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In astronomy, geology, and paleontology, bya or "b.y.a." is an acronym for billion years ago. This abbreviation is commonly^{citation needed} 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.

Like the related unit mya ("million years ago"), bya is traditionally written in lowercase. Ga or Gya has a capitalized first letter instead.

Events around 1 to 13 bya:

Astronomic light years:

• The Big Bang was 13.7 bya
• The Milky Way formed 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 (Giant impact hypothesis)
  • Oldest known mineral formed 4.404 bya
  • Oldest known life form appeared around 4.0 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 1.2 bya
  • Neoproterozoic Snowball Earth was 0.6 bya

• Supercontinents:
  • Vaalbara formed 3.6 bya
  • 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

• Myr
• annum
• Before Present
• Gigaannum or Ga
• Gya
• mya (unit)
• Orders of magnitude (time)#Years

Notes

Wikipedia content modification information:

• This page was last modified on 10 October 2008, at 04:58.

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