A short history of nearly everything (40 page)

One of the biggest surprises in the earth sciences in recent decades was the discovery of just how early in Earth’s history life arose. Well into the 1950s, it was thought that life was less than 600 million years old. By the 1970s, a few adventurous souls felt that maybe it went back 2.5 billion years. But the present date of 3.85 billion years is stunningly early. Earth’s surface didn’t becomesolid until about 3.9 billion years ago.

“We can only infer from this rapidity that it is not ‘difficult’ for life of bacterial grade to evolve on planets with appropriate conditions,” Stephen Jay Gould observed in theNew York Times in 1996. Or as he put it elsewhere, it is hard to avoid the conclusion that “life, arising as soon as it could, was chemically destined to be.”

Life emerged so swiftly, in fact, that some authorities think it must have had help—perhaps a good deal of help. The idea that earthly life might have arrived from space has a surprisingly long and even occasionally distinguished history. The great Lord Kelvin himself raised the possibility as long ago as 1871 at a meeting of the British Association for the Advancement of Science when he suggested that “the germs of life might have been brought to the earth by some meteorite.” But it remained little more than a fringe notion until one Sunday in September 1969 when tens of thousands of Australians were startled by a series of sonic booms and the sight of a fireball streaking from east to west across the sky. The fireball made a strange crackling sound as it passed and left behind a smell that some likened to methylated spirits and others described as just awful.

The fireball exploded above Murchison, a town of six hundred people in the Goulburn Valley north of Melbourne, and came raining down in chunks, some weighing up to twelve pounds. Fortunately, no one was hurt. The meteorite was of a rare type known as a carbonaceous chondrite, and the townspeople helpfully collected and brought in some two hundred pounds of it. The timing could hardly have been better. Less than two months earlier, theApollo 11 astronauts had returned to Earth with a bag full of lunar rocks, so labs throughout the world were geared up—indeed clamoring—for rocks of extraterrestrial origin.

The Murchison meteorite was found to be 4.5 billion years old, and it was studded with amino acids—seventy-four types in all, eight of which are involved in the formation of earthly proteins. In late 2001, more than thirty years after it crashed, a team at the Ames Research Center in California announced that the Murchison rock also contained complex strings of sugars called polyols, which had not been found off the Earth before.

A few other carbonaceous chondrites have strayed into Earth’s path since—one that landed near Tagish Lake in Canada’s Yukon in January 2000 was seen over large parts of North America—and they have likewise confirmed that the universe is actually rich in organic compounds. Halley’s comet, it is now thought, is about 25 percent organic molecules. Get enough of those crashing into a suitable place—Earth, for instance—and you have the basic elements you need for life.

There are two problems with notions of panspermia, as extraterrestrial theories are known. The first is that it doesn’t answer any questions about how life arose, but merely moves responsibility for it elsewhere. The other is that panspermia sometimes excites even the most respectable adherents to levels of speculation that can be safely called imprudent. Francis Crick, codiscoverer of the structure of DNA, and his colleague Leslie Orgel have suggested that Earth was “deliberately seeded with life by intelligent aliens,” an idea that Gribbin calls “at the very fringe of scientific respectability”—or, put another way, a notion that would be considered wildly lunatic if not voiced by a Nobel laureate. Fred Hoyle and his colleague Chandra Wickramasinghe further eroded enthusiasm for panspermia by suggesting that outer space brought us not only life but also many diseases such as flu and bubonic plague, ideas that were easily disproved by biochemists. Hoyle—and it seems necessary to insert a reminder here that he was one of the great scientific minds of the twentieth century—also once suggested, as mentioned earlier, that our noses evolved with the nostrils underneath as a way of keeping cosmic pathogens from falling into them as they drifted down from space.

Whatever prompted life to begin, it happened just once. That is the most extraordinary fact in biology, perhaps the most extraordinary fact we know. Everything that has ever lived, plant or animal, dates its beginnings from the same primordial twitch. At some point in an unimaginably distant past some little bag of chemicals fidgeted to life. It absorbed some nutrients, gently pulsed, had a brief existence. This much may have happened before, perhaps many times. But this ancestral packet did something additional and extraordinary: it cleaved itself and produced an heir. A tiny bundle of genetic material passed from one living entity to another, and has never stopped moving since. It was the moment of creation for us all. Biologists sometimes call it the Big Birth.

