LIFE × EVOLUTION

Life Leaves the Seafloor

~410 to 360 million years ago

Phanerozoic · Paleozoic · Devonian

Fish, ammonoids and the animals that hunted them moved off the bottom and into the open water, exchanging a surface for a volume. When that happened is genuinely unsettled. One reading makes it a Devonian revolution, driven by competition on a crowded seabed and by abundant plankton to feed on; a later count, with sampling standardised, finds swimmers already well established before the Devonian and their share of the fauna falling, not rising, during it.

Why it matters

The open ocean is the largest living space on the planet, and filling it is what made food webs three-dimensional. Whether that was a sudden move or a slow drift across the whole Palaeozoic changes what we think drives animals into empty space: pressure from below, or simply time.

Dating & uncertaintyDisputed

The dates shown follow one reading, in which animals took over the water column during the Devonian, under pressure from a seabed already full. A later analysis that corrects for uneven sampling finds swimmers well established before the Devonian and no burst during it; on that reading there is no single date to give, only a slow rise across the whole Palaeozoic.

Sources

1. Klug et al. (2010), Lethaia · The Devonian nekton revolution
2. Whalen & Briggs (2018), Proceedings of the Royal Society B · The Palaeozoic colonization of the water column and the rise of global nekton
3. De Baets et al. (2012), Evolution · Early evolutionary trends in ammonoid embryonic development

Earth history as one day

21:49:572 h 10 min before midnight

if Earth’s 4.54 billion years were compressed into a single day