The human life cycle begins at fertilization, and not before
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No, the Life Cycle Does Not Begin Before Fertilization
Chapter 2 of Developmental Biology (6th edition), Life cycles and the evolution of developmental patterns, explains it this way:
“When we consider a dog, for instance, we usually picture an adult. But the dog is a ‘dog’ from the moment of fertilization of a dog egg by a dog’s sperm. It remains a dog even as a senescent, dying hound. Therefore, the dog is actually the entire life cycle of the animal from fertilization through death.”
In other words, the life cycle of a mammal begins at fertilization, not before. A fertilized egg isn’t a “potential dog” or “pre-dog”—it’s a dog at the earliest stage of development.
Later editions of the same textbook reinforce this point. The 11th edition, in its chapter on fertilization, states:
“Fertilization accomplishes two separate ends: sex (the combining of genes derived from two parents) and reproduction (the generation of a new organism).”
So fertilization does two things at once: it unites parental genes, and it creates a new, genetically distinct organism.
That means the mammalian life cycle—including the human life cycle—begins at fertilization.
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