Evolution of primates

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Evolution_of_primates 

Phylogenetic tree of the primates

The evolutionary history of the primates can be traced back 57-90 million years. One of the oldest known primate-like mammal species, Plesiadapis, came from North America; another, Archicebus, came from China. Other such early primates include Altiatlasius and Algeripithecus, which were found in Northern Africa. Other similar basal primates were widespread in Eurasia and Africa during the tropical conditions of the Paleocene and Eocene. Purgatorius is the genus of the four extinct species believed to be among the earliest example of a primate or a proto-primate, a primatomorph precursor to the Plesiadapiformes, dating to as old as 66 million years ago.

The surviving tropical population of primates, which is seen most completely in the upper Eocene and lowermost Oligocene fossil beds of the Faiyum depression southwest of Cairo, gave rise to all living species—lemurs of Madagascar, lorises of Southeast Asia, galagos or "bush babies" of Africa, and the anthropoids: platyrrhine or New World monkeys, catarrhines or Old World monkeys, and the apes, including Homo sapiens.

Geologic timescale

Early primate evolution

Artistic rendering of Purgatorius, an early plesiadapiform

Primates are believed to have diverged from other mammals during the Late Cretaceous period of the Mesozoic Era. Fossils of Cretaceous primates are virtually non-existent, with estimates based on molecular data instead of morphological information. The oldest record of stem-primates comes from the order Plesiadapiformes in the earliest Paleocene epoch shortly following the Cretaceous-Paleogene (K/PG) extinction event. The plesiadapiform subfamily Purgatoriidae represents the earliest plesiadapiform lineage known to diversify, as fossils of the family are known from sediments across North America within ~140,000 years of the K/PG extinction. Though fossils of purgatoriids are rare and limited to fragmentary elements, they are considered to be stem-primates based on the morphology of their ankles which share several features with both Plesiadapiformes and wider members of Euarchonta. This placement has been supported in subsequent phylogenetic analyses. The morphology of purgatoriids suggests an arboreal lifestyle that has been hypothesized as ancestral to crown primates, while their tooth morphology, which possesses a mixture of pointed and rounded cusps, is theorized to have supported an insectivorous or omnivorous diet depending on the species. Other fossils in the group reveal long snouts with large upper incisors and laterally facing orbits lacking postorbital bars, also suggesting early primate traits.

Throughout the Paleocene and Eocene epochs, plesiadapiforms underwent a substantial radiation based in North America and Europe, though several species are known from Asia as well. In total, eleven different plesiadapiform families are recognized from the fossil record, encompassing over 25 million years of time and displaying a wide diversity of dietary behaviors. The oldest fossils of euprimates come from the Paleocene-Eocene transition, approximately 57 million years ago, and already demonstrate a split between haplorhines and strepsirrhines. This, in combination with molecular studies, suggests that the divergence of haplorhines and strepsirrhines came in the Paleocene, estimated to be sometime between 64 and 63 million years ago.

Evolution of strepsirrhines

Sample of Strepsirrhine Diversity

A note on "prosimians"

Strepsirrhines, the group of primates including lemurs, lorises, and galagos, is also part of the older categorization "prosimians". The "prosimian" category is a paraphyletic group which includes all strepsirrhines and tarsiers. Tarsiers are part of the monophyletic clade of Haplorhinii but share some traits with Strepsirrhines leading to the older mis-categorization creating the group "prosimians". A significant amount of fossil work still discusses Strepsirhini within the context of prosimians because of the shared morphological and locomotor traits between Strepsirhini and Tarsiformes, namely: presence of a Grooming claw, clinging and leaping locomotion, Unfused mandibular symphysis, high cusped molar teeth, and a bicornate uterus.

Adapoids: Potential early strepsirrhines

Magnadapis quercyi cranium and mandible. Original photo from Leptadapis article, using older genera classification.

The earliest strepsirrhines are known as adapiforms, a diverse group that ranged throughout Eurasia and North America. Adapoids share several traits with modern Strepsirrhines including: long snouts, relatively small eyes, a long nasolacrimal canal indicating a rhinarium or "wet nose", a gap between small incisors to access their vomeronasal organ, canines larger than their incisors, and an inflated bony auditory bulla with a suspended tympanic ring. Adapoids originated in the Eocene, and though they survived till the late miocene, the earliest known strepsirhines appear in North African fossil beds from the Eocene. This indicates that, despite the longevity of adapoids, an early branch of this clade gave rise to lemuriform primates, including lemurs and their kin.

There are currently six adapiform clades: Notharctids, Cercamoniids or Protoadapids, Caenopithecids, Adapids, Sivaladapids, and Asiadapids.

Early European fauna is also exemplified by Darwinius, a basal strepsirrhine, or haplorhine dated to 47 million years ago (early Eocene)

Darwinius masillae

Darwineus masilae is a primate fossil from an Eocene lagerstätte in Messel, Germany. Preserved in oil shale in an anoxic lake bed, the skeleton is laterally crushed but articulated, preserving the outline of the long bones and leaving irregular and short bones relatively intact. Darwinius (or "Ida," as the adolescent female has been nicknamed) has an estimated body mass of 650-1000g, and dentition and stomach contents indicating a herbivorous diet. Ida has a low intermembral index (long hindlimbs compared to forelimbs), indicating potential for clinging and leaping locomotion, and large orbits compared to skull length indicating nocturnality.

Since its discovery, Darwinius's mixture of primitive and derived traits has led to disagreements on its phylogenetic placement. Although this is fairly common within paleoprimatology, the Eocene time period and array of anthropoid and primitive traits specifically could influence the phylogenetic placement of adapoids and omomyoids depending on when the derived traits present in Darwinius evolved. It has a mixture of more basal and derived traits. Its short rostrum, fused mandibular symphysis, deep mandibular ramus, fused mandibular symphysis, spatulate incisors, quadrate lower molars, uncompressed mesocuneiform, and lack of tooth comb all align well with derived haplorhines.

Meanwhile, the lack of postorbital closure, a potentially sloping tibulo-fibular facet, and implacable grooming claw perpetuate the debate by indicating strepsirrhine phylogeny.  Authors supporting this hypothesis state that many of the traits used to associate Darwinius with haplorhines are athropoid traits, while Darwinius lacks many basal haplorhine traits. They argue that spatulate incisors could be (and often are in primates) a symplesiomorphies for diet specialization rather than an indicator of haplorhine heritage. They also argue that a quadrate molar should not be counted as evidence for haplorhine relation, as many anthropoids do not have one.

Two points on which authors illustrate inter-observer variation are the condition of the tibulo-fibular facet and the nail or claw on digit II. Williams et al. argue that the tibulo-fibular facet is crushed and cannot be adequately observed for inference, while Gingerich et al. argue that it is not too crushed or obscured to infer it is sloping but also argue this is likely a primitive condition rather than a derived one. While both Williams and Gingerich say digit II has a nail, Williams argues it is symplesiomorphic with anthropoids while Gingrich claims it indicates early haplorhine traits.

