The human biological cycle is very special. Our growth biology contains evolutionary novelties such as the slow development of somatic and neuronal maturation during childhood, in stark contrast to the equally innovative pubertal spurt seen in our species, Homo sapiens. As biologists, we ask ourselves how and when our unique way of developing came about. To answer this question we resort, first, to comparative biology, especially with other species of mammals and primates. However, the main source of information is human fossils, especially those of immature individuals. Among the fossils of human species most commonly used for such analyses are Neanderthal fossils.
During the excavation campaigns from 2009 to 2013 many pieces of bone were recovered in the Osario de la Cueva Gallery in El Sidrón (Asturias). They belonged to a young Neanderthal from about 49,000 years ago. One put together, they make up a partial skeleton we have called “El Sidrón J1”, of immense value for studying growth in this extinct species, and whose analysis we published in Science magazine in September 2017.
The age of the death of this youth was estimated by dental histology methods. The counting of the lines of enamel and dentine increase, which follow circadian rhythms for secretion and deposition, allowed us to estimate the age of death at around 7.7 years old. Having established this value, we compared it with a wide range of human skeletons whose age at death was known, collected from different museums and scientific institutions. The results were grouped in three groups. The first and most general affirms that the pattern of somatic growth of Neanderthals in indistinguishable from that of children of our own species. This means that Neanderthals’ bodies matured following the same pattern as that of anatomically modern humans. The slow evolution of growth in our bodies is a shared evolutionary trait, inherited at least from the last predecessor shared by Neanderthals and H. sapiens, maybe a million years ago.
However, the biggest surprise came from the second group of data, the skeletal maturation of the vertebrae. We saw that the Atlas vertebra and the dorsal vertebrae of our protagonist, El Sidrón J1, matured more slowly than in youths of our species. That is, it took slightly longer for the arch to fuse with the vertebral body. Curious, to say the least. Finally our third group of data led us to the conclusion that the brain may also have grown more slowly than in modern humans, albeit very slightly, but with an important biological value. Our interpretation of these facts is based on the principle that a large body and a large brain, like the Neanderthals’, would have needed a lot of energy to grow. The biological solution to this demand for energy was to slightly postpone the growth of the brain (and maybe the whole central nervous system) to share out energy consumption. In this way competition for energy between body and brain could be partly avoided.
This perspective clearly gives rise to a long series of new questions. Can we generalise this conclusion for other human species? What are the biocultural implications of the fact that a large brain slows down growth? These are open questions which we must pay attention to with new data.
Antonio Rosas is Research Professor for CSIC at the Museo Nacional de Ciencias Naturales (National Museum of Natural Science) and Director of the Paleoanthropological Group Paleoantropología MNCN-CSIC. Since 2003 he has been in charge of studying Neanderthal fossils from the site at El Sidrón, in Asturias. He has published 250 scientific journals and four popular books (among them, Los neandertales, 2010, CSIC/Los Libros de la Catarata), and has also been science commissioner for several exhibitions in national museums. The article published in Science magazine to which this Work of the Month refers can be consulted in: “The growth pattern of Neanderthals, reconstructed from a juvenile skeleton from El Sidrón (Spain)