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Hi

Steven Jay Gould have suggested that under the right circumstances

evolutionary leaps can occur in as little as 1,000 years.3 This is a tick of the clock

in evolutionary terms, but if we apply it to the evolutionary leap that is taking

place now – which has been clearly visible for around 300 years – it still means

that, at the very least, we may have to wait several more centuries for a real

transformation to occur. And when we look ahead at the ground we still have to

cover this kind of timescale seems realistic. It seems sensible to think in terms of

centuries rather than decades.

If this is true, the human race’s future looks very bleak indeed, since it’s

clear that we don’t have anything like that much time to play with. But this is

where we come in. There’s an important difference between human beings and

other forms of life, which means that this evolutionary development could happen

much faster than any previous one.

--- What does "this is where we come in" mean?
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Hi,

--- What does "this is where (eg) Tom comes in" mean?

It's a fixed expression, meaning 'this is the point where Tom enters the story, this where Tom becomes important, this is where we should give our attention to Tom'.

eg Many people thought that the USA would never have a black President. 2008 is where Barack Obama came in.

You can say it of things, too. eg

Mary: Oh, it's starting to rain. We're goiung to get wet.

Tom: This is where my pocket umbrella comes in.

The expression may be related to the world of the theatre, where an actor waits for the point in the play where he is supposed to 'come in', ie enter the stage.

Best wishes, Clive

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NewguestIf this is true, the human race’s future looks very bleak indeed, since it’s

clear that we don’t have anything like that much time to play with. But this is

where we come in.
This is where X comes in ~ This is the point in the process where X is used, where X is useful. Example:

There is a bowl on the kitchen counter. Beside it is a whisk and some eggs. I ask my wife what the whisk is for. She says, "Watch". She cracks the eggs, separates the whites, and puts them in the bowl. She says, "To make the eggs foam up, we need to incorporate air. This is where the whisk comes in." She then uses the whisk to beat the egg whites.

So, in the example you quoted, here's the meaning I'm picking up:

But this is the point (in evolutionary history) where we human beings become useful, i.e., become a significant factor. (We may be able to alter the rate of evolutionary change, which other organisms could never do.)

CJ
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Hi

So it means something like "this is where human beings appear", but how does it link with the previous sentence, i.e.:

If this is true, the human race’s future looks very bleak indeed, since it’s

clear that we don’t have anything like that much time to play with. But this is where we come in.

Maybe it says: But this is why we have an important role to play ... or something like that?
 CalifJim's reply was promoted to an answer.
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It's clear now. Thanks.