Which came first, the chicken or the egg?

Some people believe that children should be allowed to use their minds as freely and imaginatively as possible, without attention to the tedious laws of rationality. Others think that a child is never too young to get his or her first dose of logical and scientific reasoning. But in any case, a child with the intellectual maturity to ask a question like “which came first, the chicken or the egg?” is probably ready for a valuable lesson in logic and biology…more of a lesson, perhaps, than many of us are ready to give. This little essay aims to change all that, and thereby protect you and your pint-size inquisitors from the perils of ignorance (and specifically, being recognized as an incurable case thereof!).

The question “which came first, the chicken or the egg?” looks at first glance like a matter of straightforward reproductive biology. But before we can even begin to answer this question, we must define our terms. So actually, it is a classic case of semantic ambiguity…a problem of meaning and interpretation. Specifically, while the term “chicken” is biologically unambiguous – we all know what a chicken looks, sounds and tastes like – the term “egg” is somewhat more general and is therefore a possible source of ambiguity. Do we mean (1) just any egg, or (2) a chicken egg? And if we’re talking about a chicken egg, then is a “chicken egg” (2a) an egg laid by a chicken, (2b) an egg containing a chicken, or (2c) both? Reformulating the question to reflect each possible meaning of “egg” leads to four distinct versions of the chicken-or-egg question.

1. Which came first, the chicken or (just any old) egg?

2a. Which came first, the chicken or an egg laid by a chicken?

2b. Which came first, the chicken or an egg containing a chicken?

2c. Which came first: the chicken, or an egg laid by and containing a chicken?

Contrary to popular belief, there is indeed a definite answer to each of these questions. Specifically, the answers are: (1) The egg. (2a) The chicken. (2b) The egg. (2c) The chicken. Given some knowledge of logic and biology, these answers are not hard to verify. To get this show on – or should that be across? – the road, let’s go through them in order.

First, consider question 1: which came first, the chicken or (just any old) egg? This question is answered “the egg” because species that lay eggs have been around a lot longer than modern chickens. For example, we have plenty of fossil evidence that dinosaurs laid eggs from which baby dinosaurs hatched, and dinosaurs predate chickens by millions of years. Indeed, a growing body of research indicates that dinosaurs were among the biological ancestors of chickens!

Now let’s look at question 2a: which came first, the chicken or an egg laid by a chicken? The answer to this question is “the chicken” on semantic grounds alone. That is, if a chicken egg must be laid by a chicken, then before a chicken egg can exist, there must by definition be a chicken around to lay it. And question 2c – which came first, the chicken or an egg laid by and containing a chicken? – is answered the same way on the same grounds; logically, the fact that a chicken egg must be laid by a chicken precedes and therefore “dominates” the (biologically subsequent) requirement that it contain a chicken. So whereas we needed paleozoological evidence to answer question 1, questions 2a and 2c require practically no biological knowledge at all!

Having saved the best for last, let us finally consider the most interesting version, 2b: which came first, the chicken or an egg containing a chicken? This version is interesting because an egg containing a chicken might have been laid by a chicken or a non-chicken, which of course affects the answer. Thanks to modern genetic science, we can now be sure that the egg came first. This is because reproductive mutations separating a new species from its progenitor generally occur in reproductive rather than somatic DNA and are thus expressed in differences between successive generations, but not in the parent organisms themselves. While the somatic (body) cells of the parents – e.g. wing cells, drumstick cells and wishbone cells – usually contain only the DNA with which they were conceived, germ (reproductive) cells like ova and spermatozoa contain non-somatic DNA that may have been changed before or during mating by accidental deletion, insertion, substitution, duplication or translocation of nucleotide sequences. This is what causes the mutation that results in the new species.

Where an animal qualifies as a member of a given species only if its somatic DNA (as opposed to its reproductive DNA) conforms to the genotype of the species, the parents of the first member of a new species are not members of that new species. At the same time, all the biological evidence says that the ancestors of modern chickens were already oviparous or egg-laying…that a male and a female member of the ancestral species of the modern chicken, call this species “protochicken”, mated with each other and created an egg. (Could the first chicken have evolved from a viviparous or live-bearing species, and after being born alive, have started laying eggs? All the biological evidence says “no”.) But because their act of mating involved a shuffling of reproductive genes that were not expressed in the body of either parent – if they had been expressed there, the parents would themselves have been members of the new species – the fetus inside the egg was not like them. Instead, it was a mutant…a modern chicken!

Only two loose ends remain: the “gradual” and “sudden” extremes of the evolutionary spectrum. These extremes are evolutionary gradualism – Darwin’s original slow-paced timetable for natural selection – and punctuated evolution, as advocated more recently by evolutionary theorists including the controversial Stephen J. Gould.

