Chicken or egg? One zoologist’s attempt to solve the conundrum of which came first

Photograph: Olha/Getty Images/iStockphoto

The chicken or the egg? Sometimes, as a zoology author, I am asked this question by the kid at the front with the raised hand and large questioning eyes. Sometimes it’s the older guy at the back with a glint in his eye. Sometimes it’s a student who approaches the lectern at the end of a lecture while everyone else files out. The same mischievous eyes, the same wry smile. “So which came first?” they ask, beaming, unaware that this is not the first time I have been asked.

I hadn’t foreseen, years ago, when I began exploring the evolution of the animal egg and the role it has played in the long history of life on this planet, that it would become pretty much the only question I would be asked. I spent years reframing the evolution of life on Earth as a story told from the egg’s perspective, tracing this strange vessel’s adaptation to land, its movement across continents, the evolution of the umbilical cord, the evolution of the placenta, menstruation, menopause… but even now, having finally turned this journey into a book, I expect that a great deal of my dialogue with readers will be chicken-based.

Luckily, I consider chickens a fascinating gateway species for anyone who has never really stopped to think about how strange and beautiful animal eggs are when you consider them for a moment.

So, the question at hand – chicken or egg? Which really came first?

Like an egg, the question itself needs some space to breathe. The chicken and egg paradox – the classic causality dilemma – playfully expresses the difficulty that human minds have in sequencing actions where one thing depends on the other being done first and vice versa. Aristotle, writing in the fourth century BC, considered it to be an example of an infinite sequence, with no true beginning. It was a way of imagining what infinity represents. Later, Plutarch, the Greek historian and biographer, talked of the chicken and egg being a “great and weighty problem” that forced philosophers to engage in questions about whether the world had a beginning or whether it would ever end. The chicken and the egg were, in a way, precursors to modern-day questions about cosmology, deep time and physics. Later, through a series of exciting discoveries in the 19th century (particularly the ideas of Charles Darwin and Alfred Russel Wallace, co-discovers of natural selection), biologists and geologists were able to offer a more evidence-based perspective on the age-old question. And so, what follows in the next paragraph is the standard response you are likely to get should you throw a “chicken or egg” question at a modern-day zoologist.

A more thought-provoking way of approaching the question is to ask: ‘What came first, the egg or the egg tube?’

If you think of an egg as something with a hard shell that you can crack with a spoon, then the egg did arrive long before chickens. Because birds, which all lay eggs, go back a long time in history, many millions of years, whereas chickens, according to DNA studies and archaeological evidence, have been around for less than 10,000 years. So the answer to the paradox is a simple one. Egg wins. By a country mile. In fact, shelled eggs evolved in some (but not all) dinosaur groups, one of which was the ancestor of modern-day birds, about 160m years ago. Other dinosaur groups, including the earliest long-necked dinosaurs known as sauropods, may have evolved shelled eggs 195 million years ago. And so, in a very real way, there you have it: the egg, almost 200m years and counting, is considerably older than the chicken, which is, at most, around 0.01m years old.

But that doesn’t feel satisfying. My problem with this go-to zoological response is that it shortchanges the egg. Because eggs are very varied indeed. These numerous organic vessels, whose primary function is to fire genetic lineages forward through time, deserve a little more space to… cook. So, when asked this question, I like to elaborate.

A more thought-provoking way of approaching the question is to ask: “What came first, the egg or the egg tube?” For it is not chickens, but egg tubes (known as oviducts; fallopian tubes in humans) that make many eggs look the way that they do. Egg tubes abound across the animal kingdom. From egg tubes that leak milk from their walls like the eyes of holy statues (see: some flies), to egg tubes that paste cement-like glue all over the eggs, so that they can be stuck on to human hair (see: head lice). There are egg tubes where embryos wrestle and fight to the death (see: some sharks); egg tubes inhabited by blood-sucking placentae (see: some mammals); egg tubes flanked by paired vaginas (see: marsupials).

