evolution evolutionary history thought occurred indian million years ago period million years fully aquatic marine mammals belonging order branched million years ago thought evolved earlier sharing closest common ancestor mammals surface breathe air finger bones fins nurse young despite fully aquatic life style retained skeletal features terrestrial ancestors characteristics point terrestrial origin mammals terrestrial mammals come land discoveries starting late 1970s pakistan revealed stages transition land sea modern toothed thought separated million years ago second radiation occurring adaptation toothed distinguishes fully aquatic early presence occurred gradually earlier varieties having little size linked dependence subsequent increase filter feeding modern day evolution largely affected local culture social network tool use foraging certain societies affects diet allows access varieties food behavior passed mother child residing use protect foraging social bonds pod affect overall fitness habitat decreasing increasing survival chances particular individual pod social bonds different species cause results hybrids occurs early evolution aquatic lifestyle began indian million years ago period million years discovered antarctica reduce million years extinct containing ancient traditional theory evolution proposed related extinct order animals resembled wolves sister group theory arose similarities unusual triangular teeth early molecular phylogeny data indicates closely related closest living relative placed formed words proposed order containing earliest ancestors appear fossil record millions years known whale ancestor appeared implying groups diverged molecular analysis identifies closely related probably derive directly molecular data supported discovery earliest skeletons derive directly instead began water soon split retained aspects ancestry triangular teeth modern modern lost earliest ancestors mammals probably partly today later evolution retained diet prey available needed higher content order live marine warm specialized likely disadvantage large prey uncommon competed better adapted animals like later small like animal lived million years ago belongs family believed closest sister group identified trait unique size domestic cat creature shared traits modern notably bone growth pattern characteristic found species showed signs adaptations aquatic life including dense limb bones reduce stay underwater similar adaptations found modern aquatic mammals suggests similar survival strategy african water threatened bird prey water hides beneath surface minutes mammals thought earliest known closest sister group lived early million years ago fossils discovered located river far shores initial discovery fossils found mainly early deposits northern pakistan northwestern india based discovery likely lived arid environment streams moderately developed millions years ago stable oxygen analysis shown drink fresh water implying lived freshwater bodies diet probably included land animals approached water drinking freshwater aquatic organisms lived river elongated fused consistent making earliest fossils recovered period following divergence event classified mainly structure ear bone formed bone shape ear region highly unusual skull like absent stage lacks enlarged space filled fat oil receiving underwater sound modern dorsal eye facing similar eye placement helps submerged predators observe potential prey water according study teeth resemble teeth fossil like dog having triangular teeth link modern initially thought ears adapted underwater hearing expected anatomy rest creature ears specialized hearing land able listen underwater enhanced bone depending membrane like land mammals method hearing directional hearing underwater long thin legs relatively short hands feet suggest poor swimmers compensate bones unusually thick probably adaptation animal heavier counteract water according morphological analysis display aquatic skeletal adaptation instead display adaptations running jumping likely aquatic lived million years ago discovered pakistan probably amphibious resembled physical appearance inhabited bays northern pakistan fossils found near shore shallow marine deposits associated abundant marine plant fossils found marine deposits oxygen values indicate consumed range water different degree specimens having evidence sea water consumption fresh water time teeth clear tolerated wide range salt concentrations represent transition phase ancestors fresh water marine habitat increased size indicates fat pad likely housed lower jaw modern toothed fat pad extends middle ear allows sounds received lower jaw transmitted fat pad middle ear similar skull face relatively long limbs particularly strong hind legs retained tail sign structure shows ability engage terrestrial significantly limited compared contemporary terrestrial mammals skeletal structures knee ankle indicates motion restricted plane suggests land propulsion powered extension dorsal muscles walk land addition swimming clear fast environment speculated hunted like prey fish probably swam pelvic way swimming mainly utilizes hind limbs generate propulsion water caudal way swimming uses column generate force movements seals modern intermediate stage evolution modern swim caudal way swimming similar caudal energy efficient lived million years ago compared family diverse family found north central pakistan western india found shallow marine deposits obviously aquatic demonstrated recovery fossils variety coastal marine environments including near shore deposits according stable oxygen analysis fresh water lost fresh water relatively soon origin faced small suggests vision important sense nasal opening eventually modern located near tip snout position nasal opening remained unchanged