The solid-state conformations of the C-3 acetates of two isomeric hopanoids-1, isoarborinol (D:C-friedo-B1:A1-3β,5α,8α,10β,13β,14α,17β,18α,21β) and 2, motion (D:C-friedo-B1:A1-neogammacer-7(8)-en-3β-ol[3β,5α,9α,10β,13α,14β,17α,18β,21α])-have been determined by X-ray crystallography. The data show that whereas both molecules are planar, 1 orients into a chair-halfchair-chair-chair-halfchair conformation while 2 orients into a chair-sofa-twist-halfchair-halfchair conformation. To explain the biogenesis of 1 and 2 from squalene oxide, a step-wise mechanism is proposed which proceeds through the protosteroid cation (for 1) and dammarenyl cation (for 2). After ring enlargement from the corresponding 13(17)bond followed by concerted 1,2-migrations and loss of the 11β-H and 7β-H as protons, respectively, a 9,11-double bond (in 1) and a 7,8-double bond (in 2) is introduced into the nucleus. The mechanism is discussed in relation to the classical view of a non-stop cyclization process where, for example, squalene oxide folds in a chair-chair-chair-chair-boat conformation to give a cyclized product (motiol) presumably with the same conformational disposition as the cyclizing material. The three-dimensional geometry of 1 and 2 was found to be structurally dissimilar from sterols. For instance, 1 and 2 are shorter and volumetrically smaller molecules than cholesterol, and this may explain their diminished importance as membrane inserts compared with sterols in eukaryote evolution.