The earliest life forms that emerged, perhaps 4 billion years ago, were simple cells, prokaryotes (meaning that the cells lack a nucleus and other organelles). But after a further 2 billion years of evolution, eukaryotic cells emerged, in which membrane-bound organelles, including the cell nucleus, can be found. And some 600 million years ago another evolutionary transition involving further complexification took place, the one in which multicell organisms—plants and animals—appeared.