notable features canals important balancing land mammals decreased size reduction size closely accompanied radiation marine environments according study modification canal system represent crucial point return event early evolution excluded prolonged semi aquatic phase compared relatively short limbs based skeletal remains probably amphibious adapted swimming likely swim caudal form diverse group known asia europe africa north_america lived approximately million years ago fossil remains uncovered coastal unlike previous families fossils uncovered africa north_america include open marine forms probably amphibious aquatic compared leave indian disperse shallow subtropical oceans world genera family great variations aquatic adaptations exist probably able support weight land supposed amphibious nature supported discovery pregnant positioned head delivery suggesting gave birth land gave birth water positioned tail delivery avoid drowning birth unlike large oriented increasingly lateral facing eyes observe underwater prey similar eyes modern furthermore nasal openings large halfway snout great variety teeth suggests diverse feeding modes size increased large indicates fat pad present air filled present modern function ear enable better underwater hearing present external ear canal absent modern present method sound transmission present combines aspects modern toothed intermediate stage hearing development transmission airborne sound poor modifications ear underwater hearing directional underwater hearing poor compared modern short wide likely swimming limbs gave slow land possible clear adapted aquatic life style example bone land mammals fusion connects rest column divided loose connected ancestry characteristics like presence ends toes foot structure shows predominantly aquatic study oceanic environment similarly pelvic supplemented caudal terrestrial limited structure thought moved way similar seals land rotating hind forward underneath body lived late million years ago oldest known aquatic fully recognizable lived entirely ocean supported fossils usually found deposits indicative fully marine environments lacking freshwater influx probably distributed tropical subtropical seas world commonly found association closely related stomach contents indicates ate fish look like modern lacked organ allows toothed use small brains suggests solitary complex social structures modern covered entire depth lower jaw modern faced nasal opening moved higher snout closer position modern furthermore ear structures modern air filled ear skull unlike modern retained large external skeletons immediately recognizable big larger modern genera like reaching lengths long smaller genera like reaching long large size extreme tail body proportions suggest swam caudal propulsion contrast shorter powerful column unlike probably swam similarly modern caudal probably shaped external hind limbs tiny certainly involved fingers retained mobile joints relatives tiny formed hind legs probably mating pelvic bones associated hind limbs connected column essentially longer clearly distinguished relatively closely related modern belong according study barnes large size elongated body ancestral extant forms species family elongated bodies immediate ancestors extinct evolution modern modern filter place teeth exact means differs species feeding feeding members groups appeared middle filter feeding beneficial allows efficiently gain huge energy resources makes large body size modern variety possible development filter feeding result worldwide environmental change physical changes oceans large scale change ocean current temperature initiated radiation modern leading demise toothed forms earlier variety little relied mainly teeth generally speculated modern families separate origins modern whale right gray whale right whale derived characteristics presently unknown vice versa crest toothed adaptation occurred toothed split apart distinguishes modern toothed fully aquatic happened million years ago second radiation modern toothed rely sense sight hunt prey allowed toothed dive deeper search food light longer necessary navigation opened new food sources toothed creating series emitted frequencies sound emitted reflected objects retrieved lower jaw evidence appearance lived early middle middle million years ago featured modern toothed compressed room nose outward characteristic modern toothed gave appearance similar thought unlikely direct ancestors modern toothed oceanic evolved late greatly mid fossil near shallow seas inhabit found species like found california animals spreading european coasts later earliest known ancestor arctic late million years ago single fossil baja california indicates family inhabited warmer waters ancient sperm differ modern sperm tooth count shape face jaws example tapered genera teeth upper jaws differences suggest ancient species necessarily deep sea squid hunters like modern sperm whale genera mainly ate fish contrary modern sperm ancient sperm built hunt short wide measuring feet gave whale ability inflict major damage large struggling prey early species like collectively known killer sperm consist genera earlier variety probably preyed killer sperm large sharks large number fossil discovered coast south_africa confirming remaining species remnant higher diversity gone extinct studying numerous fossil researchers discovered absence functional teeth south african evidence feeding developed whale extinct robust suggesting male male interactions skeletal evolution modern internal rudimentary hind limbs reduced pelvic internal lip ear bone feature compares directly modern similar feature composition teeth contained calcium needed eating drinking aquatic animals unlike modern day toothed tooth morphology opposed tooth morphology present somewhat