Are Aliens Among Us?
In pursuit of evidence that life arose on Earth more than once, scientists are searching for microbes that are radically different from all known organisms
By Paul Davies
COVER OF SCIENTIFIC AMERICAN, DEC. 07
If, as many scientists believe, life can readily emerge under the right environmental conditions, it is possible that life arose on Earth more than once. Researchers are now seeking evidence of a second genesis by searching for exotic microbes that are biochemically different from all known organisms. In this image, artist Adam Questell has imagined an alien cell that carries its genetic material in twin nuclei.
Artist conception – SILICON LIFE-FORM
The most radically different aliens would be those based on silicon instead of carbon. Because silicon, like carbon, has a valence number of four (that is, the atom’s outermost orbital contains four electrons), silicon atoms can be arranged in rings and long chains that could form the backbones of biological molecules.
Artist conception – SILICON LIFE-FORM
Scientists and science-fiction writers have long speculated about what a silicon-based life-form would look like. In an article for the Saturday Review, H. G. Wells wrote: One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminium organisms…wandering through an atmosphere of gaseous sulphur, let us say, by the shores of a sea of liquid iron some thousand degrees or so above the temperature of a blast furnace.
Artist conception – TINY ALIENS
The smallest bacteria have a diameter of about 200 nanometers. Autonomous organisms on our tree of life cannot be much smaller because they must contain protein-building cellular structures called ribosomes, which are each about 20 to 30 nanometers across. But if alien microbes could function without ribosomes, they could be comparable in size to the smallest viruses, which are only 20 nanometers wide.
Artist conception – MIRROR LIFE
Large biological molecules can be configured into two mirror-image orientations: left-handed or right-handed. In all known life-forms, the amino acids are left-handed and DNA is a right-handed double helix. But if life started again from scratch, the amino acids could be right-handed and the DNA left-handed.
Artist conception – AMINO ACIDS LIFE-FORM
All familiar organisms use, with rare exceptions, the same 20 amino acids to construct proteins, but chemists can synthesize many others. Alien microbes could incorporate unusual amino acids such as isovaline and pseudoleucine, which have been found in meteorites.
Artist conception – ALIENS WITHIN US
Perhaps the most intriguing possibility of all is that alien life-forms inhabit our own bodies. While observing mammalian cells with an electron microscope in 1988, Olavi Kajander and his colleagues at the University of Kuopio in Finland observed ultrasmall particles inside many of the cells. With dimensions as small as 50 nanometers, these particles were about one-tenth the size of conventional small bacteria. Ten years later Kajander and his co-workers proposed that the particles were living organisms that thrive in urine and induce the formation of kidney stones by precipitating calcium and other minerals around themselves.
Artist conception – ARSENIC LIFE-FORM
Researchers have hypothesized that in alien organisms arsenic could successfully fill the biochemical role that phosphorus plays for known life-forms. Arsenic is poisonous to us because it mimics phosphorus so well; similarly, phosphorus would be poisonous to an arsenic-based organism.
Artist conception – VARIOUS LIFE-FORMS
It is possible that life arose on Earth several times, each time generating life-forms with different chemical characteristics.