Scientists believe that the egg probably preceded the chicken because of reproduction. When chickens reproduce, sperm from the rooster fertilizes an ovum from the hen, forming a new cell called a zygote. This zygote then reproduces, eventually forming all of the cells that make up a baby chick. This is why all of a chick's cells have exactly the same DNA. Before there were chickens, there were other nonchicken species, which evolved to form the modern chicken. However, any changes to the nonchicken that evolved into a chicken had to occur in the zygote. Therefore, what probably happened is that two nonchickens mated and their zygote mutated into a chicken cell, and then multiplied. Eventually, a full chicken was ready to hatch. This means that the egg probably is the answer to the age-old question of which came first. Of course, this happened millions of years ago, and chickens weren't domesticated until about 7,000 years ago [source: Furlong].
The age-old question lacks an important part of reproduction, however: sperm. Even way back in time when the animals from the nonchicken species mated, one of them had to have been a male member of its species to create the zygote that mutated into what eventually became today's chicken. A zygote is, after all, formed when a single sperm penetrates the mother's egg cell. When the species formed, it probably was not in an in vitro fertilization lab meant to help nonchickens virtually meet and create tastier test-tube versions of foul. It's way more likely that the holders of the chicken precursor's sperm and egg had sex.
Humans have known for quite some time that sexual activity between women and men creates babies, but the specifics weren’t confirmed until the 18th century -- and not by birds, bees or chickens, but by frogs. In 1777, Italian priest, biologist and physiologist Lazzaro Spallanzani got the idea to fit prophylactic pants on frogs to see what would happen. The female frogs mating with the well-dressed frogs stopped getting pregnant, but when Spallanzani exposed fresh frog eggs to the used frog pants, the tadpoles appeared. This research confirmed that male sperm was the agent for fertilizing eggs.
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