Lorisiform evolution

Lorisiforms are members of the superfamily Lorisoidea, within which are the families Galagidae (bushbabies and galagos) and Lorisidae (lorises and pottos). Loris and galago phylogeny is difficult to trace for a number of reasons, including the crypsis of both groups, arboreal lifetyles in densly forested environments, and frequent convergence on similar morphologies. Researchers continue to disagree on the phylogenetic organization of extant lorisiformes, primarily whether the subfamily Perodicticinae (angwantibos and pottos / African lorises) is truly a sister group to Lorisinae (Asian lorises) or more closely related to extant Galagidae (galagos) and subject to a surprising convergence with lorises. Galagos are characterized by leaping saltatorial locomotion aided by elongated tarsal bones and insectivory, while lorises and pottos are known for a form of slow arboreal quadrupedalism and secondarily shortened tarsal bones. However, lorises are subject to significant morphological plasticity based on diet; for example, slow lorises and pottos are primarily frugivorous and exudate eaters, requiring less stereoscopic vision, while slender lorises and angwantibos, which are nocturnal predators, have more convergent orbits and thereby more stereoscopic vision.

Based on mitochondrial cytochrome sampling of modern taxa, lorises and galagos split from lemurs has been estimated to the Late Paleocene. Continuing disagreement on the constitution of each clade makes classifying fragmentary fossil primates even more difficult. Karanisia clarki and Saharagalago, both dating to 37mya Fayum deposits in Egypt, are two of the oldest potential Lorisiformes. Saharagalago is a stem galagid from Fayum, Egypt, 37mya; although its dentition is very similar to modern galagos, it is still considered a potential stem member of Galagidae or Lorisoidae. Consisting of only a mandible fragment and a few isolated teeth, Karanisia clarki is the oldest fossil primate known to have a tooth comb. Karanisia's estimated body mass is 273grams. Dental morphology and microwear analysis of the molars indicate a potential for folivory. However, a completely folivorous diet is prohibited by the energy and metabolic requirements of its size, leading researchers to reconstruct Karanisia as an omnivore. Karanisia molars closely resembles those of the living lorisid Arctocebus, the golden potto. Although its position as the oldest tooth-combed primate fossil places it confidently within at least stem Strepsirrhini, many researchers debate if it is a crown strepsirhini, stem lorisoid, or stem lemuroid.

Wadilemur elgans is a later stem galagid from 35mya; in addition to similar dental features to modern galagos, it also has evidence of leaping traits.

Semicircular canals in primate locomotion

Semicircular canals are often used by paleontologists and paleoanthropologists to inform locomotor estimations when there is limited postcranial evidence for a species. Semicircular canals are held within the petrous portion of the temporal bone, a very thick and often well-preserved portion of the skull. Semicircular canals with a larger radius of curvature and more intraspecific consistency can indicate faster movement, as more surface area is needed for more precise perception of balance. In contrast, animals that move more slowly tend to have more variation in general, as their canals are not under pressure to maintain a certain morphology. Galagos generally utilize a clinging and leaping locomotion, while lorises tend towards a slow arboreal quadrupedal form of locomotion. As a result, there is a noticeable and quantifiable difference in their semicircular canal volumes, which can be quantified and used to assign fossil crania or partial crania to a range of speed and by extension infer each locomotion.

Malagasy radiation

Lemurs

Lemurs constitute the clade Lemuridae, the sister clade to Lorisidae. All extant lemurs are currently isolated on the island of Madagascar. Lemurs are believed to have colonized Madagascar in the early Tertiary period, about 40 mya. Recent genetic evidence suggests that lemurs are one monophyletic group distinct from lorisiformes, supporting the idea that there was one colonization event; this would mean the ancestor of all modern lemurs arrived on Madagascar once rather than several ancestors of different groups arriving at different times. One hypothesis for lemurs' diversity on Madagascar and absence in the rest of Africa is that they or their ancestors were outcompeted by monkeys and lorisoids in continental Africa but were able to radiate into similar niches in isolation on Madagascar. Recent research suggests that the adaptive radiation in Madagascar might have been comparatively gradual, rather than a sudden burst, with hybridization and genetic introgression resulting in more diversity and aiding speciation. Modern lemurs are divided into five families: daubentoniids (which likely branched off first), lemurids, lepilemurids, indriids, and cheirogalids.

Giant lemurs

In the late Pleistocene, there were a plethora of giant lemurs inhabiting Madagascar; many of these species were in genera that no longer have any living members. There were three primary groups: archeolemurs, megladapines, and paleopropithecines. All three clades are characterized by large size and slow maturation, but each is associated with a set of derived traits convergent with separate mammalian groups.

Archaeolemurines, weighing between 26 kg and 35 kg, are also called "monkey lemurs" due to their convergent traits with Cercopithecines. This family includes the genera Hadropithecus and Archaeolemur. They have a lemur-like ear and lack a postorbital closure, consistent with their status as lemurs. However, they also have broad flat incisors with no gap between the two central incisors, a fused mandibular symphysis, and bilophodont molar teeth, which are consistent with cercopitheocoid monkeys. The family also has unique adaptations such as a caniniform anterior premolar rather than a true canine tooth. They have a mixture of dental traits similar to catarrhines, including shearing premolars (indicative of foliovory), low rounded bilophodont molars (indicative of frugivory), and enamel microstructure conducive to hard seed or insect crushing. Altogether, researchers believe archeolemurs likely had an omnivorous diet. The short limbs relative to trunk length are indicative of arboreal quadrupedalism, though smaller limb features are more similar to gorillas' terrestrial quadrupedalism.

Megaladapines are referred to as "koala lemurs" due to their hypothesized posturing and folivorous diet. Containing only the genus Megaladapis, at about 70 kg, they were as large as modern gorillas but had no upper incisors. Extremely short limbs and teeth with well-developed crests indicate a highly folivorous diet. Researchers believe they likely clung to thick tree trunks much like a koala, and their lack of upper incisors indicate they may have had a horny pad or plate on their maxilla to help grind food like Artiodactyla. The buttressing extending above the nasal aperture has led some researchers to hypothesize that megaladapines might have had a highly mobile upper lip or proboscis like a giraffe or elephant.

Paleopropithecines, ranging in size between 11 kg and 160 kg, are called "sloth lemurs" because of their extremely sloth-like limb proportions and reduced dentition. They have a very high intermembral index of 144, indicating extemely long forelimbs compared to hindlimbs, which aligns well with modern primates' and sloths' suspensory behavior. Their similarity to sloths extend to their very curved phalanges, resembling suspensory apes like orangutans morphologically and sloths' curved claws ecologically. Their incisors were stubby, indicating they played little role in diet, and their molars were well-adapted to folivory. Due to their similarities to living indriids (including suspensory feeding postures, four toothed tooth comb, and reduced dentition overall), they are generally placed in a monophyletic subfamily within Indriidae. This subfamily includes the genera Mesopropithecus, Babakotia, Paleopropithecus, and Archaeoindris. Archaeoindris is the largest subfossil lemur currently known at around 160 kg.

Evolution of haplorhines

Sample of the haplorhine family Tarsioidea

Haplorhines can be separated from the more primitive strepsirrhines. Haplorhines possess a retinal fovea on the posterior surface of their eyeballs. They have an orbital plate disjoining their orbits and temporal fossa, and relatively smaller olfactory bulbs than strepsirrhines with fewer nasal turbinates. Haplorhines also have a reduced vomeronasal organ, a dry rhinarium, and a vertical nasolacrimal duct connecting the orbit to the nasal cavity.

The oldest haplorhines in the fossil record are from the Paleocene-Eocene, a period of time associated with a global warming event. The climate of the early and middle Eocene was warm, and more frequent warming events allowed for the survival and spread of early groups. Haplorhine origins are assumed to date to the Eocene in Africa, but they probably radiated from that region due to climate change. Early primates and haplorhines must have been found much farther north than they are today. Their geographic range is thought to be about 60°N latitude, close to the Arctic Circle.

