Emergence of Eukaryotes and a New Life Pattern

The first reasonable evidence for life on earth dates from approximately 3.5 billion years ago. The first cells were prokaryotic bacterialike organisms. The early prokaryotes diversified greatly over an enormous time span; their descendants now belong to two groups, the Eubacteria and the Archaea. One lineage within the ancient prokaryotes also gave rise to the first eukaryotic form. The key steps in the evolution of a eukaryotic cell from a prokaryotic ancestor involved symbiogenesis, a process whereby one prokaryote engulfed, but did not digest, another. The engulfed cell was eventually reduced to an organelle inside the host cell. The eukaryotic products of symbiogenesis include mitochondria and plastids.

A mitochondrion originated from an aerobic prokaryote capable of deriving energy in the presence of environmental oxygen. An anaerobic bacterium that engulfed such an aerobic form would gain the capacity to grow in an oxygen-rich environment. The engulfed aerobic bacterium would persist inside the cell as a mitochondrion with its own genetic material. Over evolutionary time, most, but not all, genes from the mitochondrion came to reside in the host cell nucleus. Almost all present-day eukaryotes have mitochondria and are aerobic.

The eukaryotic plastid originated when a cell engulfed a photosynthetic bacterium. When a prokaryote is engulfed and modified to become a eukaryotic organelle, we say the organelle developed via primary endosymbiosis. The chloroplasts in red algae, and in green algae and multicellular plants, arose this way. However, in some cases, a eukaryotic cell may obtain plastids from another eukaryote. This is secondary endosymbiosis. Two similar cells may have formed very differently, so it is not easy to untangle the evolutionary relationships among the diverse array of unicellular forms we now see.

The assemblage of eukaryotic unicellular organisms is collectively called protozoa. The inclusion of “zoa” in the name refers to two animal-like features: the absence of a cell wall, and the presence of at least one motile stage in the life cycle. However, the plant-animal distinction is not easily made in unicellular forms because many motile unicells carry photosynthetic plastids. The myriad of ways to live as a unicellular organism is fascinating, beguiling, and a little bewildering.