Recently I wrote about research into growing muscle cells in the lab to provide animal-free meat. At £50,000 per kilogramme and lacking blood, fat or the texture that whets carnivorous appetites, test-tube meat isn't economically viable. We still have a long way to go to compete with nature.
I was reminded of this on a recent jaunt up Ben Lomond.

My hiker's lunch comprised an excellent roast-beef sandwich. I ate it while watching a herd of cows benignly chewing away at the grass. Cows break grass down to its chemical constituents. They then reconfigure these into new molecular structures that assemble into "cow". In digesting my sandwich, I then convert cow to "man". Life forms operate through enzymes, which perform these astonishing acts of transmogrification. This is biochemistry: the constant inter-conversion of chemical forms by living entities. Grass is made of water, carbon dioxide (from air) and nitrogen, with trace amounts of other elements absorbed from the soil. Sunlight provides the energy that drives the chemical combinations of small chemicals into larger ones that yield the grass that convert to cow, then to man.

The ancient Greek notion that we all derive from earth, air, wind and fire was close to the truth. Harnessing these "classical elements" remains the greatest challenge to human sustainability.