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*'''Nakedness''': Humans are the only [[primate]] species in which [[hair]] does not cover almost the entire body. Environments known to give rise to naked [[mammal]]s are tropical (in some larger-sized mammals such as [[elephant]]s and some [[rhinoceros]] species), aquatic ([[whale]]s, [[dolphin]]s, [[walrus]], [[dugong]]s, and [[manatee]]s), semi-aquatic ([[hippopotamus]], [[babirusa]]s), or subterranean ([[naked mole rat]]).
*'''Bipedalism''': Humans are among very few [[bipedal]] mammals, none of which adopt the fully-upright, full-time human posture with a vertical vertebral column. Gorillas, chimpanzees and [[bear]]s are able to walk on two legs when they have a particular reason, but always revert to [[quadruped]]alism as their basic means of locomotion. Some prosimians such as [[indri]]s skip sideways on two legs when on the ground, because their adaptations to leaping through trees make ground-based quadrupedalism difficult. [[Kangaroo]]s and hopping rodent species use a bipedal form of locomotion with bent knees and bent hips in rest. Even [[bird]]s, with exceptions such as [[penguin]]s which have vertical vertebral columns, walk bipedally but with a horizontal vertebral column. Creatures such as [[squirrel]]s and [[meerkat]]s often adopt an upright posture when stationary, but do not walk or run bipedally. Although the posture improves the ability to use [[tools]] while [[walking]] or [[running]], bipedalism and upright posture are believed to come at a significant cost, from [[back]] and [[knee]] problems, [[varicose vein]]s, [[hemorrhoid]]s, [[hernia]]s, and problems with [[childbirth]]. AAT proponents argue that if evolution works in small steps ([[gradualism]]), it is hard to see how bipedalism could have evolved on the [[savanna]]: the mass of the [[torso]] makes it inherently unstable and inefficient for locomotion. [[Water]], however, supports the body, and [[proboscis monkey]]s as well as lowland gorillas have been observed wading bipedally in mangrove or swamp forests. It has been claimed that the one other animal known to have a pelvis adapted to bipedal walking was prehistoric ''[[Oreopithecus bambolii]]'' (commonly known as the "swamp ape" owing to its flooded [[habitat]]).
*'''Breathing''': Most land mammals have no conscious control over their [[respiratory system|breathing]]. The voluntary control humans have over their [[respiratory system]] can be compared to that of (semi)aquatic mammals which inhale as much [[Earth's atmosphere|air]] as they need for a dive, then return to the surface for air. Morgan argued that this voluntary breathing capacity was one of the preadaptations to human voluntary speech.
*'''Fat''': Humans have ten times as many [[fat]] [[cell (biology)|cell]]s under the skin as would be expected in a non-aquatic animal the same size, and have many [[Adipose Tissue|adipose cells]] even when considered slim. Mammals which [[hibernation|hibernate]] have localised seasonal fat humps; but aquatic mammals retain fat ([[blubber]]) throughout the year. Human infants are especially fat compared to apes and most other fully terrestrial mammals. The human fatty layer (panniculus adiposus) is also attached to the skin of the central body parts as is the case with most medium- or larger-sized (semi)aquatic mammals, rather than to the muscle as in almost all land mammals. Humans also lack the layer of cutaneous muscle (panniculus carnosus), possessed by land mammals including non-human primates, which allows many land animals to twitch their skin, and which is not present in aquatic mammals.
*'''Childbirth''': Dramatic increase in [[cranium size]] is a prominent theme in human evolution, making childbirth difficult and dangerous. [[Water birth|Water birthing]] is believed to facilitate childbirth and to reduce risks to mother and infant. Human infants are born covered in [[vernix caseosa]], a waterproof coating also seen in newborn harbour seals, and continue to draw oxygen through the [[umbilical cord]] while underwater.
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