Figure 1.

Tree of eukaryotes. A schematic tree summarizing current data on the relationships between major lineages of eukaryotes with selected ancient and recent examples of structural complexity and behaviour. Examples of basic behaviours (and complex behaviours emerging from them) are indicated at the root of the tree, and are predicted to be present in the last eukaryotic common ancestor (LECA). These include (left to right) gliding motility, ontogenetic development, phagocytosis, amoeboid movement, forming endosymbiotic associations and flagellar swimming. These have all existed since the LECA or soon after, and in some cases, all modern examples are homologous (e.g. flagella), whereas others are constantly reinvented in parallel (e.g. forming endosymbiotic associations or gliding motility). Indicated around the branches of the tree are selected examples of complex structures and behaviours (often resulting from strings of basic behaviours) that emerged many times in parallel. These include (left to right) creating cell walls or armour (by aggregating material, crystalization or scaffolding organic material either inside the cell or by secretion), building and expelling extrusomes, intracellular infection, sensory organelles (light sensing eye spots is shown as an example), structural complexity though symmetrical repetition and colony formation (by reproduction or aggregation). These traits have originated many times throughout the evolution of eukaryotes. (Online version in colour.)