The long standing assumption is that complex life started two billion years ago when an archaea cell fused with a bacteria cell to create the first eukaryotic cell. The journal Nature is reporting there is some preliminary evidence to support the hypothesis.
A “Missing Link” – Researchers Shed Light on the Origin of Complex Life Forms
https://scitechdaily.com/a-missing-link-researchers-shed-light-on-the-origin-of-complex-life-forms/