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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)

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

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
    • 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 3)
      • 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)4)
        • 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

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 23mya5))
                  • 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
        • 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”
                  • 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 6)
            • 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: 2020/06/26 13:08 by gary1