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What are the origins of life on Earth?

It came from outer space ...

By analysing the interplanetary detritus that has struck the Earth, we can calculate when our planet was born. And life itself? That may have been delivered in kit form by cosmic projectiles, says Nigel Henbest

03 March 2004


It was the third rock from the Sun - but not planet Earth as we know it. This was a ball of incandescent molten rock, shining like a miniature star in the blackness of space. Above its magma oceans there was no atmosphere, just a thin pall of rock vapour suffusing into the surrounding vacuum.

Yet this planet from hell would one day become a green and pleasant land, where sentient beings could work, rest and play in what - on the cosmic scale - is a veritable Garden of Eden.

Scientists of all kinds - astronomers, geologists, chemists, biologists - are now converging on a new view, both of what the early Earth was like and how it was transformed in such a remarkable way. Our planet was born from dust and rocks that were swirling around the young Sun. Some rocks from this solar nebula have survived to this day, and they fall to earth as meteorites. These provide a birth certificate for the planets.

Yuri Amelin from the Royal Ontario Museum has unlocked the natural clocks inside the oldest surviving examples of these meteorites with awesome precision. These "clocks" rely on the radioactive decay of atoms - in particular, the rate at which different kinds of uranium decay into lead. He has established that the solar system - including Earth - was born precisely 4,567 million years ago (with a possible error of less than a million years).

That's a staggeringly long time ago - too big for even the scientific mind to encompass. But we can put Earth's history in perspective by imagining that the clock that started then has been running for 24 hours. So Earth was born at midnight last night; and now we have reached midnight again. Complex life was a latecomer on the scene: the first fish at 9.22pm, dinosaurs at 10.50pm, and humans at just 30 seconds to midnight. None of these animals could appear until Earth was a favourable habitat for life. In its earliest days, it was anything but.

On our 24-hour clock, Earth formed from interplanetary rocks pretty rapidly - within the first few minutes. Less than 10 minutes after midnight, rocks in Earth's centre melted under the pressure of the overlying rocks and heat from radioactive atoms. The raw material of Earth contained a lot of iron, the most common metal in the universe. Like a giant blast-furnace, liquid iron separated from the molten rock and - being denser - flowed to the planet's very heart.

This core of liquid iron survives to this day. Throughout Earth's history, it has had a major role in keeping the planet safe for life. Electric currents in the molten core create Earth's magnetic field, which shields us from the radiation that surrounds us in space.

All this time, Earth was under bombardment by space rocks - and by larger and more dangerous bodies. At 0.18am, a wayward world as big as Mars smashed into Earth. Our planet was almost split in two. Molten rock splashed out into space, and later condensed to become the Moon. The impact stripped Earth of its original atmosphere, boiled away all its water and any organic molecules that could have been the raw material for life, and left its surface a deep sea of molten rock.

From this unpromising start, how could this planet become the Earth we know today? The answer, according to an accumulation of new evidence, is manna from heaven. The cosmic projectiles impacting our planet at that time were a motley bunch. Some were rocky; some were made mainly of ice; and others carried a lot of organic molecules. And Earth began to regenerate remarkably quickly.

The evidence comes from the barren outback of Western Australia. Here, Simon Wilde of the Curtin Institute of Technology has found the oldest fragments of Earth - tiny crystals of zircon. "When we look at the chemistry in detail," he explains, "we find it is consistent with having grown in a piece of continental crust. This is the first time we've been able to extend the history of Earth back to 4.4 billion years."

On our 24-hour clock, Earth had reached just 0.48am. And the planet had now cooled enough to have a solid crust. But something of even greater importance was to emerge from these tiny crystals.

Two rival teams - one headed by Simon Wilde and the other by Stephen Mojzsis of the University of Colorado at Boulder - probed the oxygen atoms in the zircon crystals. They found that the rocks contained an unexpectedly high proportion of rare oxygen-18. Geologists believe the only way to bump up the amount of oxygen-18 in a rock is by the action of running water.

"These zircons seem to show that the crust interacted with large volumes of liquid water," Mojzsis says. "So, by 200 million years after the formation of Earth, you can imagine a landscape of islands and small continents, bathed by a primitive ocean." As Earth had previously been boiled dry, this water must have been imported from elsewhere. And the finger of suspicion points at comets.

Comets are cosmic icebergs, several miles across. When a comet feels the heat of the Sun, some of its ice evaporates to form a giant glowing head and the trademark long tail. In the early solar system, there were certainly plenty of comets about that could collide with Earth, and drench it in water.

