The amniotic egg evolution is an adaptation that allowed the first reptiles to thrive in a dry land – the development occurred over 300 million years ago. A shell surrounds the amniotic eggs of reptiles and birds. The robust shell protects the egg from drying out, predators, pathogens, and physical damage to the delicate membranes. The shell is a poor heat conductor to help retain the optimum temperature inside an egg. One can store a healthy egg at room temperature without damage.
The shell also prevents toxins from invading the internal membranes. The shell surface has tiny pores that allow oxygen to pass into the egg. However, the pores are small enough to restrict disease-causing organisms from invading egg fluids. Smooth inflow and outflow of air in an egg are essential, as it prevents embryo suffocation. Notably, the shell and four other membranes form a secure liquid environment that allows an embryo to grow from a few cells to a mature animal. The first sac, the chorion, transports oxygen into the embryo and eliminates carbon dioxide through the pores. The second membrane is known as the amnion, and it contains moisture that keeps the egg damp on dry land. The allantois, which is the third sac, stores the waste generated in the embryo. The membrane links with the chorion to create a chorioallantoic membrane that serves as a lung, facilitating the absorption of oxygen in the embryo and removing the unwanted carbon dioxide. Overall, the shell and its adaptations allow reptiles and egg-laying mammals to inhabit drylands.
In summary, the amniotic egg evolution is crucial as it ensures animals that could only survive in wet environments can occupy other places and reproduce successfully. Some amniotes have developed unique adaptations that allow them to stay in the dry land. For instance, humans do not produce tough-shelled eggs since the ovum grows inside the womb – a controlled moist environment. Consequently, the adaptation allows amniotic mammals to reproduce successfully in various surroundings: hot, cold, dry, and wet conditions.