mitochondrion. This mitochondrial invasion (or endosymbiotic event, as biologists like to term it) made complex life possible. (In plants a similar invasion produced chloroplasts, which enable plants to photosynthesize.)
Mitochondria manipulate oxygen in a way that liberates energy from foodstuffs. Without this niftily facilitating trick, life on Earth today would be nothing more than a sludge of simple microbes. Mitochondria are very tiny-you could pack a billion into the space occupied by a grain of sand-but also very hungry. Almost every nutriment you absorb goes to feeding them.
We couldn’t live for two minutes without them, yet even after a billion years mitochondria behave as if they think things might not work out between us. They maintain their own DNA. They reproduce at a different time from their host cell. They look like bacteria, divide like bacteria, and sometimes respond to antibiotics in the way bacteria do. In short, they keep their bags packed. They don’t even speak the same genetic language as the cell in which they live. It is like having a stranger in your house, but one who has been there for a billion years.
The new type of cell is known as a eukaryote (meaning “truly nucleated”), as contrasted with the old type, which is known as a prokaryote (“prenucleated”), and it seems to have arrived suddenly in the fossil record. The oldest eukaryotes yet known, called Grypania, were discovered in iron sediments in Michigan in 1992. Such fossils have been found just once, and then no more are known for 500 million years.
Compared with the new eukaryotes the old prokaryotes were little more than “bags of chemicals,” in the words of the geologist Stephen Drury. Eukaryotes were bigger-eventually as much as ten thousand times bigger- than their simpler cousins, and carried as much as a thousand times more DNA. Gradually a system evolved in which life was dominated by two types of form-organisms that expel oxygen (like plants) and those that take it in (you and me).
Single-celled eukaryotes were once called
Eventually the eukaryotes learned an even more singular trick. It took a long time-a billion years or so-but it was a good one when they mastered it. They learned to form together into complex multicellular beings. Thanks to this innovation, big, complicated, visible entities like us were possible. Planet Earth was ready to move on to its next ambitious phase.
But before we get too excited about that, it is worth remembering that the world, as we are about to see, still belongs to the very small.
20 SMALL WORLD
IT’S PROBABLY NOT a good idea to take too personal an interest in your microbes. Louis Pasteur, the great French chemist and bacteriologist, became so preoccupied with them that he took to peering critically at every dish placed before him with a magnifying glass, a habit that presumably did not win him many repeat invitations to dinner.
In fact, there is no point in trying to hide from your bacteria, for they are on and around you always, in numbers you can’t conceive. If you are in good health and averagely diligent about hygiene, you will have a herd of about one trillion bacteria grazing on your fleshy plains-about a hundred thousand of them on every square centimeter of skin. They are there to dine off the ten billion or so flakes of skin you shed every day, plus all the tasty oils and fortifying minerals that seep out from every pore and fissure. You are for them the ultimate food court, with the convenience of warmth and constant mobility thrown in. By way of thanks, they give you B.O.
And those are just the bacteria that inhabit your skin. There are trillions more tucked away in your gut and nasal passages, clinging to your hair and eyelashes, swimming over the surface of your eyes, drilling through the enamel of your teeth. Your digestive system alone is host to more than a hundred trillion microbes, of at least four hundred types. Some deal with sugars, some with starches, some attack other bacteria. A surprising number, like the ubiquitous intestinal spirochetes, have no detectable function at all. They just seem to like to be with you. Every human body consists of about 10 quadrillion cells, but about 100 quadrillion bacterial cells. They are, in short, a big part of us. From the bacteria’s point of view, of course, we are a rather small part of them.
Because we humans are big and clever enough to produce and utilize antibiotics and disinfectants, it is easy to convince ourselves that we have banished bacteria to the fringes of existence. Don’t you believe it. Bacteria may not build cities or have interesting social lives, but they will be here when the Sun explodes. This is their planet, and we are on it only because they allow us to be.
Bacteria, never forget, got along for billions of years without us. We couldn’t survive a day without them. They process our wastes and make them usable again; without their diligent munching nothing would rot. They purify our water and keep our soils productive. Bacteria synthesize vitamins in our gut, convert the things we eat into useful sugars and polysaccharides, and go to war on alien microbes that slip down our gullet.
We depend totally on bacteria to pluck nitrogen from the air and convert it into useful nucleotides and amino acids for us. It is a prodigious and gratifying feat. As Margulis and Sagan note, to do the same thing industrially (as when making fertilizers) manufacturers must heat the source materials to 500 degrees centigrade and squeeze them to three hundred times normal pressures. Bacteria do it all the time without fuss, and thank goodness, for no larger organism could survive without the nitrogen they pass on. Above all, microbes continue to provide us with the air we breathe and to keep the atmosphere stable. Microbes, including the modern versions of cyanobacteria, supply the greater part of the planet’s breathable oxygen. Algae and other tiny organisms bubbling away in the sea blow out about 150 billion kilos of the stuff every year.
And they are amazingly prolific. The more frantic among them can yield a new generation in less than ten minutes;
About once every million divisions, they produce a mutant. Usually this is bad luck for the mutant-change is always risky for an organism-but just occasionally the new bacterium is endowed with some accidental advantage, such as the ability to elude or shrug off an attack of antibiotics. With this ability to evolve rapidly goes another, even scarier advantage. Bacteria share information. Any bacterium can take pieces of genetic coding from any other. Essentially, as Margulis and Sagan put it, all bacteria swim in a single gene pool. Any adaptive change that occurs in one area of the bacterial universe can spread to any other. It’s rather as if a human could go to an insect to get the necessary genetic coding to sprout wings or walk on ceilings. It means that from a genetic point of view bacteria have become a single superorganism-tiny, dispersed, but invincible.
They will live and thrive on almost anything you spill, dribble, or shake loose. Just give them a little moisture-as when you run a damp cloth over a counter-and they will bloom as if created from nothing. They will eat wood, the glue in wallpaper, the metals in hardened paint. Scientists in Australia found microbes known as
They have been found living in boiling mud pots and lakes of caustic soda, deep inside rocks, at the bottom of the sea, in hidden pools of icy water in the McMurdo Dry Valleys of Antarctica, and seven miles down in the Pacific Ocean where pressures are more than a thousand times greater than at the surface, or equivalent to being squashed beneath fifty jumbo jets. Some of them seem to be practically indestructible.