But this theory has had its problems. Astronomers measured the composition of three bright comets to a high precision - Halley's in 1986, and the comets Hyakutake and Hale-Bopp a decade later. They found that the hydrogen in these celestial icebergs does not match the composition of Earth's oceans: they all contain more of the rare type of hydrogen known as deuterium.

Recently, however, the pendulum has swung back. Theorists have calculated that comets come in two types, born in very two different regions of the solar system. Halley, Hyakutake and Hale-Bopp all happen to come from the cold outer region, beyond the orbit of Neptune, where cosmic chemistry would tend to increase the amount of deuterium.

On the other hand, the comets that impacted the early Earth would have been born much closer to home. In this relatively warmer region - near where Jupiter now lies - the cosmic cauldron would have produced comets containing less deuterium. They should have had a composition that is indeed like Earth's oceans.

In 2000, the Nasa scientist Michael Mumma tried to test this theory by measuring the composition of a comet that had originated in Jupiter's region of the solar system. The comet disintegrated before he could make the crucial observations of deuterium. But the rest of its chemistry fitted well with the predictions. As Mumma puts it: "For the first time we've seen a comet with the right composition to do the job."

This spring, scientists hope to test the theory once and for all. By chance, two bright comets from this same region will be homing in on the Sun. Comet Neat (named after the Near Earth Asteroid Tracking programme) will be well visible in the southern hemisphere in April and early May, while Comet Linear (Lincoln Near Earth Asteroid Research) will be a striking sight for us in the northern hemisphere in May - the best comet viewing since Hale-Bopp.

As well as water, comets probably delivered the raw materials of life to the young Earth. When Europe's pioneering comet-probe Giotto flew past Halley's comet, one of the big surprises was the comet's dust. It was rich in carbon, hydrogen, oxygen and nitrogen (nicknamed "CHON") - the ingredients of living cells. Mumma says: "Life on Earth did not have to start completely from scratch - it was delivered in kit form from space."

But could the delicate molecules of life survive their fiery entry through Earth's atmosphere, and the subsequent devastating impact with the ground? Jennifer Blank of the Lawrence Livermore National Laboratory decided that the theory wasn't sufficient to answer the question. Her answer lay in the laboratory's giant gas gun.

Blank did the impact in reverse. She fired a bullet - representing Earth's solid surface - at 10,000mph towards a fixed target containing the kind of simple molecules that are found inside comets. "We were hoping that some fraction of these molecules would survive the impact," she says, "but what we found was much more exciting."

After the deafening sound of the impact faded, Blank investigated the mixture of simple amino acids in the sample. Instead of destroying them, the impact had done the opposite - welded them together. "The energy associated with the impact built them up into larger molecules - peptides."

Peptides are simple forms of proteins, the building blocks of life. So the impacts of comets, and probably carbon-rich meteorites as well, not only provided Earth with the raw materials of life, but helped to build up the actual molecules needed to form living cells.

How these molecules came together to make the first living cells is still a matter of intense dispute. There's a lot of argument over what are the oldest fossilised remains of living cells. But Stephen Mojzsis has some strong indirect evidence for when life began. He has applied his analysis of different kinds of elements to ancient rocks from Greenland. The balance of carbon found there, he reckons, could be made only by living cells. That means that life had begun on Earth by 3,850 million years ago - or 3.45am on our 24-hour clock.

That's an astonishingly early date. Geologists studying the Moon know that our satellite was still under heavy bombardment at that time, so presumably Earth was also still being hit by cosmic projectiles. The exposed surface of Earth was a dangerous place for living cells to form. As well as the shock of direct impacts, striking comets and asteroids would boil away the oceans, filling the air with vapour creating a planetwide super-greenhouse.

Instead, Mojzsis ponders that life may have begun far beneath the surface - either near hot volcanic vents on the ocean floor, or in deep waterlogged cracks.

There is another possibility. Maybe life began on another planet further from the Sun, a safer world with a lower gravity and smaller seas. During the cosmic bombardment, a rock containing life may have travelled from that planet, and delivered its cargo of primitive bacteria to Earth. Arriving here, that life took hold and flourished, evolving into life on Earth today. In that case, we don't have to undertake space travel to find alien life - because we are all Martians...

Nigel Henbest is the scriptwriter (with Alice Harper) of the Channel 4 programme 'The Day the Earth was Born', and author (with Heather Couper) of 'Mars: the Inside Story of the Red Planet' (Headline, £14.99)


This article has been reproduced with the kind permission of Nigel Henbest and a 'The Independent'


This article was reproduced with the kind permission of the Independent and is not to be used for commercial gain.

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