possible aquatic adaptations in human
arguments for the waterside models (including the aquatic ape hypothesis)
species : Homo sapiens
proposed natural habitat : coasts & beaches
| # | Category | Hypothesis | Status |
|---|---|---|---|
| A | Swimming | Possible adaptations for floating and swimming on the water surface | |
| A | Swimming | Human evolved better swimming ability than other apes (despite the loss of instinctive swimming in all apes), with high endurance and various swimming gaits e.g. breaststroke, front crawl.[1][2] | Speculated |
| A | Swimming | Reduction of body hair and smooth skin surface evolved for reducing drag.[1] | Speculated |
| A | Swimming | Dark skin pigment evolved for blocking sunlight in tropical waters.[3] | Speculated |
| A | Swimming | Higher proportion of subcutaneous fat layer compared to visceral fat evolved for preventing heat loss in water (replace hair), increasing buoyancy, and/or streamlining the body.[1][4] | Speculated |
| A | Swimming | Fusiform, straight, streamlined body evolved for reducing drag.[1] | Speculated |
| A | Swimming | Lower bone density evolved for increasing buoyancy (c.f. higher density in Homo erectus possibly evolved for shallow bottom diving).[5] | Speculated |
| A | Swimming | Hair growth pattern possibly follows water flow lines during breaststroke, evolved for reducing drag before reduction of hair.[1] | Speculated |
| A | Swimming | Underarm & pubic hair evolved for reducing turbulent flow in the concave surfaces.[6] | Speculated |
| A | Swimming | Curly hair fibers caused by oval cross-section, evolved for reducing drag (like in seals).[6] | Speculated |
| A | Swimming | Flexible arms and shoulders, scoop-like hands with thumb webbing (thenar space) evolved for producing steering strokes in swimming and diving.[4] | Speculated |
| A | Swimming | Longer legs, paddle-like feet with fused toes evolved for producing powerful propulsion in swimming, diving and running.[4] | Speculated |
| A | Swimming | Force exerted mainly on the first and the last toes, evolved for swimming and diving (like in seals).[7] | Speculated |
| A | Swimming | Rare condition of webbed fingers and webbed toes as half-way aquatic adaptations.[4] | Speculated |
| A | Swimming | Utilization of both drag-based & lift-based propulsion in swimming and diving.[6] | Speculated |
| A | Swimming | Lower normal body temperature with low fluctuations evolved like in aquatic mammals.[7] | Speculated |
| B | Diving | Possible adaptations for apnea diving under water | |
| B | Diving | Human evolved better diving ability than land mammals, capability lasts from birth to very old ages (e.g. 90).[8] | Evidence support |
| B | Diving | Human's bimodal diving pattern similar to sea otters: serial short dives to <20m, separated by recovery intervals, in extreme cases down to 100m.[8] | Evidence support |
| B | Diving | Enhanced diving reflex approaching the level of semi-aquatic mammals:[8] - Bradycardia: heartbeat reduces during immersion to save oxygen; - Peripheral vasoconstriction: blood flow to limbs cuts down to save oxygen for important organs; - Blood shift: blood fills lung vessels and cavities in extreme depth to prevent organs crushed. |
Evidence support |
| B | Diving | Fewer & larger red blood cells, higher concentration of hemoglobin evolved for better oxygen storage under water.[7] | Speculated |
| B | Diving | Spleen contracts to release extra red blood cells during prolonged diving.[8] | Evidence support |
| B | Diving | Fine breath control evolved for diving, later as exadaption for speech:[4][3] - Descended larynx allows mouth breathing for inhaling more air before dives / during swimming; - Better voluntary breath control for conscious planning of inhalation; - Breathing pattern in diving (quick inhale & slow exhale) similar to that in speech. |
Speculated |
| B | Diving | Ablility to equalize ears at depth as a behavioral adaptation.[8] | Evidence support |
| B | Diving | Mobile neck evolved for searching resources under water.[6] | Speculated |
| B | Diving | Flexible backbone enables lift-based "dolphin kick" under water.[4] | Speculated |
| B | Diving | Dexterous, sensitive hands evolved for collecting and hunting underwater, led to first pebble tool-making and fire-making began near the shore.[1] | Speculated |
| B | Diving | More efficient countercurrent veins in limbs evolved for reducing heat loss in water.[6] | Speculated |
| B | Diving | Multi-pyramidal kidneys evolved for excreting excess salt from saline water.[8] | Speculated |
