Humans do not lay eggs; instead, human reproduction involves a complex internal process resulting in live birth. This method, shared by most mammals, offers advantages in offspring protection and nourishment compared to egg-laying animals.
How Human Babies Develop
Human reproduction begins with internal fertilization, where a sperm fertilizes an egg inside the female’s body. The resulting fertilized egg, or zygote, then travels to the uterus. There, it implants itself in the uterine wall, marking the start of internal embryonic development.
The developing embryo and later fetus receive all necessary nutrients and oxygen directly from the mother’s bloodstream through a specialized organ called the placenta. This organ also removes waste products from the fetus, maintaining a stable internal environment for growth. This entire developmental period, known as gestation, occurs within the mother’s uterus. At the end of this period, the fully developed baby is born live, a reproductive strategy known as viviparity.
How Some Animals Lay Eggs
Many animals reproduce by laying eggs, a method known as oviparity. In oviparous species, the female produces an egg, which may be fertilized either internally or externally. After the egg is expelled from the mother’s body, the embryo develops outside, relying on the yolk within the egg for nourishment.
The eggs of different oviparous animals vary in structure; birds, for instance, lay hard-shelled eggs, while many reptiles lay eggs with softer, leathery shells. Common examples of egg-laying animals include birds, most reptiles, amphibians, fish, and the vast majority of insects. Once development within the egg is complete, the young hatch from the egg.
Why Humans Don’t Lay Eggs
Humans, like most mammals, do not lay eggs because our reproductive system evolved to support internal gestation and live birth. This viviparous strategy provides continuous protection and nourishment for the developing offspring within the mother’s body. The placenta plays an important role, allowing for a constant supply of nutrients, oxygen, and waste removal, which supports human development.
Egg-laying, by contrast, involves external development where the embryo is vulnerable to environmental threats and predators once laid. While some egg-laying animals provide parental care, the developing embryo in an egg lacks the direct, stable maternal connection that viviparity offers. The human body is adapted for internal development, with the uterus providing a protective environment and the placenta facilitating nutrient exchange. This makes egg production and laying biologically incompatible with our species.