The chicken’s egg tube really is astonishingly beautiful. Every chicken’s egg you have ever held was first dressed in a dizzying, constricted, complicated life-corridor. Every egg you’ve ever cracked into a mixing bowl or boiled and served with soldiers graduated from it. Deep within the chicken, the egg you held in your hand began as a gloopy, slimy blob. As it passed through the egg tube, it was tended to by glands in the walls of the egg tube which sprayed different chemicals on to the egg, almost as if it were a vehicle passing through a car wash. Some nozzles sprayed a foamy calcium-rich layer that hardened into shell. Some sprayed tiny pencil-like markings on the eggshell; others painted constellations of dots and spots. In some birds, the eggs can be made all manner of blues and greens by these tiny nozzles. The blackbird egg (laid in spring and early summer in a shrub near you) looks almost as if it has been carved in jade. There are even pores in the wall of the chicken’s egg tube that secrete a waxy layer to the external shell of the egg, to protect it from microbes. And then the egg is delivered, like a shiny executive wagon on a car forecourt, polished and ready to go.

Which came first, the egg or the tube that made it? Why would an egg tube evolve if there were no egg for it to serve? How could there be an egg if there were no egg tube? Deeper we go. The truth is that the egg came a long way before the evolution of the egg tube, and by an extensive margin – many millions of years, clearly visible in the fossil record. In jellyfish, among the very first animals thought to have evolved, eggs are grown in the body and then shed directly into the water, often in their thousands. Perhaps the earliest eggs were shed this way.

Eggs are truly ancient. They go back 600m years or more, as documented by discoveries of sphere-like specimens found in slabs of ancient sea floors. Barely a millimetre or so across, some appear surprisingly intact. Some even have what are argued to be primitive cells within them – two, four, eight, 16 – dividing to become new life: an embryo, a hatchling, a generation. The truth is that we don’t yet know much about the animals that hatched from these mysterious prehistoric eggs. Some are claimed to be jellyfish; others may have been primitive marine worms. Either way, these eggs are very old. Far older than chickens or egg tubes. These fossil eggs go back to the Ediacaran period, about 100m years before animals (as we know them) really got going. The very idea of the existence of a chicken – a walking, squawking, feathered thing with an internal mineral-enriched skeleton, eyes and a beak – would have been unimaginable to anything capable of imagination back then. Yet, incredibly, the egg probably goes back further in time even than this.

If you expand the parameters of the question to allow the inclusion of sex cells (gametes), eg ova and sperm, then eggs beat chickens by, give or take, 1bn years. The uniformity and commonality of sex among distantly related modern-day groups, such as algae, plants and animals (then mostly little more than single-celled specks, hoovering detritus from rocks), suggests that eggs and sperm likely evolved at some time around 1bn years ago. This leads us to conclude that there were eggs and sperm on this planet long before animals as we know them today evolved. This was long, long, before egg tubes.

And so, in this great paradox of recent millennia, it’s the egg. Always the egg. The egg is older than the chicken. That’s what I’ll say next time I am asked, before readying myself for a final flourish. Because the paradox, like the egg, still has many fascinating layers that continue to attract human minds.

There is the genetics to consider, for instance. There must have been a moment when the chicken’s ancestor, wild jungle fowl laid a fertilised egg, within which were the exact combination of mutations that gave rise to the lineage that was then given the spoken label “chicken” (or its early language equivalent). And what is a “chicken”, exactly? The chicken of old, striding around back yards pecking at grain? Or the modern-day broiler, the monstrous perversion bred into existence by the poultry industry? What we call a “chicken” is really, when viewed across millennia, a tumbling river of genes and genetic lineages flowing forwards in time, shuffling in and out of novel combinations as generations pass, chiselled and finessed by the whims of unthinking planetary surface forces or, more commonly for this species, the sculpting, selective hands of industry. Like countries upon continents, the concept of “chicken” only exists because there is an upright ape on this planet with a kink for categories and a fondness for labelling things as they stand at this precise geological moment in Earth’s history. And what are animals, really? Are animals organisms that produce eggs in order to make more animals? Or are animals the vessels that eggs use, in an evolutionary way, to make more eggs?

Chicken or egg? Eggs or egg tubes? Eggs or animals? An enduring paradox, dreamed up 2,000 years ago, remains, in my eyes at least, as delicious and thrilling as ever to consider. We are living in an age of science, of rigorous evidence and journals and discoveries galore, yet still this simple question has the potential to exercise the mind in a very satisfying way. And so, long live the egg, the leftmost bookend to every animal life. Modern graduate of the egg tube. A truly marvellous thing.

Infinite Life: A Revolutionary Story of Eggs, Evolution and Life on Earth by Jules Howard is published by Elliott & Thompson (£20). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply