No. 85: Jan-Feb 1993
The two giant planets, Jupiter and Saturn, are 318 and 95 times more massive than the earth, respectively. Being so weighty they strongly perturb the orbits of comets, deflecting many away from the inner solar system, where we reside. Calculations by G. Wetherill, at the Carnegie Institution, reveal that if Jupiter and Saturn were only 15 times the mass of the earth, the earth would have been devastated every 100,000 years by giant comets, instead of about every 100,000,000 years, as indicated by the geological record. Under such intense bombardment, it would probably have been difficult for advanced life forms to develop. (Croswell, Ken; "Why Intelligent Life Needs Giant Planets," New Scientist, p. 18, October 24, 1992.)
Comment. Reasonable as the foregoing assertion sounds, we do not really know what stimulates the development of new life forms. Actually, the fossil record reveals that some biological "radiations" occurred soon after great geological upheavals. That the Jupiter-Saturn "shield" was and is not completely effective is indicated by the heavy debris traffic mentioned above.