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Scientists are still trying to answer the age-old chicken or the egg question

DON GONYEA, HOST:

It's a question that has scrambled our brains for years. What came first, the chicken or the egg? Well, a team of scientists at the University of Bristol in England thinks it has cracked the code. One of those researchers is Michael Benton. He's a professor at the university's School of Earth Sciences, and he joins us now. Welcome to the show.

MICHAEL BENTON: Thank you very much.

GONYEA: Mild apologies for the puns there. You've heard them all, I'm sure.

BENTON: Not at all.

GONYEA: We are all itching to know. Was it the chicken or the egg? What did you find?

BENTON: Yeah, well, the quick answer is the egg, because the chicken is just one of many birds. And of course, all birds, as we well know, lay an egg with a hard shell. So if you're familiar with your breakfast egg, that's pretty much the same for all birds. And therefore, that's likely to be the egg that was laid by the very first bird all the way back in time. So one quick answer is the egg comes first. However, our research suggests that actually if you go deeper in time, that's not the whole story.

GONYEA: OK. Let's dive into the whole story, though. I mean, your initial answer was the egg, but then you said, wait a minute.

BENTON: Yes.

GONYEA: What's the wait-a-minute part?

BENTON: Many listeners may be aware of the fact that we know of eggs from dinosaurs. There are many nests of football-sized eggs. But a few years ago, some colleagues pointed out that the first dinosaurs very likely produced parchment-shelled eggs. Those are soft-shelled eggs. And the secret is something that's been known to lizard aficionados for a long time, which is that many of them possess an ability to retain the embryos inside. So the mother holds the young and releases them at a particular point. This is called extended embryo retention, EER. What we discovered is that property of extended embryo retention goes all the way back to the start of the reptiles. So it was not the hard-shelled egg. It was extended embryo retention that they were doing.

GONYEA: So help me put this in the simplest terms possible. We know hard-shell eggs. We even know softer-shell eggs where it's more like a membrane. But you're saying these eggs themselves weren't always there as part of the birth or as part of the protection...

BENTON: Yeah.

GONYEA: ...Of the embryo. So they evolved as well.

BENTON: So I think the easiest way to contrast it is that the standard textbook story is that the first reptile was hugely successful because it laid a hard-shelled egg like a chicken egg, and that acted as a private pond. This is a phrase people have often used, a private pond, contrasting with the amphibians that came before. We think of frogs and salamanders. They lay their eggs in water. They tend to abandon them, and the young grow up like little fishy things in the water. And then they eventually leave, which is the reason that amphibians have to stay close to water. So the whole business in the story of the evolution of life was sometime in the Carboniferous, 300 million years ago, some of the earliest reptiles got this ability to get away from the water and therefore to conquer the whole of the landscape. Wrong. It wasn't the private pond. It was the extended embryo retention and live birth. That seems to be the primitive state for reptiles.

GONYEA: Why are these findings important?

BENTON: I think it's important because the origin of reptiles and then the later origin of dinosaurs and birds and mammals really changed the world. You know, we look around the landscape, and everywhere you look, you see birds and mammals. And, you know, one way or another, it's not just us saying that we're important. Therefore we pay attention to these creatures.

GONYEA: Do you think the old expression - what came first, chicken or the egg? - will go away? It feels like one of those things that's going to persist.

BENTON: No, I don't think so. It's a classic pub quiz question. So whether it's like the why did the chicken cross the road, I've got no idea. But I expect people will keep asking and keep answering it in different ways.

GONYEA: That was my next question, by the way (laughter).

BENTON: That's one I can't answer.

GONYEA: All right. Michael Benton is a professor at the University of Bristol's School of Earth Sciences. Thank you for being with us and for explaining all this.

BENTON: Thank you very much.

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Hadeel Al-Shalchi
Hadeel al-Shalchi is an editor with Weekend Edition. Prior to joining NPR, Al-Shalchi was a Middle East correspondent for the Associated Press and covered the Arab Spring from Tunisia, Bahrain, Egypt, and Libya. In 2012, she joined Reuters as the Libya correspondent where she covered the country post-war and investigated the death of Ambassador Chris Stephens. Al-Shalchi also covered the front lines of Aleppo in 2012. She is fluent in Arabic.