Metazoans

Approximately 1 billion years ago, individual cells started aggregating to form colonies of cells that were the precursors of all animal and plant life that we recognize today.  One billion years ago was Neoproterozoic era (the Tonian period, to be precise).

Other than unicellular fungi, bacteria, archeans, and a few multicellular algae and possible primitive metazoans, there was no life during the Tonian period. These primitive organisms probably inhabited the land immediately around water sources. In general, oxygen levels were lower in the Tonian than during the subsequent Cryogenian and Ediacaran, making the evolution of complex life difficult. Microbes gathered into large, thick colonies called microbial mats. These microbial mats have no modern-day analogues, as any exposed mats today would quickly be devoured by animals.

Early life was most probably single celled. Multicellularity has evolved independently dozens of times in the history of Earth, for example in plants and animals[1]. Multicellularity exists in both prokaryotes and eukaryotes, and first appeared several billion years ago in cyanobacteria. In order to reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cells (i.e. sperm and egg cells), an issue that is studied in developmental biology. Therefore, the development of sexual reproduction in unicellular organisms during the Mesoproterozoic is thought to have precipitated the development and rise of multicellular life

Age estimates prior to this time are tentative.  Before cells aggregated, there was single celled life for probably two billion years and before that, life consisted of replicating strands of RNA.  We are all now aware of how damaging these strands can be as Coronaviruses that usually cause the common cold and other respiratory diseases in humans belong to the family of RNA viruses.