Earth’s biodiversity has never been static. Over 3.5 billion years of life’s history, species have appeared through speciation and disappeared through extinction, with the balance between these processes determining biodiversity at any given time.
The fossil record provides our primary evidence of past biodiversity and its changes:
The record shows:
- Life began ~3.5 billion years ago (single-celled prokaryotes)
- Multicellular life emerged ~600 million years ago
- Biodiversity has generally increased over time, punctuated by dramatic collapses
A mass extinction is an event in which a large proportion of species goes extinct in a geologically short timeframe (usually defined as >75% of species lost within ~2 million years).
| Event | Time (MYA) | Species Lost | Likely Cause |
|---|---|---|---|
| End Ordovician | 443 | ~85% | Ice age + sea level fall |
| Late Devonian | 375 | ~75% | Multiple causes, possibly volcanism |
| End Permian | 252 | ~96% | Volcanic eruptions (Siberian Traps) |
| End Triassic | 201 | ~80% | Volcanism, climate change |
| End Cretaceous (K-Pg) | 66 | ~76% | Asteroid impact + volcanism |
The End Permian extinction was the most severe — Earth came close to losing all complex multicellular life.
Following each extinction, surviving lineages radiate into vacated ecological niches — a process called adaptive radiation:
Current extinction rates are estimated at 100–1000 times the background rate, leading many scientists to designate the present as the beginning of a sixth mass extinction, driven primarily by human activities rather than natural events.
REMEMBER: The fossil record is incomplete and biased towards organisms with hard parts that lived in depositional environments. When interpreting fossil evidence of past biodiversity, acknowledge these limitations. VCAA expects critical engagement with the evidence.
