Physical constraints during Snowball Earth drive the evolution of multicellularity
William W. Crockett, Jack O. Shaw, Carl Simpson, Christopher P. Kempes;
Physical constraints during Snowball Earth drive the evolution of multicellularity
Proc Biol Sci 1 June 2024; 291 (2025): 20232767. https://doi.org/10.1098/rspb.2023.2767
This was a difficult paper for me to get through for some reason. I kept getting diverted into different rabbit holes. The following review and notes reflect that.
Crockett and Co. derive an energy balance for a single cell, and for a “choanoblastula”, a hypothetical spherical cluster of cells. They vary parameters relating to energy expenditure and calorie consumption, and find that oceanic conditions during a snowball or slushball earth period could conceivably have encouraged spherical cell clusters to get bigger, and become true metazoans. All that seems OK, I did not grind through the differential equations.
Something about this paper seems out-of-alignment. The ages for apparent snowball earth episodes are 654.5 million years ago to 632.3 ± 5.9 million years ago (Marinoan glaciation and 717 million years ago to 660 million years ago (Sturtian glaciation).
I can easily find papers like Phylogenomics, Origin, and Diversification of Anthozoans (Phylum Cnidaria) that place “divergence of Anthozoa” at 648–894 Ma. I believe that’s when the common ancestor of corals and sea anemones diverged from the rest of Cnidaria. That common ancestor was a non-colonial polyp, already a decently multi-cellular animal with a gelatinous body.
If the divergence was at 648 million years ago, animals would have to have attained multi-cellularity during the Sturtian glaciation. On the older side, 894 million years ago is well before the Sturtian.
Another paper puts arthropod-chordate divergence at 993 ± 46 million years ago, way before the Sturtian glaciation/snowball earth. It also puts Porifera, Cnidaria and Ctenophora diverging at 1.2 to 1.5 billion years ago.
There’s a lot of uncertainty around when the Marinoan started, and begin and end dates of the Sturtian, but it seems like there’s a good chance multi-cellularity evolved before snowball earth episodes.
This paper’s introduction makes some interesting assertions:
- “Molecular and fossil evidence suggests that complex eukaryotic multicellularity evolved during the late Neoproterozoic ear, coincident with Snowball Earth glaciations…”
- “…other work suggests that the evolution of more complex eukaryotes, including multicellular organisms, could have led to ocean oxygenation…”
- “… an increased abundance of eukaryotes and an increase in mean organism size have been proposed as a mechanism for the end of the Neoproterozoic glacial events, owing to an increase in organic matter sinking rates…”
- More viscous cold water during Snowball Earth could have encouraged evolution of multicellularity.
- “Because of the multiple contemporaneous origins of eukaryotic multicellularity an environmental driver is likely. However, an
environmental driver cannot be universal because only a few of the many co-occurring eukaryotic lineages evolved multicellularity,
such that the driver must also sort between adaptive strategies."
Poking around, it seems that multi-cellularity has evolved many times, some paper say 50 times, and complex multi-cellularity has evolved 5 or 6 times.
There’s a reference to the Oxygen Control Hypothesis, and the bibliography has another Mills and Canfield paper, Oxygen requirements of the earliest animals.
This paper has a roughly 2-page, 74-item bibliography. It looks like the bibliography is a rich source of papers on snowball earth and eukaryotic evolution.