- I'Beth

- And I'm Phil

- Phil, do you believe there's life on other planets?

- Well, there's a lot of other planets, so, yeah, I think there must be.

- Yeah, I agree. Well, it's a question that interests American astrophysicist Adam Frank, who discussed it with his collegue Gavin Schmidt, director of Nasa's Institute for Space Studies. Here, Adam recalls their conversation for BBC World Service programme CrowdScience.

- We know that there's been no other civilization on Earth, and he stop me and said, "How do you know that?" And my jaw just dropped down to the floor.

- Adm's jaw dropped, and idom for when someone looks shocked and surperised. Could there have been a technologically advanced civilization before us, here on Earth?

- It may sound weird but this idea has a sicentific name. Here is Caroline Steel, presenter of BBC's CrowdScience, to explain:

- The Silurian hypothesis proposes that if there was a technologically advanced civilisation hundreds of millions of years ago, we wouldn't be able to find traces of it.

- A hypothesis is an idea which explains how somethings happens and can be tested to find out if it's correct. In this episode, we'll be discussing the Silurian hypothesis: the idea that a technogically advanced civilisation existed before us on Earth but vanished without leaving a trace. As usual, we'll be learning some useful new words and idoms, and remember, you'll find all the vocabulary on our website, bbclearningenglish.com.

- But first, I have a question for you, Phil. Human civilisation is thousands of years old but the planet itself is much older. So, how old do scientists think the Earth is? Is it a) 3.5 billion years, b) 4.5 billion years, or c) 5.5 billion years?

- I'm going to guess c) 5.5 billion years.

- OK. Well, we'll find the answer out later in the programme. To believe a technologically advanced civilisation existed on Earth hundreds of millions of years ago, most people would probably want evidence. We have evidence about other species from the past, such as the dinosaurs, from fossils, the remains of prehistoric plants or animals that have been preserve in rock for a very long time.

- But according to Adam Frank, fossils couldn't help prove Silurian hypothesis, as he explains to BBC World Service programme CrowdScience:

- Most things are not fossilised. It's only a tiny fraction of Earth's life that has ever become fossilised. So image that you have a 10,000-year-long civilisation, which is a blink of the eye for geology, that's too short to create a lot of fossils.

- Adma argues that only a tiny fraction, meaning a very small amount of life on Earth has turned into fossil.

- In geological time, even a 10,000-year-od civilisation is the blink of an eye, an idiom meaning a very short period of time. In other words, older civilisation might have existed on Earth but not for long enough to leave fossilised evidence.

- However, not everyone is convinced by Adam's ideas. Evidence of our own civilisation, including plastics and man-made materials like concrete, is already being layered into the Earth's crust, and these are going to last a very long time. So, surely a technologically advanced civilisation from prehistory would have left similar marks.

- Well, here's Adam Frank again, answering these objections on BBC Wrold Service programme, CrowdScience:

- What happens is somebody else will pick up on their work, either affirming it and showing new evidence for it or pushing back on it, and once you get to, like 20, 30 or 40 papers, then you have a consensus. You're like, "OK, we've really really looked at this and now we know."

- Adam welcomes new evidence, even evidence which contradicts his ideas, as part of the scientific method needed to prove a hypothesis right or wrong. It's scientists form a consensus, meaning a general agreement, about the issue.

- Personally, I think the idea of prehistoric civilisation on Earth is so mysterious, it's OK to keep an open mind. Now, Isn't it time you reveal your answer to you question Beth?

- I think it is. I asked you, "How old do scientists think the Earth is?" You said, "5.5 billion years," and I'm afriad you were wrong, Phil. It's 4.5 billion years. OK. Let's recap the vocabulary we've learnt, strating with the idiom my jaw dropped, which is used to say some looks very shocked and surperised.

- A hypothesis is a suggestion that's proposed to explain something, which can then be tested to see if it's true.

- Fossils are the remains of prehistorical plants or animals that have been preserved in rock for a very long time.

- A tiny fraction of something is a very small amount of it.

- The idiom the blink of an eye means a very short period of time.

- And finally, a consensus is a general agreement. Once again, our six minutes are up, but if you keen for more, then head over to our website, bbclearningenglish.com, where you'll find a worksheet and a quiz for this episode. See you again soon. But for now, it's goodbye!

- Goodbye!