Two tracks toward independence
About 300 million years ago, a major evolutionary event occurred that paved the way for terrestrial vertebrates, including birds. Animals began to lay “land eggs” in a dry terrestrial environment, thereby breaking the hold that water had on fish and amphibians, the vertebrates at that time.
The land egg had a hard shell (think turtle or chicken) that allowed air to pass through the porous outer covering but prevented water from moving through. The embryo produced membranes that were not part of
