key concept5.5Eukaryotic Cells Evolved in Several Steps

Life on Earth was entirely prokaryotic for about 2 billion years—from the time when prokaryotic cells first appeared until about 1.5 billion years ago, when evidence of eukaryotic cells appears in ancient rocks. The advent of compartmentalization—the hallmark of eukaryotes—was a major event in the history of life. It permitted many more biochemical functions to exist in the same cell than had previously been possible. Compared with a typical eukaryote, a single prokaryotic cell is often biochemically specialized, limited in the resources it can use and the functions it can perform.

106

focus your learning

  • Biologists theorize that the endomembrane system and the cell nucleus originated as the result of infoldings of the cell membrane.

  • The theory of endosymbiosis proposes that organelles arose when cells engulfed other cells, and this arrangement led to symbiotic relationships.

What is the origin of compartmentalization? We will describe the evolution of eukaryotic organelles in more detail in Key Concept 25.2. Here we outline two major themes in this process.