Mammalian Evolution and Early Primate Evolution

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Mammalian Evolution and Early Primate Evolution
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Vertebrate Evolutionary History A Brief Summary

• The geological time scale divides the earths
  history into eras, periods, and epochs.
• The first vertebrates appear in the fossil record
  in the early Paleozoic (500 m.y.a.).
• Mammal-like reptiles diversified at the close of
  the Paleozoic (250 m.y.a.).
• These forms are probably ancestors of modern
  mammals.

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• Continental drift had profound evolutionary
  effects during the Paleozoic and the Mesozoic
  groups of animals were separated from each other
  and mammals and reptiles were redistributed
  around the globe.
• During the late Paleozoic, the continents came
  together to form Pangaea.
• Gondwanaland, consisting of South America,
  Africa, Antarctica, Australia, and India, began
  to split off from Pangaea during the early
  Mesozoic.
• The other continents (North America, Greenland,
  Europe, and Asia) formed Laurasia.
• The continents continued to move and began to
  assume their present positions by the end of the
  Mesozoic (c. 65 m.y.a.).

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• The dominant form of land vertebrates during the
  Mesozoic was the dinosaur.
• Most went extinct at the end of the Mesozoic due
  to collisions of asteroids or comets with the
  earth (K/Tr).
• The earliest mammals also existed during the
  early Mesozoic.
• The first placental mammals appeared
• about 70 m.y.a. and were extremely
• successful during the Cenozoic.
• The Cenozoic is divided into seven epochs the
  Paleocene, Eocene, Oligocene, Miocene, Pliocene,
  Pleistocene, and Holocene.

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Mammalian Evolution

• The Cenozoic is known as the Age of Mammals.
• The diversification of the mammals followed the
  extinction of the dinosaurs, since new ecological
  niches became available.
• Mammals were successful because they evolved a
  complex brain and had flexible behaviors.
• Mammals are viviparous.
• Reptiles have homodont dentition, whereas mammals
  have heterodont dentition that allows them to
  process a wide variety of foods.
• The primitive mammalian dental formula is
  3.1.4.3.
• Mammals (and birds) maintain a constant internal
  body temperature (they are endothermic).

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The Emergence of Major Mammalian Groups

• The three major subgroups of mammals are
  monotremes, marsupials, and placental mammals.
• The monotremes are the most primitive mammals
  because they lay eggs.
• In marsupials, the young are born extremely
  immature and complete their development in the
  mothers pouch.
• Placental mammals nourish the fetus with a
  specialized tissue, the placenta.
• The placenta permits a longer gestation period
  resulting in a more completely developed central
  nervous system in the fetus.

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Processes of Macroevolution

• An adaptive radiation is the rapid expansion and
  diversification of new life forms into new
  ecological niches.
• A species will diverge into as many variations as
  allowed by its adaptive potential and the
  adaptive opportunities of the ecosystem.
• For example, the evolution of the reptilian egg
  caused the adaptive radiation of the reptiles by
  opening new adaptive niches on land.
• The diversity of the living and sub-fossil lemurs
  of Madagascar are a result of an adaptive
  radiation that began about 50 m.y.a.

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Generalized and Specialized Characteristics

• An adaptive radiation involves the transformation
  of generalized characteristics into specialized
  characteristics.
• Traits that are adapted for many functions are
  generalized.
• Generalized and specialized can also refer to the
  entire organism.
• Species that are generalized have the greatest
  adaptive potential.

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Proto-primates

• The first primate-like mammals, or
  proto-primates, were roughly similar to squirrels
  and tree shrews in size and appearance. 
• The existing, very fragmentary fossil evidence
  (mostly from North Africa) suggests that they
  were adapted to an arboreal way of life in warm,
  moist climates.
• They probably were equipped with relatively good
  eyesight as well as hands and feet with pads and
  claws for climbing.
• These proto-primates will remain rather shadowy
  creatures for us until more fossil data becomes
  available.

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Early Prosimians

• The first primates may have evolved during the
  last part of the Paleocene Epoch.
• Their bones have been found in 60 myo geological
  deposits in Morocco, but they probably lived in
  other areas at this time as well.
• They looked different from the primates today.
• They were still somewhat squirrel-like in size
  and appearance, but apparently they had grasping
  hands and feet that were increasingly more
  efficient in manipulating objects and climbing
  trees.
• It is likely that they were developing effective
  stereoscopic vision.

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• The beginning of the Eocene Epoch coincides with
  the appearance of early forms of most of the
  placental mammal orders that are present today.
• Among them were primate species that somewhat
  resemble modern prosimians such as lemurs,
  lorises, and possibly tarsiers.
• This was the epoch of maximum prosimian adaptive
  radiation.
• There were at least 60 genera of them that were
  mostly in two families--the Adapidae (similar to
  lemurs and lorises) and the Omomyidae (possibly
  like galagos and tarsiers).

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• This is nearly four times greater prosimian
  diversity than today.
• Eocene prosimians lived in North America, Europe,
  Africa, and Asia.
• It was during this epoch that they reached the
  island of Madagascar.
• The great diversity of Eocene prosimians was
  probably a consequence of the fact that they did
  not have competition from monkeys and apes since
  these latter more advanced primates had not yet
  evolved.
• However, by the end of the Eocene Epoch, many of
  the prosimian species had become extinct. 
• This may be connected with cooler temperatures
  and the appearance of the first monkeys during
  the transition to the next geologic epoch, the
  Oligocene (about 35.4 million years ago).

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• Major evolutionary changes were beginning in some
  of the Eocene prosimians that foreshadow species
  yet to come.
• Their brains and eyes were becoming larger, while
  their snouts were getting smaller.
• At the base of a skull, there is a hole through
  which the spinal cord passes. This opening is
  the foramen magnum.
• During the Eocene, the foramen magnum in some
  primate species was beginning to move from the
  back of the skull towards the center.
• This suggests that they were beginning to hold
  their bodies erect while hopping and sitting,
  like modern lemurs, galagos, and tarsiers.

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