The primate superfamilies Tarsioidea and Adapiformes were the major primate groups of the early and middle Eocene. Early Tarsioidea and Adapoidea are thought to be stem or crown groups of haplorhines because of their comparable fossil dentitions. Tarsioidea is now maintained by the living genus Tarsius. Adapoidea members have no living relatives, but are thought to have been precursors of apes and monkeys. Other early haplorhine groups include Ceboidea and Hominoidea from the Oligocene and Cercopithecoidea from the early Miocene. Living haplorhines are tarsiers or anthropoids.

Omomyids: Potential early haplorhines

The oldest fossil haplorhine primates are the omomyids, which resemble modern-day tarsiers. Omomyids first appear at the beginning of the Eocene, around 56 million years ago. Like the strepsirrhine adapiforms, omomyids were diverse and ranged throughout Eurasia and North America. The phylogeny of omomyids, tarsiers, and simians is currently unknown.

One of the oldest known omomyids in the fossil record is an articulated but incomplete skeleton of the species Archicebus achilles. The fossil was uncovered in the lower Eocene Yangxu Formation in China. It is thought to be from the early Eocene of China, around 55 million years ago. A. achilles is very small with long limbs and a long tail. It has a rounded braincase, short snout, vertically facing upper canine, four premolars in each jaw quadrant, long hindlimbs, long feet, and a long tail. A. achilles has features similar to those of the anthropoid and tarsiiform groups within haplorhines: its calcaneal shape and metatarsal proportions in the foot resemble those of anthropoids, and its skull, dentition, and parts of its appendicular skeleton resemble those of tarsiiforms.

Evolution of color vision

Main article: Evolution of color vision in primates

Some of the primates' vertebrate ancestors were tetrachromats, but their nocturnal mammalian ancestors lost two of their four cones during the Mesozoic. Most modern primates, however, have evolved to be trichromats. All Old World monkeys and apes are trichromats, but New World monkeys are polymorphic trichromats, meaning that males and homozygous females are dichromat while heterozygous females are trichromats (with the exceptions of howler monkeys and night monkeys, which have more and less robust color vision respectively).

There are four prevailing theories as to what the evolutionary pressure was for primates to develop trichromatic vision. The fruit theory suggests that it was easier for trichromatic primates to find ripe fruit against a green background. While there is data supporting the fruit theory, there is some dispute about whether or not trichromacy was more advantageous for determining how ripe fruit was up close or spotting fruit from afar. The young leaf hypothesis suggests that primates with more advanced color vision could better spot younger and more nutritious leaves during fruit shortages. Another theory suggests that more advanced color vision was better for recognizing changes in skin tone, allowing primates to better determine the blood oxygen saturation of others. The fourth theory postulates that primates' color vision evolved alongside their sense of smell, though research has shown no direct correlation between concentration of olfactory receptors and acquisition of color vision.

Evolution of platyrrhines (New World monkeys)

A selection of extant Platyrrhini

Following the emergence of Simiiformes in Africa, the pavorder Platyrrhini (New World Monkeys) split from Catarrhini during the Eocene period, sometime around 40 million years ago on the African continent. Their present distribution in South America is theorized to have resulted from rafting across the Atlantic Ocean, with dispersal events facilitated by large mats of vegetation capable of transporting small populations to South America. Transportation of vegetation rafts were aided by a relatively narrower Atlantic, which is hypothesized to have been ~1,000 km narrower than today based on calculated rates of mid-ocean ridge spreading at 2.5 mm per year, and differing paleocurrents which produced westwards movement of currents. Given the favorable currents and limited width of the Atlantic, rafting events are estimated to have taken only eight to fifteen days depending on when they occurred. Similar dispersal mechanisms have also been proposed in Caviomorph rodents. Three separate transoceanic dispersal events between Africa and South America are theorized to have occurred within primates due to the presence of several non-platyrrhine primates from fossil deposits in South America.

The exact timing of platyrrhine dispersal is debated. Historically, a mid-to-late Oligocene dispersal was viewed as likely based on fossil and molecular data. Bayesian estimates of divergence time have indicated the most recent common ancestor of New World monkeys existed between 27-31 million years ago. Additionally, the platyrrhine species Branisella boliviana, found in the Salla Luribay basin and dated to the Late Oligocene (26 million years ago), was believed to represent the oldest known stem platyrrhine. However, new evidence suggests an earlier origin, as fossils of the species Perupithecus ucayaliensis from the Peruvian Amazon date to approximately 36 million years ago. This pushes the date of platyrrhine dispersal to the late Eocene, approximately 37 million years ago and 10 million years older than previously considered.

Fossils of platyrrhines are infrequent and difficult to access, mainly due to the dense forest cover of South America and the poor probability of fossilization in rainforest habitats. The best material from stem platyrrhines belongs to the Oligocene genus Branisella, whose anatomy suggests it was a semiterrestrial omnivore that occupied woodland habitats. The record of platyrrhines in the Miocene is more extensive. Platyrrhines from the earliest Miocene (20-15 million years ago) are known primarily from fossils in the Southern Cone of South America, an area that rests outside their present distribution. The southern platyrrhines show adaptations for frugivory and granivory, particularly seeds and hard fruits that reflect the dry habitats occupied by several taxa, and show anatomical affinities with the platyrrhine subfamily Pitheciidae, though this relationship is debated. Fossils from the middle-to-late Miocene are distributed in northern countries in South America, particularly the La Venta fossil site of Colombia, and show the evolution of several modern platyrrhine subfamilies, including relatives of howler monkeys, cebids, night monkeys, callitrichids, and titi monkeys, among others.

Evolution of Old World monkeys

The earliest known catarrhine is Kamoyapithecus from uppermost Oligocene at Eragaleit in the northern Kenya Rift Valley, dated to 24 million years ago. Its ancestry is thought to be species related to Aegyptopithecus, Propliopithecus, and Parapithecus from the Faiyum depression, at around 35 million years ago. In 2010, Saadanius was described as a close relative of the last common ancestor of the crown catarrhines, and tentatively dated to 29–28 million years ago, helping to fill an 11-million-year gap in the fossil record. Notable species also include Nsungwepithecus gunnelli and Rukwapithecus fleaglei of the Oligocene.

Reconstructed tailless Proconsul skeleton

In the early Miocene, about 22 million years ago, the many kinds of arboreally adapted primitive catarrhines from East Africa suggest a long history of prior diversification. Fossils dated to be 20 million years old include fragments attributed to Victoriapithecus, believed to be the earliest Old World monkey. Among the genera thought to be in the ape lineage leading up to 13 million years ago are Proconsul, Rangwapithecus, Dendropithecus, Limnopithecus, Nacholapithecus, Equatorius, Nyanzapithecus, Afropithecus, Heliopithecus, and Kenyapithecus, all from East Africa.

The presence of other generalized non-cercopithecids of the middle Miocene age from sites far distant—Otavipithecus from cave deposits in Namibia, and Pierolapithecus and Dryopithecus from France, Spain and Austria—is evidence of a wide diversity of forms across Africa and the Mediterranean basin during the relatively warm climatic regimes of the early and middle Miocene. The youngest of the Miocene hominoids, Oreopithecus, is from coal beds in Italy that have been dated to 9 million years ago.

Molecular evidence indicates that the lineage of gibbons (family Hylobatidae) diverged from Great Apes some 12 to 18 million years ago, and that of orangutans (subfamily Ponginae) diverged from the other great apes at about twelve million years; there are no fossils that clearly document the ancestry of gibbons, which may have originated in a so-far-unknown South East Asian hominoid population, but fossil proto-orangutans may be represented by Sivapithecus from India and Griphopithecus from Turkey, dated to around 10 million years ago.

