Pilularia globulifera

How far back this lineage goes — and how we know. Everything here is measured in Ma, short for “mega-annum”: millions of years ago. The chart reads left to right like a calendar of the Earth, from the deep past on the left to today at the right edgetop to bottom like a core drilled through the Earth, from the deep past at the top down to today at the bottom.

When this lineage existed

How to read this: the coloured strip along the bottomdown the left is the geological calendar — the standard periods (Jurassic, Cretaceous…) every museum uses, shown so you can see which chapter of Earth's history this lineage lived in. The dashed rules marked ✦ are the five great mass extinctions. The solid bar is the fossil range: the span between the oldest and the youngest fossil that palaeontologists have assigned to Pilularia globulifera. Above itBeside it, each dot is one dated fossil find — few enough to count, so they are drawn individually rather than as a graph. The orange marker is the DNA clock: DNA accumulates mutations at a roughly steady rate, so comparing this species' DNA with its relatives estimates when the lineage split off — independently of any fossil.

The two clocks disagree here. The fossil record reaches back to 101 Ma, but the molecular clock dates the lineage to only 7.13 Ma — about 93.4 Myr younger. A fossil cannot be older than the lineage it belongs to, so one of the two is off: either the fossil is assigned to the wrong species, or the clock is running fast.