In 2004, scientists digging in the Canadian Arctic unearthed the petrified remains of a half-fish, half-amphibian
that all but confirmed paleontologists' theories about how land-dwelling tetrapods–four-limbed animals,
evolved from fish. It is a classic example of a transitional form, one that bridges a so-called evolutionary gap
between different types of animal.
*Preserved remains provide evidence that, over time, organisms of increasing complexity appeared on the
earth. Bacteria and blue-green bacteria are the first life that were preserved from the Precambrian era. During
the beginning of the Paleozoic era, complex multi-cellular invertebrates dominated life in the oceans. By the
end of the Paleozoic era, plants and animals had colonized the land surface of the earth.
The image to the left shows that the fetal
stages of certain species, such as fish,
chickens, rabbits, and humans, develop
almost identically, especially in the early
stages.
*In
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the case of vertebrates, the early stages of development are nearly identical. In fact, they're virtually
indistinguishable. They all have:
a notochord (developing backbone)
a dorsal hollow nerve cord
a tail
a series of paired brachial grooves
Some of those characteristics, such as the tail, do not develop fully in all organisms- yet oddly they appear in
early development. These similarities indicate that these traits and developmental processes were inherited from
a common ancestor.
*When considering the forelimbs of mammals, on the outside the forelimbs of say, a human, a bat and a whale,
look and function very differently, yet inside, they’re structurally quite similar. All three contain the ulna,
radius, and humerus bone. Each of these organisms also has carpals, and a total of five digits extending from
their limb. The similarity in build indicates that although these organisms look very different now, they shared a
common ancestor.
The wings of flightless birds such as ostriches and emus are vestigial, nonfunctional structures. The eyes of
many cave dwelling species of salamanders are also considered to be vestigial because they are useless; the
amphibians are actually blind. Because other organisms have these same organs, only in a functional form, its
evident that both evolved from a common ancestor.
The more that we learn about the evolution of specific species lines, the more that these so-called gaps or
"missing links in the chain of evolution" are filled with transitional specimens. One of the first of these gaps to
be filled was between small bipedal dinosaurs and birds. Just two years after Darwin published On the Origin
of Species, the 150-145 million year old remains of Archaeopteryx were found in southern Germany. It had
jaws with teeth and a long bony tail like dinosaurs, broad wings and feathers like birds, and skeletal features of
both. This discovery verified the assumption that birds had reptilian ancestors.
*All of the tens of thousands of types of proteins in all living things are made of only 20 amino acids.
Despite
the great diversity of life on our planet, the simple language of the DNA code (A,T,G,C) is the same for all
living things. This indicates that all living things are related to each other and to a common ancestor.
The many anatomical similarities of wolves, dogs, and other members of the genus Canis are due to the fact that
they are descended from the same ancient canine species and still share 99.8% of their genes.
These structures function differently, but have similar internal structures, indicating common ancestry.
These structures are nonfunctional today in some organisms, but are believed to have been useful in these
organisms’ past, indicating that an organism evolved and that it shares a common ancestor with other organisms
who have these same structures, only in a functional form.
These structures have similar functions, but their internal structure is very different. These structures do not
indicate evolution from a common ancestor.