Alternative biochemical life

This month's Scientific American contains an article by Paul Davies on the possibility that life may have arisen on multiple independent occasions upon the surface of the Earth. A nice summary is provided of the biochemical differences which might exist between life forms of independent origin:

• "Large biological molecules possess a definite handedness: although the atoms in a molecule can be configured into two mirror-image orientations—left-handed or right-handed—molecules must possess compatible chirality to assemble into more complex structures. In known life-forms, the amino acids—the building blocks of proteins—are left-handed, whereas the sugars are right-handed and DNA is a right-handed double helix. The laws of chemistry, however, are blind to left and right, so if life started again from scratch, there would be a 50–50 chance that its building blocks would be molecules of the opposite handedness. Shadow life could in principle be biochemically almost identical to known life but made of mirror-image molecules. Such mirror life would not compete directly with known life, nor could the two forms swap genes, because the relevant molecules would not be interchangeable."
• "Another possibility is that shadow life might share the same general biochemistry with familiar life but employ a different suite of amino acids or nucleotides (the building blocks of DNA)...chemists can synthesize many other amino acids that are not present in known organisms...Some of these unfamiliar amino acids might make suitable building blocks for alternative forms of life."
• "Another popular conjecture concerns the basic chemical elements that make up the vital parts of known organisms: carbon, hydrogen, oxygen, nitrogen and phosphorus. Would life be possible if a different element were substituted for one of these five? Phosphorus is problematic for life in some ways. It is relatively rare and would not have existed in abundance in readily accessible, soluble form under the conditions that prevailed during the early history of Earth. Felisa Wolfe-Simon, formerly at Arizona State University and now at Harvard University, has hypothesized that arsenic can successfully fill the role of phosphorus for living organisms and would have offered distinct chemical advantages in ancient environments."
• "Some astrobiologists have speculated about the possibility of life arising from silicon compounds instead of carbon compounds." [Carbon, like silicon, possesses four valence electrons, enabling it to form rings and chains which form the backbone of biological molecules.]