Complex sentence: Archaea is a domain of organisms that are prokaryotic, single-celled, and are though to have developed 4 billion years ago, sharing a common ancestor with bacteria, and are more closely related to eukaryotes in comparison to bacteria.
Split into simpler sentences: Archaea is a domain of organisms that are prokaryotic, single-celled, and are thought to have developed 4 billion years ago. They share a common ancestor with bacteria, but are more closely related to eukaryotes in comparison to bacteria.