Such differences include: (i) their membrane lipids (isoprene ethers versus fatty acid esters) [
123], (ii) their cell walls (peptidoglycan versus S-layer) [
124], (iii) their DNA maintenance machineries [
116,
125], (iv) the 31 ribosomal proteins that are present in archaebacteria but missing in eubacteria [
126,
127] (v) small nucleolar RNAs (homologues found in archaebacteria but not eubacteria) [
128], (vi) archaebacterial versus eubacterial-type flagellae [
129], (vii) their pathways for haem biosynthesis [
130,
131], (viii) eubacterial- versus archaebacterial-specific steps in the shikimate pathway [
132,
133], (ix) a eubacterial-type methylerythrol phosphate isoprene pathway versus an archaebacterial-type mevanolate isoprene pathway [
134], (x) a eubacterial-type fructose-1,6-bisphosphate aldolase and bisphosphatase system versus the archaebacterial bifunctional aldolase-bisphosphatase [
135], (xi) the typical eubacterial Embden–Meyerhoff (EM) and Entner–Doudoroff (ED) pathways of central carbohydrate metabolism versus the modified EM and ED pathways of archaebacteria [
136], (xii) differences in cysteine biosynthesis [
137], (xiii) different unrelated enzymes initiating riboflavin (and F
420) biosynthesis [
138], and (xiv) in very good agreement with
figure 2b, different, unrelated, independently evolved enzymes in core pterin biosynthesis [
139], to name a few examples. The pterin biosynthesis example is relevant because the cofactors H
4F, H
4MPT and MoCo, which are central to the eubacterial and archaebacterial manifestations of the Wood–Ljungdahl pathway are pterins (
figure 2d), suggesting that methyl synthesis occurred geochemically (non-enzymatically) for a prolonged period of biochemical evolution.