Neanderthals had a wider rib cage and pelvis than Homo sapiens, which researchers say could be down to their adoption of a high-protein diet.
It is believed Neanderthals lived in Ice-Age Europe, co-existing alongside modern-day humans, or Homo sapiens, until around 40,000 years ago.
In terms of appearance, Neanderthals were very similar to Homo sapiens, and the two species even mated. However, there were some notable differences. As well as being shorter and stockier, Neanderthals had a wider rib cage, or thorax, and a larger pelvis than our modern ancestors.
In the American Journal of Physical Anthropology, researchers from Tel Aviv University (TAU) in Israel suggest a change in diet during severely cold periods contributed to such differences.
In a previous study, the researchers noted that when it comes to converting protein into energy, humans have a limited ability; protein makes up only 30% of the human diet. As such, Homo sapiens increase their intake of fat and carbohydrates to meet their nutritional needs.
However, they hypothesize that the Neanderthals were forced to eat a high-protein diet due to lack of available carbohydrates during glacial winters.
An 'evolutionary adaptation to high-protein diet'
Using a nutritional model, the researchers estimated that around 74-85% of Neanderthals' calorie intake would have had to come from animal fat during cold snaps.
"During harsh Ice-Age winters, carbohydrates were scarce and fat was in limited supply. But large game, the typical prey of the Neanderthal, thrived," explains study coauthor Miki Ben-Dor, of the Department of Archaeology and Ancient Near Eastern Cultures at TAU, adding:
"This situation triggered an evolutionary adaptation to a high-protein diet - an enlarged liver, expanded renal system and their corresponding morphological manifestations. All of these contributed to the Neanderthal evolutionary process."
The liver is the organ that converts protein into energy, so the team says this is likely to enlarge with a high-protein diet - something that has been demonstrated in animal studies. Furthermore, the increase in metabolism triggered by a high-protein diet would put more strain on the bladder and kidneys to remove toxic urea, which may lead to their enlargement.
"Early indigenous Arctic populations who primarily ate meat also displayed enlarged livers and the tendency to drink a lot of water, a sign of increased renal activity," adds Ben-Dor.
As well as offering insight into how diet may have affected human evolution, the researchers say their results pave the way for future research into how Neanderthals became extinct.
They note that their extinction occurred around the same time as the arrival of giant animals, or "megafauna," in Europe. Whether Neanderthals' dependence on large animals for their nutritional needs contributed to their extinction is something they plan to investigate.
Last year, Medical News Today reported on a study in which researchers analyzed the DNA of a 40,000-year-old Romanian human jawbone and found that it was between 6-9% Neanderthal. This suggests that some of the earliest Homo sapiens in Europe interbred with Neanderthals.