“The internal structures of Maba 1 show a combination of morphological features found in various species,” scientists stated in the American Journal of Biological Anthropology. That one sentence, based on new micro-CT evidence, encapsulates a challenge to decades of paleoanthropological taxonomy. For a farmer discovery in 1958 of a fossil in a limestone cave in Maba Village, China, the Maba 1 cranium has provided a site of contention for arguments regarding the mosaic nature of late Middle Pleistocene human evolution.
The tale starts not in a lab, but in a trench, where villagers who were digging for fertilizer exposed bone fragments that would puzzle future generations of scientists. The Maba 1 skull, some 300,000 years old, was first characterized as Neanderthal. But as scientists looked closer at its surface with state-of-the-art micro-CT imaging, the familiar categories of Homo erectus, Neanderthal, and Homo sapiens started to get mixed up.
Micro-CT scanning, employing high-resolution X-rays to generate three-dimensional reconstructions of fossils, has been a game-changer for the analysis of ancient remains. In contrast to older, potentially damaging approaches to analyzing irreplaceable specimens, micro-CT enables non-destructive investigation of internal morphologies. As outlined in a recent technical guide to best practices in micro-CT, stringent packing of specimens, scan parameter calibration, and advanced post-processing, including deep learning segmentation, are now routine to yield maximum anatomical detail. These advances not only sped up the analysis of fossils but also opened up access to previously concealed morphologies so that evolutionary hypotheses could be subjected to rigorous statistical test.
When the Maba 1 cranium was scanned and reconstructed, a number of features appeared that could not be classified in simple terms. The frontal sinuses, when subjected to linear discriminant analysis, grouped with Neanderthals. Yet, the diploic vessels canals within the spongy bone anastomosed with the parietal foramen, a rare Neanderthal characteristic. The endocast, or internal mold of the braincase, showed a frontal lobe different from Homo erectus, but closer to H. erectus overall than to Neanderthals or present-day humans. The bregma, where significant cranial sutures converge, was significantly thicker than in most H. erectus samples, and the cranial capacity was near that of Neanderthals and Homo sapiens.
This mix of characteristics part Neanderthal, part H. erectus, part neither positions Maba 1 within a group of equivocal fossils like LH18 from Tanzania, Djebel Irhoud from Morocco, and Broken Hill 1 from Zambia. These specimens as a whole demonstrate the great morphological diversity of Asian hominins during the late Middle Pleistocene, an era now known for its evolutionary dynamism instead of stasis. As entailed in a recent review of Middle Pleistocene fossils from China, “specimens are not easily allocated to existing taxonomic groups and reveal that populations in Middle Pleistocene China were highly variable after the period of around 300 ka.”
The taxonomic implications of this diversity go beyond taxonomy. Climatic instability in Marine Isotope Stages 12–10, about 500,000 to 340,000 years ago, probably forced population dispersals and admixture, to fuel evolutionary experimentation. The outcome: a mosaic of cranial and dental morphology, some primitive, some derived, and many novel. The Maba 1 cranium, with its mix of traits, is representative of this trend and highlights the insufficiency of categorical species boundaries in the explanation of hominin diversity in East Asia (see further discussion).
Micro-CT also uncovered another tale inscribed into the bone a semicircular lesion on the right frontal squama. This trauma, with exocranial depression and endocranial bulge, resulted from localized blunt force trauma. The remodeled state of the bone demonstrates that the person lived through the injury for several months, possibly years, a testament to the biological resilience as well as possibly social assistance. As described in a paleopathological evaluation, “the Maba 1 lesion, as with the other identified Pleistocene cranial lesions, was healed long before death. Whatever the ultimate cause of the lesion, it provides further evidence of long-term Pleistocene human survival of potentially debilitating conditions.”
The etiology of the trauma is still unclear. Differential diagnosis excluded anemia, tumors, and infections, but accidental injury or interpersonal violence are still the most likely explanations. The lesion’s pattern, with concentric ridges and radiating cracks, is very similar to other Middle and Late Pleistocene hominin injuries, like those of Dolní Věstonice 11/12 and Qafzeh 11. Interestingly, cranial trauma in Pleistocene humans tends to occur on the right side, which is not entirely expected from modern human conflicts, implying a multifaceted interaction of behavior, handedness, and possibly weapon use.
The survival of seriously injured or disabled individuals, recorded in both Maba 1 and comparative European fossils such as Aubesier 11 (source), suggests the existence of social care and support mechanisms long before the advent of anatomically modern humans. This then poses questions on the evolution of empathy, cooperation, and group bonding among archaic Homo populations.
The technical breakthroughs that enabled these observations are a reflection on the fast pace of fossil imaging advancements. Micro-CT scanners, with resolutions below 20 microns, enable researchers to digitally uncover fossils trapped in matrix, rebuild internal vascular and sinus architecture, and quantify bone thickness accurately. Current workflows leverage high-throughput scanning and deep learning segmentation to greatly accelerate the isolation of target features (details here). These techniques have been revolutionary not only for vertebrate fossils such as Maba 1 but also for invertebrate paleontology, expanding the field of evolutionary study.
As the Maba 1 skull continues to be re-examined with increasingly advanced methods, it remains both a scientific enigma and a reminder of the complexity in mapping our evolutionary history. The mosaic anatomy, indication of healed trauma, and technological advancements that unlock them, all serve to remind us that human evolutionary history is never linear, nor ever so tidy as the categories we attempt to place upon it.
