The fossil record spans three-quarters of Earth history but forms a coherent whole only since the time that animals with skeletons appeared at the beginning of the Cambrian period. Both the origin of life and the origin of the major groups of animals remain unknown. The seafloor became heavily populated with animals in Cambrian time, and the lands were colonized in the Silurian Period. Organic communities have become more complex through geologic time, but not in a linear fashion. Extinction of species is normally counterbalanced by speciation, but at times it has prevailed, and during the great biotic crises some communities were reduced to low levels throughout the world, and major groups of organisms, such as the dinosaurs and ammonites, were lost. That part of the record is a reminder that organisms exist at the tolerance of the environment, and that this environment, throughout geologic time, has exhibited variations of a nature and magnitude outside the realm of human experience.