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. The thirty-seven others are unresolved: a few read as possible eukaryotes, a few as probable prokaryotes, and for most of them nothing settles the question.

Why it matters

That stretch has a reputation for being uneventful, and these cysts are much of the reason: there is little else to look at. Yet each newly described assemblage raises 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, since no known prokaryote builds a wall like that, but putting a lineage to any one cyst is usually out of reach.

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