As Easter approaches I naturally am coloring some eggs, which made me think about what an incredible advancement in evolution the egg really is! Without a shell around the egg, life on earth was essentially confined to the water. Amphibians were able to move about on land, but they needed water to reproduce. The evolutionary advance that created the membranes that surround the egg to create the cleidoic egg (also called amniotic egg) meant the egg would not dry out and thus brought about the invasion of life onto land! This opened up new opportunities for life and created an explosion in species diversity including all the animals in class Reptilia and Mammalia; together these are known as the amniotes (reptiles, birds, mammals).
There are four very important membranes that are required for an embryo to develop inside a shell:
- The amnion – this surrounds and protects the embryo
- The yolk sac – this holds the nutrients for the embryo in the yolk
- The allantois – this holds metabolic waste
- The chorion – works with the allantois to exchange gas between the embryo and air sac
The other evolutionary advances that were required to successfully invade land were the evolution of skin that does not dry out, a new mechanism of breathing with the rib cage instead of the mouth, the development of kidneys to conserve water, and internal fertilization (before the egg shell is produced). You can find more information and see a good diagram of an egg with the parts labeled here.
So now you know, which came first, the chicken or the egg? Enter your answer below by leaving a comment!
Reference: Booker, RJ, EP Widmaier, LE Graham, and PD Stiling. 2008. Biology. McGraw Hill, Boston
Cornell Laboratory of Ornithology has put together a really cool, interactive experience exploring the diversification of the Reptilia, with an emphasis on birds, of course! The Wall of Birds is a must-see experience and easy to navigate for anyone interested in exploring.