Bone is formed through a process of bone deposition by osteoblasts (bone-forming cells) and resorption by osteoclast (bone-absorbing) cells, which break down bone. In humans, the outermost layer of bone in the face consists of large resorptive fields, but in Neanderthals, the opposite is true: In the outermost layer of bone, there is extensive bone deposition.
The team used an electron microscope and a portable confocal microscope, developed by co-author Dr. Timothy Bromage of NYUCD’s Department of Biomaterials, himself a pioneer in the study of facial growth remodelling in fossil hominins, to map for the first time the bone-cell growth processes (resorption and deposition) that had taken place in the outer layer of the facial skeletons of young Neanderthals.
“Cellular processes relating to growth are preserved on the bones,” says Bromage. “Resorption can be seen as crater-like structures – called lacunae – on the bone surface, whereas layers of osteoblast deposits have a relatively smooth appearance.”
The study found that in Neanderthals, facial bone-growth remodelling – the process by which bone is deposited and reabsorbed, forming and shaping the adult skeleton – contributed to the development of a projecting (prognathic) maxilla (upper jawbone) because of extensive deposits by osteoblasts without a compensatory resorption – a process they shared with ancient hominins. This process is in stark contrast to that in human children, whose faces grow with a counterbalance action mediated by resorption taking place especially in the lower part of the face, leading to a flatter jaw relative to Neanderthals.
The team studied several well-preserved Neanderthal child skulls unearthed in 1926 in the British territory of Gibraltar and from the La Quina site in southwestern France, also excavated in the early 1900s. They also compared Neanderthal facial growth remodelling with that of four Middle Pleistocene (about 400,000 years ago) hominin faces of teenagers from the fossil collection of the Sima de los Huesos in north-central Spain. The Sima fossils are considered likely Neanderthal ancestors based on both anatomical features and genomic DNA analysis.
“We always considered Neanderthals to be a very different category of hominin,” said Lacruz. “But in fact, they share with older African hominins a similar facial growth pattern. It’s actually humans who are developmentally derived, meaning that humans deviated from the ancestral pattern. In that sense, the face that is unique is the modern human face, and the next phase of research is to identify how and when modern humans acquired their facial growth development plan.”
Moreover, Lacruz says, understanding the process of facial ontogeny can help explain the variation in facial size and shape among modern humans.