Biology Today — December 2017

(c) Vestigial organs in other animals are :

• Splint bones are vestigial 2nd and 4th digits of forelimb as well as hindlimb in horse (Equus) due to cursorial mode of
  locomotion.


• Vestiges of hindlimbs and pelvic girdles, e.g., femur and ilium in snakes like python and sand boas due to fossorial
  living and creeping locomotion. This proved evolution of snakes from limbed ancestors.


• Vestiges of hindlimbs and pelvic girdle bones, e.g., femur and ischium inside the body of Greenland whales showing
  evolution of whales from limbed ancestors. These have completely disappeared in other whales.

(d) Atavism or reversion

• It is reappearance or refunctioning of some ancestral organs which have either completely disappeared, or are present as
  vestigial organs.


• Examples :

(i) Birth of a human baby with a small tail. (ii) Development of power of moving pinna in some persons. (iii) Long and dense
hair. (iv) Presence of additional mammae in some human beings. (v) Appearance of homodont dentition in many aquatic
mammals. (vi) Reduced leaflets of a trifoliate leaf of Citrus which becomes unifoliate with winged petiole.

B. Embryological Evidences

• These are evidences based on comparative developmental studies of embryos of various animals.


• Examples :


• Similar early development : All triploblastic animals from flatworms to mammals show similar early developmental
  processes like cleavage (mitotic divisions of zygote), blastulation (formation of blastula) and gastrulation (formation
  of gastrula) to transform one-celled zygote into a three-layered gastrula larva. Gastrula has three primary germ layers
  (ectoderm, mesoderm and endoderm). These germ layers form similar structures in various animals. This shows their common
  ancestry.


• Similar vertebrate embryos : The early embryos of the vertebrates like fish, salamander, tortoise, chick, rabbit and human
  being resemble one another so closely that it becomes difficult to distinguish them. These are similar in having similar form
  and structures like gill clefts, notochord, tail, eye and ear rudiments, etc. Notochord is replaced by vertebral column.


• Temporary embryonic structures : The embryos of certain animals have some temporary structures which disappear or
  reduce before hatching or birth, e.g., young one of bird with egg-tooth though tooth is not required as embryo feeds upon
  soft yolk absorbed with vitelline blood vessels but repeats the ancestral characters. This indicates that “birds are glorified
  reptiles” which means that birds have evolved by the modification of toothed reptiles but teeth are lost to reduce the body
  weight for flight. The presence of egg tooth in their embryos confirms the repetition of ancestral characters.


• Recapitulation theory proposed by Von
  Baer stated that the generalised features like
  brain, spinal cord, axial skeleton, aortic arches,
  etc., that are common to all the vertebrates,
  appeared earlier than the special features like
  hair (only in mammals), feathers (only in birds),
  etc., which distinguish various vertebrates that
  appeared later. Later recapitulation theory was
  modified as “Biogenetic Law” by Ernst
  Haeckel. It states that “Ontogeny repeats
  phylogeny” which means that an individual
  during its own development (ontogeny) passes
  through its ancestral history (phylogeny) in
  an abbreviated form. Thus, the phylogenetic
  relationship is well exhibited in the embryology.


• Example: Presence of fish characters like gills,
  gill slits, tail with a tail fin, lateral line sense
  organs, etc., in the tadpole larva of frog. It also
  shows that in evolution there is trend of moving of life from water to land.

Gillslits

Gill

slits

Gills Forelimb

bud

Hindlimb bud

Fish Salamander Tortoise Chick Rabbit Man

Tail Tail

Fig.: Embryos of different vertebrates