“Wherever you go in the world, whatever animal, plant, bug, or blob you look at, if it is alive, it will use the same dictionary and know the same code. All life is one,” says Matt Ridley. We are all the result of a single genetic trick handed down from generation to generation nearly four billion years, to such an extent that you can take a fragment of human genetic instruction, patch it into a faulty yeast cell, and the yeast cell will put it to work as if it were its own. In a very real sense, itis its own.

The dawn of life—or something very like it—sits on a shelf in the office of a friendly isotope geochemist named Victoria Bennett in the Earth Sciences building of the Australian National University in Canberra. An American, Ms. Bennett came to the ANU from California on a two-year contract in 1989 and has been there ever since. When I visited her, in late 2001, she handed me a modestly hefty hunk of rock composed of thin alternating stripes of white quartz and a gray-green material called clinopyroxene. The rock came from Akilia Island in Greenland, where unusually ancient rocks were found in 1997. The rocks are 3.85 billion years old and represent the oldest marine sediments ever found.

“We can’t be certain that what you are holding once contained living organisms because you’d have to pulverize it to find out,” Bennett told me. “But it comes from the same deposit where the oldest life was excavated, so itprobably had life in it.” Nor would you find actual fossilized microbes, however carefully you searched. Any simple organisms, alas, would have been baked away by the processes that turned ocean mud to stone. Instead what we would see if we crunched up the rock and examined it microscopically would be the chemical residues that the organisms left behind—carbon isotopes and a type of phosphate called apatite, which together provide strong evidence that the rock once contained colonies of living things. “We can only guess what the organism might have looked like,” Bennett said. “It was probably about as basic as life can get—but it was life nonetheless. It lived. It propagated.”

And eventually it led to us.

If you are into very old rocks, and Bennett indubitably is, the ANU has long been a prime place to be. This is largely thanks to the ingenuity of a man named Bill Compston, who is now retired but in the 1970s built the world’s first Sensitive High Resolution Ion Micro Probe—or SHRIMP, as it is more affectionately known from its initial letters. This is a machine that measures the decay rate of uranium in tiny minerals called zircons. Zircons appear in most rocks apart from basalts and are extremely durable, surviving every natural process but subduction. Most of the Earth’s crust has been slipped back into the oven at some point, but just occasionally—in Western Australia and Greenland, for example—geologists have found outcrops of rocks that have remained always at the surface. Compston’s machine allowed such rocks to be dated with unparalleled precision. The prototype SHRIMP was built and machined in the Earth Science department’s own workshops, and looked like something that had been built from spare parts on a budget, but it worked great. On its first formal test, in 1982, it dated the oldest thing ever found—a 4.3-billion-year-old rock from Western Australia.

“It caused quite a stir at the time,” Bennett told me, “to find something so important so quickly with brand-new technology.”

She took me down the hall to see the current model, SHRIMP II. It was a big heavy piece of stainless-steel apparatus, perhaps twelve feet long and five feet high, and as solidly built as a deep-sea probe. At a console in front of it, keeping an eye on ever-changing strings of figures on a screen, was a man named Bob from Canterbury University in New Zealand. He had been there since 4A.M., he told me. SHRIMP II runs twenty-four hours a day; there’s that many rocks to date. It was just after 9A.M.and Bob had the machine till noon. Ask a pair of geochemists how something like this works, and they will start talking about isotopic abundances and ionization levels with an enthusiasm that is more endearing than fathomable. The upshot of it, however, was that the machine, by bombarding a sample of rock with streams of charged atoms, is able to detect subtle differences in the amounts of lead and uranium in the zircon samples, by which means the age of rocks can be accurately adduced. Bob told me that it takes about seventeen minutes to read one zircon and it is necessary to read dozens from each rock to make the data reliable. In practice, the process seemed to involve about the same level of scattered activity, and about as much stimulation, as a trip to a laundromat. Bob seemed very happy, however; but then people from New Zealand very generally do.