Human evolution

Main article: Human evolution

See also: Homo sapiens § Evolutionary history of Primates

Human evolution is the evolutionary process that led to the emergence of anatomically modern humans, beginning with the evolutionary history of primates, in particular the genus Homo, and leading to the emergence of Homo sapiens as a distinct species of the hominid family, the great apes. This process involved the gradual development of traits such as human bipedalism and language.

The study of human evolution involves many scientific disciplines, including biological anthropology, primatology, archaeology, paleontology, neurobiology, ethology, linguistics, evolutionary psychology, embryology and genetics. Early genetic studies suggested that primates diverged from other mammals about 85 million years ago, but newer research questions this, suggesting a date around 55 million years ago in the Paleocene, consistent with the earliest fossils.

Within the superfamily Hominoidea (apes), the family Hominidae diverged from the family Hylobatidae (gibbons) some 15 to 20 million years ago; African great apes (subfamily Homininae) diverged from orangutans (Ponginae) about 14 million years ago; the tribe Hominini (humans, Australopithecines and other extinct biped genera, and chimpanzee) parted from the tribe Gorillini (gorillas) between 9 million years ago and 8 million years ago; and, in turn, the subtribes Hominina (humans and biped ancestors) and Panina (chimpanzees) separated about 7.5 million years ago to 5.6 million years ago.

Evolution of the pelvis

In primates, the pelvis consists of four parts: the left and the right hip bones, which meet at the ventral mid-line; the sacrum, which connects the hip bones dorsally; and the coccyx. Each hip bone consists of the ilium, the ischium, and the pubis, and at the time of sexual maturity these bones become fused together, though there is never any movement between them. In humans, the ventral joint of the pubic bones is closed.

In bipedal primates, the ilium is shortened and widened, and the blades are located more laterally than in other primates. These morphological changes are due to the stresses involved in bipedal locomotion, in which the Rectus femoris muscle activates on the stationary leg to counterbalance as the other leg moves forward.

The shape of the pelvic outlet is remarkably small in bipedal primates compared to that of quadrupedal primates because a smaller pelvis provides better balance for bipedal locomotion. As such, modern humans must have fully rotational births. Though it is hypothesized that Australopithecus afarensis had semi-rotational births, no other primates require assisted birth.

Evolution of cetaceans

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Evolution_of_cetaceans 

Species of the infraorder Cetacea

A phylogenetic tree showing the relationships among cetacean families.

The evolution of cetaceans is thought to have proceeded in modern day Pakistan during the Eocene epoch (56–34 mya), the second epoch of the Paleogene period of the present-extending Cenozoic Era. Specifically, cetaceans are thought to have transitioned from land to water at the tailend of the Paleocene–Eocene transition about 56 Mya, which was marked by increased global temperatures of 5.6°C, warming the earth’s oceans.

Cetaceans are fully aquatic mammals belonging to the order Artiodactyla. Molecular and morphological analyses suggest that they share a relatively recent closest common ancestor with hippopotamuses – their sister group, diverging about 55.5 Mya. Cetacea completely branched off from other artiodactyls around 50 mya. Research conducted in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea. Being mammals, they surface to breathe air; they have five finger bones (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retain many skeletal features from their terrestrial ungulata ancestors.

The two modern parvorders of cetaceans – Mysticeti (baleen whales) and Odontoceti (toothed whales) – are thought to have separated from each other around 28–33 mya in a second cetacean radiation, the first occurring with the archaeocetes. The adaptation of animal echolocation in toothed whales distinguishes them from fully aquatic archaeocetes and early baleen whales. The presence of baleen in baleen whales occurred gradually, with earlier varieties having very little baleen, and their size is linked to baleen dependence (and subsequent increase in filter feeding).

Early evolution

Main article: Archaeoceti

Cladogram showing the position of Cetacea within Artiodactylamorpha

Evolution of cetaceans

Indohyus (Raoellidae cca 49–48 Ma)

 

Pakicetus (Pakicetidae cca 49–48 Ma)

 

Ambulocetus (Ambulocetidae cca 49–48 Ma)

 

Kutchicetus (Remingtonocetidae cca 48 Ma)

 

Remingtonocetus (Remingtonocetidae cca 48 Ma)

 

Maiacetus (Protocetidae cca 47.5 Ma)

 

Rodhocetus (Protocetidae cca 45 Ma)

 

Dorudon (Basilosauridae cca 35 Ma)

 

Squalodon (Squalodontidae, Odontoceti cca 25 Ma)

 

Kentriodon (Kentriodontidae. Odontoceti cca 20 Ma)

 

Aulophyseter (Physeteridae, Odontoceti cca 20 Ma)

 

Brygmophyseter (Physeteridae, Odontoceti cca 15 Ma)

 

Aetiocetus (Aetiocetidae, Mysticeti cca 27 Ma)

 

Janjucetus (Mammalodontidae, Mysticeti cca 25 Ma)

 

Cetotherium (Cetotheriidae, Mysticeti cca 18 Ma)

The aquatic lifestyle of cetaceans first began in the Indian subcontinent from even-toed ungulates 50 million years ago, with this initial stage lasting approximately 4-15 million years. Archaeoceti is an extinct parvorder of Cetacea containing ancient whales. The traditional hypothesis of cetacean evolution, first proposed by Van Valen in 1966, was that whales were related to the mesonychians, an extinct order of carnivorous ungulates (hoofed animals) that resembled wolves with hooves and were a sister group of the artiodactyls (even-toed ungulates). This hypothesis was proposed due to similarities between the unusual triangular teeth of the mesonychians and those of early whales. However, molecular phylogeny data indicates that whales are very closely related to the artiodactyls, with hippopotamuses as their closest living relative. Because of this observation, cetaceans and hippopotamuses are placed in the same suborder, Whippomorpha. Cetartiodactyla (formed from the words Cetacea and Artiodactyla) is a proposed name for an order that includes both cetaceans and artiodactyls. However, the earliest anthracotheres, the ancestors of hippos, do not appear in the fossil record until the Middle Eocene, millions of years after Pakicetus, whereas the first known whale ancestor appeared during the Early Eocene; this difference in timing implies that the two groups diverged well before the Eocene. Molecular analysis identifies artiodactyls as being very closely related to cetaceans, so mesonychians are probably an offshoot from Artiodactyla, and cetaceans did not derive directly from mesonychians, but the two groups may share a common ancestor.

The molecular data are supported by the discovery of Pakicetus, the earliest archaeocete. The skeletons of Pakicetus show that whales did not derive directly from mesonychians. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychians. Archaeocetes retained aspects of their mesonychian ancestry (such as the triangular teeth) which modern artiodactyls, and modern whales, have lost. The earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, and today's artiodactyls and perissodactyls became herbivores later in their evolution. Whales, however, retained their carnivorous diet because prey was more available and they needed higher caloric content in order to live as marine endotherms (warm-blooded). Mesonychians also became specialized carnivores, but this was likely a disadvantage because large prey was uncommon. This may be why they were out-competed by better-adapted animals like the hyaenodontids and later Carnivora.

