In this case, the egg is not assumed to be a chicken egg. In
effect this changes the question to: "Which came first, a chicken
or any egg".
From a cellular biology point of view this question can be answered
quite easily. The egg came first because any female sex cell is
called an egg.
If the egg is defined structurally as the hard shelled thing, and
the chicken a feather covered animal, the answer is still simple.
Evolutionary scientists believe the first hard shell egg was the
amniotic egg laid around 300 million years ago, and was laid by the
animal who was the link between amphibians and reptiles. One of the
first dinosaurs that we know had feathers was the Archaeopteryx,
and came much later. Modern birds would not arise until 150 million
years ago, descending from theropod dinosaurs.
In this case, the first chicken must have been the mutated
offspring of a proto-chicken that laid the egg containing the first
true chicken. In any case, this creature hatched from a
recognizable egg. After all, the question is purposefully ambiguous
-- it is not, "Which came first, the chicken or the chicken
egg?"
The crux of the matter is how to biologically define "a chicken".
What level of genetic similarity or structural similarity determine
whether an organism is a chicken? One can only define what was the
first chicken after the fact, thus any definition of the first
chicken becomes arbitrary. The question 'which came first?' ignores
the complicated reality of speciation. The concept of species is an
abstraction intended to categorize a broad swath of genomes and
their subsequent phenomes. If one were to do away with approximate
categories, each individual 'chicken' actually represents a unique
genotype. Under this definition, if a 'chicken' possessing genome A
were to lay an egg possessing genome B, then an egg of genome B is
antecedent to an animal possessing genome B and that the
parent--genome A--is antecedent to, yet different from the egg of
genome B. Hence, in an absolute sense, the egg came before the
'chicken.'
According to the principles of speciation, neither the chicken nor
the egg came first, because speciation does not occur in simple,
obvious units. In fact, evolution is about a slow transition in an
overall population. What qualifies as “chicken” (ignoring the many
diverse modern types of chicken) involves a wide range of genetic
traits (alleles) that are not encompassed in a single individual
and continue to be modified from generation to generation.
The transition from non-chicken to chicken is a grey area in which
several generations are involved, and therefore which includes many
many chicken-and-egg events, with no one step representing the
whole. Since the result of the process is an incomplete transition
into various new characteristics rather than one single blueprint,
a new species, "chicken", is only identified in hindsight when the
species can be obviously identified as different from its ancestral
stock.
Possibly, if life originated from an ooze or protozoa of some type,
at first there may only have been cellular life that used division
as a reproductive method but as multicellular creatures evolved,
mutation led to sexes differentiating. Division of the reproductive
task into sexual roles took the form of an ovum / fertilization
sequence. The egg was therefore present at the same time as the
creature that gestated/layed it, speciation into birds or turtles
happens much later with such a scenario.
http://en.wikipedia.org/wiki/Chicken-and-egg_problem
