Eukaryotes are a type of organism that contains complex cells, in which the genetic material is enclosed within a nuclear membrane. Eukaryotes include all plants, animals, fungi and protists, but exclude bacteria and archaea.
The defining feature of eukaryotic cells is their membrane-bound nucleus, which contains the cell’s DNA. The nucleus is surrounded by a double membrane called the nuclear envelope, which separates the chromosomes from the cytoplasm. Other features of eukaryotic cells include mitochondria (organelles that generate energy for the cell) and Golgi apparatus (an organelle involved in lipid synthesis and secretion). Eukaryotic cells also have a cytoskeleton, which provides structure and support for the cell.
Eukaryotes are generally larger and more complex than prokaryotes (bacteria and archaea). They can range in size from single-celled organisms to multicellular organisms such as humans. Multicellularity has evolved independently several times in different groups of eukaryotes.
The first evidence for eukaryotic life comes from fossils of cyanobacteria that date back 2.7 billion years ago. These fossils show structures that are similar to modern-day cyanobacteria, including internal membranes that may have been used to separate different functions within the cell. The first unambiguous fossil evidence for eukaryotes dates back 1.6–1.8 billion years ago and comes from microfossils known as acritarchs. Acritarchs are thought to represent early protists, although their exact identity is still debated by scientists.
The study of eukaryotes is known as eukariology or protistology (depending on whether one focuses on unicellular or multicellular organisms). Eukariologists have made significant progress in understanding the diversity and evolution of these fascinating creatures over the past few decades, thanks to advances in molecular techniques such as DNA sequencing