A species
Gut methane-maker
Methanobrevibacter smithii
The tiny gut guest that turns leftovers into methane.
You and the gut methane-maker last shared an ancestor about 3.5 billion years ago (on 2 March, if life's history were one year).
Living in the guts of most people is a tiny methane-maker that is not a bacterium at all. It is an archaeon, from a separate branch of life. Surprisingly, the way it copies DNA and builds proteins is closer to yours than to bacteria, which is why its branch sits beside the eukaryotes, the group that includes you, on the tree of life.
It lives without oxygen in the large intestine, mopping up hydrogen and carbon dioxide left over when gut bacteria digest tough plant sugars. It joins them together to give off methane. Each cell is a colourless oval rod, far too small to see without a microscope, often sitting in pairs or short chains.
About a thousand cells in a row would span one millimetre, yet in some people they build up into billions of cells.
Farmyard cousins
Close cousins live in cattle and sheep, while this one specialises in people.
Breath clue
The methane it makes leaves the body in breath and wind from the bowel, so a breath test for methane can hint that these methane-makers are present.
Tough walls
Its wall uses a material called pseudomurein, not the material in bacterial walls, so many common antibiotics that attack bacterial walls leave it unharmed.