Neanderthal anatomy is characterised by a long, flat skull and a stocky body plan. When first discovered, Neanderthals were thought to be anatomically comparable to Aboriginal Australians, in accord with historical race concepts. As more fossils were discovered in the early 20th century, French palaeontologist Marcellin Boule defined them as a slouching, apelike species; a popular image until the middle of the century. Neanderthal features gradually accreted in European populations over the Middle Pleistocene, driven by natural selection in a cold climate, as well as genetic drift when populations crashed during glacial periods. This culminated in the "classical Neanderthal" anatomy by the Last Interglacial.

The Neanderthal skull is distinctive by namely a rounded supraorbital torus (brow ridge), large orbits (eye sockets) and nose, and an occipital bun at the back of the skull. The jaws and teeth are strong, which may have been a response to habitual heavy loading of the front teeth. The body is typically short and stocky, with an average size of and for males, and and for females. Short limbs may be an adaptation to the cold climate (Allen's rule) or to improve sprinting efficiency.

The brain is large, averaging in males and in females, larger than the average of any living population. The Neanderthal brain was organised much differently than the modern human brain, especially in regions related to cognition and language, which may be implicated in Neanderthal behaviour and the poorer evidence of material culture compared to Cro-Magnons.

Neanderthals may have developed mesopic vision in low-light conditions, and a stronger respiratory system to fuel a comparatively faster metabolism. It is unclear if Neanderthals could produce speech at the same level as modern humans. Neanderthal skin and hair colour may have ranged from dark to light. Red hair seems to have been a rare trait. Neanderthals may have had a faster growth rate than modern humans. Neanderthals suffered extensively from traumatic injury and major physical trauma, possibly as a consequence of risky hunting strategies and animal attacks. They also maintained a low population and genetic diversity, leading to inbreeding depression.

Research history

When the first Neanderthal fossil, Neanderthal 1 (a skullcap), was discovered in 1856, initial reactions characterised it as belonging to a brutish, savage race of man. The low braincase and flattened forehead were often cited as evidence of its primitiveness, as classic markers of the lower races, in accord with historical race concepts. Consequently, the Neanderthal skull was often anatomically compared to most notably Aboriginal Australians, who were considered the most primitive race alive. German pathologist Rudolf Virchow interpreted Neanderthal characteristics as evidence of senility, disease, and malformation instead of archaicness, which stalled Neanderthal research until the end of the century.

By the early 20th century, numerous other Neanderthal discoveries were made, establishing H. neanderthalensis as a legitimate species. The most influential specimen was La Chapelle-aux-Saints 1 ("The Old Man") from La Chapelle-aux-Saints, France. French palaeontologist Marcellin Boule described him as a hairy, slouching, ape-like creature; a reconstruction which would endure until around the middle of the 20th century. By this point, several fossils from across the Old World were classified as "Neanderthaloid", a type of transitional fossil in human evolution halfway between Homo erectus and Homo sapiens. The modern anatomical definition of "Neanderthal" was formalised with the popularisation of cladistics in the late 1970s.

Skeleton Skull

The Neanderthal skull is distinguished namely by a flat and broad skullcap, rounded supraorbital torus (the brow ridges), high orbits (eye sockets), a broad nose, mid-facial prognathism (the face projects far from the base of the skull), an "en bombe" (bomb-like) skull shape when viewed from the back, and an occipital bun at the back of the skull. The occipital bun, or "chignon", is within the range of variation for modern humans who have it. In Neanderthals, it is caused by the high and anterior (more forward) positions of the cranial base and temporal bones, in combination with a flatter skullcap.

Jaws In Neanderthals, the zygomatic arches (cheekbones) are positioned in a rearward location relative to modern humans, while the maxilla (upper jaw) and nasal bones are positioned in a more forward direction. The front (anterior) teeth are characterised by their large size, strong and bulging tooth roots, and tooth wearing (especially on the lower front teeth). The upper incisors are shovel-shaped. There is a large retromolar space (gap behind the molars). Especially in Europe, Neanderthals had a high frequency of taurodontism, a condition where the molars are bulkier due to an enlarged pulp (tooth core). In modern populations the trait has an incidence rate of about 5%, but a weaker form of taurodontism (hypotaurodontism) is somewhat common in some European H. heidelbergensis populations, especially at the Sima de los Huesos site.

These observations are typically explained as a response to habitual heavy loading of the front teeth (anterior dental loading hypothesis), either to process mechanically challenging or attritive foods, or because they regularly used the mouth as a third hand. The robusticity of the front teeth and the rest of the face has sometimes been ascribed to the production of a high bite force on the front teeth, but biomechanical studies have typically concluded that Neanderthals could produce similar or even smaller bite forces than modern humans.

