- Most of the organisms that we see today are alive thanks to the microscopic germs that evolved the process for creating oxygen
- Scientists believe changes in oxygen levels are related to two major increases in the size of organisms during the history of life on Earth
Billions of years ago, tiny life forms called cyanobacteria evolved a process called oxygenic photosynthesis which was a turning point for Earth. Photo: Wikimedia Commons
From towering trees to the smallest bugs, life on Earth today has been affected by many factors, such as changes in the environment and in our genes. But did you know that billions of years ago, tiny bacteria played a massive role in changing the course of life as we know it?
Why cyanobacteria were the key
Earth has changed significantly since its early days. For over 2 billion years, our planet’s air had no oxygen. Instead, it had lots of methane, a gas that would poison us today. The first life forms were microscopic organisms that were anaerobic, meaning they did not need oxygen to survive.
A turning point came about 3.4 to 2.9 billion years ago when tiny life forms called cyanobacteria evolved a process called oxygenic photosynthesis. This process releases oxygen as cyanobacteria use sunlight to convert carbon dioxide and water into sugar for food.
The elements that build our bodies
Oxygenic photosynthesis is one of the most significant evolutions in Earth’s biological history because it caused oxygen to accumulate in the atmosphere and oceans. The rise of oxygen levels led to the evolution of aerobic respiration, a process that uses oxygen to convert food into energy. Today, most land and aquatic organisms – including humans – need aerobic respiration to survive.
Scientists believe changes in oxygen levels are related to two major increases in the size of organisms during the history of life on Earth (see graphic).
Most organisms on our planet rely on oxygen to create cellular energy, which is crucial for growth and reproduction. Thus, scientists think the abundance of oxygen in the atmosphere triggered a surge in the potential of life on Earth to evolve.
Not only was cyanobacteria key for Earth’s oxygen levels, but they also played a role in the evolution of plants. Plant cells’ chloroplasts, which are responsible for photosynthesis, are thought to have originated from cyanobacteria. So, in a way, cyanobacteria gave plants the ability to make their own food and produce oxygen.
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Where are they now?
These days, cyanobacteria are no longer the major source of oxygen as they have been replaced by plankton, algae and plants on land. In fact, more than 50 per cent of the atmosphere’s oxygen that we breathe today comes from the ocean.
However, the oxygen in oceans and lakes is decreasing because of global warming, as warm water holds less oxygen than cold water does. This can be especially harmful to algae, which rely on oxygen for photosynthesis and respiration. When oxygen levels fall below a certain level, algae become stressed and may eventually die.
To protect algae and all the organisms that rely on them, including humans, we must reduce our carbon emissions, which lead to global warming. After all, even the tiniest organisms can leave a big impact on the world.
Young Post has partnered with Hong Kong Science Museum and Hong Kong Space Museum to encourage your pursuit of science. Every month, the museums answer questions about the world around us, the cosmos and beyond.