Summary

What is geobiology? Geobiology is the study of how organisms have influenced and been influenced by Earth’s physical environment.

What is the biosphere? The biosphere is the part of our planet that contains all of its living organisms. Because the biosphere intersects with the lithosphere, hydrosphere, and atmosphere, it can influence or even control basic geologic and climate processes. The biosphere is a system of interacting components that exchanges energy and matter with its surroundings. Organisms use inputs of energy and matter to function and grow. In the process, they generate outputs such as oxygen and certain sedimentary minerals.

How do organisms interact with their physical environment? The activities of organisms influence the concentrations of gases in the atmosphere and the cycling of elements through Earth’s crust. Organisms contribute to the weathering of rocks by releasing chemicals that help break down minerals, precipitate minerals in sedimentary environments, and modify the composition of the oceans. The oxygen in Earth’s atmosphere is the result of the metabolism of photosynthetic microorganisms that evolved billions of years ago. In a similar way, the physical environment influences life. Geologic barriers such as mountains, deserts, and oceans help determine how ecosystems are divided. Some geologic processes can cause mass extinction events that permanently change life.

What is metabolism? Metabolism is a process that organisms use to convert inputs to outputs. Photosynthesis is a metabolic process in which organisms use energy from sunlight to convert water and carbon dioxide into carbohydrates, releasing oxygen as a by-product. Respiration is a metabolic process in which organisms use oxygen to release the stored energy of carbohydrates. Many organisms take up oxygen from the atmosphere and release carbon dioxide and water as by-products of respiration. Others, such as microorganisms that live in environments where oxygen is absent, must obtain oxygen by breaking down oxygen-containing compounds, producing substances such as hydrogen, hydrogen sulfide, or methane as by-products of respiration.

What are some ways in which metabolism affects the physical environment? When organisms produce oxygen, it is released into the atmosphere, where it can react with other elements and compounds. When organisms release carbon dioxide or methane, which are both greenhouse gases, they contribute to global warming. Conversely, when organisms consume these gases, they contribute to global cooling.

How do microorganisms interact with the physical environment? Microorganisms are the most abundant and the most diverse organisms on Earth. Some microorganisms, called extremophiles, can live in extremely hot, acidic, salty, oxygen-depleted, or otherwise inhospitable environments. Microorganisms are involved in many geologic processes, such as weathering, mineral precipitation, mineral dissolution, and the release of gases into the atmosphere. In these ways, they play critical roles in the flux of elements through the Earth system in biogeochemical cycles.

How did life originate? Experiments show that compounds thought to be abundant on early Earth, such as methane, ammonia, and water, could have combined to form amino acids, which could then have combined to form proteins and genetic materials. These results have been supported by the finding of meteorites that are rich in amino acids and other carbon-bearing compounds. The oldest potential fossils on Earth are 3.5 billion years old and appear to be the remnants of microorganisms, based on their shape and size. Chemofossils from about 2.7 billion years ago suggest that photosynthetic bacteria and eukaryotes were both present at that time. Banded iron formations, red beds, and the appearance of eukaryotic algae testify to an initial rise in atmospheric oxygen by about 2.1 billion years ago. A second, more dramatic rise in oxygen occurred near the end of Precambrian time and may have triggered the evolution of animals.

What is the difference between radiation and extinction? When groups of organisms are no longer able to adapt to changing environmental conditions or to compete with more successful groups, they become extinct. In a mass extinction, many groups of organisms become extinct at the same time. An evolutionary radiation is the relatively rapid evolution of new types of organisms from a common ancestor. Radiations may be stimulated by the availability of new habitats when a mass extinction eliminates highly competitive and established groups. The greatest radiation of animals in Earth’s history occurred during the early Cambrian period, when all the animal phyla living today evolved. Several mass extinctions have occurred throughout the Phanerozoic eon. A major mass extinction occurred at the end of the Cretaceous period, when an asteroid hit Earth and 75 percent of all species were wiped out. Global warming resulting from a release of methane caused a mass extinction at the Paleocene-Eocene boundary. The cause of the greatest mass extinction of all time, which wiped out 95 percent of all species at the end of the Permian period, is unknown.

How can we search for life on other worlds? Astrobiologists searching for extraterrestrial life recognize that life as we know it on Earth is based on carbon-containing compounds and liquid water. There is ample evidence that carbon compounds are common throughout the universe, so astrobiologists search for evidence of the presence of liquid water, today or in the past. There is a habitable zone at a certain distance from every star where liquid water could be stable. If a planet is within the habitable zone, there is a chance that life might have originated there. There is unambiguous evidence that Mars had liquid water on its surface, and thus may have been habitable, at some time in the past.