In his new book, Air-Borne: The Hidden History of the Life We Breathe, Carl Zimmer charts a 4 billion-year history of the aerobiome, the collection of organisms, spores, bacteria, algae, viruses, and on and on that live in the air.
Carl Zimmer
Zimmer was kind enough to provide Sequencer with an outtake from the book. We pick up after life has arisen in the ocean, and the waves spray that life into the air. "Here, I chart the rise of terrestrial life, which came to dominate the aerobiome," said Zimmer.
Some of the microbes that rose from the ocean fell on land instead of water. Lying on the bare continents, they no longer had sea water to shield them from direct sunlight. Many likely died as the ultraviolet radiation ravaged their genes and proteins. Meanwhile, the atmosphere was sucking out the water from their interiors, causing their molecules to stick together and collapse into toxic shapes.
Over time, however, life adapted to land. The earliest signs of its spread are 3.2-billion-year-old fossils from South Africa. They preserve microbial mats that grew in a braidplain of streams woven across an arid landscape. The water in the streams would have periodically dried up, exposing the mats to dry air. Mutations that helped the microbes survive longer out of water would have allowed them to reproduce more, shaping future generations. Instead of relying on water to shield them, these microbes grew pigments that could absorb the deadly ultraviolet rays. They also relied on cooperation to survive in the air. Terrestrial microbes worked together to build rubbery films around themselves. These biofilms soaked up rain and water vapor from the air and held onto it during dry weather.
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Three billion years later, these living films still exist. Known as biological soil crusts, they can be as thin as butter on toast, or as thick as the toast itself. Biological soil crusts today cover about 12 percent of dry land—roughly equivalent to the area of South America. They thrive where younger forms of life cannot. They coat the Dry Valleys of Antarctica. They grow in the Atacama Desert of Chile, which gets less than an inch of rain a year.
The first biological soil crusts probably started off as ribbons of life that hugged tide pools, river banks, and the edges of lakes. As they grew more adapted to dry air, they spread further inland. And they began traveling by air. When the wind blew over a dried crust, it could peel up bits and deliver them back to the ground miles away. Violent updrafts could rip up larger chunks. The paper meteorites** that Christian Ehrenberg first recognized as crusts were falling from the sky for billions of years. Terrestrial microbes could also get lofted into the air on the grains of dust scoured from the ground by winds. Today, dust storms deliver billions of tons of dust into the atmosphere. Each grain can carry its own menagerie. In 2018, German scientists inspected individual grains of sand they picked up from a beach. A single grain could harbor 100,000 cells, belonging to 6,000 different species. When these microbes landed on new ground, they might be first living things to ever alight there. They spread biological soil crusts across entire continents, giving the land a living skin.
While bacteria crusts spread on land, algae were also evolving into cellular collectives. In some species, a few dozen cells stuck together, looking like microscopic clusters of grapes. Other species became yards long, evolving anchors to hold fast to the bottom of oceans and rivers and flat blades that caught sunlight filtering through the water.
These algae probably hosted some of the first fungi. The fungi stuck to their surface and released enzymes that destroyed their cells. The fungi could then soak up the remains. Some fungi attacked living algae, while others waited for them to die. The fungi also evolved bodies of their own—threads of cells that wove themselves into webs as they grew.
By about 450 million years ago, green algae spread ashore. Its descendants became the first plants. At first they had no roots to tap the ground for water. Instead, they grew on damp surfaces where they could form moist carpets. Mosses and other tiny plants are the closest living analogies to the species that first turned the world green. Paleobotanists suspect that fungi came ashore with them. Moss today plays host to fungi. Fungal fossils are preserved alongside some of the oldest fossils of plants. It’s likely that some fungi lived on land as parasites of living plants, while others broke them down after death. But some fungi also evolved a new relationship with plants: a beneficial partnership. The fungi used their versatile enzymes to break down minerals from the underlying rocks, and then delivered them to plants. The plants gave the fungi some of the carbohydrates they built with help from the sun.
Underground, fungal threads grew into vast networks that stretched below forests for miles. They supplied trees with nutrients, and even began to shuttle carbon from one tree to another
To picture what the greening of the Earth with plants and fungi looked like, it helps to look to Iceland. New volcanoes emerge there form from time to time, spreading fields of lava that cool and crack. Within a few years, the bare rock becomes coated with moss and other tiny plants, accompanied by their fungi. When the plants die, the fungi take the first step of breaking them down, using their enzymes to tear apart the tough cells. Bacteria decompose the plants further, producing a layer of soil measuring a few inches deep. What’s most impressive about these new ecosystems is how they come into being. They do not spring to life through spontaneous generation. Many of the species arrive on Iceland’s cooling lava fields by air.
Both plants and fungi did more than just evolve the ability to withstand being exposed to the air. They harnessed the air, exploiting it to advance through their life cycles. Moss, for example, can reproduce sexually, producing male and female cells. When cells of the opposite sex fuse, they develop into a stalk that rises an inch or more above the moss mat. The stalk cracks open, releasing spores like a puff of smoke. Sebastian Sundberg, an ecologist at Uppsala University, once laid out sheets of cotton cloth in a Swedish bog. He left them there for the summer. Sundberg returned to the bog, collected the sheets, and counted the spores of moss that had landed on them. From that census, he estimated that as many as six million moss spores fell on every square meter of the bog.
