The Tonian (Greek: "stretch") period is the first geologic period of the last billion years, stretching from 1,000 million to 850 million years ago. The Tonian is the first period of the longer Neoproterozoic era, which also includes the Cryogenian and the Ediacaran. Unlike many geologic periods which come after it, the Tonian is not defined based on rock layers (stratigraphy), but radiometric dating.
The Tonian period, like the rest of the Neoproterozoic, was in general a cold period. The average temperature may have been 10 °C (41 °F) colder than today, especially in continental interiors. Possible trace fossils of small metazoans, such as 1-mm nematode-like worms, have been found dating to the Tonian and even 200 million years before, in the Stenian period. Although no body cast fossils of animals date to the Tonian period, this may be because organisms were very small, soft, and fossilized poorly. Multicellular algae fossils have been recovered from the Tonian period.
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.
The most common fossils from the Tonian period are acritarchs, mostly remains of planktonic algae and other unknown but related organisms. Acritarchs appear in the fossil record over 1400 million years ago, blooming in diversity over the subsequent 600 million years. Around 800 million years ago, shortly after the Tonian, the Sturtian-Varanger glacial event took place, covering the planet in a layer of ice and sharply curtailing acritarch diversity. By the time the planet started to warm up 630 million years ago, the first complex multicellular organisms began to emerge.