| B | Diving | High output of water and salt due to abundance of both elements in habitat: abundant sweat & tears, saturated expiration, dilute urine, watery faeces.[7] | Speculated |
| B | Diving | Sweat (perhaps also tears) evolved for excreting excess salt.[4] | Withdrawn |
| C | Pregnancy | Possible adaptations in females for pregnancy, laboring and nursing near the water surface | |
| C | Pregnancy | Water birth proposed as the natural way of laboring in human: less painful and low risk, preferred by women, infant mortality & infection rates no higher than land births, practiced in primitive cultures.[9] | Evidence support |
| C | Pregnancy | Immersion relieves pain, reduces stress hormones (adrenaline) and facilitates "love" hormone (oxytocin).[8] | Evidence support |
| C | Pregnancy | More body fat, darker and smoother skin during pregnancy evolved for immersion in open sea.[6] | Speculated |
| C | Pregnancy | Longer scalp hair (becomes thicker and stronger during pregnancy) evolved so that the floating infant could cling on.[4] | Speculated |
| C | Pregnancy | Protruding buttocks (in extreme case steatopygia) evolved as a platform for carrying baby.[7] | Speculated |
| C | Pregnancy | Estrogen causes fat stored around buttocks, thighs and hips, evolved for forming a more fusiform body.[6] | Speculated |
| C | Pregnancy | Large floating breasts evolved to help feeding while immersed in water.[7] | Speculated |
| C | Pregnancy | Umbilical cord evolved long enough for the newborn to reach the water surface.[6] | Speculated |
| C | Pregnancy | No practice of eating placenta (placentophagy) as not possible in water.[8] | Speculated |
| D | Infancy | Possible adaptations in infants for living near the water surface | |
| D | Infancy | Infants are able to swim/dive before being able to crawl/walk, no fear or harm caused by immersion.[4][9] - 0-4 months: infant diving reflex: opens eyes, holds breath, rhythmic limb movements to propel forward; - 4-12 months: infant floating reflex: rolls onto the back to float & breathe; - > 1 year: voluntary movements, starts learning to swim, dive, walk. |
Evidence support |
| D | Infancy | Brown fat tissues evolved for generating heat in water.[3] | Speculated |
| D | Infancy | Abundant body fat evolved for preventing heat loss and increasing buoyancy.[4] | Speculated |
| D | Infancy | Multi-lobed kidney (reniculi) evolved as in marine mammals (the lobes are fused later).[8] | Speculated |
| D | Infancy | Strong muscles in chin, cheek and lips evolved for close contact suckling, avoid water leaks in.[6] | Speculated |
| D | Infancy | Vernix caseosa (waxy substance on newborns, with squalene) evolved for waterproofing & antibacterial properties, similar to other semi-aquatic mammals.[2][8] | Evidence support |
| D | Infancy | Newborns able to hold breath until reaching the water surface.[9] | Evidence support |
| E | Sex | Possible sexual selection and adaptations in a waterside context | |
| E | Sex | Facial hair and baldness in males evolved for further streamlining the head and neck during diving (alternative: facial hair evolved as a sexual signal above the water surface).[5] | Speculated |
| E | Sex | Foreskin in males evolved for avoiding infection in sea water.[6] | Speculated |
| E | Sex | Longer vaginal canal in females evolved for further isolating the uterus from sea water; longer penis in males evolved in response to deeper vagina.[2] | Speculated |
| E | Sex | Menstruation synchronized with circalunar (tidal) cycle.[2] | Feature disputed |
| E | Sex | Labia majora, hymen & vaginal trasverse ridges evolved for waterproofing the vagina, low pH (~ pH 4.5) and lactobacillus colony in the vaginal canal developed to inhibit waterborne pathogens.[2][9] | Speculated |
| E | Sex | Concealed ovulation (hidden estrus) evolved due to visual (genital swelling) and scent (pheromone) signals became inefficient in water.[2] | Speculated |
| E | Sex | Practice more face-to-face sex due to straight body plan for diving, e.g. vagina directed towards the front, as in other aquatic mammals.[4] | Speculated |
| E | Sex | Sex differences evolved due to different gender roles: females more engaged in floating/swimming and males more engaged in diving.[citation needed] | Speculated |
| F | Bipedalism | Bipedalism possibly originated and/or enhanced in a waterside context | |
| F | Bipedalism | Bipedalism facilitated by wading: for shallow water foraging, supported by water buoyancy, could overcome various disadvantages.[1][10] | Evidence support |
| F | Bipedalism | Obligatory bipedalism and upright posture evolved due to longer legs and straight body plan for diving.[4] | Speculated |
