The physical traits of the Neanderthal, Homo neanderthalensis, represent a distinctive pattern of human evolution shaped by hundreds of thousands of years in Eurasia. Their phenotype encompasses the observable physical characteristics resulting from their genetic makeup and the environment they inhabited. Studying this phenotype is a primary method for understanding how this extinct human species adapted to the challenging, often glacial, climates of the Middle to Late Pleistocene epoch. The Neanderthal body and skull structure provides a clear morphological record of their lifestyle, and their preserved DNA reveals their biological connection to modern humans.
Defining Cranial Characteristics
The Neanderthal skull possesses features that immediately distinguish it from that of modern Homo sapiens. The cranium is long, low, and elongated, unlike the high, rounded vault of living humans. This shape accommodated a large brain, which was often similar to or larger than that of modern humans, though its internal organization differed.
The most recognizable feature is the supraorbital torus, a prominent double-arched brow ridge projecting significantly above the eye sockets. The mid-face also exhibits mid-facial prognathism, a pronounced forward projection with the cheekbones angled backward. This morphology is unique among hominins.
The nasal cavity was considerably large and wide, interpreted as a specialized adaptation for conditioning the cold, dry air of their environment. A large internal volume helped warm and humidify the air before it reached the lungs, protecting internal tissues from cold stress. At the back of the skull, many Neanderthal crania exhibit an occipital bun, a rounded bulge of bone that anchored the large neck musculature required to support their massive head.
The Robust Neanderthal Body Plan
Moving below the neck, the Neanderthal post-cranial skeleton reveals a body plan optimized for strength and heat retention in cold climates. Their bones were remarkably thick and dense, suggesting a heavily muscled physique capable of immense power. Analysis of muscle attachment sites indicates that Neanderthals possessed significantly larger muscles, particularly in the shoulders, arms, and hands, compared to modern humans.
This skeletal robustness is consistent with a physically demanding lifestyle involving close-quarters hunting of large prey. The high frequency of healed fractures in their fossil record, comparable to that of modern rodeo professionals, supports frequent, high-impact encounters with animals. Their overall body proportions adhere closely to principles of thermoregulation known as Allen’s and Bergmann’s rules.
The Neanderthal torso was short and stocky with a wide, barrel-shaped chest, minimizing the surface area-to-volume ratio. This decreased ratio is an effective mechanism for reducing heat loss in frigid environments. Their limbs were also relatively short, particularly the forearms and shins, further reducing the body’s exposed surface area and contributing to their compact, powerful build.
Genetic Influence on Modern Human Traits
The legacy of the Neanderthal phenotype extends into the genetic makeup of modern non-African human populations due to an admixture event that occurred roughly 50,000 to 60,000 years ago. Individuals of Eurasian descent carry approximately one to four percent of Neanderthal DNA scattered throughout their genome. While much of the Neanderthal genome was lost through natural selection, the remaining segments often conferred an adaptive advantage to early modern humans migrating into new environments.
Neanderthal genes have had a noticeable influence on the human immune system, particularly a cluster of genes involved in recognizing and fighting pathogens. These archaic variants included genes for Toll-like receptors, which are crucial components of the innate immune response. They likely helped modern humans quickly adapt to the new array of diseases in Eurasia, though some variants may contribute to increased risk for certain allergies in contemporary populations.
Segments of Neanderthal DNA also significantly affect the skin and hair phenotypes of modern humans by influencing the production of keratin. Certain inherited variants are linked to lighter skin tones, which aided in Vitamin D synthesis in regions with low sunlight. However, these same variants can increase susceptibility to sun-induced skin lesions like actinic keratosis. The Neanderthal genome also contributed to a person’s chronotype, with specific variants associated with being an “evening person.”
The genetic inheritance also carries implications for neurological and psychiatric traits, including a subtle influence on mood regulation. Neanderthal alleles have been identified that are associated with an increased risk for depression and nicotine addiction in modern humans. Other health-related traits include a Neanderthal variant that promotes faster blood clotting. This may have been beneficial for wound healing in a high-risk environment but today increases the risk of conditions like deep vein thrombosis.