 In their work The Life and Death of Planet Earth, authors Peter D. Ward and Donald Brownlee have also argued that some form of animal life may continue even after most of the Earth's plant life has disappeared. Ward and Brownlee use fossil evidence from the Burgess Shale in British Columbia, Canada, to determine the climate of the Cambrian Explosion, and use it to predict the climate of the future when rising global temperatures caused by a warming Sun and declining oxygen levels result in the final extinction of animal life, as well as comparing the rising and eventually falling diversity of complex life to a cannonball rising to its highest point when going to the present and falling back down again when going into the future. Initially, they expect that some insects, lizards, birds and small mammals may persist, along with sea life. Without oxygen replenishment by plant life, however, they believe that the animals would probably die off from asphyxiation within a few million years. Even if sufficient oxygen were to remain in the atmosphere through the persistence of some form of photosynthesis, the steady rise in global temperature would result in a gradual loss of biodiversity. As temperatures continue to rise, the last animal life will inevitably be driven back toward the poles, and possibly even underground. They would become primarily active during the polar night, aestivating during the polar day due to the intense heat. Much of the surface would become a barren desert and life would primarily be found in the oceans; however, due also to a decrease of the amount or organic matter coming to the oceans from the land as well as oxygen in the water, life would disappear there too following a similar path to that on Earth's surface with invertebrates being the last living animals, and of them those that do not depend on living plants such as termites or those near hydrothermal vents such as worms of the genus Riftia As a result of these processes, multi-cellular lifeforms may be extinct in about 800 million years, and eukaryotes in 1.3 billion years, leaving only the prokaryotes.
