Impossible Problem Solved: the Shelled Egg

November 13, 2010

: Rob Sheridan
Summary Author: Rob Sheridan

One of the many amazing things we take for granted in our world is the hard-shelled egg. A solution arrived at by millennia of evolutionary trial-and-error, the egg solved a seemingly insoluble problem. As complex multi-cellular animals evolved in the marine environment, gametes were simply released and joined by happenstance. The resulting zygote, if not consumed by predators or destroyed by the harsh environment, continued the organism's life-cycle. Fish and amphibians modified this with directed external fertilization of eggs laid in a jelly-like mass, still in an immersed or moist environment. It's believed that reptiles evolved from amphibians, as they began to explore the terrestrial world. Two linked problems blocked amphibian terrestrial colonization: How were gametes to join together to form zygotes in a terrestrial environment? How were the resulting zygotes to develop in a dry world? Reptiles solved these seemingly insurmountable problems by evolving internal fertilization and the hard-shelled egg.

Terrestrial reptiles evolved a strategy in which zygotes are created by internal fertilization. Subsequently, the fertilized egg develops protective layers (amnion), the outer of which calcifies. The zygote then grows in the egg, which is laid in the environment. The layers of amnion perform the remarkable, seemingly conflicting, tasks of allowing gas exchange while maintaining a moist protected environment. The developing zygote is supplied with oxygen by diffusion through the semi-permeable shell, while waste carbon dioxide escapes by diffusing out its concentration gradient. Species evolving from reptiles modified these remarkable adaptations. Mammals that evolved after monotremes (eg platypus and echinda) retained internal fertilization, but not the shelled egg, becoming a viviparous species. Birds retained the shelled egg, but modified size, shape, and pigmentation based on environmental pressures. The above photo shows a set of Herring Gull (Larus argentatus) eggs sitting in a ground nest. Like the eggs of many ground-nesting birds, they've evolved a conical shape to help prevent them from rolling far from the nest and a pigment pattern to hide them in the stippled light of the ground environment in which they are typically laid. Photo taken from Boston Harbor, Massachusetts on September 21, 2009.

Photo details: Camera Maker: Canon; Camera Model: Canon PowerShot SD1000; Focal Length: 14.4mm; Aperture: f/4.5; Exposure Time: 0.0063 s (1/160); ISO equiv: 400; Exposure Bias: none; Metering Mode: Matrix; White Balance: Auto; Flash Fired: No (Auto); Color Space: sRGB.