LIFE × EVOLUTION

Six Hundred Million Years of Spiny Cysts

~1.6 to 1.0 billion years ago

Proterozoic · Mesoproterozoic · Calymmian

Microscopic spheres of tough organic wall, some smooth, some ornamented, some bristling with spines, and many pierced by a single neat round opening: the hatch the occupant left by. From roughly 1.6 to 1.0 billion years ago they are almost the entire fossil record of complex cells. In one thoroughly described assemblage from Mauritania, eleven forms out of forty-eight can be called eukaryotic with confidence. What the others were, nobody can say.

Why it matters

Six hundred million years of the history of complex life come down to these cysts. That stretch is usually called uneventful; but the newest assemblages keep raising the count of recognisable eukaryotes, which suggests the quiet may belong to the record rather than to the life.

Dating & uncertainty

Acritarch is a name for a shape, not a lineage: organic-walled cysts whose owners are unknown. Ornamented and spine-bearing forms are accepted as eukaryotic because no prokaryote builds walls like that, but assigning them to a group is mostly impossible.

Sources

1. Beghin et al. (2017), Precambrian Research · Microfossils from the late Mesoproterozoic to early Neoproterozoic Atar/El Mreiti Group, Mauritania
2. Loron et al. (2019), Precambrian Research · Organic-walled microfossils from the late Mesoproterozoic to early Neoproterozoic lower Shaler Supergroup, Arctic Canada
3. Huntley et al. (2006), Precambrian Research · 1.3 billion years of acritarch history: an empirical morphospace approach

Earth history as one day

15:32:308 h 27 min before midnight

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