Impacts by asteroids and comets can create cataclysmic events that destroy life witness the demise of the dinosaurs 65 million years ago, widely believed to have been brought about by a "killer" asteroid impact. Without Jupiter, instead of being hit with a killer asteroid every hundred million years or so, wed get one every 10,000 years. This reduction in impacts enabled the Earth to develop both simple and complex forms of life.

But gas giant planets like Jupiter dont always help in the development of complex life. Consider, for instance, that while Jupiter deflects many asteroids away from Earth, it also is responsible for most of the asteroids in the first place. When planetesimals were clumping together to form the terrestrial planets, the gravitational influence of Jupiter prevented a fifth planet from forming. Instead, today there is the asteroid belt that lies between Mars and Jupiter a sad echo of the planet-that-never-was.

"Only a small fraction of the original material was left behind to become the asteroid belt," says Alan Boss of the Carnegie Institution of Washington. He says that Jupiter excited this material to high orbital eccentricities, causing much of it to collide with the Sun and the growing terrestrial planets.

"Jupiter does not prevent asteroid impacts on the Earth it causes them," says Boss. "Asteroids can still be kicked out of their orbits and sent on paths that intersect the terrestrial planets."

Mars also was affected by Jupiters gravity during the early formative years. Mars is only half the size and one-tenth the mass of Earth, prevented by Jupiter from accumulating enough mass to become an Earth-like world capable of sustaining complex life. The lower mass of Mars, in combination with its low magnetism, prevented the planet from retaining an atmosphere a vital necessity for life to exist and thrive.

If Jupiter were a little more massive, or if it was closer to the Sun, the Earth might have suffered the same fate as Mars. If Jupiter were close enough to the Sun to reside in the habitable zone 0.8 AU to 1.7 AU for our solar system, or roughly from the orbit of Venus to the orbit of Mars - then the Earth and the other terrestrial planets would never have formed at all. Jupiters gravitational influence would have prevented rocky debris from coalescing into planets. The inner solar system instead would have become a barren asteroid belt.

"Habitable zones tend to be only a few AU in width, so if there is a Jupiter-mass planet inside the habitable zone, it is highly unlikely that another planet of any mass could form or exist so close by," says Boss. "Instead, the only hope for habitability in such systems would be on moons orbiting the Jupiter-mass planet."

Could it be that a solar system devoid of gas giants might have the best chance for developing life? If gas giants like Jupiter and Saturn had never formed in our solar system, three Earth-like planets might have developed instead of just one. Lacking the gas giants, Uranus and Neptune-like icy planets likely would have formed in the Jupiter-Saturn region. This system would have been relatively asteroid-free, since most of the rocky debris in the early solar system wouldve coalesced into planets.

But a Jupiter-free solar system would still have to contend with comets small bodies of water ice that, in our solar system, come from places like the Kuiper Belt and the Oort cloud. Comet impacts can be as disastrous to life as asteroid impacts.

"In systems without asteroid belts, impactors come from the comet belts," says Donald Brownlee, an astronomer at the University of Washington in Seattle. "There is evidence that most stars form distant comet belts around them."

George Wetherill of the Carnegie Institution of Washington has suggested that without Jupiter and Saturn, there would be many more comets in the Kuiper Belt. These comets would wander into the inner solar system so frequently that the Earth would be struck by a major comet impact every 100,000 years.

Despite the devastation caused by asteroid and comet impacts, a solar system free of these bodies may not be the best thing for the evolution of life.

For one thing, comets may be necessary for life to emerge on terrestrial planets. It is thought that comets act as delivery systems for organic material and water, elements that were necessary for the origin and development of life on Earth. According to Brownlee, Jupiters scattering of cometary debris probably delivered most of our oceans.

Although asteroid and comet impacts can destroy life, they also set the stage for new evolutionary life forms to emerge. By clearing a planet of the more dominant organisms, other life forms can move to fill in the recently vacated ecological niches.

"If Jupiter was not there, or if it was smaller or further away, then there might not be an asteroid belt and thus no asteroid impacts," says Brownlee, "This can be good or bad depending on the situation. The dinosaurs were certainly not fans of the asteroids, but then they might not ever have evolved without the asteroids."

The evolution of complex life on Earth owes a lot to happenstance situations like asteroid or comet impacts. Not only did dinosaurs probably owe their very existence to such impacts, but so too do humans. If the asteroid impact that killed the dinosaurs had never occurred, perhaps our ancestors would not have been able to evolve beyond the small rodents that constituted the Mammalian branch 65 million years ago.