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An archaic Homo juvenile appears surprisingly modern in its dental

development

A research led by Song Xing from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), Chinese Academy of Sciences, with colleagues from France, America, Spain and South Africa, was published in Science Advances on Jan.16. This project investigated the dental growth and development of an archaic Homo juvenile from the East Asian Middle-Late Pleistocene transition (148,000 to 248,000 years or 104,000 and 125,000 years before present). The study revealed that most aspects of this juvenile’s dental development fell within modern human ranges. This was surprising because the individual is otherwise characterized by many more archaic (older) morphological traits. According to this study, several facets of modern human dental growth and development evolved prior to the appearance of fully modern human morphology in East Asia.

Compared to other primates, modern humans take a very long time for their physiological systems grow and develop. This is associated with characteristics like prolonged period of childhood dependency, delayed ages at first reproduction, and long lifespans. Overall, modern humans “live slow and die old”. In addition to the morphological, genetic, and behavioral standards, the growth and development in a slow pace is also a critical attribute to define modern humans. Understanding when and in which earlier hominins the modern human-like growth and development (or life history) first appeared is a subject of ongoing investigation and of strong interest. Teeth, one of the most abundant materials within fossil collection, are irreplaceable in tracing human life history due to its specialty. First of all, its growth is rhythmic and recorded as daily (short-period) as well as longer-period incremental lines in its hard tissues that can be imaged non-destructively in precious fossils. Secondly, several dental developmental traits (e.g. prolonged crown formation times and delayed molar eruption ages) are typical of modern humans, and were proved to be associated with their prolonged life histories.

According to previous studies, earlier hominins, Australopithecus and Paranthropus, were quite variable in their developmental rates. However, none of them displays the prolonged periods of dental growth and development that characterize modern humans. H. erectus also has an accelerated pace of dental growth. Some aspects of Neanderthals dental development appear to be encompassed within the modern human range of variation, although they generally fall at the advanced end of that range. How is the archaic Homo (referred as archaic H. sapiens sometimes) like in the dental development? Archaic H. sapiens was proposed to be a transitional form connecting H. erectus and modern human.

Within the fossil collections from China, it is of great fortunate to happen to have a juvenile maxilla of archaic Homo from Xujiayao site, northern China. This kid just had its first molar erupted for a very short while at the age of death, with incisor, canine, premolars and second molars are still buried inside the jaw. A direct U-series dating on the associated mammal teeth

suggested at least 104,000-years age for the hominin living time. The deposits containing the Xujiayao fossils were dated to be 148,000-248,000 years before present (B.P.) using Optically Stimulated Luminescence (OSL). This age corresponds to the transition of Middle Pleistocene to Late Pleistocene, and is a very critical time for the origin of H. sapiens. At the Xujiayao site, three systematic excavations were carried out in 1970s, and 20 pieces of hominin fossils were unearthed, including cranial and mandibular fragments, as well as isolated teeth. The juvenile maxilla, numbered as Xujiayao 1 within the hominin fossils, was recovered in 1976. A series and comprehensive studies have been performed in the past few years on the Xujiayao hominin fossils and consistently reveal its complex mosaic state of morphologies, including characteristics found in H. erectus, modern humans, and Neanderthals.

To assess various features of dental growth and development, the permanent teeth of the Xujiayao juvenile was scanned at European Synchrotron Radiation Facility (ESRF), France, by Paul Tafforeau. With this high-resolution and non-destructively virtual histology, the researchers were able to trace the short-period and long-period incremental lines without physically sectioning the valuable fossil teeth. Through a systematic counting, cross-matching and calculation, the initiation time, crown and root formation time, first molar eruption time, age at death were obtained with high certainty. After comparing these developmental variables with those of extinct and extant human samples, most aspects of the Xujiayao teeth were found to fall into the range of variation of modern human, except for a relatively fast average extension rate of the roots. Especially, Xujiayao juvenile has a prolonged crown formation time, delayed first molar eruption time, and more perikymata within the cervical half of the crown.

This work is the first systematic assessment of dental growth and development in an archaic Hominin (genus, Homo) from East Asia. Its findings suggest that the presence of a slow life history comparable to that of modern humans might have appeared prior to the fully modern human morphology.

Contacts: XING Song (xingsong@ivpp.ac.cn)

Figure 1. The Xujiayao maxilla (left) and three-dimensional (3D) reconstruction of the teeth (right).

Figure 2. The perikymata distribution of Xujiayo incisor (left) and canine (right).

Figure 3. The neonatal line in the paracone of M1 indicated by the dashed lines and the 10-days periodicity in M2 indicated by the pink and cyan solid lines.