The spores of mosses and other primitive plants are not just abundant: they can also travel long distances. In a study on Sphagnum moss, scientists found that more than three-quarters of a plant’s spores travel over ten feet before landing. Breezes will carry them further—sometimes much, much further. After World War II, the Netherlands unwittingly launched an experiment in how far plant spores can travel. They built embankments along their coast to reclaim land from the ocean. At first, the land was a featureless expanse of mud flats. Over several years, mosses began growing over them. The closest sources for some of the species of mosses were over 50 miles away. Spores may travel even further on rare occasion, giving rise to new species on distant continents and islands. Over millions of years, their travels have left their mark on the locations of bryophytes, which often track the directions of winds across the planet.
Spores likely accelerated the spread of the first plants. Most landed in places where they had no chance of growing, but their sheer numbers increased the odds of winding up on a new lava field or some other place where they could establish a new mat. And by merging male and female genes in each spore, plants could increase their odds even further, since the spores—like human siblings—carried different combinations of genes. If one combination couldn’t survive in a new home, another might.
Moss-like plants did not have land to themselves for long. New kinds of plants evolved, with stems that could grow a few inches high. Some plants evolved roots, which could reach into the deepening soil to draw up water and nutrients. Plants evolved in parallel with algae in the ocean, growing flat green blades. Their leaves let them capture more sunlight, and fueled their evolution to greater heights. Wood and other tough tissues held up their trunks. New plumbing systems let them suck water high above ground. The tallest plants began competing with each other for sunlight, escaping from shadows by growing even higher. By about 380 million years ago, forests had emerged.
The plants of the new forests found new ways to use the air to their advantage. The moss-like plants had released their sex cells into the water that surrounded them, and the cells had swum to find each other. Now the new plants turned their male sex cells into pollen grains. They amassed the grains in cones or flowers, from which they could waft away in clouds. A single plant might make a million pollen grains, in order that a few might find a female match. The lucky grains fertilized female cells, which developed into seeds.
Unlike spores, seeds were generally too heavy to soar. Many species just dropped their seeds to the ground. But some species evolved ways to keep their seeds in the air longer, so that they could travel further. Maple trees evolved samaras, with blades that spun like helicopter blades. Dandelions evolved parachute-like pappi, that could ride even a faint breeze.
As plants came to dominate the land, the fungi that depended on them thrived as well. Underground, fungal threads grew into vast networks that stretched below forests for miles. They supplied trees with nutrients, and even began to shuttle carbon from one tree to another. Fungi also formed a coat on the leaves of plants along with bacteria. This living skin kept the plants in good health, protecting them from diseases and helping their metabolism. But fungi also found success in attacking plants. A single honey mushroom in Oregon has been attacking trees for several thousand years. Its network of parasitic threads now extends across 3.4 square miles of forest soil, weighing perhaps as much as 35,000 tons.
Fungi became especially adept at using the air to spread. Some fungi pack spores into little tufts, called conidia, that blow away to produce clones of themselves. Others, like honey mushrooms, wait to have sex before making their spores. Fungal sex occurs when the threads from two individual organisms cross paths. If they belong to different mating types, they may fuse their cells together to create a joint offspring. (Some fungi have two mating types, but others have dozens.) These cells may then multiply into a structure that rises towards the surface, pushing through the dirt and the leaf litter, unfurling itself into a mushroom. The gills of the mushrooms are lined with spores, which can get carried away by currents of air.
Many species of fungi have evolved adaptations to improve the odds that the spores they release rise up from the ground and can float away. Some fungi launch their spores from microscopic catapults. Others fill tiny balloons with water, which they then squeeze like cannons to fire their spores into the air. Others shoot up an initial wave of spores with so much force that they pull a wake of air up behind them. A second wave of spores drafts behind it and rises even higher.
The spores of some species appear to only drift a short distance, creating genetically distinct populations that take up small ranges. But others journey further. One wandering fungus is known as Aspergillis fumigatus. It lives in Arctic soil and underneath tropical forests, as a single worldwide species. It’s likely that airborne spores keep the species bound together, bringing genes with new mutations from one continent to another. Aspergillis fumigatus came to the attention of scientists because it can live not just in the ground, but in the lungs of people with weakened defenses, killing hundreds of thousands of them every year. It’s likely that other species also straddle the globe, soaring from continent to continent without drawing notice, because they don’t put people in hospitals.
**Ed: Christian Ehrenberg was a 19th century biologist and the founder of the field of aerobiology. In his lifetime, "curiosity chambers," a sort-of forerunner to natural history museums, displayed what they called "paper meteorites," leathery balls of unknown origin. Some scientists at the time — erroneously — found traces of a variety of different metals within them and concluded they came from space, like any other meteorite. Ehrenberg analyzed a paper meteorite under a microscope and saw blue-green algae and diatoms typical of the North Sea in them. Ehrenberg concluded that the meteorites were microbial mats that had been picked up by storms and transported upwards of 1000 miles.