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A respiração

Once, nothing breathed. Life originated in anoxia, a complete absence of oxygen, and persisted this way for almost 2 billion years: minuscule, microbial, ocean-bound. It might have carried on this way for 2 billion more years, but for the emergence of an algae called cyanobacteria. This was the first organism to produce oxygen through oxygenic photosynthesis – the conversion of light into energy, with oxygen expelled as waste. Feeding off the limitless energy source of the Sun, cyanobacteria bloomed blue-green across the surface of the oceans. The upstart, previously minor gas oxygen proliferated. Once surface stores of iron and sulphur couldn’t soak it up anymore, oxygen flooded the atmosphere, and this new abundance triggered a fall in methane levels that plunged the planet into an ice age that lasted 300 million years.

This drastic change in the makeup of our atmosphere didn’t, as previously thought, trigger a mass extinction. But it did radically alter the nature of organic life. Aerobic respiration releases 16 times more energy than older forms of metabolism. It produced so much energy – was so ‘exergenic’ – that it enabled multicellular life. Fuelled by oxygen, an unknown bacterial ancestor evolved into mitochondria, the oxygen-processing component of the complex cells that make up almost all eukaryotes. From here on out, ‘gas exchange’ would define advanced life. Anaerobic organisms retreated to low-oxygen niches in the extreme deep of the ocean. Higher up, the seas blossomed with sponges, anemones, molluscs and the rest of that otherworldly bazaar that, right this second, somewhere, is captivating a scuba diver.

As the biology of gas exchange evolved, skin was superseded by gills, and gills were superseded by rudimentary lungs. After this long underwater gestation, around 500 million years ago, ‘aquatic-to-terrestrial transition’ began. Eukaryotic life moved from ocean to land, and proto-reptiles evolved, with stunning slowness, into mammals and birds. Birds evolved their own method of gas exchange, involving a series of air sacs lodged throughout the body and bones. In mammals, the lungs became the powerhouse, existing at the centre of a process that goes like this: on inhale, the diaphragm flattens downward and the intercostal muscles lift up the ribs, expanding the volume of the lungs. As volume increases, air pressure decreases relative to the atmosphere, and air rushes in. Mammalian lungs are covered with millions of microscopic balloons called alveoli; through their infinitesimally thin walls, the oxygen in air is picked up by the red blood-cell protein haemoglobin and carried to the ever-ravenous cells. Carbon dioxide travels in the opposite direction, transferred by the alveoli to the soon-to-be-exhaled air. On exhale, the diaphragm and intercostal muscles relax. The decrease in lung volume results in increased pressure relative to atmosphere, and so the air rushes out. Thus, a single breath. Repeat until death.

— M.M. Owen, Breathtaking