(PHYSORG)- In the second report of our Egg Cetera series on egg-related research, let’s begin with the age-old question: which came first, the chicken or the egg? Armed with knowledge of evolution, the answer is straightforward. Eggs came first.
Dinosaurs, the animal group that includes birds and their ancestors, laid eggs. This means that the ancestors of birds laid eggs long before chickens came about. Because we know so much about the evolution of life on Earth, this answer now seems obvious. But without fossils, and without understanding how evolution works, we could have got it wrong.
My research focuses on the evolution of dinosaurs, and what that tells us about the evolution of the many distinctive features of birds, such as hollow air-filled bones, warm-bloodedness, feathers and flight.
Archaeopteryx, which lived 150 million years ago, shows a mosaic of bird-like features such as feathery wings, alongside distinctly non-birdy features such as teeth and a long bony tail. Fossil remains of archaeopteryx were first found in 1861. However, until recently, there was no consensus about where, evolutionarily speaking, birds came from. Scientists were confident that, among today’s animals, they formed a group with crocodiles, but how they fitted in with the vast diversity of extinct groups was not clear.
The idea that birds are descendants of dinosaurs was first proposed by Thomas Henry Huxley in 1869, but received little support until discoveries of the bird-like dinosaur Deinonychus (Jurassic Park’s ‘Velociraptor’) by John Ostrom in the 1960s. More recent discoveries include dinosaur fossils from China, preserved in ancient lake beds, complete with feathers.
Many features thought to be characteristic of birds have an ancient evolutionary history. For example, ‘pneumatic’ air-filled bones first appeared in Late Triassic dinosaurs, 200 million years ago, and were common in saurischians like Tyrannosaurus rex and Diplodocus.
We can also look at the evolution of eggs from the earliest fossil evidence right up to the egg we might eat for breakfast today. Like crocodiles and some turtles, birds lay hard-shelled eggs, enclosed by a calcitic, mineralised layer. These are unlike the primitive, leathery eggs of lizards, snakes, many turtles, and egg-laying mammals such as the platypus. Bird eggs also have a number of unique features not found in other reptile eggs. They form long shapes, not spheres, and they taper towards one end rather than being symmetrical. Furthermore, birds incubate their eggs directly using body heat, and they ovulate and lay one egg at a time, whereas crocodiles and other reptiles have two functional oviducts.