tianyuan Cave Man in Beijing, are modern humans who spread across Eurasia after 60,000 years ago and lived in East Asia between 40,000 and 30,000 years ago, rather than representatives of modern humans who left Africa and arrived in East Asia earlier than 60,000 years ago.

In fact, the skull morphology of Liujiang people is closer to that of today's modern humans than that of Shangdingdong people, and they lived younger than Shangdingdong people (39,000-36,000 years ago), which may reflect a morphological evolution of early modern humans to today's modern humans.

At the same time, the skull morphology of Liujiang people is very similar to that of Cro-Magnon people in Europe about 28,000 years ago, but molecular anthropology research shows that modern humans on the east and west sides of the Eurasian continent should have diverged between 50,000 and 40,000 years ago.

This similarity in skull morphology means that in the early stages of the divergence of modern humans in the east and west of Eurasia, there may not have been obvious morphological differences between the two, which may be related to the rapid spread of modern humans in Eurasia.

Today, there are obvious differences in physical features such as facial features between Western and Eastern populations in Eurasia. This may be the result of the evolution of different populations about 30,000 years ago, including the harsh environment during the Last Glacial Maximum (26,500-19,000 years ago), which caused natural selection and genetic drift in modern humans in different regions, and the resulting population migration and expansion events. The Neolithic Agricultural Revolution and the evolution of populations in historical periods also had a certain impact.

French Cromanon 1 skull.

(Image credit: Thilo Parg/Wikimedia Commons)

At the same time, the age of the Liujiang Man fossils is 33,000-23,000 years, which coincides with the period when the global climate was changing to dry and cold. The stone tools in southern my country at this time continued to be mainly gravel tools, but on the other hand, small stone flakes and other small stone tools such as flint scrapers and arrowheads began to appear in large numbers, and the use of bone tools became more common.

This shift in stone tool technology and tools reflects the behavioral and cultural adaptation of local humans in the face of environmental changes, but it may also be related to the introduction of new culture and technology by modern humans migrating "like Liujiang people."

Some "advanced" tools found at the Bailian Cave site in Guangxi. 1,7: Ring-shaped stone tools; 2,3: Polished stone tools; 4,5: Tools made of deer antlers; 6: Perforated stone ring; 8: Grinding stone with traces of ochre

(Image source: Reference 4)

Future Outlook

In the past decade, as molecular anthropology has become an important means of studying modern human fossils in western Eurasia, the changes of modern human populations in the region have begun to become gradually clear, but this also relies on the fact that the modern human fossils in the region have a good chronological research foundation.

In eastern Eurasia, the research situation is weaker. According to statistics, more than 50 modern human fossil sites have been discovered in East Asia, but the origin and age of many fossils are unknown, which seriously weakens the research value of these fossils.

Only by establishing an accurate chronological framework for these fossils and combining them with physical anthropology and molecular anthropology studies on these modern human fossils can we understand the origin and diffusion pattern of modern humans in East Asia. The chronological study of Liujiang Man undoubtedly provides an excellent model for future chronological research.

We hope that in the future there will be more modern human fossils in China whose ages can be determined, telling us the stories of our ancestors who have lived on this land for tens of thousands of years.

References:

1. Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences. New progress in the comprehensive study of Liujiang Man stratigraphy and chronology

2. Ge, J., Xing, S., Grün, R. et al. New Late Pleistocene age for the Homo sapiens skeleton from Liujiang southern China. Nat Commun 15, 3611 (2024).

3. Xing Song. Fossil evidence for the emergence and evolution of modern humans[J]. Acta Anthropologica Sinica, 2022, 41(06): 1069-1082.

4. Zhou Y, Jiang Y, Liang G, et al. A technological perspective on the lithic industry of the Bailiandong Cave (36–7 ka) in Guangxi: An effort to redefine the cobble-tool industry in South China[J]. Comptes Rendus Palevol, 2019, 18(8): 1095-1121.