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history:evolution_mammals

evolution of mammals

Introduction

  • mammals evolved 201-227 million years ago and are group under the synapsids which evolved separately to the reptiles and dinosaurs (and thus separate to birds) but from a common basal amniote ancestor
  • basal amniotes evolved from tetrapod amphibian ancestors around 315mya and are characterized by having eggs fertilized within the mother or laid on ground
  • mammals are characterized by:
    • the females having breasts which produce milk
    • brain has a neocortex
    • middle ear has 3 bones
    • skin has fur or hair
    • having only 16 nicotinic acetylcholine receptors (nAChR) instead of 19 as in ancestral vertebrates 1)
    • PACAP precursor (and the new PRP gene) is now on a different gene to the GHRH precursor gene (and the new C-peptide gene) whereas in non-mammals GHRH-like peptide and PACAP are located on the same precursor 2)
    • Placental development and genomic imprinting co-evolved with parental conflict over resource distribution to mammalian offspring (monotremes do not have genomic imprinting of IGF-2 gene but other mammals do)
  • endothermy - a key characteristic of mammals that is shared with birds is warm-bloodedness or endothermy which allowed them to be active day and night
    • endothermy was likely the starting point where mammalness evolved:
      • the acquisition of an insulating fur coat (probably via mutation of MSX2 occurred at the evolutionary root of the Probainognathia some 245 million years ago); the evolution of a larger brain, supplied with warmer blood; a faster reproduction rate; and a more active life are all defining mammalian traits that evolved because of warm-bloodedness.
      • endothermy appears to have appeared in mammalian ancestors some 33 million years prior to the origin of mammals, and endothermy evolved very quickly in geological terms, in less than a million years according to studies of the semicircular canal geometry:3)
      • The viscosity of endolymph in the semi-circular canals is critical to the balance organ’s ability to efficiently detect head rotation and aid balance. The endolymph’s viscosity would normally be altered by the evolution of a higher body temperature - in mammals, the canals adapt to higher body temperature by changing their geometry.
      • the earliest mammals were nocturnal and couldn’t rely on their eyes to feel their environment. So, together with hearing and smell, the presence of tactile hairs on the face and body of our nocturnal ancestors played an important role in their survival and stimulated the development of the neocortex. The development of the brain, an expensive organ that needs a lot of energy and produces a lot of heat, in turn played a role in the evolution of endothermy.4)
  • mammalian mucins mainly evolved via protein DNA gaining repeated sequences ('PTS repeats') with a high content of the amino acids proline, threonine and serine which converts it from a non-mucin protein to a mucin protein5)

brief ancestral tree

  • primordial RNA cell
    • bacteria
      • bacterial precursor of mitochondria
        • eukaryocytes evolve from endosymbiosis of procaryocytes with mitochondrial DNA creating complex cells oxidative mitochondrial capacity as well as anaerobic pathways (1,450 mya)
          • multi-celled organisms
            • Choanozoa (950mya)
              • Animals (760mya) - nearly all animals have a dramatically shrunken mitochondrial gene count of only 13 protein coding genes the other thousands have been relocated into the host nucleus DNA
                • Eumetazoa
                  • Parahoxozoa (680mya)
                    • Bilateria (bilaterally symmetric body plan arose c650mya)
                      • Nephrozoa (650mya; excretory organs and nerve cords - eg. early marine worms)
                        • Protostomia (610mya) (the first opening (the blastopore) becomes the mouth)
                          • Deuterostomia (the first opening (the blastopore) becomes the anus)
                            • Chordata (notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, a post-anal tail)
                              • two basal vertebrate tetraploidization events 1R/2R duplicating the genome
                                • Vertebrata (525mya)
                                  • jawed fish / gnathostomes
                                    • lobed fish
                                      • amphibia (c370mya)
                                        • tetrapod amphibia (c367mya) Devonian mass extinction 360mya
                                          • Reptiliomorpha
                                            • basal amniotes (c312mya eggs fertilized within the mother or laid on ground)
                                              • synapsids (earliest known is 318mya)
                                                • mammals (201-227mya)
                                                  • prototheria (monotremes)
                                                    • metatheria (marsupials)
                                                      • eutherian mammals
  • synapsids (earliest known is 318mya) the largest terrestrial vertebrates in the Permian period, 299-251mya
    • characterized by having differentiated teeth (canines, molars, incisors);
    • Eupelycosauria
      • Sphenacodontia
        • Therapsida
          • Anomodontia
            • Chainosauria
              • Dicynodont (270-201mya) survived Permian extinction; 70 genera of herbivores with tusks - the most abundant and diverse vertebrates on land just before dinosaur times but became extinct in late Triassic
          • Neotherapsida
            • Theriodontia
              • Eutheriodontia
                • Therocephalia - survived Permian extinction but became extinct in the Triassic
                • Cynodontia - survived Permian extinction
                  • Probainognathia
                    • mammals (201-227mya)
                      • prototheria (monotremes)
                        • metatheria (marsupials)
                          • eutherian mammals

