By studying the remains of infant Neanderthals, researchers have found that even at the age of just a few weeks old, the ancient human babies were much stockier than modern human children. This, say the researchers, proves that the adaptations seen in our ancient cousins were genetic, and not caused by behavior, habits, or the environment. 
When the early Neanderthal ancestors split from ours around 600,000 years ago, they stuck to the chilly forests and grasslands of Europe, so that when the first recognizable Neanderthals appeared around 400,000 years later, they showed a whole raft of adaptations that some think are due to the cold environment they settled in.
For example, their bodies were much shorter and stockier than those of our ancestors, with large barrel-shaped chests. They also had differently proportioned limbs, robust bones, and are thought to have been much stronger than modern humans.
But the question remained about whether these observed physiological differences were due to the environment, or genetics. To answer this, the researchers turned to the incredible remains discovered of two infant Neanderthals. One of the skeletons was discovered in Mezmaiskaya Cave in the Caucasus of Eastern Europe in 1993, after Neanderthals are thought to have buried the two-week old baby close to 70,000 years previous. The second comes from Le Moustier in southwest France, and belonged to an infant no longer than four months old.
They are exceptionally well-preservered skeletons, being the some of the best examples of Neanderthal remains ever discovered. Their wide geographic distribution has meant that the researchers were able to get a broader picture of Neanderthal development, rather than basing their studies on a restricted group of the ancient humans. From these two skeletons, they then compared the physiology with the skeletons of 68 newborn modern humans.
“The study of exceptional specimens such as those of Mezmaisakya and Le Moustier show us that the Neanderthal 'bauplan' was mostly likely determined by their genes and not by their environment or behaviour,” explained Jean-Jacques Hublin, Director at the Max Planck Institute for Evolutionary Anthropology in Leipzig, which investigated the neonatal skeletons, and published their results in the Proceedings of the National Academy of Sciences. They found that the babies were already showing characteristics associated with the adults, including a wide body, long pubis, and robust long bones.
So while our ancient cousins were like us in many ways, developing tools, displaying culture, and even building structures deep underground, it seems that they were fundamentally different physiologically from us, starting at birth.
Main image: Gianfranco Goria/Flickr CC BY-NC-ND 2.0
Image in text: The remains of the four-month infant found in France, and one of the best-preserved skeletons of any Neanderthal ever discovered. Musée national de Préhistoire, Les Eyzies-de-Tayac Sireuil/Ph. Jugie
