LIFE × EVOLUTION

Fossils Caught Mid-Division

~609 million years ago

Proterozoic · Neoproterozoic · Ediacaran

A few tenths of a millimetre across, and caught in the act: thousands of spheres in the phosphate rock of South China lie frozen part-way through dividing into two, four, eight cells. X-ray tomography has sorted them into four distinct division patterns and found late stages in which some cells are set aside from the rest: the separation of germ from body. One pattern, in which the cell divides only on one side, occurs in animals and nowhere else.

Why it matters

This is the only place in the record where multicellular development can be watched rather than inferred, and it is more than thirty million years older than the first Ediacaran fossils. What these spheres were is still argued, whether algae, stem animals or something else, but the argument is now about which multicellular eukaryote, not whether.

Dating & uncertaintyDisputed

What these microscopic spheres are has been argued for twenty-five years: sulfur bacteria, protists, algae, or animal embryos. Tomography now shows cell differentiation and several distinct cleavage patterns, which narrows the field without closing it.

Sources

1. Chen et al. (2014), Nature · Cell differentiation and germ-soma separation in Ediacaran animal embryo-like fossils
2. Yin et al. (2022), Philosophical Transactions of the Royal Society B · Diverse and complex developmental mechanisms of early Ediacaran embryo-like fossils from the Weng'an Biota
3. Yin et al. (2016), Geology · Meroblastic cleavage identifies some Ediacaran Doushantuo (China) embryo-like fossils as metazoans

Earth history as one day

20:46:503 h 13 min before midnight

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