"Extremophiles" is a bit of a parochial term - this is the name for organisms that live happily in environments that we consider extreme. Too cold, too hot, too acid, too something to support life, in our terms. Studying the lifeforms that disagree with us on what is a good place to live has started a new field of astrobiology and a new appreciation of the possibility of life existing in the wider universe.
In the end nearly all life on Earth depends on sunlight for its energy source. Heterotrophs like ourselves are a step or two away from the sunlight, but ultimately it's the process of photosynthesis that fuels our food and thus ourselves. Photosynthesis also, as a byproduct, provides the air we breathe. The three experts who talked about it on In Our Time were Nick Lane (University College London), Sandra Knapp (Natural History Museum) and John Allen (Queen Mary, University of London).
This week we listened to the In Our Time programme on the cell while we had our Sunday morning breakfast. This is a subject about which I know rather more than the average educated layperson*, so I was curious to see if it'd hold up as still being interesting. It did :)