| F | Bipedalism | Endurance running only evolved recently, with exadaptations from aquatic locomotion (strong legs, good balancing...) and new adaptations for running (eccrine sweating, foot arches, short toes...).[5] | Speculated |
| G | Head | Possible adaptations for waterproofing & thermoregulation in the head and upper body (as usually being out of water, in contrast to insulation in lower body) | |
| G | Head | Greasy hair evolved for waterproofing the head.[3] | Speculated |
| G | Head | Scalp hair evolved for blocking sunlight in the tropical waters.[1] | Speculated |
| G | Head | Round-shaped head (reduced brow ridge, flat face, flat ears) evolved for reducing drag.[6] | Speculated |
| G | Head | Ear wax (contains oily sebum) evolved for preventing water from entering the ears.[11] | Speculated |
| G | Head | Sebaceous glands secreting sebum (with squalene) evolved for waterproofing the upper body (head, upper body, back).[3] | Speculated |
| G | Head | Sweat evaporates through the upper body (highest rate in head, upper body, back) which is above water during aquatic activities.[6] | Speculated |
| G | Head | Remnant brown fat tissues generate heat in the upper body (neck, upper chest).[6] | Speculated |
| G | Head | Vestigial air sacs (laryngeal saccule) once evolved in ape ancestors as floating aid.[8] | Speculated |
| G | Head | Eyebrows and eyelashes evolved for preventing water from entering the eyes.[6] | Speculated |
| G | Head | Paranasal sinuses evolved for assisting the head to float above the water surface.[7] | Speculated |
| G | Head | Downward nostrils evolved to prevent water from entering the nose.[4] | Speculated |
| G | Head | Upper lip and philtrum evolved to enable sealing of the nostrils during diving.[2][8] | Speculated |
| H | Ingestion | Possible adaptations in eating and drinking for aquatic diet | |
| H | Ingestion | Specialized oral cavity (small mouth, round jaw & palate, reduced teeth, round versatile tongue) evolved for suction feeding of slippery seafood, later as exadaptions for speech.[8] | Speculated |
| H | Ingestion | Blunter molars with thick enamel evolved for cracking hard shellfish.[8] | Speculated |
| H | Ingestion | Low drinking capacity, low tolerance to dehydration developed in a watery context.[7] | Speculated |
| H | Ingestion | No salt hunger and high tolerance in salty taste since salt is everywhere at the coast.[3] | Speculated |
| I | Diet & Brain | Brain evolution and the origin of language possibly enabled by freshwater and marine diet | |
| I | Diet & Brain | Freshwater and marine diet with their high availability, high reliability, and abundance in essential omega-3 fatty acids (e.g. DHA) and micronutrients (e.g. iodine, zinc, selenium) enabled brain enlargement & reorganization, led to higher intelligence, creativity and syntactic ability.[12] | Evidence support |
| I | Diet & Brain | Vocal learning, speech and singing enabled by fine breath control (orignally for diving) and enhanced articulating apparatus (orignally for ingestion of aquatic food).[8] | Speculated |
| I | Diet & Brain | Vocal expressions (speech and singing) replaced body expressions as the more efficient choice of communication channel above the water surface.[4] | Speculated |
| I | Diet & Brain | Acquisition of syntax and speech (originally enabled by aquatic diet) led to the origin of human language.[12] | Speculated |
| I | Diet & Brain | Recent brain shrinkage due to migration from coastal to riverine, terrestrial habitats.[12] | Speculated |
| J | Senses | Possible adaptations in senses attuned to a semi-aquatic environment | |
| J | Senses | Enlarged semicircular canals evolved for better balance under water and on land (bipedalism).[4] | Speculated |
| J | Senses | Good underwater vision by maximally constricting pupils evolved.[8] | Evidence support |
| J | Senses | Myopia and astigmatism evolved for correcting vision affected by light refraction in water.[11] | Speculated |
| J | Senses | Color vision and color blindness evolved due to the less colored environment under water (even monochromatic in deep waters).[8] | Speculated |
| J | Senses | Peculiar color terms (e.g. combined word for blue-green, for red-yellow) originally developed in watery (blue-green) and sunny (red-yellow) environment.[8] | Speculated |
| J | Senses | Reduction in smelling sense as being useless in or above water (alternative: smelling sense not reduced but specialized to aquatic diet).[4] | Speculated |
