The rise of complex animals was physically impossible until ocean oxygen levels rose

All members of the kingdom Animalia are obligate aerobes, meaning they require oxygen to fuel their cellular processes. During the Cryogenian period, Earth's oceans were largely anoxic, restricting life to simple, anaerobic forms. It was only after the 'Snowball Earth' glaciations, when oxygen levels finally spiked, that animals could evolve the high-energy lifestyles required for movement, predation, and complex tissue development.