The microbes once lumped as Excavata may hold clues to how complex cells began
Excavata was named for a scooped-out feeding groove on the underside of many single-celled organisms, including the parasites Giardia and Trichomonas. Genetics has since shattered it as a natural group, but some of its members, which lack ordinary mitochondria, now feature in a bold rethink of eukaryote origins.
Alastair Simpson and David Patterson proposed the group in 1999, and Thomas Cavalier-Smith gave it a formal Latin name and rank in 2002. Membership rested on details seen under the electron microscope: many excavates are flagellates carrying two, four or more whips and a microtubule-supported ventral groove used to feed. Some organisms lacking those features were added on genetic grounds. Nearly all are non-photosynthetic; only certain euglenozoans capture light. The Acrasid slime moulds are the only ones that ever cluster into loose multicellular bodies.
Genome-wide analyses split the assemblage into three widely separated lineages: discobids, metamonads and malawimonads. Discoba unites euglenozoans, heteroloboseans and jakobids, and some trees still pair it with the metamonads, while the malawimonads keep wandering to other branches. Scientists therefore now regard Excavata as paraphyletic, an obsolete label rather than a true family branch.
The metamonads are strange. Many anaerobic gut parasites among them have shrunk their mitochondria into hydrogen-producing hydrogenosomes or tiny mitosomes, and the oxymonad Monocercomonoides reportedly has no such organelle at all. The textbook story says eukaryotes began when an archaeon swallowed an alphaproteobacterium that became the mitochondrion.
Caesar Al Jewari and Sandra Baldauf offer a different sequence. Their 2023 tree, built from 186 taxa, places the metamonad Parabasalia near the root of all eukaryotes. They argue an earlier partnership with a delta- or gammaproteobacterium explains odd anaerobic enzymes in metamonads, meaning hydrogenosomes and mitosomes could be older than mitochondria rather than stripped-down versions of them. The rest of the eukaryotes, animals included, would then have acquired the alphaproteobacterium later. Rooting such trees is notoriously hard, and the related Anaeramoebae might yet prove even more basal.
Source: Excavata