Brain

The Neanderthal braincase averages for males and for females, which is significantly larger than the averages for all groups of recent humans; for example, recent European males can average about and females . Modern human brain size seems to have decreased since the Upper Palaeolithic, with a sample of 28 modern human specimens from 190,000 to 25,000 years ago averaging about disregarding sex. The largest Neanderthal brain, Amud 1, was calculated to be , one of the largest ever recorded in humans. Both Neanderthal and modern human infants measure about .

The Neanderthal brain had different growth and development rates than modern humans, especially in the orbitofrontal cortex (associated with decision making), parietal and temporal lobes (language processing and memory), and the cerebellum (motor functions). All of these regions are proportionally smaller in Neanderthals, and diverge in growth pattern at what would be a critical period in modern human neurological development. Altogether, such differences, while slight, may underlie the differences in Cro-Magnon and Neanderthal behaviour including sociality, technological innovation, and artistic output in the archaeological record.

Postcranium

The neck vertebrae of Neanderthals are thicker from the front to the rear and transversely than those of (most) modern humans. This may have improved stability of the larger head.

The Neanderthal chest was deep and wide, with a proportionally expansive thoracic cavity, and possibly better lung performance. This was caused by longer, straighter ribs, and more dorsally (towards the head) orientated transverse processes on the vertebrae (where the rib connects to the spine). The latter caused the vertebrae to protrude farther into the ribcage (invagination). To support the wider lower thorax, the sacrum (where the pelvis connects to the spine) was more vertically inclined, and the pelvis was wider. This also caused the lumbar vertebrae (lower spine) to be less curved (hypolordosis, "flatback"), except for possibly the 5th lumbar vertebra. Though the skeleton of ancient Homo is poorly documented, because H. ergaster specimens over 1 million years old (namely Turkana Boy) seem to have had a similar physiology, it is possible a wide thorax is normal for Homo. The narrower thorax of modern humans may be a unique adaptation associated with the adoption of endurance running.

The limbs are proportionally short, which has traditionally been explained as a "hyper-arctic" adaptation (Allen's rule). A similar trend is also observed in modern Inuit and Siberian Yupik populations. Neanderthals in more temperate climates—such as Iberia—still retain the "hyperarctic" physique. The shorter limbs, combined with evidence of a stronger respiratory system and a faster metabolism fueling more fast-twitch muscle fibres, could altogether also be explained as adaptations for sprinting. Neanderthals were probably not as adept at endurance running as modern humans given the expanded thorax and long heel bones (causing a long moment arm at the Achilles' tendon, possibly more conducive for acceleration than distance).

Neanderthals had proportionately shorter thumbs and index fingers, but this does not seem to have negatively impacted dexterity. Bone trauma indicates Neanderthals habitually made use of a strong, squeezing grip. Like modern humans, Neanderthals seem to have exhibited handedness, generally preferring the right hand.

Size In a sample of 45 Neanderthal long bones from 14 men and 7 women, the average height was for males and for females. The fossil record shows that adult Neanderthals varied from about in height.

For comparison, the average height of 20 male and 10 female Cro-Magnons is, respectively, and , although this decreases by nearer the end of the Upper Palaeolithic based on 21 males and 15 females. The average height in the year 1900 was and , respectively.

For Neanderthal weight, a sample of 26 specimens found an average of for males and for females. Using , the body mass index for Neanderthal males was calculated to be 26.9–28.3, which in modern humans correlates to being overweight. This indicates a very robust build. The Neanderthal LEPR gene concerned with storing fat and body heat production is similar to that of the woolly mammoth, and so was likely an adaptation for cold climate.

Biology Vision The orbits (eye sockets) of Neanderthals, and possibly also the eyeballs, were bigger than those of Cro-Magnons and modern humans. It is unclear how this could relate to vision. In modern humans, eye socket size does not correlate with eyeball size; their volumes instead seem to be most influenced by the frontal lobe pushing into the area. In Neanderthals, the separation of the orbits from the frontal lobe is about midway what is seen in H. heidelbergensis and modern humans. Neurologically, Neanderthals had a proportionally large occipital lobe compared to Cro-Magnons, associated with visual processing. Genetically, colour blindness (which may enhance mesopic vision in low-light conditions) is typically correlated with high-latitude populations, and the Neanderthals from Vindija Cave, Croatia, had some substitutions in the Opsin genes which could have influenced colour vision.

The functional implications of these traits are inconclusive, but altogether, they could indicate a preference for activity in dimmer light conditions. This may have been advantageous in northerly latitudes where daylight hours are much shorter especially in winter, and low-light or nighttime hunting would enhance ambush tactics when pursuing large game. Neanderthal-derived alleles near ASB1 and EXOC6 are associated with being an evening person, narcolepsy and day-time napping.

Airways

Breathing