Excavata

Lore & Background

Excavata was proposed based on ultrastructural features, particularly a conspicuous ventral feeding groove supported by microtubules, which gives the organisms their name. Most excavates are unicellular, heterotrophic flagellates, though some Euglenozoa are photosynthetic. Many have two, four, or more flagella, and some anaerobic intestinal parasites have greatly reduced mitochondria, with some lacking classical mitochondria entirely and instead possessing hydrogenosomes or mitosomes. The Acrasidae slime molds are the only excavates to exhibit limited multicellularity, assembling into larger clusters at times.

Reader's Guide

Excavata's significance lies in its role as a proposed grouping for a diverse set of unicellular eukaryotes, including important human parasites. However, the group was always considered uncertain in its relationships, and phylogenomic analyses eventually showed it to contain three widely separated groups: discobids, metamonads, and malawimonads. The group is now considered paraphyletic, with some analyses placing malawimonads on a different branch. Despite its obsolescence, the concept of Excavata has influenced the study of eukaryotic evolution, particularly regarding the origin of mitochondria and the relationships among basal flagellate lineages. The group's diversity in mitochondrial forms—from classical mitochondria to hydrogenosomes and mitosomes—has provided insights into the evolution of these organelles. The ongoing uncertainty about the placement of malawimonads and ancyromonads highlights the complexity of early eukaryotic phylogeny.

Did You Know?

• Excavata was first suggested by Simpson and Patterson in 1999 and named by Thomas Cavalier-Smith in 2002.
• The group includes important human parasites such as Giardia and Trichomonas.
• Most excavates are heterotrophic flagellates, but some Euglenozoa are photosynthetic.
• The Acrasidae slime molds are the only excavates to exhibit limited multicellularity.