Holozoa is the clade that includes all animals and their closest single‑celled relatives, but excludes fungi.

On the animal side of the split, our ancestors stayed “soft” and flexible to keep moving. They evolved Cadherins: calcium‑dependent adhesion proteins. These specialized proteins acted like Velcro to snap cells together into complex, multicellular bodies. Instead of becoming rigid like a fungus, animal cells used this glue to build muscles and tissues that could contract and expand. While the “body” became a massive, crawling, or swimming machine, they kept the ancient posterior flagellum in a time capsule, the sperm cell, ensuring that the ancestral rear-engine motor would always be what drives the next generation forward.

Possible Snowball Earth Link

Snowball Earth may have favored multicellularity or tighter cell cooperation in some eukaryotes, but we do not currently have clear evidence that the glaciations specifically selected for cadherins themselves. Cadherin-like adhesion machinery appears to predate animals and to have evolved in a unicellular holozoan context before true animals arrived.