TABLE 5.3 The Organic Content of the Tagish Lake Meteorite

Aliphatic hydrocarbons

5 ppm

Dicarboximides

5.5 ppm

Aromatic hydrocarbons

≥1 ppm

Sulfonic acids

20.0 ppm

Dicarboxylic acids

17.5 ppm

Amino acids

≥0.1 ppm

Carboxylic acids

40 ppm

Amines

<0.1 ppm

Pyridine carboxylic acids

7.5 ppm

Amides

<0.1 ppm

SOURCE: Data from Pizzarello, S., Huang, Y.S., Becker, L., Poreda, R.J., Nieman, R.A., Cooper, G., and Williams, M. 2001. The organic content of the Tagish Lake meteorite. Science 293:2236.

We do not know the extent to which the Murchison organics reflect what was available on early Earth before life emerged. The rich inventory of amino acids does not appear to be universal in carbonaceous chondrites (although the number that have been examined in detail is very small). For example, only a few amino acids (glycine, alanine, α-aminoisobutyric acid, α-amino-n-butyric acid, γ-aminobutyric acid) are found in the meteorite that fell in 2000 on Tagish Lake, Canada (Table 5.3).7 The near absence of complex amino acids is significant, inasmuch as the meteorite was captured in a pristine condition soon after it fell.

It is also significant that no discovery of a dipeptide in meteorites has yet been reported. Joining two amino acids is the first step toward the synthesis of proteins, such as those found in contemporary terran life. If the meteorite organics analyzed to date are representative of planetary processing of primitive organic compounds, the process of assembling amino acids into polypeptides (short strings) may have been carried out first within living cells.

5.2.2
Biological-like Molecules from Planetary Processes

Current research is showing the interactions between organic molecules and a wide array of minerals. These include the formation of carboxylic acids in thermal vent chemistry and the formation of reduced chemical species through photochemistry involving semiconducting minerals.8

5.2.3
The Origin of Phosphorus

Phosphorus is an important component of terran life, but its synthesis in stars is not simple. It is produced as phosphorus-31 in stars with 15 protons and 16 neutrons and hence is an “odd-Z” element. Odd-Z elements are more difficult to produce than “even-Z” elements having an equal number of protons and neutrons; these can be produced from other even-Z elements via an “alpha chain” from helium. Odd-Z elements like P-31 are generally produced in abundance only when there is an excess of neutrons to protons. This excess emerges only as the universe ages, implying that life based on phosphorus cannot have emerged early in the life of the universe. Odd-Z elements are also less abundant in the Sun than common elements, such as carbon and oxygen, by factors in excess of 100, although current models with sophisticated stellar nucleosynthesis account rather well for the observed phosphorus abundance in the Sun.9

Phosphorus is abundant on Earth, both as an element (the 11th-most abundant atom in Earth’s crust) and as phosphate. Meteorites hold a variety of phosphate-containing minerals and some phosphide minerals.10 Scientists at the University of Arizona have recently suggested that Fe3P, the mineral schreibersite, leads to the formation of phosphate and phosphite when corroded in water. Although phosphorylation of alcohols was not demonstrated, mechanistic considerations suggest that it should be possible. It is noteworthy that a clear prebiotic pathway for the chemical incorporation of phosphate into RNA or DNA has not been found. No nucleosides (nucleobases joined to sugars) have been reported from meteorites. Nor has evidence been found in any meteorite of the presence of nucleosides or nucleotides (nucleosides attached to phosphates). That suggests that nucleic acids were first formed as products of metabolism.



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