Although largely unseen, fungi are found virtually everywhere in the biosphere. Indeed, fungi are estimated to make up about 25% of Earth’s biomass. Fungi play very important roles in ecosystems. Along with bacteria, they are decomposers: they feed on dead plant and animal material.


For 90% of Earth’s history, plants did not exist. That’s hard to picture at a time when plants carpet most of this green planet, but it’s true. Yet once they evolved from algae and began to spread, the rise of plants became the single most important step in the development of terrestrial life as we know it.


Microbial mats, layered sheets of bacteria and other microorganisms, had dwelled on land for more than a billion years by the time plants arrived. Lichens, mutualistic combinations of a fungus and a green alga were likely there too. Both emitted chemicals that broke down rock into the particles found in soil. The excretions and remains of burrowing animals, including worms and insects, mixed in with the particles. The buildup of organic materials served as nutrients that plants would use to survive.


Prokaryotes account for more than half of all the biomass on Earth. By current estimates, about 90% of prokaryotes are bacteria, and the rest are archaea.


We are fascinated by disaster. Yet whatever the cataclysm that marks the “end of life on Earth,” we can be fairly sure it won’t end microbial life. Prokaryotes are so well adapted to every habitat on the planet that at least some are sure to find a place where they can survive. And they are, by themselves, most of the life on Earth anyway.


The most frightening aspect of viruses is the possibility of epidemics or pandemic. An epidemic is an outbreak of disease that affects a large number of individuals in a population. A pandemic is an epidemic on a very large scale, both in terms of how many people are infected and how widespread the infection is geographically.


The quantity of viruses on the planet is staggering. We don’t have very reliable estimates for the mass on land, but when assays were done on the number of viruses in seawater, the results were stunning. There are about 100B virus particles in a liter of seawater. In an assay of 200 liters of seawater, 5,000 different viruses were detected, 90% of them previously unknown.


From about 230M years ago to 145M years ago, dinosaurs dominated Earth. Most died off about 65M years ago in a mass extinction that made room for mammals. Technically, dinosaurs are not extinct, because this group also includes birds.


Fossils tell us that as mammals evolved, their brains got bigger. On average, a mammal’s brain today is about 10 times larger than that of a modern reptile of similar body size. And, in comparison with modern reptiles and other vertebrates, the cerebrum in mammals is much more complex.


Many mammals use their hair to communicate aggression or to court a mate. Hair can also serve a sensory function. Humans, who evolved in a warm climate, have lost much of their body hair. However, thick hair remains on our heads, where it protects us from the sun’s UV rays and helps us retain heat.