The Earth Sciences compound was an odd combination of things—part offices, part labs, part machine shed. “We used to build everything here,” Bennett said. “We even had our own glassblower, but he’s retired. But we still have two full-time rock crushers.” She caught my look of mild surprise. “We get through alot of rocks. And they have to be very carefully prepared. You have to make sure there is no contamination from previous samples—no dust or anything. It’s quite a meticulous process.” She showed me the rock-crushing machines, which were indeed pristine, though the rock crushers had apparently gone for coffee. Beside the machines were large boxes containing rocks of all shapes and sizes. They do indeed get through a lot of rocks at the ANU.

Back in Bennett’s office after our tour, I noticed hanging on her wall a poster giving an artist’s colorfully imaginative interpretation of Earth as it might have looked 3.5 billion years ago, just when life was getting going, in the ancient period known to earth science as the Archaean. The poster showed an alien landscape of huge, very active volcanoes, and a steamy, copper-colored sea beneath a harsh red sky. Stromatolites, a kind of bacterial rock, filled the shallows in the foreground. It didn’t look like a very promising place to create and nurture life. I asked her if the painting was accurate.

“Well, one school of thought says it was actually cool then because the sun was much weaker.” (I later learned that biologists, when they are feeling jocose, refer to this as the “Chinese restaurant problem”—because we had a dim sun.) “Without an atmosphere ultraviolet rays from the sun, even from a weak sun, would have tended to break apart any incipient bonds made by molecules. And yet right there”—she tapped the stromatolites—“you have organisms almost at the surface. It’s a puzzle.”

“So we don’t know what the world was like back then?”

“Mmmm,” she agreed thoughtfully.

“Either way it doesn’t seem very conducive to life.”

She nodded amiably. “But there must have been something that suited life. Otherwise we wouldn’t be here.”

It certainly wouldn’t have suited us. If you were to step from a time machine into that ancient Archaean world, you would very swiftly scamper back inside, for there was no more oxygen to breathe on Earth back then than there is on Mars today. It was also full of noxious vapors from hydrochloric and sulfuric acids powerful enough to eat through clothing and blister skin. Nor would it have provided the clean and glowing vistas depicted in the poster in Victoria Bennett’s office. The chemical stew that was the atmosphere then would have allowed little sunlight to reach the Earth’s surface. What little you could see would be illumined only briefly by bright and frequent lightning flashes. In short, it was Earth, but an Earth we wouldn’t recognize as our own.

Anniversaries were few and far between in the Archaean world. For two billion years bacterial organisms were the only forms of life. They lived, they reproduced, they swarmed, but they didn’t show any particular inclination to move on to another, more challenging level of existence. At some point in the first billion years of life, cyanobacteria, or blue-green algae, learned to tap into a freely available resource—the hydrogen that exists in spectacular abundance in water. They absorbed water molecules, supped on the hydrogen, and released the oxygen as waste, and in so doing invented photosynthesis. As Margulis and Sagan note, photosynthesis is “undoubtedly the most important single metabolic innovation in the history of life on the planet”—and it was invented not by plants but by bacteria.

As cyanobacteria proliferated the world began to fill with O2to the consternation of those organisms that found it poisonous—which in those days was all of them. In an anaerobic (or a non-oxygen-using) world, oxygen is extremely poisonous. Our white cells actually use oxygen to kill invading bacteria. That oxygen is fundamentally toxic often comes as a surprise to those of us who find it so convivial to our well-being, but that is only because we have evolved to exploit it. To other things it is a terror. It is what turns butter rancid and makes iron rust. Even we can tolerate it only up to a point. The oxygen level in our cells is only about a tenth the level found in the atmosphere.

The new oxygen-using organisms had two advantages. Oxygen was a more efficient way to produce energy, and it vanquished competitor organisms. Some retreated into the oozy, anaerobic world of bogs and lake bottoms. Others did likewise but then later (much later) migrated to the digestive tracts of beings like you and me. Quite a number of these primeval entities are alive inside your body right now, helping to digest your food, but abhorring even the tiniest hint of O2. Untold numbers of others failed to adapt and died.