History

Birds appeared on Earth over 150 million years ago, during the Mesozoic Era ("dinosaur time").  The Tertiary Period saw birds diversify into modern groups.

 

Geologic Time Scale

Era

Period

Epoch

Began (MYA*)

Cenozoic

   (age of birds and

    mammals)

Quaternary

Recent

0.01

Pleistocene

1.5 - 3.5

Tertiary

Pliocene

7

Miocene

26

Oligocene

26-28

Eocene

53-54

Paleocene

65

Mesozoic

   (age of reptiles)

Cretaceous

Late

100

Early

135

Jurassic

Late

155

Middle

170

Early

180-190

Triassic

230

*MYA = Million Years Ago

 

Birds and Reptiles

Birds evolved from reptiles - probably from theropod dinosaurs.  Evidence can be seen when comparing birds to both reptiles and mammals.

Birds & Reptiles

Mammals

  • Single ball and socket joint to attach skull to spine.
  • Double ball and socket joint to attach skull to spine.
  • Simple middle ear with one bone.
  • Three bones in the middle ear.
  • Lower jaw (mandible) has 5 or 6 bones on either side.
  • Lower jaw (mandible) has only one bone.
  • Ankles located in tarsal bones.
  • Ankles located between lower leg bones and tarsal bones.
  • Scales on bird legs are similar to reptile body scales.

 

  • Lay eggs with large amounts of yolk, with the embryo forming on the surface (within the shell).
  • No yolk; the egg IS the embryo.
  • Females have two different chromosomes (Z and W), males have two of the same.
  • Males have two different chromosomes (X and Y), females have two of the same.
  • Blood cells have nuclei.
  • Blood cells do not have nuclei.

The evolutionary link between birds and reptiles is Archaeopteryx  lithographica.  This was the earliest known bird, and shows characteristics of both birds and reptiles.  It is the "missing link" between the two groups.

Characteristics of Archaeopteryx include...

  • wing bones with flight feathers
  • pairs of feathers attached to each vertebra of the tail
  • a long tail (bones, not just feathers)
  • a blunt snout with many small, reptilian teeth
  • large furcula (wishbone) to anchor strong pectoral muscles
  • asymmetrical feathers - exactly the same as modern birds
  • brain larger than reptiles of similar size, but not as large as modern birds
  • enhanced visual, hearing, and spatial-orientation centers

Birds & Dinosaurs

There are two schools of thought as to where birds evolved from:

  1. Birds evolved from thecodonts - primitive reptiles that gave rise to dinosaurs, pterosaurs, and crocodilians.
  2. Birds evolved from theropod dinosaurs.  (Theropod means "beast foot" and refers to 3-clawed, 3-toed, bipedal dinosaurs such as T-Rex and Velociraptor.)

Recent evidence, much of which has come in the form of extremely well-preserved fossils from the Gobi Desert in Mongolia, has swayed the debate in favor of the theropod dinosaurs as ancestors of birds.

One example of an area where scientists still disagree is comparing the hand bones of theropod dinosaurs to birds.  Both show three fingers (although bird fingers are tiny - like our appendix), but these seem to be different fingers.  Dinosaurs kept fingers I, II, and III (fingers IV and V were reduced and/or lost - like our appendix).  Birds seem to have kept fingers II, III, and IV - although some scientists argue that these are really I, II, and III (there is some genetic evidence of this).

 

Early Evolution of Birds

After Archaeopteryx, many fossils were found that "fill in the gap" between Archaeopteryx and modern birds.  These showed many intermediate steps between dinosaur structures and bird structures.  Two major groups arose during the Cretaceous Period:

  1. Enantiornithes was the earlier of the two groups.  They were able to fly, and lived in many ecosystems, worldwide.  They laid eggs on the ground, and ranged in size from sparrow-sized to vulture-sized.  Many were arboreal.  Advances include: modification of hand/forearm for flight, the ability to raise wings high and to fold them, a large pygostyle (supports tail feathers), and a hallux (for perching).  This group died out at the end of the Mesozoic Era, along with the dinosaurs.
  2. Ornithurae were toothed birds the later evolved into modern birds.  These showed more advanced wing structure and flight ability, and a fully developed perching foot.  This group diversified into wading birds, diving birds, perching birds, and some flightless land birds (evolved from flying ancestors).  Most of these died out at the end of the Mesozoic Era - the survivors evolved into modern birds.

Development of Feathers

Feathers have been discovered on more than a dozen species of theropod dinosaurs, including ones not closely related to Archaeopteryx.  There were two types of dinosaur feathers:

  • Soft, downey, filamentous feathers - "dino-fuzz".
  • Modern-type feathers.

Scientists believe that feathers started out as way to help regulate body temperature, and that flight came as a later adaptation.

 

Evolution of Flight

There are two schools of thought on how flight evolved in birds:

  1. The Arboreal Theory argues that bird ancestors living in trees would glide and parachute from tree to tree.  This was aided by longer arms and (what are now) flight feathers.  As the animals' bodies adapted to this life, flight was a logical extension of gliding.
  2. The Cursorial (Running) Theory argues that small theropods would run and jump to catch prey.  Longer arms would help to control and extend leaps.  The development of feathered wings and tails would help control the body's position and, later on, would give longer jumps and greater maneuverability.  Flight was an extension of this behavior.

Since Archaeopteryx spent time running and climbing trees, the actual evolution of flight was probably influenced by both factors.  The key development was a wing stroke powerful enough to provide both lift and thrust.  This required changes in both wrist and shoulder - including the development of the alula.

The behavior that would cause such changes in the birds' anatomy is demonstrated by modern chickens.  They use their wings when running up steep inclines (including tree trunks).

 

 Modern Birds

 Modern groups of birds appeared on Earth in two waves:

  1. The Nonpasserines - including ratites (large, flightless birds) - began appearing at the beginning of the Tertiary Period (65 MYA).
  2. The Passerines (perching songbirds) began appearing in the Miocene Epoch (26 MYA), as the evolution of flowers and insects offered new food sources.

Geologic Time Scale

Era

Period

Epoch

Began (MYA*)

Birds Appearing

Cenozoic

(age of birds and

mammals)

Quaternary

Recent

0.01

Human Civilization

Pleistocene

1.5 - 3.5

Humans

Tertiary

Pliocene

7

 

Miocene

26

Passerines (perching songbirds)

Oligocene

26-28

Hummingbirds, rollers, kingfishers, hornbills.

Eocene

53-54

Loons, auks, gulls, ducks, cranes, petrels, woodpeckers.

Paleocene

65

Ratites (ostriches, emus, kiwi, cassowary, etc.) and Tinamous

*MYA = Million Years Ago

 

The Earth was much different during the Tertiary Period.  The global climate was much warmer, with tropical ecosystems in northern Europe, Asia, and North America.  The continents were still in the process of separating - Pangaea had just begun to break up during the Mesozoic Era - and this allowed ancestral groups to move between continents much more easily than they could today.

 

Subpages (1): Files for History