main trees

  • prototheria
    • monotremes probably evolved around 220mya - perhaps they were the 1st mammals? (oldest fossil is 123mya)
      • egg laying mammals with bird-like ZW sex chromosomes (although echidna only has 4 Y chromosomes) and reptilian features including venomous spurs, and reptilian gait
    • eutherian sex chromosomes lie on autosomal chromosome 6, while sex determination appears to be more like birds
  • mammals with 2 pairs of XY sex chromosomes, a new SRY gene, a placenta, nipples, and give birth
    • mammals replace baby teeth with adult teeth only once unlike most other vertebrates
      • mammal teeth are attached to the jaw by gomphosis which is a ligament attachment whereas other vertebrate teeth are attached to the jaw by ankylosis, which is a hard-tissue fusion of bone to tooth
      • mammal's reduced rates of tooth replacement and a flexible ligament attaching the tooth to the jaw are needed for true tusks to evolve
    • theria c166mya
      • not egg laying; lost the coracoid bone
      • mammalian placentae are unique and characterized by trophoblast syncytialization (syncytial trophoblast), where adjacent cells fuse to produce cells with more than a single nucleus, which is accompanied by expression of an endogenous, retrovirus-derived syncytin protein
      • metatherians and eutherians probably diverged 148mya 6)
      • metatherians c125mya in Asia
        • give birth to relatively undeveloped young
        • marsupials (?100mya in Utah) 65mya (young are carried in a pouch)
          • in common with reptiles and monotremes, marsupials lack a corpus callosum and have a lower body temperature than therian mammals (marsupials are only 35degC, monotremes are 31degC)
          • unlike eutherians, they lack homologous regions in X and Y chromosomes
          • although the genes on the marsupial X are all homologous with genes on the human X, about a third of the X genes in eutherians are autosomal in marsupials
          • The Y chromosome of marsupials is very tiny because it is derived from the smaller X of ancient mammals which did not include the added region which eutherian X have, but it retains several genes, some shared with the human Y, others unique to marsupials
          • in those with karyotypes X0 and XXY, the formation of testes is determined by the Y-dominant mechanism, whereas the other traits characteristic of males and females depend on the X-chromosome dose
          • formation of a pouch in female or a scrotum in male depends on the X-chromosome dose
      • eutherian mammals c161mya?
        • amniotic sac surrounds the fetus, lack epipubic bones
        • have a corpus callosum and body temperature is higher at 37degC
        • presence of homologous regions in X and Y chromosomes - the so-called (“pseudoautosomal region”)(Burgoyne, 1982), in which conjugation and recombination take place during meiosis
        • the X is completely conserved in mammals with same order of genes in humans as with other eutherians
        • SRY on the Y chromosome is the mammalian sex determining gene, and drives the usual autosomal sex determination mechanisms in non-mammalian vertebrates (SOX9 - is highly conserved and is pivotal in sex determination in all vertebrates)7)
        • three ancestral blocks (different autosomal regions) that merged with the marsupial X / ancient X during the formation of the eutherian sex X chromosome have been described:
          • those arranged in the same order that in the chicken chromosome 1
          • genes in the ancient conserved region of the human X chromosome correspond to the genes in chromosome 4p of the chicken and microchromosomes of other birds as well as to the short arm of chromosome 8 of marine turtles
          • genes near ZFX on the short arm of the human X map, not to the X, but to chromosome 5 in kangaroos and chromosome 3 on the dunnart

