杏吧原创

OF THE 200,000 shards of rock that has retrieved from Australia since the mid-1980s, only one contained what he was looking for. Two flecks of graphite, each barely the size of a red blood cell. Small, perhaps, but capable of overturning everything we know about life on Earth. Harrison, a geologist at the University of California, Los Angeles, remembers thinking to himself: 鈥淏y golly, they鈥檙e a dead ringer for a biogenic origin.鈥� Biogenic means made by life 鈥� but how? These graphite flecks were found in a zircon crystal that had lain trapped deep in the Jack Hills of Western Australia for 4.1 billion years. So they seem to imply our planet was inhabited at least 300 million years earlier than anyone had previously imagined.

What鈥檚 more, these first living organisms would date from a time before our planet was thought capable of harbouring any life at all. In these early years, Earth was supposedly a molten hellhole racked by volcanism and bombarded by space debris, zinging around a solar system yet to find inner peace. If Harrison鈥檚 fossils are all they seem, they wouldn鈥檛 only rewrite the history of life and Earth 鈥� but the entire solar system鈥檚 as well.

When it came to explaining how these things all got started, we thought we had it more or less worked out. Some 4.6 billion years ago, a vast cloud of dust and gas in some corner of an unremarkable galaxy began to collapse into a dense ball of matter. As more and more surrounding material was pulled towards it, the temperature and pressure at its core increased, to the point where nuclear fusion kicked in. This released vast quantities of energy and marked the moment our sun became a star.

As the newborn star slowly began to spin, smaller bodies started to coalesce in orbit around it. Close in, vast quantities of water ice were boiled away, leaving only metallic compounds behind to form the smaller rocky planets. Further out, cooler temperatures allowed giant worlds of ice and gas to form. All in a single plane along smooth, near-circular tracks.

It was a nice story, but as further details emerged, it became apparent that this picture was incomplete. For one thing, it struggled to explain the quantity and distribution of the so-called Trojan asteroids, thousands of tiny bodies that chase after Jupiter in its orbit. The Kuiper belt, the icy band beyond Neptune that Pluto belongs to, was equally difficult to justify: many of its bodies orbit at far greater angles to the planetary plane than the conventional picture would allow. Perhaps most perplexing of all, however, was the evidence our cosmic neighbourhood had once been under heavy bombardment. Rocks returned to Earth by the Apollo astronauts suggested the widespread cratering on our own moon was the result of a protracted assault which took place 3.9 billion years ago 鈥� a ruction the conventional model found hard to explain.

The solution, named after the city in France where it was devised in 2005, was the Nice model. In this refinement of the traditional story, our solar system鈥檚 four giant planets started out much closer together than they are today. This configuration was unstable, leading to hundreds of millions of years of gravitational tussling, during which the giant planets migrated into their current positions, disturbing the millions of tiny bodies littering the ancient solar system. Many fell under Jupiter鈥檚 gravitational influence, becoming its Trojan followers, while others settled in the solar system鈥檚 outer regions as highly angled denizens of the Kuiper belt.

Meanwhile, asteroids in the band between Mars and Jupiter were dislodged from orbit, many going on to collide with the innermost planets. This period of intense activity, known as the Late Heavy Bombardment (LHB), would have left deep craters on the moon and given our fledgling planet a serious knock during the turbulent early stages of its development.

The small number of surviving solid rocks from this period have led us to picture early Earth as a fiery world covered in volcanoes bursting through a molten crust. The LHB鈥檚 few hundred million years of constant collisions contributed to a nightmarish landscape so extreme that the geological period is known as the Hadean, after the Greek god of the underworld. The existence of life in such a hellscape was considered preposterous. Instead, the first traces of biogenic carbon, dated at 3.8 billion years old, neatly coincide with the time Earth was finally at peace and the bombardment from outer space had slowed.

Hence the excitement if Harrison鈥檚 fleck of graphite really is what it appears to be: evidence not only of our planet鈥檚 oldest known life form, but one that emerged at an impossible time. His smoking gun was the ratio of isotopes carbon-13 and carbon-12, within the sample. 鈥淚f you were looking at this carbon ratio today, you would say it was biogenic,鈥� he says.

Astonishing as it is, from the University of Oxford cautions against getting carried away. 鈥淚t is one inclusion in one zircon,鈥� he says. 鈥淏ut this sets the bar for people to find more and really show there was life around back then.鈥�

Life or no life, it鈥檚 just the latest piece of evidence from the Jack Hills suggesting Earth鈥檚 hellish youth was more short-lived than astronomers thought possible. As far back as 1999, geologists uncovered other zircons in this astonishing terrain that indicated part of Earth鈥檚 surface had cooled and solidified 4.4 billion years ago. What鈥檚 more, measurements of how much oxygen the rocks contained suggested that Earth had been mild enough to support liquid water.

