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When it comes to determining the age of stuff scientists dig out of the ground, whether fossil or artifact, “there are good dates and bad dates and ugly dates,” says paleoanthropologist John Shea of Stony Brook University.

The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results.

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Before more precise absolute dating tools were possible, researchers used a variety of comparative approaches called relative dating.

These methods — some of which are still used today — provide only an approximate spot within a previously established sequence: Think of it as ordering rather than dating.

Biostratigraphy: One of the first and most basic scientific dating methods is also one of the easiest to understand.

Layers of rock build one atop another — find a fossil or artifact in one layer, and you can reasonably assume it’s older than anything above it.

Paleontologists still commonly use biostratigraphy to date fossils, often in combination with paleomagnetism and tephrochronology.

A submethod within biostratigraphy is faunal association: Sometimes researchers can determine a rough age for a fossil based on established ages of other fauna from the same layer — especially microfauna, which evolve faster, creating shorter spans in the fossil record for each species.

Paleomagnetism: Earth’s magnetic polarity flip-flops about every 100,000 to 600,000 years.

The polarity is recorded by the orientation of magnetic crystals in specific kinds of rock, and researchers have established a timeline of normal and reversed periods of polarity.

Paleomagnetism is often used as a rough check of results from another dating method.

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