It's not easy being Green: Zack Polanski

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  The Economist.

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  Hello and welcome to The Intelligence from The Economist.

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  I'm Rosie Blaw.

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  Today on the show, the first cells with no biological ancestors,

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  and why we still use human travel agents.

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  But first,

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  Biology is the study of life.

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  But what really is life?

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  Oliver Morton is our planetary affairs editor.

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  Animals are one recognizable form, as are plants, fungi, bacteria, other sorts of single-celled organisms.

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  And that's the clue.

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  Single-celled organisms.

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  At the simplest level, life is a cell.

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  A bubble of biological membrane full of molecules with purpose.

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  This single unit of life can grow and reproduce.

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  Over time, populations of cells can evolve under natural selection.

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  That opens up a crucial connection.

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  Once you add that to the origin of species and to Darwin,

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  you get this extraordinary vertiginous feeling

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  that all cells are descended from earlier cells,

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