Cells make up the basic part of every living organism. The first cells on Earth were prokaryotic, meaning they lacked a nucleus and cell membrane. They then evolved into eukaryotic cells, or complex cells. In the world of astrobiology, many theorists consider this split from simple celled organisms to complex as “the great filter” that has prevented life from evolving into complex intelligent lifeforms, which explains why we don't see alien life spread throughout the galaxy.
