This is why scientists cannot be certain about how life began on Earth.

The idea

Fossils give evidence about past life, but the earliest record is patchy. Most organisms that ever lived left no fossil because decay, scavengers and geological processes destroyed their remains, and soft-bodied organisms rarely fossilise. Conditions must usually be right: rapid burial, low oxygen, or preservation in ice, amber or acidic peat. The oldest rocks have been recycled by plate movement, heat and pressure, erasing early traces. Even where fossils exist, dating them and deciding whether a structure is a true fossil or a mineral pattern can be difficult. So several plausible ideas, such as life forming in deep-sea vents or shallow pools, remain hypotheses rather than certainties.

What earns the marks

• Fossil formation is rare because it needs specific conditions such as rapid burial and little oxygen, so the fossil record is incomplete.
• Soft-bodied organisms and very early life forms decay quickly and seldom leave fossils, removing key evidence about life's origins.
• Geological processes such as plate movement, erosion, heat and pressure destroy or recycle the oldest rocks and their fossils.
• Fossils can be fragmentary or hard to date, so interpreting them involves uncertainty and competing hypotheses.
• Scientific explanations of how life began are hypotheses supported by indirect evidence, not proven facts, because the events cannot be repeated or directly observed.

Common mistakes and how to put them right

• Thinking that every organism that ever lived becomes a fossil.

  Put it right by stressing that fossilisation is rare and needs particular conditions.

• Assuming the fossil record is complete and simply not yet fully collected.

  Put it right by explaining that many fossils have been destroyed by geological processes.

• Treating one hypothesis about the origin of life as established fact.

  Put it right by describing the evidence as indirect and the explanations as provisional.

Exam tips

• Link each reason for uncertainty to a specific process, such as decay or plate movement, rather than writing only that evidence is missing.
• Use words such as hypothesis, incomplete record and indirect evidence to show understanding of scientific uncertainty.
• If asked to suggest why scientists disagree, give at least two distinct reasons and explain how each one removes or obscures evidence.