All life affects the environment. All organisms remove material they need, and they all release material that they do not need. During the evolution of life on Earth, the planet has been greatly modified by both non-biological factors and also by life itself.
The most dramatic event was the removal of carbon dioxide from the atmosphere, and its replacement by oxygen. Driven by photosynthesis, it perpetrated for the original life forms – which evolved in strictly anaerobic conditions, with no oxygen – what could be viewed as the greatest environmental pollution event to affect Earth. The removal of carbon dioxide also decreased the levels of this greenhouse gas, which lowered the planet’s temperature.
Where did all that carbon go? One sink was into organic carbon that we now extract as gas and oil. Much of this material originated from the burial of microscopic photosynthetic organisms, the phytoplankton.
Phytoplankton form the base of the marine foodweb: they produce fatty acids, some of which, once they get transferred into fish, form an important part of the human diet. If they become buried for millions of years, the fatty acids are converted into oil. Millions of years of phytoplankton growth have produced the oil that humanity is so rapidly extracting and converting back to atmospheric carbon dioxide.
Most of this CO₂ is not in this form, however – most is phytoplankton buried as chalk, a form of limestone; think of the white cliffs of Dover. Perhaps fortunately for Earth, humans have no great need for chalk, so this reservoir of carbon dioxide remains safely locked up. But why did biology make limestone in the first place?
Many organisms, including ourselves, use calcium carbonate for their skeletons. Even microscopic organisms make hard parts made of this material, and some of those structures could certainly be viewed as having a skeletal function, to protect the organism, or to hang components from. However, much of the limestone represents deposits made by phytoplankton that have no obvious need for a skeleton. So why did they go through the effort of making calcium carbonate; indeed why are they still doing it, and will climate change affect the process?