For any mass extinction, the ultimate question is, of course, how? What was the trigger and chain of events that caused such a massive upset of Earth’s biota? Though this question may be the most compelling of the lot, it is also the most difficult to answer. To get a handle on it, scientists figure out what sorts of observations they would expect to make today if a particular hypothesis about the cause of a mass extinction were true and then see if those expectations are borne out. For an example, consider the idea that an asteroid impact contributed to the end-Cretaceous extinction. If this hypothesis were true, we’d expect:

• To observe dust from the meteorite deposited in the rocks of the end-Cretaceous
• To discover a crater from the impact dating to the end-Cretaceous
• To observe debris from the impact dating to the end-Cretaceous
• To find evidence of shockwaves from the impact dating to the end-Cretaceous
• To find glass particles that were produced by the heat of the impact dating to the end-Cretaceous
• Models to predict large-scale environmental disturbances resulting from the impact of a massive asteroid

All of these expectations were borne out and constitute strong evidence that an asteroid impact occurred and was a cause of the end-Cretaceous mass extinction. When multiple lines of evidence (e.g., fossils and patterns of fossil preservations, trace chemicals in different strata, dating techniques, and models) all point to the same cause or causes, we gain confidence in these hypotheses and can work towards refining them.

Of course, all the evidence doesn’t always line up so neatly. For example, when one scientist found trapped helium (of a sort that is common in extraterrestrial objects) in sediments dating to the end of the Permian, she interpreted it as evidence that an asteroid had triggered the end-Permian extinction.⁵ Other scientists rushed to investigate, but they could not reproduce the finding⁶ — nor could they find a crater that could be reliably dated to the same time period.⁷ Though one line of evidence pointing towards an asteroid impact fails to stand up to scrutiny, this does not necessarily mean that the hypothesis is entirely flawed. Scientists are still investigating the causes of the end-Permian mass extinction, including the idea of an asteroid impact.

It is worth noting that determining the dates of ancient events is critically important in establishing how and why a mass extinction happened. If an event that is thought to be a cause of a mass extinction (e.g., an asteroid impact) turns out to have occurred after the mass extinction began, it cannot have been the trigger for the extinction.