| J | Senses | Thicker lips evolved for testing allergens in seafood.[6] | Speculated |
| J | Senses | More importance of umami taste since it is the most prevalent in seafood.[6] | Speculated |
| K | Scenario | Ecological, behavioral and theoretical considerations of semi-aquatic/waterside scenarios | |
| K | Scenario | Most of modern human population living near coasts and rivers as remnant of original waterside habitat.[2] | Speculated |
| K | Scenario | Psychological attraction to water and coastline due to semi-aquatic past (e.g. high-value properties with seaview, beaches, bathing, aquatic sports).[1][10] | Speculated |
| K | Scenario | Semi-aquatic lifestyle and underwater foraging (procurement of shellfish/fish for food and precious shells/pearls for trading) is possible and sustainable in modern humans, as examplified by Sea Gypsies and Ama/Haenyeo divers, possibly also important in early Homo sapiens.[8] | Evidence support |
| K | Scenario | Auditory exostosis (surfer's ear) in modern human swimmers and fossils of more recent Homo (H. sapiens, H. neanderthalensis and H. erectus) suggests diving activities.[13] | Evidence support |
| K | Scenario | Several species of human-specific aquatic parasites (e.g. Dracunculus, Schistosoma) co-evolved with human ancestors.[10] | Speculated |
| K | Scenario | Aquatic ape model (Hardy/Morgan) - Single semi-aquatic phase in a Miocene "fossil gap" before Australopithecus led to human-chimp split, possibly on an isolated island in East Africa, later returned to land.[1][2] | Disproved |
| K | Scenario | Saci LCA model (Bender) - Loss of instinctive swimming (i.e. dog paddle) in early hominoids (Miocene apes) due to intense adaptation to arboreal life, led to intrinsic risk of drowning in all extant apes including human.[14] | Speculated |
| K | Scenario | Amphibian generalist model (Niemitz) - Bipedalism and upright posture in early hominoids (Miocene apes) triggered and maintained by wading in wooded wetland.[10] | Speculated |
| K | Scenario | River apes model (Kuliukas) - Bipedalism triggered and maintained by wading with very slight levels of selection.[8] | Speculated |
| K | Scenario | Aquarboreal ancestors model (Verhaegen) - Preliminary freshwater adaptations in early hominoids (Miocene apes, e.g. Morotopithecus, Oreopithecus):[8][15] - Larger, wider body and tail loss evolved for reducing drag and preventing heat loss in water; - Flexible shoulders and rigid, centered spine evolved for hanging and swinging; - Frugivorous and freshwater diet, collecting food by wading & climbing; - Partial bipedalism and upright posture aided by buoyancy and/or grasping branches |
Speculated |
| K | Scenario | Littoral dispersal model (Verhaegen) - Gradual coastal adaptations reached maximum in Homo erectus (Pleistocene) as shallow bottom divers along Indo-Pacific coasts:[8][16] - Denser bones (pachyosteosclerosis) evolved to aid bottom diving in shallow water to collect sessile foods, causing auditory exostosis; - Thick back of skull evolved for back-floating between dives. |
Speculated |
| K | Scenario | Shore-based model - freshwater (Cunnane et al.) - Gradual freshwater adaptations in early hominins (Pliocene):[12] - Spead along East African coastal forests and riverine corridors - Aridity refugium model (Joordens);[17] - Freshwater diet enabled moderate levels of brain expansion; - Thicker enamel, polished molars in Australopithecus due to eating tough aquatic plants (e.g. papyrus sedge); may also ate catfish; - Thicker enamel, large molars, heavy skull in Paranthropus evolved for eating hard-shelled invertebrates, comparable to otters; - Thinner enamel in early Homo (e.g. Homo habilis) due to more reliance on stone tools. |
Evidence support |
| K | Scenario | Shore-based model - coastal (Cunnane et al.) - Gradual coastal adaptations in Homo (Pleistocene):[12] - Hunting fish and collecting shellfish using advanced technologies (e.g. spears, nets); - Seafaring to islands, e.g. Homo erectus to Crete, Homo neanderthalensis to Flores, Homo sapiens to Australia; - Marine diet enabled further brain expansion; - Behavioral modernity in Homo sapiens first emerged in South Africa coasts, with intense exploitation of coastal resources.[18] |
Evidence support |
| K | Scenario | Coastal migration model - Homo sapiens migrated along Indo-Pacific coasts from Africa down to South America:[19][20] - Hopping among estuaries[21], islands and possibly kelp forests[22], travelling by boats and/or swimming (Pleistocene); - Later more humans settled along rivers and inland habitats with the advance of agriculture (Holocene). |
Widely accepted |