BBC: Did a civilisation exist on Earth before humans?

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 colleague 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 stopped me and said. 'How do you know that?' Any my jaw just dropped dwon to the floor. 

Adam's jaw dropped- an idiom for when someone looks shocked and surprised. Could there have been a technologically advanced civilization before us, here on Earth?

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

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

A hypothesis is an idea which explains how something 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 technologically advanced civilization existed before us on Earth but vanished without leaving a trace. As usual, we'll be learning some useful new words and idioms, and remember- you'll find all the vocabulary on our website, bbclearningenglish.com.

But first, I have a question for you, Phil. Human civilization 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 will find the answer out later in the programme. To believe a technologically advanced civilization 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 preserved in rock for a very long time. 

But according to Adam Frank, fossils won't help prove the Silurian hypothesis, as he explains here 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, imagine that you have a 10,000 year-long civilization, which isa blink of the eye for geology- that's too short to really create a lot of fossils.

Adam agrues 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-old civilization is the blink of an eye- an idiom meaning a very short period of time. In other words, older civilizations 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 civilization, 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 time. So, surely a technologically advanced civilization from prehistory would have left similar marks. 

Well, here's Adam Frank again, answering these objections on BBC World 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 how scientists form a consensus- meaning a general agreement- about the issue. 

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