Because free @entity1522 were most probably available in primitive abiotic environments, their metabolism is likely to have provided some of the very first metabolic pathways of life. What were the first enzymatic reactions to emerge? A cladistic analysis of metabolic pathways of the 16 aliphatic @entity1522 and 2 portions of the Krebs cycle was performed using four criteria of homology. The analysis is not based on sequence comparisons but, rather, on coding similarities in enzyme properties. The properties used are shared specific enzymatic activity, shared enzymatic function without substrate specificity, shared coenzymes, and shared functional family. The tree shows that the earliest pathways to emerge are not portions of the Krebs cycle but metabolisms of @entity492 , @entity2734 , @entity2227 , and @entity1160 . The views of Horowitz (Horowitz, N. H. (1945) Proc. Natl. Acad. Sci. U. S. A. 31, 153-157) and Cord  n (Cord  n, F. (1990) Tratado Evolucionista de Biologia, Aguilar, Madrid, Spain), according to which the upstream reactions in the catabolic pathways and the downstream reactions in the anabolic pathways are the earliest in evolution, are globally corroborated; however, with some exceptions. These are due to later opportunistic connections of pathways (actually already suggested by these authors). Earliest enzymatic functions are mostly catabolic; they were deaminations, transaminations, and decarboxylations. From the consensus tree we extracted four time spans for @entity535 metabolism development. For some @entity1522 catabolism and biosynthesis occurred at the same time ( @entity2574 , @entity367 , Lys, Leu, Ala, @entity2596 , Ile, Pro, @entity534 ). For others ultimate reactions that use @entity1522 as a substrate or as a product are distinct in time, with catabolism preceding anabolism for @entity2734 , @entity7647 , and @entity2569 and anabolism preceding catabolism for Ser, Met, and Thr. Cladistic analysis of the structure of biochemical pathways makes hypotheses in biochemical evolution explicit and parsimonious.

Evolution of XXXX metabolism inferred through cladistic analysis.

@entity1522 :: ('CHEBI:33709', 'Chemical') :: ['amino acids'] @entity2227 :: ('CHEBI:29987', 'Chemical') :: ['glutamate'] @entity1160 :: ('CHEBI:28300', 'Chemical') :: ['glutamine'] @entity535 :: ('CHEBI:33704', 'Chemical') :: ['amino acid'] @entity2569 :: ('CHEBI:17561', 'Chemical') :: ['Cys'] @entity2734 :: ('CHEBI:17196', 'Chemical') :: ['asparagine', 'Asn'] @entity534 :: ('CHEBI:16467', 'Chemical') :: ['Arg'] @entity7647 :: ('CHEBI:32668', 'Chemical') :: ['Gln'] @entity492 :: ('CHEBI:29995', 'Chemical') :: ['aspartate'] @entity367 :: ('CHEBI:18237', 'Chemical') :: ['Glu'] @entity2596 :: ('CHEBI:16414', 'Chemical') :: ['Val'] @entity2574 :: ('CHEBI:22660', 'Chemical') :: ['Asp']
@entity535 :: (CHEBI:33704,Chemical) :: ['amino acid']