Raoellidae

Main article: Raoellidae

Raoellidae (/reɪoʊˈɛlɪdeɪ/) is a family of extinct semiaquatic artiodactyls from the middle Eocene (about 48 million years ago) that were not cetaceans, but closely related to cetaceans. Raoellidae has been recovered as the family most closely related to Cetacea in multiple phylogenetic analyses. They are known primarily from northern India and Pakistan, although potential remains of the raoellid Khirtharia have been found in northern China. They are relatively small animals; on average, raoellids were the size of a red fox. However, Khirtharia major, at about twice the size of an average-sized raoellid, would have been approximately the size of a coyote. Meanwhile, the smallest raoellid, Metkatius, was roughly the size of a house cat. Raoellidae is of particular importance to the understanding of cetacean evolution due to representing a transitional form between fully-terrestrial artiodactyls and the semi-aquatic pakicetids. The raoellid that has been most thoroughly studied and most heavily contributed to the knowledge of Raoellidae is Indohyus. It showed signs of adaptations to aquatic life, including dense limb bones that reduce buoyancy so that they could stay underwater, which are similar to the adaptations found in modern aquatic mammals such as the hippopotamus. This suggests a similar survival strategy to the African chevrotain or water chevrotain which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.

Possible relationships between cetaceans and other ungulate groups.

Pakicetidae

Main article: Pakicetidae

The pakicetids were digitigrade hoofed mammals that are thought to be the earliest known cetaceans, with Raoellidae being the closest sister group. They lived in the early Eocene, around 50 million years ago. Their fossils were first discovered in North Pakistan in 1979, located at a river not far from the shores of the former Tethys Sea. After the initial discovery, more fossils were found, mainly in the early Eocene fluvial deposits in northern Pakistan and northwestern India. Based on this discovery, pakicetids most likely lived in an arid environment with ephemeral streams and moderately developed floodplains millions of years ago. By using stable oxygen isotopes analysis, they were shown to drink fresh water, implying that they lived around freshwater bodies. Their diet probably included land animals that approached water for drinking or some freshwater aquatic organisms that lived in the river. The elongated cervical vertebrae and the four, fused sacral vertebrae are consistent with artiodactyls, making Pakicetus one of the earliest fossils to be recovered from the period following the Cetacea/Artiodactyla divergence event.

Pakicetids are classified as cetaceans mainly due to the structure of the auditory bulla (ear bone), which is formed only from the ectotympanic bone. The shape of the ear region in pakicetids is highly unusual and the skull is cetacean-like, although a blowhole is still absent at this stage. The jawbone of pakicetids also lacks the enlarged space (mandibular foramen) that is filled with fat or oil, which is used in receiving underwater sound in modern cetaceans. They have dorsal orbits (eye sockets facing up), which are similar to crocodiles. This eye placement helps submerged predators observe potential prey above the water. According to a 2009 study, the teeth of pakicetids also resemble the teeth of fossil whales, being less like a dog's incisors, and having serrated triangular teeth, which is another link to more modern cetaceans. It was initially thought that the ears of pakicetids were adapted for underwater hearing, but, as would be expected from the anatomy of the rest of this creature, the ears of pakicetids are specialized for hearing on land. However, pakicetids were able to listen underwater by using enhanced bone conduction, rather than depending on the tympanic membrane like other land mammals. This method of hearing did not give directional hearing underwater.

Pakicetids have long thin legs, with relatively short hands and feet which suggest that they were poor swimmers. To compensate for that, their bones are unusually thick (osteosclerotic), which is probably an adaptation to make the animal heavier to counteract the buoyancy of the water. According to a 2001 morphological analysis by Thewissen et al., pakicetids display no aquatic skeletal adaptation; instead they display adaptations for running and jumping. Hence pakicetids were most likely aquatic waders.

Ambulocetidae

Main article: Ambulocetidae

Ambulocetus, which lived about 49 million years ago, was discovered in Pakistan in 1994. They were vaguely crocodile-like mammals, possessing large brevirostrine jaws. In the Eocene, ambulocetids inhabited the bays and estuaries of the Tethys Sea in northern Pakistan. The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and littoral mollusks. Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed a range of water with different degrees of salinity, with some specimens having no evidence of sea water consumption and others that did not ingest fresh water at the time when their teeth were fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations. Hence, ambulocetids represent a transition phase of cetacean ancestors between fresh water and marine habitat.

The mandibular foramen in ambulocetids had increased in size, which indicates that a fat pad was likely to be housed in the lower jaw. In modern toothed whales, this fat pad in the mandibular foramen extends posteriorly to the middle ear. This allows sounds to be received in the lower jaw, and then transmitted through the fat pad to the middle ear. Similar to pakicetids, the orbits of ambulocetids are on the top of the skull, but they face more laterally than in pakicetids.

Ambulocetids had relatively long limbs with particularly strong hind legs, and they retained a tail with no sign of a fluke. The hindlimb structure of Ambulocetids shows that their ability to engage in terrestrial locomotion was significantly limited compared to that of contemporary terrestrial mammals, and likely did not come to land at all. The skeletal structures of the knee and ankle indicates that the motion of the hindlimbs was restricted into one plane. This suggests that, on land, propulsion of the hindlimbs was powered by the extension of dorsal muscles. They probably swam by pelvic paddling (a way of swimming which mainly utilizes their hind limbs to generate propulsion in water) and caudal undulation (a way of swimming which uses the undulations of the vertebral column to generate force for movements), as otters, seals and modern cetaceans do. This is an intermediate stage in the evolution of cetacean locomotion, as modern cetaceans swim by caudal oscillation (a way of swimming similar to caudal undulation, but is more energy efficient).

A recent study suggests that ambulocetids were fully aquatic like modern cetaceans, possessing a similar thoracic morphology and being unable to support their weight on land. This suggests that complete abandonment of the land evolved much earlier among cetaceans than previously thought. However the scientists involved in the study cautioned that the study was limited by a lack of information on the exact density of the bone, the location of the centre of mass, and the reliance of false ribs for thoracic support.

Remingtonocetidae

Main article: Remingtonocetidae

Remingtonocetids lived in the Middle-Eocene in South Asia, about 49 to 43 million years ago. Compared to family Pakicetidae and Ambulocetidae, Remingtonocetidae was a diverse family found in north and central Pakistan and western India. Remingtonocetids were also found in shallow marine deposits, but they were obviously more aquatic than ambulocetidae. This is demonstrated by the recovery of their fossils from a variety of coastal marine environments, including near-shore and lagoonal deposits. According to stable oxygen isotopes analysis, most remingtonocetids did not ingest fresh water, and had hence lost their dependency on fresh water relatively soon after their origin.

The orbits of remingtonocetids faced laterally and were small. This suggests that vision was not an important sense for them. The nasal opening, which eventually becomes the blowhole in modern cetaceans, was located near the tip of the snout. The position of the nasal opening had remained unchanged since pakicetids. One of the notable features in remingtonocetids is that the semicircular canals, which are important for balancing in land mammals, had decreased in size. This reduction in size had closely accompanied the cetacean radiation into marine environments. According to a 2002 study done by Spoor et al., this modification of the semicircular canal system may represent a crucial 'point of no return' event in early cetacean evolution, which excluded a prolonged semi-aquatic phase.

Compared to ambulocetids, remingtonocetids had relatively short limbs. Based on their skeletal remains, remingtonocetids were probably amphibious cetaceans that were well adapted to swimming, and likely to swim by caudal undulation only.

