LIFE × EVOLUTION

The First Backbones

~518 million years ago

Phanerozoic · Paleozoic · Cambrian

Small swimming animals, a few centimetres long, with a stiffening rod running the length of the body, a pair of camera eyes, paired nasal sacs and muscle blocks arranged in zigzag. No jaws, no bone, no fins to speak of. Two of them are known from South China and one from the Burgess Shale, which shows the group was already living across the world's oceans.

Why it matters

Fish, dinosaurs, whales and readers of this page are variations on this animal. What made the variation possible is not the backbone but a sheet of migrating cells that these creatures had already: it is what builds the face, the sense organs and, later, the jaws of every vertebrate.

Dating & uncertainty

Soft-bodied fossils a few centimetres long from South China and the Burgess Shale. A notochord, paired eyes, nasal sacs and W-shaped muscle blocks are visible; whether one or another is already a craniate or still a stem chordate is argued case by case.

Sources

1. Shu et al. (2003), Nature · Head and backbone of the Early Cambrian vertebrate Haikouichthys
2. Conway Morris & Caron (2014), Nature · A primitive fish from the Cambrian of North America
3. Holland & Chen (2001), BioEssays · Origin and early evolution of the vertebrates

Earth history as one day

21:15:422 h 44 min before midnight

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