monotremes

  • monotremes probably evolved around 220mya (oldest fossil is 123mya) and are egg laying mammals sharing some genes with birds (the vitellogenin genes, and they have 5 pairs of sex chromosomes and that one of the X chromosomes resembles the Z chromosome of birds, suggesting that the two sex chromosomes of marsupial and placental mammals evolved after the split from the monotreme lineage)
  • structural differences in their brains, jaws, digestive tract, reproductive tract, and other body parts compared to the more common mammalian types
    • in common with reptiles, they have a single duct (the cloaca) for their urinary, defecatory, and reproductive systems - only semen passes through the penis which is similar to the turtle; urine is excreted through the cloaca
    • lactation is via skin openings as they do not have nipples and their milk contains a highly expressed antibacterial protein not found in other mammals
    • in common with reptiles and marsupials, monotremes lack a corpus callosum - L&R hemisphere communication is via the anterior commissure which carries all the commissural fibers arising from the neocortex
    • have extra bones in the shoulder girdle, including an interclavicle and coracoid, which are not found in other mammals
    • retain a reptile-like gait, with legs on the sides of, rather than underneath, their bodies
    • the monotreme leg bears a spur in the ankle region; the spur is not functional in echidnas, but contains a powerful venom in the male platypus (many non-monotreme archaic mammal groups also possess venomous spurs)
    • remarkably low metabolic rate by mammalian standards with body temperature only 31degC although this may be a later adaptation to harsh environments
    • extant monotremes lack teeth as adults
  • ancestral venomous monotreme
    • platypus (17-89mya)
    • echidna (17-89mya)

metatherians

  • evolved c125mya in Asia (give birth to relatively undeveloped young)
  • marsupials (?100mya in Utah) 65mya (young are carried in a pouch)
    • Laurasian possums ⇒ North America ⇒ South America (connected to Nth America until 65mya)
      • Ameridelphia (American marsupials - oppossums)
      • Australidelphia (arose in Sth America, there is one species in Sth America, then migrated across a temperate Antarctica to Australia c55mya)
        • Australian megafauna (now mostly extinct)
        • Diprotodontia earliest fossil is 23-28mya but probably evolved long before this
          • Vombatiformes
            • Mukupirna (25mya); koala, wombats, marsupial lions, marsupial tapirs, giant wombats
          • Phalangeriformes
            • possums
          • Macropodiformes
            • potoroos, kangaroos, wallabies
        • Peramelemorphia omnivorous bandicoots and bilbies
        • Notoryctemorphia marsupial moles
        • Dasyuromorphia
          • most of the Australian carnivorous marsupials
          • quolls, dunnarts, the numbat, the Tasmanian devil, and the extinct thylacine

eutherian mammals

  • evolved around 161mya
  • characterized by having an amniotic sac which surrounds the fetus and a lack epipubic bones
  • from 55mya to 34mya, Western Europe and Eastern Asia formed two distinct land masses with very different mammalian faunas:
    • European forests were home to endemic fauna such as Palaeotheres (an extinct group distantly related to present-day horses, but more like today’s tapirs)
    • Asia was populated by a more diverse fauna including the mammal families found today on both continents
  • 34mya, Western Europe was colonised by Asian species, leading to a major renewal of vertebrate fauna and the extinction of its endemic mammals, a sudden event called the ‘Grande Coupure’ perhaps via traversing “Balkanatolia”, a forgotten continent which today covers the present-day Balkans and Anatolia

atlantogenata (105mya)

  • originated and radiated in the South American and African continents
  • afrotheria: aardvarks, elephants, sea cows, dugongs
  • xenarthra: anteaters, armadillos in the Americas 59mya

