Bya

 ( Units Of Time ) 

BYA, an abbreviation for "billion years ago", is a unit of time commonly used in disciplines such as geology, astronomy, paleontology, and cosmology to denote events that occurred billions of years in the past. It is primarily used in the context of describing the age of the Earth, Solar System, Universe, and major geological or biological milestones in Earth's history.

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Usage The term "BYA" is often used to express time in billions of years before the present (where the "present" is generally considered to be 1950 CE, according to radiocarbon dating conventions).

Gary Brent Dalrymple (2025). 9780804723312, Stanford Univ. Pr. ISBN 9780804723312

For example:

• The formation of the Earth is estimated to have occurred approximately 4.54 BYA.
• The Big Bang is estimated to have taken place around 13.8 BYA.
  Committee on the Origins and Evolution Life (2025). 9780309096270, National Academies Press. ISBN 9780309096270
• Life is believed to have originated on Earth around 3.5–4.0 BYA.
  Andrew H. Knoll (2025). 9780691165530, Princeton University Press. ISBN 9780691165530

The abbreviation may also appear in scientific literature as Ga (from the Latin gigaannum, meaning "billion years") or Gya (giga years ago), though these variants are more common in international and SI unit-compliant literature.

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Alternative Notation

BYA	Billion Years Ago	Non-SI, widely used in English-speaking regions
Ga	Gigaannum	SI-compliant, used in formal scientific literature
Gya	Giga years ago	Less common variant, similar to BYA

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Context in Scientific Fields

• Geology: Used to mark eras and events such as continental formation, mass extinctions, and plate tectonics.
• Astronomy: Used to describe the age of celestial bodies or cosmological events.
• Paleontology: Applied to the timeline of biological evolution, including the rise of multicellular life or the Cambrian explosion.

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See also

• Geologic time scale
• Year
• Before Present

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