Birds are related to reptiles and their ancestry can be traced all the way back to the dinosaurs 150+ million years ago. In the fossil record we find transitional species with feathers, teeth, plus older reptilian features..
We can
look back genetically and find the “instruction manual” that the previous bird
ancestors used. We glanced into the genetic past of birds and we found that
they retain the instruction manual their ancestors used to make teeth. So why
don’t we see birds with teeth outside the window if they have the genes?
30 years
ago: Scientists took a layer of cells from the mouth of a mouse embryo and
placed it in the beak region of a chick embryo and the cells combined, causing
the chick to grow rudimentary teeth in its beak! The bird embryo had the
“instruction manual” for teeth already and just needed the “go” signal, which
they received from the mouse cells. The mouse and chick cells combined and they
used chemical “language” to talk to each other. The genes for the “go” signal
chemically turned on the genes for instructions for teeth, and together they
developed teeth on the chick’s beak!
2006: We
discovered a small mutation in the DNA of a chick embryo that affected its
development and caused it to develop rudimentary teeth. This mutation is
important because it changed the arrangement of tissue layers in the beak. In
modern birds their beak tissues develop too far apart to exchange their
“chemical language” and thus, never talk about making teeth. This small
mutation caused the layers of tissue to move closer and they began
communicating again. We then looked at these rudimentary teeth in the chick’s
beak to look at how they were developing and we then saw that the teeth were
being built just like reptiles and not mammals.
Why still
“rudimentary” teeth?
If you
visualize genes lined up along chromosomes like close neighbors living next
door in a neighborhood, we find that in birds the houses/genes for making
enamel are in the same “neighborhoods” in reptiles and mammals. We looked for
the houses/enamel making genes in the chick DNA and the genes are still in
their “neighborhood” but they are but badly mutated...
Visualize
visiting an old neighborhood where several houses are broken down and have been
abandoned for 80 million years! Who cares if you throw a brick through a window
of an broken down abandoned house? Well natural
selection doesn't care what you do with abandoned genes either! If
there is a mutation in a gene that is not needed, it will be passed on because
it doesn't harm anything. Reproduction will continue to pass on old
genes as they accumulate mutation, like an old house falling apart through
time. These old genes will be useless just like the old house unless it can
become renewed…which for a house could be a new tenant and a paint job, but for
genes its mutation. Mutation will continue in the genes and sometimes (quite
often actually) old useless genes gain a new function because the right
mutation.
This is
the story of birds and their teeth. Birds have broken down genes for their “go”
signal that “talks” to the instructional genes for teeth. This allowed
mutations in the neighborhood of enamel making genes to accumulate unnoticed
and eventually cause the weak “rudimentary teeth” in the chick embryo. Perhaps
these old one day will mutate and become the signal for something brand new.
So if
birds did not evolve from the reptilian-like ancestors we find in the fossil
record, why then do birds have the ancient instruction for their teeth?
http://www.pnas.org/content/97/18/10044.short
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