In the history of life on Earth, a dramatic and revolutionary change in the nature of the sea floor occurred in the early Cambrian (541–485 million years ago): the agronomic revolution. This phenomenon was coupled with the diversification of marine animals that could burrow into seafloor sediments. Previously, the sea floor was covered by hard microbial mats, and animals were limited to standing on, resting on, or moving horizontally along those mats. In the agronomic revolution, part of the so-called Cambrian Explosion of animal diversity and complexity, vertical burrowers began to churn up the underlying sediments, which softened and oxygenated the subsurface, created new ecological niches, and thus radically transformed the marine ecosystem into one more like that observed today.