Living in a microbial world

Sarah Lloyd examines the importance of bacteria in the origin of life

Long before humans arrived on Earth, the planet was ruled by microbes. Although individually insignificant, when taken together, these organisms make a formidable force. It was bacteria that shaped the planet to create the current world we now live in; life as we know it would not be possible without them.

When bacteria first emerged over 3.5 billion years ago, the world’s atmosphere was a far cry to how it is today. A thick, black layer of dried lava covered the ground, while sweeping gusts of acidic vapours swirled under an orange sky – in other words, very inhospitable. However, within the hot hydrothermal vents of the newly formed oceans, bacteria were born, and life began.

For over a billion years, bacteria continued to grow and evolve. As the bacteria replicated, copying errors within their genetic code led to mutations. One such mutation – believed to have happened 2.4 billion years ago – created a new species of bacteria that could harness energy from the sun to split apart water molecules, releasing oxygen into the atmosphere. This process, known as photosynthesis, enabled oxygen-dependent life forms to exist. All animals and plants owe their existence to this chance mutation in a single bacterium billions of years ago.

Even today we rely on bacteria to ensure our planet is hospitable. Nitrogen, the most abundant gas in our atmosphere, makes up 78% of all the air we breathe and all organisms need it to grow. However, in its natural form, it is useless to both animals and plants. Instead, we rely on nitrogen-fixing bacteria to convert it into different forms that we can use, like ammonia and nitrates.

As well as looking after our planet, bacteria also aid human survival. This statement may sound strange given the multitude of bacteria-borne diseases. However, of the numerous bacterial species on the planet, only around 150 are pathogenic. The rest are relatively harmless, and some have even been seen to benefit humans.

Trillions of bacteria reside within our body. These bacteria have developed a mutual relationship with humans: they rely on us for a space to live protected from the outside world, and in return they provide humans with many functions that we need but are unable to do ourselves. And not just humans, bacteria have evolved an astounding range of intimate relationships with almost every animal on the planet.

The best characterized example of this mutually beneficial relationship, known as symbiosis, is seen in mammals that eat a high-fibre diet, such as cows. Unable to break down the tough cellulose wall of the grass they eat, they rely on bacteria inside their gut to do this for them, and then utilise the digested fragments for energy. A similar reliance has also been reported in humans, it is estimated that 10% of our calories are acquired through bacteria breaking down food that would otherwise be unusable.

Bacteria have also been implemented in coordinating the human immune response and determining our susceptibility to certain diseases. They can do this both passively (by growing in large numbers, limiting space for pathogenic bacteria to colonize) and actively (by killing pathogenic strains and stimulating the immune response). A recent study conducted by researchers at the Brain-Body Institute, Ontario also suggests that bacteria can influence human behaviour. Mice grown to be germ-free show a significantly different response to stress, implying that bacteria are partially responsible for an animal’s mood.

And these examples are just the tip of the iceberg! Scientists have only recently begun to realise the importance of the bacteria that live inside us. Referred to as our ‘microbiome’, it is likely that further research on this aspect of the body will reveal a longer list of roles bacteria play within us. Even our ‘human’ cells contain the remnants of bacteria – the mitochondrion, often referred to as the ‘powerhouse of the cell’ is responsible for providing energy for animals to survive. It’s believed to be the descendent of a bacterium that was engulfed by a larger bacterium billions of years ago, giving rise to the first eukaryotic cell. Put simply, humans (and all other life) wouldn’t be able to exist without bacteria.

Recent revelations have begun to reveal the importance of these friendly bacteria. It is becoming increasingly clear that our lives are inextricably linked with bacteria, with these small organisms playing an incredibly important role in both our species’ past and present. We really are living in a microbial world.

From Issue 22: The Dark Side of Science