Why Do Some Mammals Lay Eggs?

What Exactly Defines An “Egg?”

Eggs are the reproductive body produced by most multicellular organisms. Its evolution is still considered somewhat of a scientific mystery. The earliest single-celled organism would have reproduced using binary fission, where the organism split itself into two. However, as evolution progressed, multicellular organisms had developed a specialized group of cells that was capable of cumulation with another organism, leading to the formation of the very first “egg”. It would have resembled soft-bodied eggs, such as the ones laid by modern fish.

Live Birth v/s Laying Eggs

Today, most animals are categorized into one of two types – oviparous and viviparous animals. In essence, oviparous animals lay eggs, while viviparous animals give birth to live young. Most higher vertebrates, such as birds, reptiles and amphibians, are oviparous. Mammals are exclusively viviparous, except for the platypus and echidna. Nevertheless, why do these organisms lay eggs despite being a mammal?

A platypus

A Relic from the Past

Platypus and the echidna belong to one of the three major orders of mammals called the Monotremes. The other two major orders are the placentals and the marsupials.

The platypus exhibits unusual characteristics for a mammal, such as a reptilian gait, the ability to lay eggs, and a venomous stinger on its webbed feet. These characteristics can be traced back to ancestors that diverged from the reptilian lineage to form mammals.

However, the ancestors of monotremes diverged again from the main mammalian lineage before mammals had evolved the placenta (which is the organ that develops during pregnancy). As a result, the monotremes such as the platypus still lay eggs, reminiscent of their reptilian ancestry.

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