LIFE × EVOLUTION

The First Skeletons

~550 million years ago

Proterozoic · Neoproterozoic · Ediacaran

Cloudina built itself a shell by stacking calcareous funnels one inside the next, and some of those shells are pierced by boreholes. Skeletons and the first direct evidence of animals eating animals arrive together, in shallow tropical seas whose water was so saturated with carbonate that a body could grow a wall almost by letting it happen.

Why it matters

Armour implies attackers, and cheap chemistry decides whether armour is affordable. Both conditions were met in the last ten million years of the Precambrian, and mineralised skeletons then appear in several unrelated groups at once: the opening of the arms race that fills the Cambrian.

Dating & uncertainty

Mineralised plates made by protists are known from about 810 Ma, but large animal skeletons appear only near 550 Ma, with Cloudina. Whether the trigger was seawater chemistry, rising oxygen or predation is unresolved, and the three are not exclusive.

Sources

1. Wood (2018), Emerging Topics in Life Sciences · Exploring the drivers of early biomineralization
2. Becker-Kerber et al. (2017), Scientific Reports · Ecological interactions in Cloudina from the Ediacaran of Brazil: implications for the rise of animal biomineralization
3. Cui et al. (2016), Geobiology · Environmental context for the terminal Ediacaran biomineralization of animals

Earth history as one day

21:05:332 h 54 min before midnight

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