Protocetidae

Main article: Protocetidae

The protocetids form a diverse and heterogeneous group known from Asia, Europe, Africa, and North America. They lived in the Eocene, approximately 48 to 35 million years ago. The fossil remains of protocetids were uncovered from coastal and lagoonal facies in South Asia; unlike previous cetacean families, their fossils uncovered from Africa (e.g., Phiomicetus, Protocetus, Aegyptocetus, Togocetus) and North America (e.g., Georgiacetus) also include open marine forms. They were probably amphibious, but more aquatic compared to remingtonocetids. Protocetids were the first cetaceans to leave the Indian subcontinent and disperse to all shallow subtropical oceans of the world. There were many genera among the family Protocetidae. There were different degrees of aquatic adaptations in this group, with some able to support their weight on land, and others not. The discovery of the southeastern Pacific Peregocetus indicates they crossed the Atlantic and achieved circum-equatorial distribution by 40 mya. Their amphibious nature is supported by the discovery of a pregnant Maiacetus, in which the fossilised fetus was positioned for a head-first delivery, suggesting that Maiacetus gave birth on land. If they gave birth in the water, the fetus would be positioned for a tail-first delivery to avoid drowning during birth.

Unlike remingtonocetids and ambulocetids, protocetids have large orbits which are oriented laterally. Increasingly lateral-facing eyes might be used to observe underwater prey, and are similar to the eyes of modern cetaceans. Furthermore, the nasal openings were large and were halfway up the snout. The great variety of teeth suggests diverse feeding modes in protocetids. In both remingtonocetids and protocetids, the size of the mandibular foramen had increased. The large mandibular foramen indicates that the mandibular fat pad was present. However the air-filled sinuses that are present in modern cetaceans, which function to isolate the ear acoustically to enable better underwater hearing, were still not present. The external auditory meatus (ear canal), which is absent in modern cetaceans, was also present. Hence, the method of sound transmission that were present in them combines aspects of pakicetids and modern odontocetes (toothed whales). At this intermediate stage of hearing development, the transmission of airborne sound was poor due to the modifications of the ear for underwater hearing while directional underwater hearing was also poor compared to modern cetaceans.

Some protocetids had short, wide fore- and hindlimbs that were likely to have been used in swimming, but the limbs gave a slow and cumbersome locomotion on land. It is possible that some protocetids had flukes. However, it is clear that they were adapted even further to an aquatic life-style. In Rodhocetus, for example, the sacrum (a bone that, in land-mammals, is a fusion of five vertebrae that connects the pelvis with the rest of the vertebral column) was divided into loose vertebrae. However, the pelvis was still connected to one of the sacral vertebrae. The ungulate ancestry of these archaeocetes is still underlined by characteristics like the presence of hooves at the ends of the toes in Rodhocetus.

The foot structure of Rodhocetus shows that protocetids were predominantly aquatic. A 2001 study done by Gingerich et al. hypothesized that Rodhocetus locomoted in the oceanic environment similarly to how ambulocetids pelvic paddling, which was supplemented by caudal undulation. Terrestrial locomotion of Rodhocetus was very limited due to their hindlimb structure. It is thought that they moved in a way similar to how eared seals move on land, by rotating their hind flippers forward and underneath their body.

Basilosauridae

Main articles: Basilosauridae and Dorudontinae

Archaeocetes (like this Basilosaurus) had a heterodont dentition

Basilosaurids and dorudontines lived together in the late Eocene around 41 to 33.9 million years ago, and are the oldest known obligate aquatic cetaceans. They were fully recognizable whales which lived entirely in the ocean. This is supported by their fossils usually found in deposits indicative of fully marine environments, lacking any freshwater influx. They were probably distributed throughout the tropical and subtropical seas of the world. Basilosaurids are commonly found in association with dorudontines, and were closely related to one another. The fossilised stomach contents in one basilosaurid indicates that it ate fish.

Although they look very much like modern cetaceans, basilosaurids lacked the 'melon organ' that allows toothed whales to use echolocation. They had small brains; this suggests they were solitary and did not have the complex social structures of some modern cetaceans. The mandibular foramen of basilosaurids covered the entire depth of the lower jaw as in modern cetaceans. Their orbits faced laterally, and the nasal opening had moved even higher up the snout, closer to the position of the blowhole in modern cetaceans. Furthermore, their ear structures were functionally modern, with the insertion of air-filled sinuses between ear and skull. Unlike modern cetaceans, basilosaurids retained a large external auditory meatus.

Both basilosaurids have skeletons that are immediately recognizable as cetaceans. A basilosaurid was as big as the larger modern whales, with genera like Basilosaurus reaching lengths of up to 60 ft (18 m) long; dorudontines were smaller, with genera like Dorudon reaching about 15 ft (4.6 m) long. The smallest basilosaurid whale is Tutcetus and measures 8.2 ft (2.5 m) long. The large size of basilosaurids is due to the extreme elongation of their lumbar vertebrae. They had a tail fluke, but their body proportions suggest that they swam by caudal undulation and that the fluke was not used for propulsion. In contrast, dorudontines had a shorter but powerful vertebral column. They too had a fluke and, unlike basilosaurids, they probably swam similarly to modern cetaceans, by using caudal oscillations. The forelimbs of basilosaurids were probably flipper-shaped, and the external hind limbs were tiny and were certainly not involved in locomotion. Their fingers, however, retained the mobile joints of their ambulocetid relatives. The two tiny but well-formed hind legs of basilosaurids were probably used as claspers when mating. The pelvic bones associated with these hind limbs were not connected to the vertebral column as they were in protocetids. Essentially, any sacral vertebrae can no longer be clearly distinguished from the other vertebrae.

Both basilosaurids and dorudontines are relatively closely related to modern cetaceans, which belong to parvorders Odontoceti and Mysticeti. However, according to a 1994 study done by Fordyce and Barnes, the large size and elongated vertebral body of basilosaurids preclude them from being ancestral to extant forms. As for dorudontines, there are some species within the family that do not have elongated vertebral bodies, which might be the immediate ancestors of Odontoceti and Mysticeti. The other basilosaurids became extinct.

Evolution of modern cetaceans

Baleen whales

Main article: Baleen whale § Evolutionary history

Artistic impression of two Eobalaenoptera pursued by the giant shark Otodus megalodon

All modern baleen whales or mysticetes are filter-feeders which have baleen in place of teeth, though the exact means by which baleen is used differs among species (gulp-feeding within balaenopterids, skim-feeding within balaenids, and bottom plowing within eschrichtiids). The first members of both groups appeared during the middle Miocene. Filter feeding is very beneficial as it allows baleen whales to efficiently gain huge energy resources, which makes the large body size in modern varieties possible. The development of filter feeding may have been a result of worldwide environmental change and physical changes in the oceans. A large-scale change in ocean current and temperature could have contributed to the radiation of modern mysticetes. The earlier varieties of baleen whales, or "archaeomysticetes", such as Janjucetus and Mammalodon had very little baleen and relied mainly on their teeth.

There is also evidence of a genetic component of the evolution of toothless whales. Multiple mutations have been identified in genes related to the production of enamel in modern baleen whales. These are primarily insertion/deletion mutations that result in premature stop codons. It is hypothesized that these mutations occurred in cetaceans already possessing preliminary baleen structures, leading to the pseudogenization of a "genetic toolkit" for enamel production. Recent research has also indicated that the development of baleen and the loss of enamel-capped teeth both occurred once, and both occurred on the mysticete stem branch.

