Era· The columns of time

Cenozoic

66 Ma → Today

In the Cenozoic, the last sixty-six million years, mammals filled a world emptied by an impact, grasslands spread, ice sheets formed and a lineage of primates appeared.

Era covering the last sixty-six million years: mammals occupy a world emptied by an impact, grasslands spread, ice sheets form, and a lineage of primates appears within it.

What the evidence shows

It opens at the Chicxulub boundary, 66 million years ago. What makes it possible is not any superiority of mammals but the vacating of ecological niches: the impact removed the animals that had occupied the large body sizes through almost the whole preceding era, and the ecological roles so vacated were taken up by the surviving lineages. What follows is prolonged cooling, from a world without permanent ice to one with two ice sheets, and the expansion of grasses, which opens the interiors of the continents. Homo sapiens appears in the last 300,000 years of the era, less than half a per cent of its duration.

Bounds and conventions

Only one of its limits is fixed. The older is among the sharpest on the chart; the younger remains open, the era being current, and whether the last centuries warrant a division of their own is the substance of the Anthropocene debate, which the chart has not settled.