A scheme for cell evolution. Polypeptides are in green. a) The earliest cells, formed at cool hydrothermal vents: a leaky iron sulfide membrane, its metabolism assisted by phosphate and FeS-binding intracellular peptide nests shown as semicircles. b) More advanced cells, but now superceded: an amyloid peptide membrane shown as a mixture of apolar β-sheet and polar α-sheet; a semipermeable membrane with H+ or K+ channels at the edges of α-sheets as in Fig a stabilises the environment; intracellular nests also occur, some attached to amyloid. c) Current cells: a phospholiplid or etherlipid membrane plus, often, an outer cell wall. Ion channels resembling those of the earlier α-sheet are visible in the integral membrane transporter proteins. Nests occur as part of proteins.