by Whit Gibbons

October 19, 2008

The answer is "the egg." I was reminded of this during a recent visit by Brian Todd, a postdoctoral scholar in ecology at Virginia Tech. That was the response he once gave in a class at the University of Georgia. We were discussing evolutionary ecology and someone had asked, the age-old question, "Which came first, the chicken or the egg?"

Archaeopteryx, "the first bird," appeared on earth about 150 million years ago, during the Jurassic period. Chickens followed several million years later. However, eggs of various sorts had already been around on the evolutionary stage hundreds of millions of years before any bird, let alone a chicken, appeared. Many intriguing ecological and evolutionary theories and discoveries revolve around eggs and the mechanisms for giving birth. Some animals are live-bearers, which means they do not lay eggs. Live-bearers, including humans, rattlesnakes, and mosquito fish, actually have eggs, but the embryos develop inside the mother instead of in a shelled egg outside the body.

Live-bearers vary greatly in how they carry out the process of development and birth. The level of nourishment a mother gives to an embryo during development varies greatly, depending on the type of organism. Most mammals provide a continual and direct supply of food to the developing baby through a placenta before birth. The embryos of most live-bearing reptiles obtain their proteins and energy for growth from an attached yolk sac while inside the mother, much like that of an embryo developing inside a shelled egg. All mammals are live-bearers, with two exceptions, duckbilled platypuses and spiny anteaters, both of which live in Australia. Those mothers stay with their eggs, and after birth the babies drink milk that flows out onto the mother's body from a milk-producing gland.

Among reptiles, some groups, such as turtles and crocodilians, lay eggs and are never live-bearers. Most snakes and lizards also lay eggs, but approximately one-fifth are live-bearers. Biologists have determined that early reptiles were egg layers but that some have evolved to a point that they give birth directly after the young hatch inside the body. To appreciate the variation among egg-laying reptiles, consider that some lay eggs that take more than a year to hatch whereas others lay eggs that hatch within a week. Evolutionary biologists assume that the shortened incubation periods of the eggs in some species are indicative of how some reptiles became live-bearers. Thus, eggs that hatch soon after being laid must have undergone extensive embryonic development inside the mother. Hence, delaying egg laying for a few more days would create a situation in which using energy and resources to produce an egg shell was no longer necessary. Evolving to become a live-bearer would allow the young to complete development inside the mother's body.

Most amphibians, which include frogs, salamanders, and the tropical wormlike creatures known as caecilians, lay eggs, but the eggs have no shells. The array of behaviors associated with how amphibians, especially tropical frogs, care for their eggs is intriguing: some females carry their eggs on their back; others lay their eggs in trees so that the tadpoles fall into water; others actually swallow the eggs and keep them in their stomach until the young hatch and come out their mouth. Unfortunately, the gastric brooding frog of Australia that did this last trick is now extinct. True to the rule of evolutionary exceptions, a few frogs, salamanders, and caecilians give live birth without laying eggs.

Fish have so many unusual modes of egg laying and live-bearing that complete books have been written about them. Some marine catfish keep their eggs and babies in their mouth, and the big freshwater fish known as a bowfin lays eggs in a nest guarded by the male, who stays with and protects the baby fish after they begin to school.

All birds lay eggs, but clearly the first eggs on earth came way before the chicken. Bird eggs are associated with some captivating parental behaviors, but eggs have been associated with fascinating behaviors of other organisms for eons.

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