32

key concepts

32.1

Deuterostomes Include Echinoderms, Hemichordates, and Chordates

32.2

Echinoderms and Hemichordates Are Restricted to Marine Environments

32.3

Chordates Have a Dorsal Nerve Cord and a Notochord

32.4

Life on Land Contributed to Vertebrate Diversification

32.5

Humans Evolved among the Primates

Which Came First, the Chicken or the Egg?

When people ponder the chicken and egg problem, it is supposedly a riddle without an answer. The first chicken must have hatched from an egg, but then didn’t another chicken have to lay that egg? But if you understand evolutionary biology, then the answer to the question of whether the chicken or the egg came first is obvious, and it isn’t even close. Shelled eggs originated hundreds of millions of years before the first chicken walked on Earth.

The lineage of birds that we now know as chickens is only a few million years old. If we followed that lineage back through time, the birds in it would look less like modern chickens with each preceding generation. But all birds lay shelled eggs (a phenomenon called oviparity, or “egg birth”), so we can logically infer that eggs existed long before the first chicken did. Birds are the only living group of dinosaurs, but fossil eggs for many groups of extinct dinosaurs have been discovered. The closest living relatives of birds are the crocodilians, and they also lay shelled eggs, as do turtles. Many (but not all) lizards and snakes also lay shelled eggs, and so do the earliest branching lineages of mammals, including the echidnas and duck-billed platypus. Other mammals modify the shelled egg into structures that allow the embryo to develop within the mother, so that the young are born after “hatching” inside the mother (viviparity, or “live birth”).

By comparing reproductive biology across species, biologists can trace the origin of the shelled egg to the origin of amniotes. Amniotes include all living mammals and reptiles (including birds). The group derives its name from the amnion, one of four membranes that surround the embryo within the shelled egg of amniotes.

Most amphibians also lay eggs, but their eggs have no shell. The soft, jellylike coating of amphibian eggs can easily dry, threatening the developing embryo. So amphibian eggs need to be deposited in water, or at least in moist places where desiccation is not a problem. This limitation kept the earliest terrestrial tetrapods from fully exploiting drier terrestrial environments. The shelled egg was thus an important evolutionary innovation that resulted in a great terrestrial explosion of the amniotes.

What are the advantages and disadvantages of egg laying versus live birth?