Generally it is speculated the four modern mysticete families have separate origins among the cetotheres. Modern baleen whales, Balaenopteridae (rorquals and humpback whale, Megaptera novaengliae), Balaenidae (right whales), Eschrichtiidae (gray whale, Eschrictius robustus), and Neobalaenidae (pygmy right whale, Caperea marginata) all have derived characteristics presently unknown in any cetothere and vice versa (such as a sagittal crest). Mysticetes are also known for their gigantism, as baleen whales are among the largest organisms to ever have lived; they reach lengths greater than 20 m and weigh more than 100,000 kg. This gigantism is directly related to their feeding mechanism – mysticete size has been found to be dependent on the amount of baleen a mysticete can use to filter its prey. Additionally, size is a positively selected trait that gives mysticetes a boost in fitness. Mysticete populations will therefore slowly become even more gigantic as whales with larger amounts of baleen are selected.

Toothed whales

The adaptation of echolocation occurred when toothed whales (Odontoceti) split apart from baleen whales, and distinguishes modern toothed whales from fully aquatic archaeocetes. This happened around 34 million years ago in a second cetacean radiation. Modern toothed whales do not rely on their sense of sight, but rather on their sonar to hunt prey. Echolocation also allowed toothed whales to dive deeper in search of food, with light no longer necessary for navigation, which opened up new food sources. Toothed whales echolocate by creating a series of clicks emitted at various frequencies. Sound pulses are emitted, reflected off objects, and retrieved through the lower jaw. Skulls of Squalodon show evidence for the first hypothesized appearance of echolocation. Squalodon lived from the early to middle Oligocene to the middle Miocene, around 33–14 million years ago. Squalodon featured several commonalities with modern toothed whales: the cranium was well compressed (to make room for the melon, a part of the nose), the rostrum telescoped outward into a beak, a characteristic of the modern toothed whales that gave Squalodon an appearance similar to them. However, it is thought unlikely that squalodontids are direct ancestors of modern toothed whales.

The first oceanic dolphins such as kentriodonts, evolved in the late Oligocene and diversified greatly during the mid-Miocene. The first fossil cetaceans near shallow seas (where porpoises inhabit) were found around the North Pacific; species like Semirostrum were found along California (in what were then estuaries). These animals spread to the European coasts and Southern Hemisphere only much later, during the Pliocene. The earliest known ancestor of arctic whales is Denebola brachycephala from the late Miocene around 9–10 million years ago. A single fossil from Baja California indicates the family once inhabited warmer waters.

Acrophyseter skull

Ancient sperm whales differ from modern sperm whales in tooth count and the shape of the face and jaws. For example, Scaldicetus had a tapered rostrum. Genera from the Oligocene and Miocene had teeth in their upper jaws. These anatomical differences suggest that these ancient species may not have necessarily been deep-sea squid hunters like the modern sperm whale, but that some genera mainly ate fish. Contrary to modern sperm whales, most ancient sperm whales were built to hunt whales. Livyatan had a short and wide rostrum measuring 10 feet (3.0 m) across, which gave the whale the ability to inflict major damage on large struggling prey, such as other early whales. Species like these are collectively known as killer sperm whales or macroraptorial sperm whales.

Beaked whales consist of over 20 genera. Earlier varieties were probably preyed upon by killer sperm whales and large sharks such as megalodon. In 2008, a large number of fossil ziphiids were discovered off the coast of South Africa, confirming the remaining ziphiid species might just be a remnant of a higher diversity that has since gone extinct. After studying numerous fossil skulls, researchers discovered the absence of functional maxillary teeth in all South African ziphiids, which is evidence that suction feeding had already developed in several beaked whale lineages during the Miocene. Extinct ziphiids also had robust skulls, suggesting that tusks were used for male-male interactions.

Skeletal evolution

Dolphins (aquatic mammals) and ichthyosaurs (extinct marine reptiles) share a number of unique adaptations for fully aquatic lifestyle and are frequently used as extreme examples of convergent evolution

Modern cetaceans have internal, rudimentary hind limbs, such as reduced femurs, fibulas, and tibias, and a pelvic girdle. Indohyus has a thickened ectotympanic internal lip of the ear bone. This feature compares directly to that of modern cetaceans. Another similar feature was the composition of the teeth, which contained mostly calcium phosphate which is needed for eating and drinking by aquatic animals, though, unlike modern day toothed whales, they had a heterodont (more than one tooth morphology) dentition as opposed to a homodont (one tooth morphology present) dentition. Although they somewhat resembled a wolf, the fossils of pakicetids showed the eye sockets were much closer to the top of their head than that of other terrestrial mammals, but similar to the structure of the eyes in cetaceans. Their transition from land to water led to reshaping of the skull and food processing equipment because the eating habits were changing. The change in position of the eyes and limb bones is associated with the pakicetids becoming waders. The ambulocetids also began to develop long snouts, which is seen in current cetaceans. Their limbs (and hypothesized movement) were very similar to otters.

The skeleton of a bowhead whale with the hind limb and pelvic bone structure circled in red. This bone structure stays internal during the entire life of the species.

Limblessness in cetaceans does not represent a regression of fully formed limbs nor the absence of limb bud initiation, but rather arrest of limb bud development. Limb buds develop normally in cetacean embryos. Limb buds progress to the condensation phase of early skeletogenesis, where nerves grow into the limb bud and the apical ectodermal ridge (AER), a structure that ensures proper limb development, appears functional. Occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs (atavism).

Pakicetus attocki skeleton

Pakicetus had a pelvic bone most similar to that of terrestrial mammals. In later species, such as Basilosaurus, the pelvic bone, no longer attached to the vertebrae and the ilium, was reduced. Certain genes are believed to be responsible for the changes that occurred to the cetacean pelvic structure, such as BMP7, PBX1, PBX2, PRRX1, and PRRX2. The pelvic girdle in modern cetaceans were once thought to be vestigial structures that served no purpose at all. The pelvic girdle in male cetaceans is different in size compared to females, and the size is thought to be a result of sexual dimorphism. The pelvic bones of modern male cetaceans are more massive, longer, and larger than those of females. Due to the sexual dimorphism displayed, they were most likely involved in supporting male genitalia that remain hidden behind abdominal walls until sexual reproduction occurs.

Early archaeocetes such as Pakicetus had the nasal openings at the end of the snout, but in later species such as Rodhocetus, the openings had begun to drift toward the top of the skull. This is known as nasal drift. The nostrils of modern cetaceans have become modified into blowholes that allow them to break to the surface, inhale, and submerge with convenience. The ears began to move inward as well, and, in the case of Basilosaurus, the middle ears began to receive vibrations from the lower jaw. Today's modern toothed whales use their melon organ, a pad of fat, for echolocation.

Genetics insights

The time of early cetacean evolution is consistent with dN/dS analysis estimates of cetacean UCP-1 gene inactivation, likely relaxing selection pressures against UCP-1, leading to pseudogenization. UCP-1 inactivation in cetaceans initiated the shift from heat production to heat conservation through blubber formation/thermal insulation as evidenced by a transgenic mouse model simulating cetacean-like UCP1 gene inactivation. While UCP1 pseudogenization initiated the development of these phenotypes, they weren't solidified until subsequent ADRB3 inactivation during the Eocene-Oligocene transition, which was marked by an estimated 4°C drop in benthic temperatures. ADRB3 is expressed primarily in adipocytes and functions primarily as a key regulator of lipolysis. Global cooling during the Eocene-Oligocene transition likely selected against ADRB3, favoring more robust thermal insulation provided by dense blubber formation and gigantism, and driving the modern cetacean phenotype.