resembled wolf fossils showed eye closer head terrestrial mammals similar structure eyes transition land water led skull food processing equipment eating habits changing ultimately change position eyes limb bones led began develop long seen current limbs movement similar represent fully formed limbs absence limb initiation arrest limb development limb develop normally embryos limb progress phase early nerves grow limb ridge structure ensures proper limb development appears functional occasionally genes code longer cause modern whale develop miniature legs pelvic bone similar terrestrial mammals later species pelvic bone longer attached reduced certain genes believed responsible changes occurred pelvic structure pelvic modern thought structures served purpose pelvic male different size compared females size thought result sexual pelvic bones modern male massive longer larger females sexual displayed likely involved supporting male remain hidden abdominal walls sexual reproduction occurs early nasal openings end snout later species openings begun drift skull known nasal drift nostrils modern modified allow break surface convenience ears began inward case middle ears began receive vibrations lower jaw today modern toothed use organ pad fat ongoing evolution culture culture group specific behavior transferred social learning tool use aid foraging example uses tool affects eating behavior causes differences diet tool allows new niche new prey open particular differences fitness levels change population causes evolution occur long run culture social networks played large role evolution modern concluded studies showing preferring mates socially learned behaviors songs breeding areas particularly largest non genetic effects evolution culture social structure based study population divided sea snout protective means sharp objects toxic organisms help target fish swim detect fish easily complex background specifically forage deep channels found foraging deep shallow channels foraging behavior mainly passed mother child group behavior passed social learning tool use considered cultural trait researchers study found acid analyses west populations differ areas having different food sources comparing data deep channels different acid results habitat deep shallow channels similar data suggests cause different data deep shallow channels opened new niche allowed access new prey caused long term diet changes producing different food sources population competition resources showing character displacement result carrying capacity increases entire population depend food source fitness levels population change allowing culture evolve social structure social structure forms groups individuals interact allows cultural traits emerge exchange evolve relationship especially seen populations southwestern australia known food fishermen begging behavior spread population individual spending time boats social spending time express begging behavior learning culture impact social structure causing behavior matching mating individuals certain culture likely mate individuals behaviors random individual influencing social groups structure example stick australia followed feed debris population associated individuals behavior lived habitat later longer present integrated social network couple years social networks affect cause evolution impending fitness differences individuals according study male lower survival rate stronger bonds juvenile males age sex classes tested survival rate significantly change suggests juvenile males impose social stress younger counterparts fact documented juvenile males commonly perform acts aggression dominance intimidation male according study certain populations varying levels fitness success social learning mother passed knowledge reproductive ability strong association mother population sticking group individual mother need time predators genetic studies conducted focused natural histories results origin species actually outcome hybrid spinner striped north_atlantic caused constant habitat sharing species relationships species speculated according notable spinner resulting regarded subspecies possibility hybrid spinner striped come question based similarities species environmental factors sequences revealed lacks single history species researchers found major decrease genetic diversity occurred likely event event time period sea levels rising global temperatures decreasing historical climate events matched history shows global local climate change drastically affect leading changes fitness survival evolution species european population common mediterranean differentiated types eastern western according study recent drastically decreased size eastern mediterranean population lack population structure western eastern regions contradictory distinct population structures regions mediterranean area physical barrier regions differentiated types ecology biology differences eastern western likely stems highly specialized niche choice physical barriers environment plays large role evolution species divergence largely climate environmental changes history according research divisions genus periods rapid climate change example changing temperatures cause coast landscape change opportunities separation appear specifically genetic evidence suggests differentiated coastal types divergence likely founding event large group separated following event separate groups adapted accordingly formed niche social structures differences caused groups remain separated endemic distinctive types pilot whale larger northern type smaller southern type found japanese archipelago types overlap oceanic border easternmost point thought local extinction long pilot 12th century triggered appearance causing short pilot colder ranges long variant
Evolution of cetaceans