In astronomy, geology, and paleontology, bya is an acronym for billion years ago and is used as a unit of time to denote length of time before the present. Specifically, one bya is equal to 10⁹ years ago.

The "billion" in bya is, by convention, the short-scale billion (10⁹) of American preference, not the long-scale billion (10¹²) of British usage. For this reason, there is some potential for confusion; and some scientists prefer the unit Gya, while others prefer Ga (Giga annum). However, bya remains in more widespread use.

It should be noted that like the related unit mya ("million years ago"), bya is traditionally written in lowercase. This is different from Gya, which capitalizes the first letter.

Events around 1 to 13 bya:

Astronomic light years:

• The Big Bang was 13.7 bya, starting astronomy
• The Milky Way was "born" 13.6 bya
• The Hubble Ultra Deep Field image light started >13 bya
• The Hubble Deep Field images light started ~12 bya
• The solar system sun was born 4.59 bya

Geologic Precambrian eons and Paleontology:

• Hadean 4.567 to 3.80 bya
  • The Earth formed 4.56717 bya, starting geology (Giant impact hypothesis)
  • Oldest known mineral formed 4.404 bya
• Archean 3.8 to 2.25 bya
  • Cyanobacteria possibly started photosynthesis ~3.8 bya
  • Prokaryotes (Bacteria) definitely formed 3.5 bya
• Proterozoic 2.25 to 0.5 bya
  • Eucaryotes formed 2.1 to 1.6 bya
  • First multicellular life started 0.65 bya
  • Neoproterozoic Snowball Earth was 0.6 bya

• Supercontinents:
  • Ur formed ~3.0 bya
  • Kenorland formed 2.6-2.0 bya
  • Hudsonia formed 1.8-1.4 bya
  • Rodinia formed ~1 Bya and broke up ~0.75 bya
  • Pannotia formed ~0.5 bya
  • Pangaea formed ~0.25 bya

See also

• annum
• Before Present
• Gya
• mya (unit)

Units of time

Bya