In absolute evolutionary terms, the egg came about millions of years before chickens. The oldest fossilized eggs ever found are about 600 million years old, and probably belonged to a tiny species similar to modern coral. Many invertebrate and all vertebrate females produce eggs in order to reproduce. Most mammals (excluding the platypus and echidnae, which lay eggs) and even a few other species, including some sharks, skinks, and snakes, have live births after their eggs are fertilized internally and their embryos develop in the womb. The eggs of fish and amphibians are soft and jelly-like and are not surrounded by a shell, whereas reptilian eggs have a leathery shell and bird eggs have hard shells in different textures. Eggshells first evolved in reptiles and allowed them to lay eggs away from the water, a major step in the evolution of land animals. Dinosaurs laid eggs, and since birds descended from dinosaurs, so do they. So from that standpoint, eggs clearly win the debate.
But what about if we just look at the evolution of the chicken? Speciation is really cool, but it can be a little confusing. There’s never a strict dividing line where you can say with certainty that the mother is a different species from her offspring. Species diverge via minute genetic changes that add up over time until there are two or more groups of descendants from a single ancestor that either will not mate if given the opportunity or will produce sterile offspring such as zedonks (half zebra, half donkey). It can take countless generations for a new species to emerge, with each generation being slightly less like their ancestral species and more like the new one that will eventually be recognized. That’s why it’s so hard to find “missing links” in evolution and can sometimes be difficult to determine the exact species of transitional fossils. Thousands of years ago there was a long-gone bird species that slowly evolved into the chickens we recognize today. On average, each egg contained a chick that was slightly more chicken-like than its parents. Once again, the egg came first!
Any other fun science or math questions you’d like to see answered in a future post? Leave ‘em in the comments below. And no, so far as I know there is no scientific reason that I know of that caused the chicken to cross the road, so don’t even try that one, wiseasses!
Eggs and Their Evolution by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.
Image via Wikimedia Commons, unattributed.