340 Million B.C.T. - The Egg Evolved

Around 340 million years ago, the first egg bearing animals -- the first amniotes -- appeared on the Earth.

The amniotes are a group of tetrapods (four-limbed animals with backbones or spinal columns) that have an egg equipped with an amnios, an adaptation to lay eggs on land rather than in water as anamniotes do. They include synapsids (mammals along with their extinct kin) and sauropsids (reptiles and birds), as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In eutherian mammals (such as humans), these membranes include the amniotic sac that surrounds the fetus. These embryonic membranes, and the lack of a larval stage, distinguish amniotes from tetrapod amphibians.

The first amniotes (referred to as "basal amniotes"), such as Casineria, resembled small lizards and had evolved from the amphibian reptiliomorphs about 340 million years ago, in the Carboniferous geologic period. Their eggs could survive out of the water, allowing amniotes to branch out into drier environments. The eggs could also "breathe" and cope with wastes, allowing the eggs and the amniotes themselves to evolve into larger forms.

The amniotic egg represents a critical divergence within the vertebrates, one enabled to reproduce on dry land—free of the need to return to water for reproduction as required of the amphibians. From this point the amniotes spread across the globe, eventually to become the dominant land vertebrates.
Very early in the evolutionary history of amniotes, basal amniotes diverged into two main lines, the synapsids and the sauropsids, both of which persist into the modern era. The oldest known fossil synapsid is Protoclepsydrops from about 320 million years ago, while the oldest known sauropsid is probably Paleothyris, in the order Captorhinida, from the Middle Pennsylvanian epoch (ca. 306-312 million years ago).

The first amniotes, such as Casineria kiddi, which lived about 340 million years ago, evolved from amphibian reptiliomorphs and resembled small lizards. Their eggs were small and covered with a leathery membrane, not a hard shell like those of birds or crocodiles. Although some modern amphibians lay eggs on land, with or without significant protection, they all lack advanced traits like an amnion. This kind of egg became possible only with internal fertilization. The outer membrane, a soft shell, evolved as a protection against the harsher environments on land, as species evolved to lay their eggs on land where they were safer than in the water. The ancestors of the amniotes probably laid their eggs in moist places, as such modest-sized animals would not have difficulty finding depressions under fallen logs or other suitable places in the ancient forests; and dry conditions were probably not the main reason the soft shell emerged. Indeed, many modern day amniotes are dependent on moisture to keep their eggs from desiccating.

Amniotes can be characterized in part by embryonic development that includes the formation of several extensive membranes, the amnion, chorion, and allantois. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium, rather than first entering a feeding larval tadpole stage followed by metamorphosis as in amphibians. In amniotes the transition from a two-layered periderm to cornified epithelium is triggered by thyroid hormone during embryonic development, rather than metamorphosis. The unique embryonic features of amniotes may reflect specializations of eggs to survive drier environments; or the massive size and yolk content of eggs may have evolved to allow direct development of the embryo to a larger size.

Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell and an allantois evolved to facilitate respiration while providing a reservoir for disposal of wastes. Their kidneys and large intestines are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures may be found inside the placenta.