Age· The columns of time

Northgrippian

6,237 BCE → 2,251 BCE

The Northgrippian, from 8,200 to 4,200 years ago, is the second age of the Holocene, opened by the sharpest climate swing of the last ten millennia, and lactase persistence spread within it.

Second age of the Holocene, opened by the sharpest climatic oscillation of the last ten millennia, and in which lactase persistence spreads.

What the evidence shows

Its base is defined in the Greenland core NGRIP1 at 8,236 years before the year 2000, on the 8.2 ka event: about a century and a half of cooling, one to three degrees across the northern hemisphere. That cooling was long attributed to the abrupt drainage of the glacial lakes Agassiz and Ojibway into the sea, but no model produced a cooling of that duration from so brief an input; the meltwater now proposed is slower and larger, spread over a century, and proceeds from the collapse of the ice saddle linking the domes over Hudson Bay. In this age lactase persistence spreads, the retention into adulthood of the enzyme that digests milk sugar, due to the alteration of a single DNA base. Milk consumption long predates the spread of that variant: it therefore did not open milk to adults but protected those who consumed it when conditions deteriorated. Some seven thousand pottery residues and the trajectories read in ancient genomes make famine and disease a better explanation of its spread than the availability of milk.

Bounds and conventions

Layer counting again gives a limit accurate to the year, but the event it marks is a regional signal treated as global: how far the 8.2 ka cooling extended beyond the North Atlantic is reconstructed from proxies that do not all agree. Estimates of the spread of lactase persistence vary with method, and the selective pressure involved was long assumed rather than demonstrated.

GSSP NGRIP1, 8,236 years b2k.