Gradualism says that mutations are biologically random, but subject to a selection process determined by environmental (external) conditions to which species must adapt over the course of many generations. Taken to the limit, it implies either that each minor mutation that occurs during the evolutionary change of one species into another is random and independent of any other mutation, in which case a useful combination of mutations is highly improbable, or that each individual mutation confers a selective advantage on the mutant…that every evolutionary advantage of a new species over its precursor decomposes into smaller advantages combined in a more or less linear way. Unfortunately, this makes it almost impossible to explain complex biological structures that do not break down into smaller structures useful in their own right…structures like bacterial cilia and flagella, and even the human eye.

The hypothetical gradualistic evolution of one species into another via mutations accumulated over many generations leads to the following question: when does the quality and quantity of mutations justify a distinction between “species”…when does a protochicken become a chicken? It’s a good question, but our chicken-or-egg answers remain valid no matter how we answer it.

At the other extreme, evolution sometimes appears to progress by leaps and bounds, moving directly from the old to the new in “punctuated” fashion. And to complicate matters, this sometimes seems to happen across the board, affecting many species at once. The most oft-cited example of punctuated evolution is the Cambrian Explosion. Whereas sedimentary rocks that formed more than about 600 million years ago are poor in fossils of multicellular organisms, slightly younger rocks contain a profusion of such fossils conforming to many different structural templates. The duration of the so-called “explosion”, a mere geological eyeblink of no more than 10 million years or so, is inconsistent with gradualism; new organs and appendages must have been popping out faster than the environment alone could have selected them from a field of random mutations. Clearly, the sudden appearance of a new appendage would leave little doubt about the evolutionary demarcation of ancestral and descendant species.

But the kind of punctuated evolution that occurs between generations is not the end of the line in sheer biological acceleration. Sometimes, an evolutionary change seems to occur within the lifespan of a single organism! For example, in the spirit of “ontogeny recapitulates phylogeny”, insect metamorphosis almost seems to hint at an evolutionary process in which an ancient grub or caterpillar underwent a sudden transformation to something with wings and an exoskeleton…or alternatively, in which a hard-shelled flying bug suddenly gave birth to an egg containing a soft and wormy larva. While that’s not what really happened – as is so often the case, the truth lies somewhere in the middle – what occurred was just as marvelous and just as punctuated.

What seems to have happened was this. Due to a reproductive mutation, a whole sequence of evolutionary changes originally expressed in the fetal development of an ancestral arthropod, and originally recapitulated within the womb and egg it inhabited, were suddenly exposed to the environment, or at least to the hive, in a case of “ovum interruptus”. A fetal stage of morphogenesis that formerly occurred within womb and egg was interrupted when the egg hatched “prematurely”, making the soft fetus into an equally soft larva and giving it a valuable opportunity to seek crucial nourishment from external sources before being enclosed in a pupa, a second egg-like casing from which it later hatched again in its final exoskeletal form. So metamorphosis turns out to be a case of biological common sense, providing the fetus-cum-larva with an opportunity to acquire the nourishment required for the energy-consuming leap into adulthood.

Does this affect our answer to the chicken-or-egg question? Not really. For even where the life cycle of an organism includes distinct morphological stages, the DNA of egg-laying insects does not change after conception. And since it is reproductive and not somatic DNA modification that distinguishes one species from the next in line, our answers stand firm. (Of course, this says nothing of science fiction movies in which something bizarre and insidious causes runaway mutations in the somatic DNA of hapless humans, causing them to evolve into monsters before our very eyes! Such humans have either undergone a random or radiation-induced “meta-mutation” whereby their genetic code suddenly rearranged itself to incorporate a self-modification routine that is executed somatically, within their own cells, or they are the victims of a space virus which inserted such a routine into their DNA for its own nefarious purposes.)

OK…perhaps there’s yet another loose end. Asking which of two things came first implies that time flows in a straight line from past to future (those are the “loose ends”). But what if time were to flow in either direction, or even to loop around, flowing in what amounts to a circle? No more loose ends. In fact, loops have no ends at all! But in this case, the answer depends on whether we’re on the forward or reverse side of the loop, heading towards the future or the past. Another way to formulate this question: does the cause lead to the effect, or is there a sense in which the effect leads to the cause? Suffice it to say that no matter which way we choose to go, the original answers to the four versions (1, 2a, 2b and 2c) of the chicken-or-egg question are all affected the same way. They are either all unchanged or all reversed, with no additional ambiguity save that pertaining to the direction of time (not a problem for most non-physicists and non-cosmologists).

Now that we’ve tied up every last loose end, what about the most important question of all, namely what to tell a curious child? The answer: take your pick of versions. Some kids will prefer the dinosaur angle of version 1; some kids will prefer the “birds and bees” reproductive biology lesson of version 2b. In my opinion, if we limit ourselves to one version only, the most valuable explanation is probably that of 2b; but due to its relative complexity, a younger child can probably derive greater benefit from a T. Rex-versus-Triceratops embellishment of version 1. To exhaust the golden opportunities for logical and scientific instruction, one should of course answer all four versions. But no matter which way you go, make sure the child knows exactly which version(s) of the question you’re answering. If you leave out the one he or she had in mind, you’ll no doubt be egged on until it gets answered!


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