Radiation events

There are three major radiation events that mark diversification and speciation in the evolutionary history of Cetacea. The first occurred around the middle Eocene (40 Mya) when these early cetaceans abandoned riverine and shallow coastal habitats, setting the scene for Protocetidae – the first fully marine cetacean. With the oceans and its nutrients at their disposal, rapidly diversifying protocetids were also responsible for the first major geographic expansion, dispersing throughout North Africa, Europe, and North America. The second of three major radiation events occurred near the start of the Oligocene (~34 Mya) when Neoceti diverged from Basilosauridae. This radiation event concurrently occurs with the breakup of Gondwana and the opening of the Southern Ocean, wildly changing ocean ecosystems, productivity, and temperature gradients. The timing of this second radiation event is not coincidental, as the following diversification of cetaceans was likely due to new ecological opportunities the change in oceans gave them. The final major radiation event, occurring throughout the middle Miocene and into the Pliocene (12 Mya to 2 Mya), was not due to a specific event but is associated with widespread generic expansion of odontocetes and mysticetes. Some modern genera of cetaceans began to emerge, including Balaenoptera, a genus of rorquals that includes the blue whale. Delphinidae, ocean dolphins, also arose during this radiation event in the late Miocene.

Ongoing evolution

Culture

Culture is group-specific behavior transferred by social learning. Tool use to aid with foraging is one example. Whether or not a dolphin uses a tool affects its eating behavior, which causes differences in diet. Also, using a tool allows a new niche and new prey to open up for that particular dolphin. Due to these differences, fitness levels change within the dolphins of a population, which further causes evolution to occur in the long run. Culture and social networks have played a large role in the evolution of modern cetaceans, as concluded in studies showing dolphins preferring mates with the same socially learned behaviors, and humpback whales using songs between breeding areas. For dolphins particularly, the largest non-genetic effects on their evolution are due to culture and social structure.

Based on a 2014 study, the population of Indo-Pacific bottlenose dolphins (Tursiops sp.) around Shark Bay of Western Australia can be divided into spongers and nonspongers. Spongers put sea sponges on their snouts as protection against abrasions from sharp objects, stingray barbs, or toxic organisms. The sponges also help the dolphins target fish without swim bladders, since echolocation cannot detect these fish easily against a complex background. Spongers also specifically forage in deep channels, but nonspongers are found foraging in both deep and shallow channels. This foraging behavior is mainly passed on from mother to child. Therefore, since this is a group behavior being passed down by social learning, this tool use is considered a cultural trait.

Researchers in a 2014 study in Shark Bay found the fatty acid analyses between the West and East Gulf populations to differ, which is due to the two areas having different food sources. However, when comparing data from within the West Gulf, the spongers vs. the nonspongers in the deep channels had very different fatty acid results even though they are in the same habitat. Nonspongers from deep and shallow channels had similar data. This suggests that sponging was the cause of the different data and not the deep vs. shallow channels. Sponging opened up a new niche for the dolphins and allowed them access to new prey, which caused long-term dietary changes. By producing different food sources within a population, there is less intrapopulation competition for resources, showing character displacement. As a result, the carrying capacity increases since the entire population does not depend on one food source. The fitness levels within the population also change, thus allowing this culture to evolve.

Social structure

Social structure forms groups with individuals that interact with one another, and this allows for cultural traits to emerge, exchange, and evolve. This relationship is especially seen in the bottlenose dolphin populations in southwestern Australia, which have been known to beg for food from fishermen. This begging behavior was spread through the population due to individual (dolphins spending time around boats) and social (dolphins spending time with other dolphins who express begging behavior) learning.

Culture can, however, impact social structure by causing behavior matching and assortive mating. Individuals within a certain culture are more likely to mate with individuals using the same behaviors rather than a random individual, thus influencing social groups and structure. For example, the spongers of Shark Bay preferentially stick with other spongers. Also, some bottlenose dolphins in Moreton Bay, Australia followed prawn trawlers to feed on their debris, while other dolphins in the same population did not. The dolphins preferentially associated with individuals with same behavior even though they all lived in the same habitat. Later on, prawn trawlers were no longer present, and the dolphins integrated into one social network after a couple of years.

Social networks can still affect and cause evolution on their own by impending fitness differences on individuals. According to a 2012 study, male calves had a lower survival rate if they had stronger bonds with juvenile males. However, when other age and sex classes were tested, their survival rate did not significantly change. This suggests that juvenile males impose a social stress on their younger counterparts. In fact, it has been documented that juvenile males commonly perform acts of aggression, dominance, and intimidation against the male calves. According to a 2010 study, certain populations of Shark Bay dolphins had varying levels of fitness and calf success. This is either due to social learning (whether or not the mother passed on her knowledge of reproductive ability to the calves), or due to the strong association between mother dolphins in the population; by sticking in a group, an individual mother does not need to be as vigilant all the time for predators.

Genetic studies conducted on Clymene dolphins (Stenella clymene) focused on their natural histories, and the results show that the origin of the species was actually an outcome of hybrid speciation. Hybridization between spinner dolphins (Stenella longirostris) and striped dolphins (Stenella coeruleoalba) in the North Atlantic was caused by constant habitat sharing of the two species. Relationships between these three species had been speculated according to notable resemblances between anatomies of the Clymene and the spinner dolphins, resulting in the former being regarded as subspecies of the latter until 1981, and the possibility of the Clymene dolphin as a hybrid between the spinner and the striped dolphins have come to question based on anatomical and behavioral similarities between these two species.

Environmental factors

Genome sequences done in 2013 revealed that the Yangtze river dolphin, or "baiji" (Lipotes vexillifer), lacks single nucleotide polymorphisms in its genome. After reconstructing the history of the baiji genome for this dolphin species, researchers found that the major decrease in genetic diversity occurred most likely due to a bottleneck event during the last deglaciation event. During this time period, sea levels were rising while global temperatures were increasing. Other historical climate events can be correlated and matched with the genome history of the Yangtze river dolphin as well. This shows how global and local climate change can drastically affect a genome, leading to changes in fitness, survival, and evolution of a species.

The European population of common dolphins (Delphinus delphis) in the Mediterranean have differentiated into two types: eastern and western. According to a 2012 study, this seems to be due to a recent bottleneck as well, which drastically decreased the size of the eastern Mediterranean population. Also, the lack of population structure between the western and eastern regions seems contradictory of the distinct population structures between other regions of dolphins. Even though the dolphins in the Mediterranean area had no physical barrier between their regions, they still differentiated into two types due to ecology and biology. Therefore, the differences between the eastern and western dolphins most likely stems from highly specialized niche choice rather than just physical barriers. Through this, environment plays a large role in the differentiation and evolution of this dolphin species.

The divergence and speciation within bottlenose dolphins has been largely due to climate and environmental changes over history. According to research, the divisions within the genus correlate with periods of rapid climate change. For example, the changing temperatures could cause the coast landscape to change, niches to empty up, and opportunities for separation to appear. In the Northeast Atlantic, specifically, genetic evidence suggests that the bottlenose dolphins have differentiated into coastal and pelagic types. Divergence seems most likely due to a founding event where a large group separated. Following this event, the separate groups adapted accordingly and formed their own niche specializations and social structures. These differences caused the two groups to diverge and to remain separated.

Two endemic, distinctive types of short-finned pilot whale, Tappanaga (or Shiogondou) the larger, northern type and Magondou the smaller, southern type, can be found along the Japanese archipelago where distributions of these two types mostly do not overlap by the oceanic front border around the easternmost point of Honshu. It is thought that the local extinction of long-finned pilot whales in the North Pacific in the 12th century could have triggered the appearance of Tappanaga, causing short-finned pilot whales to colonize the colder ranges of the long-finned variant. Whales with similar characteristics to the Tappanaga can be found along Vancouver Island and northern US coasts as well.