Even if you do not know the word ‘eukaryote’, you are already familiar with what they are, because you and nearly all other life forms that you experience with your unaided eyes are eukaryotes. The vast majority of eukaryotes that we knowingly interact with each day, mainly land plants and animals, are large – macroscopic – organisms, usually consisting of trillions of individual cells (Fig. 1). Even using our rather limited senses, we can immediately tell that macroscopic eukaryotes represent enormous diversity on many different levels. However, the true diversity of eukaryotes is far greater than ordinary experiences would lead you to appreciate; most of the many millions of eukaryotic species on Earth are hidden from view, because most eukaryotic life forms are microscopic (Fig. 2; also see the middle four cells in the title image bar). The diversity of these microbial eukaryotes must be discovered and explored with powerful equipment and techniques such as electron microscopy and molecular biology.
Fig. 1. Examples of multicellular macro-eukaryotes, namely animals and land plants. From left to right: a lion (© 2004 Tambako The Jaguar); a brown seaweed, Fucus (© Patrick Keeling); a fern fiddlehead (Sadleria cyatheoides, © 2004 Eric Schuettpelz); a seastar (Ophioderma rubicundum) resting on a coral ( © 1998 M. Benjamin Cowan, Ocean Images); a bee pollinating a flower (© Patrick Keeling); a green vine snake (Ahaetulla nasuta, © 2006 Jayanth Sharma). By the way, one of these pictures is actually not really an animal or plant, can you guess which one? Click here to find out.
Eukaryotes (also referred to as the Eukaryota or the Eukarya) comprise one of the three recognized domains of cellular life, the other two being the Archaea (or Archaebacteria) and the Eubacteria (or Bacteria) (Cavalier-Smith, 1998; Gogarten et al., 1989; Iwabe et al., 1989; Woese, 1987; Woese and Fox, 1977; Woese et al., 1990). Eukaryotes are distinguished from Archaea and Eubacteria in many different ways, but most importantly, the cells of eukaryotes display a much greater degree of structural organization and complexity. Archaeal and eubacterial cells generally lack internal structural organization (with a few notable exceptions, like the cyanobacteria). Eukaryotic cells, by contrast, share several complex structural characteristics. Most of these are parts of two interrelated systems: the cytoskeletal system and a system of membrane-delimited compartments. The cytoskeleton is an elaborate and highly organized internal scaffolding of proteins, such as actin-based microfilaments and tubulin-based microtubules. It also includes several molecular motors, such as kinesins and dyneins that provide the dynamic forces necessary for import and export mechanisms and many different modes of cell locomotion. Internal membrane-delimited compartments include mitochondria and plastids as well as different elements of the endomembrane system: the endoplasmic reticulum, Golgi bodies, vacuoles, and the nuclear envelope. The word ‘eu-karyote’ literally means ‘true kernel’, in reference to the sequestering of the genome into the membrane-bounded compartment called the nucleus.
Fig. 2. Examples of micro-eukaryotes. From left to right: Selenidium serpulae, a gregarine apicomplexan parasite of polychaete worms (© Brian S. Leander); Eucomonympha imla, a parabasalian (© Patrick Keeling); two views of Diploneis, a diatom (© David G. Mann); Polykrikos lebourae, a dinoflagellate (© Mona Hoppenrath, Sarah Sparmann and Brian S. Leander); Trepomonas sp., a diplomonad (© Ivan Cepicka).
With these basic building blocks, eukaryotes have evolved an amazing array of structural and behavioral characters. One of the most significant innovations is the ability to engulf and internalize particles and other cells, a process called endocytosis or phagocytosis (literally meaning ‘cell eating’). This mode of nutrition opened up many new predatory niches that ultimately facilitated the formation of permanent associations between very different life forms via endosymbiosis (Stanier, 1970). Endosymbiotic associations have provided eukaryotes with much of their central metabolism, which has remained relatively conserved throughout the group's history. Overall, Archaea and Eubacteria show tremendous diversity in their metabolic capacities, but fairly limited morphological and behavioral diversity; conversely, eukaryotes share relatively similar (albeit sloppy) metabolic machinery but have undergone tremendous evolutionary diversification in morphology and behavior.