boreoeutheria

  • “northern beasts” evolved 80-100mya most males have external testes;
  • Laurasiatheria evolved 99mya on the continent of Laurasia
    • Eulipotyphla (hedgehogs, shrews, moles)
    • Scrotifera (males have a scrotum)
      • Chiroptera: bats and flying foxes
      • Ferungulata
        • Ungulates
          • Cetartiodactyla (even-toed ungulates)
            • suinea (pigs, hippopotamus)
              • Cetacea (45-53mya): 4 legged aquatic whale-like species
                • toothed whales (40mya - mass extinction toothed baleen whales 23mya8))
                  • dolphins, porpoises
                • baleen whales (25mya)
            • Selenodont (camels, giraffe, deer, antelope, cattle)
              • Giraffoidea (giraffes)
              • cervoidea (deer)
              • caprinae
                • sheep
                • goats
                • ibex
              • bovinae
                • antelopes (16-18mya)
                • African buffalo
                • Bubalina genus (13,7mya)
                  • water buffalo (India)
                • Bovina genus (13.7mya)
                  • steppe bison Bison priscus (2-5mya)
                    • European bison (wisent) theoretically the result of hybridization between Bison priscus and the ancestors of the aurochs (Bos primigenius) which apart from the lowland European bison, became extinct in 1927
                    • yak
                    • cross to America via Bering land bridge 1-0.22mya to evolve into:
                      • Bison latifrons (giant bison or longhorn bison) 0.5mya but became extinct during the Quaternary extinction event 21,000-30,000yrs ago
                      • Bison antiquus 0.25mya
                        • B. occidentalis the modern American bison 5,000-10,000yrs ago
                  • Eurasian auroch (2mya in India then migrated to the Near East 270,000yrs ago and to Africa c25,000yrs ago; by the 13thC, their range was reduced to around the Poland region where they became extinct in 1627, but were the ancestor of all domesticated cattle and possibly, the European bison)
                    • human domestication in southern Asia results in the drought-tolerant South Asian subspecies of zebu cattle Bos indicus c8,000-10,000 yrs ago
                    • human domestication in the Near East leads to the Eurasian taurine subpecies c8,000-10,000 yrs ago
                    • (presumably) humans cross Bos taurus cattle of European origin with zebu cattle to form modern Middle Eastern cattle herds 4,000yrs ago
            • Perissodactyla: odd-toed ungulates, including horses, donkeys, zebras, rhinoceros
              • humans first domesticated donkeys (E. africanus) c5000BC in NE Africa
                • the oldest human-bred hybrid animal was the sterile kunga in Sumerian Mesopotamia c3200BC, a donkey cross-bred with a male Syrian wild arse (E. hemionus), the kunga hybrid was up to 6x more valuable than a donkey as they were stronger for towing vehicles, whereas the arse could not be domesticated
                • the mule, is the sterile offspring of male donkeys and female horses (the offspring of female donkeys and male horses are called hinnies)
                • Syrian wild arse went extinct in 1929
              • humans first domesticated horses in east Asia
        • Ferae:
          • Hyaenodontidae (c66mya extinct) the 1st mammalian carnivores in Africa and lacked post-carnassial crushing molar teeth which limited foods to meat - the 22mya Simbakubwa kutokaafrika was a hyena-like apex predator in Africa larger than a polar bear
          • Oxyaenids (c55mya now extinct) - specialized climbing carnivores with teeth designed for crushing and preyed on other terrestrial vertebrates, eggs and insects, and may be related to pangolins
          • Pholidota (pangolins)
          • Carnivora (42-50mya; evolved in Eurasia and moved south to Africa; cats, hyenas, dogs, bears, seals)
            • caniforms
              • Arctoidea evolved c46mya
                • Ursoidea
                  • Ursidea (bears)
                  • Hemicyonidae (extinct dog-bears)
                • Pinnipedia (seals)
                • Musteloidea (pandas, weasels, skunks, raccoons)
              • Leptocyon a fox-like genus 34mya
              • Canidae
                • Vulpini (foxes) 11.9mya
                • Canini (11.9mya) wolf-like canids and the South American canids
                  • Cerdocyonina
                    • wolves and some “foxes”
                    • dire wolf (separate ancestor to grey wolf perhaps ~6mya; extinct 13,000yrs ago)
                  • Canina (?1mya)
                    • jackal
                    • wild dogs and wolves
                      • grey wolf
                        • new world wolves
                          • Mexico
                          • North America/Hokkaido
                        • old world wolves
                          • Asian wolves
                          • Middle East wolves
                          • European wolves
                        • domestic dog ancestors (20,000-40,000yrs ago, domestication started over 15,000yrs ago with Asian and European lineages)
                          • dingo / New Guinea singing dog (evolved in Asia partly from the Tibet wolf and earliest Australian fossils date to around 3,300yrs ago)
                          • domestic dog 9)
            • feliformia
              • Felidae
                • Panthera big cats (6.37mya) eg. Panthera blytheae 4.1−5.95mya in Tibet
                  • snow leopard and tiger ancestor (3.9mya)
                    • tiger (3.2mya)
                  • jaguar (3.6mya)
                  • lion (2mya)
                  • leopard (2mya)
                • Felinae smaller cats, can't roar but can purr as vocal folds shorter than 6mm and have restractile claws and diverged from Pantherinae around 11.5mya
                  • Leptailurus - serval
                  • Leopardus - spotted American small cats eg. ocelot
                  • Lynx
                  • Acinonyx (cheetah)
                  • Puma
                  • Felis - wildcats and domesticated cats
  • euarchontoglires supraprimates 85-95mya
    • glires (rabbits, rodents)
    • euarchonta “true ancestors” (excludes rabbits, rodents) 88mya
      • Scandentia or treeshrews 86mya
      • Plesiadapiformes (extinct)
      • primatomorpha 86mya
        • Dermoptera or colugos 80mya
        • primates - 80mya
          • Strepsirrhini
          • haplorhines (dry nosed) - 63mya
history/evolution_mammals.txt · Last modified: 2022/09/09 08:52 by gary1

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