Characteristics of Eukaryotes
Data from these fossils, as well as from the study of living genomes, have led comparative biologists to conclude that living eukaryotes are all descendants of a single common ancestor. Mapping the characteristics found in all major groups of eukaryotes reveals that the following characteristics are present in at least some of the members of each major lineage, or during some part of their life cycle, and therefore must have been present in the last common ancestor.
- Cells with nuclei surrounded by a nuclear envelope with nuclear pores: This is the single characteristic that is both necessary and sufficient to define an organism as a eukaryote. All extant eukaryotes have cells with nuclei.
- Mitochondria: Most extant eukaryotes have "typical" mitochondria, although some eukaryotes have very reduced mitochondrial “remnants” and a few lack detectable mitochondria.
- Cytoskeleton of microtubules and microfilaments: Eukaryotic cells possess the structural and motility components called actin microfilaments and microtubules. All extant eukaryotes have these cytoskeletal elements.
- Flagella and cilia: Organelles associated with cell motility. Some extant eukaryotes lack flagella and/or cilia, but their presence in related lineages suggests that they are descended from ancestors that possessed these organelles.
- Chromosomes organized by histones: Each eukaryotic chromosome consists of a linear DNA molecule coiled around basic (alkaline) proteins called histones. The few eukaryotes with chromosomes lacking histones clearly evolved from ancestors that had them.
- Mitosis: A process of nuclear division in which replicated chromosomes are divided and separated using elements of the cytoskeleton. Mitosis is universally present in eukaryotes.
- Sexual reproduction: A meiotic process of nuclear division and genetic recombination unique to eukaryotes. During this process, diploid nuclei at one stage of the life cycle undergo meiosis to yield haploid nuclei, which subsequently fuse together (karyogamy) to create a diploid zygote nucleus.
- Cell walls: It might be reasonable to conclude that the last common ancestor could make cell walls during some stage of its life cycle, simple because cell walls were present in their prokaryote precursors. However, not enough is known about eukaryotes’ cell walls and their development to know how much homology exists between those of prokaryotes and eukaryotes. If the last common ancestor could make cell walls, it is clear that this ability must have been lost in many groups.