To get to the answer to this question, we need to understand a few things about the development of life on Earth. First we can make the observation that there are variations among individual animals (or plants)—even within the same species. Just as no two people are identical (although identical twins come close), no two whales are identical, no two pine trees are identical, and so on. There are variations among organisms (see Figure 1). In this article I’ll use the term organisms to cover all kingdoms of life—plants, animals, fungi, etc., but you can use terms that are appropriate for your students. So, where I say “organisms,” you might say “animals” or “plants” to your students. Where I say “reproduce,” you can say, “have babies.” Where I say “variations,” you can say that there are always differences among animals, even for the same type of animal. And, since a chicken is a bird, and birds are in the animal kingdom, it’s animals that we’re talking about when discussing the chicken-and-egg question.

One thing you can do with students is have them make observations of plants and animals to compare the diversity of life in different habitats. Looking at organisms within the same species, they can look for traits that are similar, and they can look for differences. How did they come to have those differences? At your grade level, you might not be teaching much about DNA, but you can say in simple terms that our bodies contain instructions that tell it how to grow, what it can do, what color our eyes are, and so on. And the important point for understanding why organisms have variations is that these instructions can change as they are passed from parents to babies. By around third grade, students start to learn that many characteristics of organisms are inherited from their parents. So you have some similarities to your parents. But you also have some differences.

As an analogy for seeing how easy it is for instructions to change, I’m sure you remember the childhood game of “telephone.” Ask the first student to whisper instructions to the next student for performing some task, who will then whisper the instructions to the next student, and so on. At higher grade levels, the instructions should be at least a few sentences long. For example, you could ask the student to provide instructions for tying a shoe or to give directions for getting from home to school. A single word or sentence might work for younger students. A sentence like “Dogs dig holes for big bones” is sure to end up mangled. The discussion after the final, garbled instructions are compared to the original should include the fact that the changes in the information occurred gradually. Each student might have made only a very slight change, but the cumulative effect is that you end up with something very different. You can then relate that to the gradual changes that can occur when information about growth and development is passed from parent animals to their babies and then, eventually to their babies, and so on (see Figure 2). These small changes in the body’s instructions are called mutations. After many mutations occur over many, many generations, a different kind of animal (i.e., a new species) can result that doesn’t resemble the original. Scientists have seen this happen!

While most mutations are either harmful or inconsequential, occasionally an organism is born with some difference that is advantageous. That organism will be better able to survive and reproduce, which means that the helpful mutation becomes more widespread. In this way, variations in organisms that are beneficial can result in species gradually becoming bigger, more complex, or smarter. Indeed, when people look at older fossils, found deeper in the ground, they see that life on Earth started out simpler, and larger, more complex organisms made their appearance as time went on (see Figure 3).

One last thing to understand is that there is competition between organisms and between species. In any environment, there’s only so much space, food, water, etc. Plants or animals will reproduce and increase their numbers until there’s not enough of these resources for them to increase their numbers any further. And then there starts to be a lot of competition for those resources. (Sometimes humans start wars over access to limited resources.) Most species end up pushing the limits of how much they can reproduce and spread out. They fight for survival, and only the organisms with the most favorable traits will survive and continue to reproduce. In the plant kingdom, sometimes an invasive species better equipped to compete will drive out another species. This kind of fighting for survival can end with a species becoming extinct. Ninety-nine percent of all species that ever lived on Earth are now extinct. Caution: Don’t describe this as “survival of the fittest,” which can lead to misconceptions.

We can summarize this discussion with two key statements and a conclusion:

• Key Statement #1: More organisms are produced than can survive.
• Key Statement #2: Organisms have variations, and these variations are inherited.
• Conclusion: The organisms with the most favorable variations are the ones that will survive. For this reason, life changes over time.

You now know enough to answer the chicken-and-egg question. But, to answer the question, we need to be precise about our terminology. The answer depends on how you define egg. Let’s say we’re talking about a chicken egg. The term chicken egg could mean two different things. It could mean (1) an egg layed by a chicken or (2) an egg out of which a chicken will hatch. You can see that, by definition 1, the chicken must come first. However, by definition 2, the egg came first. In that case, an “almost-chicken” layed an egg, which had the right mutation so that when that egg hatched, there was a baby chicken! So, tell me how you define chicken egg, and then we can say which one came first.

Isn’t it “egg-celent” how the methods of science allow us to answer so many questions?! Do you have a science question that you haven’t been able to answer? Send it to me, and we’ll see if we can answer it.

Never stop learning. ●

Matt Bobrowsky is the lead author of the NSTA Press book series, Phenomenon-Based Learning: Using Physical Science Gadgets & Gizmos. You can let him know if there’s a science concept that you would like to hear more about. Contact him at DrMatt@msb-science.com .