Between roughly 541 and 520 million years ago the fossil record floods with nearly every major animal phylum, eyes, limbs, shells, predators, after billions of mostly microbial years. Darwin worried the suddenness undermined his theory. Documented candidate triggers: rising oxygen, the Hox toolkit coming online, ecological feedback, and improved preservation. Modern consensus stretches the explosion across tens of millions of years, fast for geology, slow for a miracle.
Andrew Parker's thesis, published in 1998 and expanded in In the Blink of an Eye (2003): the trigger was vision. Once the first good eyes opened, prey had to hide, harden, or flee, predators escalated, and the arms race invented armor, burrowing, camouflage, and speed. Parker's signature evidence: the proliferation of hard shells at exactly that horizon is what a vision arms race would leave behind.
The claims that arrive whenever the record accelerates: panspermia seeding, directed intervention, and the 2018 Steele et al. paper in Progress in Biophysics floating cometary delivery of the Cambrian payload, cephalopod embryos included, to broad skepticism. Fringe literature files the explosion beside the other impossible accelerations in the archive.
Did vision itself detonate the Cambrian, the first arms race triggered the moment anything could see?