The chicken or egg problem

2008/08/25 § 1 Comment

A recent argument with a friend made me realise, that I should make a note about the so called “chicken or egg” or also called “hen or egg” problem. 

In short: what was there first? Did a first egg emerge which let a first chicken hatch? Or was there a first chicken which laid the first egg? [1]

Maybe you think this is a stupid question? Yes, it is. And we were not even actually talking about this primarily back then. But we got into the issue somehow anyway, probably helped by the pub environment we were in at the time 😉

However, my friend was sure that the only logical solution must be that the egg came first. He is not alone with this assumption since an expert group also ruled that the egg came first [2]. However, I replied that this is possible but no necessarily so, and since I came across a posting on the web the other day, claiming the same, I felt it is time to clarify [3].


First off: how to define a chicken?

What is a chicken? Where does the proto-chicken end and the modern chicken as we know it start? Domestication has led to today’s chicken. So we would have to assume that our great grandfathers are responsible for the chicken as we know it. 

Genetic research has indicated that the modern chicken is a cross of two wild species. Since there are many different types of domesticated chicken today the question is not easily addressed. However, let us assume that we take one subtype of the modern chicken. We would have to agree on which traits we want to include in our definition. Not few geneticists agree that this is a ludicrous task and that you simply cannot give a tight definition of the modern chicken.

Despite these difficulties, let us assume we could agree on a clear cut definition. Then we would assume that there must have been one pre-chicken who lacked at least one protein coding gene or gene-function of the modern chicken (also here we run into trouble if we would consider single nucleotide polymorphisms without effect on the aminoacid sequence as differences- but let’s not pick nits now 😉 [4]). 

We do know that mutations which are relevant for being passed on to subsequent generations do only occur in gametes, i.e. the male or female cells that are merged during fertilisation (egg or sperm cells). Mutations here occur due to cross-over events of the chromosomes but also due to mutations caused by random processes or mutation enhancing agents (like chemicals or radiation).

In this case clearly we would have an egg at some point which is the product of a sperm and an egg-cell which have produced the exact genetic information that we have agreed on to constitute a chicken.

This follows the assumption that we accept the definition:

– the first egg to carry the complete genetic (DNA) information of a chicken we accept as chicken egg

Then the egg comes before the chicken!


Chicken egg or no chicken egg?

You may wonder why I said, “[if] we accept the definition”; what other definition could there be for a chicken egg? Let us look at what an egg contains. 

The inner part of the egg contains the genetic information for the production of a new chicken, but it also contains some organelles, liquids and plenty of other stuff. Similarly to mammal eggs (including human) there also is the cell membrane that contains the mothers’ proteins and lipids. In the case of a chicken egg there is a hard calcified part additionally protecting the egg.

These proteins are coded by the nucleus. If the last mutation we were waiting for in order to see the first chicken-DNA to be completed occurred in a membrane protein, the egg we are talking about (containing complete chicken-DNA) would not have exclusively chicken proteins in its membranes.

Considering the calcified outer shell of the chicken we can safely assume that the structure changed with evolution. More precisely, the chicken shell developed from softer predecessors to the modern hard shell structure. So even worse; if we would (only as a brain-game of course, since we know that the wild type birds chicken derive from have hard shell eggs already) assume that the last and final mutation making a modern chicken happened in a protein controlling aspects of shell production, the first egg to contain chicken DNA was not a chicken egg. 

Exaggerating to make the point more transparent, one could imagine a gel-like egg that looks nothing like a chicken egg, but more like a soft reptile of fish egg. From this proto-egg a chicken would hatch which would then lay the first generation of chicken eggs.

Again: the first egg to carry all the nuclear genetic info to produce a chicken would not necessarily carry all proteins necessary to fulfil the definition of a proper chicken egg. Only the second generation of eggs would carry all chicken proteins and would be a proper chicken egg.

So, then we would have to define:

– the first chicken egg to be laid has to be laid by a chicken. 

The chicken comes before the egg!


Solving the mystery

Without knowledge of the exact definition the question of what came first: chicken or egg, cannot be answered (as so often definition is all, isn’t it?).

If one runs with the definition mentioned above, i.e. one is content to call anything an egg, that lets a chicken hatch (even if it is purple, cube-shaped and with spikes on its surface) then the egg came first.

If you insist that an egg has to contain all the ingredients of a chicken (not only the chicken DNA) then the chicken might have come first. Again: this is not necessarily the case but a fair possibility!

It all depends. And I guess it is clear that there are many philosophical attempts to answer the riddle out there. However, I wanted to answer the biological basis here. And I feel that chicken and eggs are primarily a biological problem to be solved.

The philosophical answers mostly come to the conclusion that the egg came first [3]. But I am not sure that philosophers necessarily know about DNA, mitochondria, ribosome, calcification and membrane proteins, to name only a few aspects one should be aware of.



It is more likely that the egg came first, which may be why “experts” have decided that the egg came first [2]. However, it would surprise me if they had not addressed the above thoughts and I would be curious to hear what they had to say about it.

However, the question itself is so extremely flawed in terms of its exact definition that it has to be treated rather theoretical of course. Admittedly, following our assumption that we would manage to define it in terms of genetics, the chance that some crucial gene for egg production was the “last gene” in the process of the chicken development is very small. This may be the simple reason the “experts” think the egg came first.

I would like to have you leave from this page keeping in mind that depending on our definition in combination with the right circumstances the chicken might have been there before the egg! And maybe next time you have the discussion coming up you can send them here 🙂










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§ One Response to The chicken or egg problem

  • chrisfong says:

    They should get people to vote for the “definition of a chicken egg” just as they did for the “definition of a planet”. Haha, I dunno. I guess it would be one of the strangest vote in the history of science.

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