Further evidence that not all was right in the established picture of Earth and the solar system came in 2013, when , then at the University of Bonn, Germany, was analysing another contender for the planet鈥檚 oldest rock 鈥� on the other side of the world in Greenland. There she found evidence that Earth contained significant quantities of gold and platinum as far back as 4.1 billion years ago 鈥� even though these metals were thought to have been delivered only later by the Late Heavy Bombardment.

Yet more contention came last year, when from the University of Oklahoma, along with , published the results of their latest simulations of solar system formation. What they found seemed to sound the death knell for the Nice model. In 85 per cent of cases, the inner solar system ended up with fewer than the four rocky worlds it has today. 鈥淢ore often than not you lose Mercury,鈥� says Kaib. Only 1 per cent of the time could they create a solar system that looked like the one we recognise. It would not be the first time the Nice model has been modified to take account of problems (see 鈥�Mystery of the missing planet鈥�), but this was a problem of a different magnitude. 鈥淚t seems very unlikely that you can get the outer solar system architecture and protect the inner planets,鈥� he says.

鈥淩ewriting our history makes the notion of life elsewhere that much more likely鈥�

Kaib has a surprisingly simple solution. The giant planets still migrated, producing the Jovian Trojans and the Kuiper belt, but they did so much earlier 鈥� while the innermost planets were still forming. By turning up to the party fashionably late, Earth dodged a bullet. The early migration of the giant planets would have scattered most of the larger impactors by the time Earth鈥檚 formation was complete. That works well, says , from the University of Bristol, UK. 鈥淭he latter part of Earth鈥檚 formation would have been calmer, as opposed to having formed and then being smacked upside the head.鈥�

It鈥檚 an appealing theory, explaining not only why the solar system looks the way it does, but how Earth became friendly to life so early. But one final mystery remains. If the giant planet migration happened before Earth and the moon had formed, then something else must have been responsible for the craters on the lunar surface. But what?

from Purdue University thinks the answer lies closer to home. 鈥淚n the Nice model, most of the LHB impactors come from the asteroid belt,鈥� he says. 鈥淏ut the distribution of crater sizes on the moon and the distribution of asteroids don鈥檛 match.鈥� of the SETI Institute agrees. 鈥淚f the LHB really was just asteroids being thrown at the moon en masse, there should be a lot more big lunar basins, and there aren鈥檛,鈥� he says. Minton believes he might have found an alternative source for the LHB: Mars.

He鈥檚 still working on the finer details, but he presented the concept to the American Astronomical Society鈥檚 Division on Dynamical Astronomy at their meeting in May 2015. One fact working in its favour is that the Red Planet鈥檚 northern hemisphere is low-lying and considerably flatter than the highlands in the south. 鈥淢any have suggested that鈥檚 because the northern area is a giant basin formed by a 2000-kilometre impactor,鈥� says Minton. Debris thrown up by the formation of this so-called Borealis basin could have bombarded the moon, and Earth, 3.9 billion years ago.

Cuk has an even more radical explanation. 鈥淭o me it is not clear at all that there was a spike in lunar bombardment 3.9 billion years ago,鈥� he says. The Apollo samples that led to the assumption were returned from several different sites on the moon, with many showing evidence of impacts clustered around that time. But Cuk believes the Apollo samples all came from the impact or impacts that formed the Imbrium basin 鈥� one of the large, dark patches that makes up the 鈥淢an in the Moon鈥�. Rocky shrapnel from this event could have contaminated disparate parts of the lunar surface, meaning that what at first looked like a host of simultaneous impacts might have only been a handful. 鈥淭he idea of the carpet-bombing of the moon 3.9 billion years ago has gone away,鈥� he says. If you could prove the impacts that caused the cratering on the moon were less of a spike and more of a steady drip, then the Nice model could be saved after all. Just as crucially, it would have profound implications for conditions on our infant planet. 鈥淚f the impacts were more smeared out, early Earth wouldn鈥檛 have been total hell,鈥� says Cuk.

Either way, with relative calmness kicking in sooner in Earth鈥檚 history, life could have emerged more quickly to leave its mark in the Jack Hills zircon. 鈥淧ushing giant planet migration back earlier would be consistent with what we found,鈥� says Harrison. Future work will look at cementing this idea. Harrison has already identified another graphite inclusion in a separate Jack Hills zircon and will be analysing the ratio of its carbon isotopes within the next few months.

If Harrison鈥檚 hunch is right, then the life forms we had previously thought of as our earliest ancestors, dating from 3.8 billion years ago, weren鈥檛 the beginning of the evolutionary tree at all. Instead life on Earth began hundreds of millions of years earlier, almost as soon as the planet was ready for it. Such a scenario would raise hopes for the speed and ease with which biology can take hold, and of its aptitude for sticking around in an unfriendly cosmos. According to Harrison, 鈥渋t makes the notion of life elsewhere in the universe that much more likely.鈥� Our revised history could point to a more interesting future.

This article appeared in print under the headline 鈥淟ife, the solar system and everything鈥