Addendum 



2078 



Abelson, Philip H. 1956. 



Paleobiochemistry. Sci. Am. 195(1): 83-92. 



Mereenaria mereenaria has been common for more than 25 million yrs , and its 

 fossil shells appear identical with their modern counterparts. It was pos- 

 sible to study the content of protein and amino acids in the shells of living 

 clams, and in clams one thousand, 500 thousand, and 25 million years old. 

 Laminated sheets of protein in modern clam shells aire colorless and have some 

 mechanical strength. Amino acids found in this protein are typical of amino 

 acids found in other animals. Some 15 were actually identified. The thousand 

 yr old shell was aged by the carbon-14 method, and had been buried in moist 

 soil. Its protein content was undiminished and its amino acids were identical 

 with those of the modern specimen. However, the sheets of protein had turned 

 brown and had lost all mechanical strength. The 500,000 yr old shell, which 

 because of the uncertainty of the dating method may have been as young as 

 100,000 yrs or as old as a million yrs, contained no protein at all. In place 

 of the protein was a black, tarlike substance. The amino acid content of the 

 shell had diminished to about one-tenth of that of the modern shell. About 

 half of the amino acid was in the form of peptide chains consisting of two 

 or more amino acids. Only individual amino acids remained in the 25 million 

 yr old shell. Where the modern shell contains the usual protein building 

 blocks, the ancient specimen consists predominantly of alanine, glutamic 

 acid, glycine, isoleucine, proline, and valine. The sequence of steps in the 

 decay of protein seems clear. Water penetrates the shell and reacts with the 

 protein so that it breaks down into peptide chains and individual amino acids. 

 These smaller molecules are more soluble in water, and some of them may drain 

 out of the shell. Of the amino acids that remain in the shell, some (such as 

 serine, threonine, and tyrosine) are unstable and tend to break down more 

 rapidly. After a few million years they will tend to vanish altogether. This 

 explains why older fossils contain the same unusual assortment of amino acids. 

 Originally they all contained proteins composed of the same amino acids we 

 find in proteins today, but only the stable amino acids have survived. Is it 

 possible that these specimens are contaminated? We find similar amino acids 

 in fossils of the same species collected from many different formations, which 

 strongly supports the assumption that the amino acids were there during the 

 whole life of the animal. Certain amino acids are adsorbed on calcium carbon- 

 ate precipitated in the laboratory. Aspartic acid and glutamic acid tend to 

 be adsorbed on such a precipitate, others are not. The fossil material is 

 composed of calcium carbonate, and if amino acids had been adsorbed in water 

 from the ground, we would expect to find an abundance of aspartic and glutamic 

 acids, but we do not. By laboratory tests it can be demonstrated that at 450°C 

 it took only about a second for alanine in water to decrease by 63%, but at 

 room temp alanine could last for billions of yrs. The most stable amino acids 

 are alanine, glutamic acid, glycine, isoleucine, proline, and valine, and the 

 least stable were arginine, aspartic acid, lysine, phenylalanine, serine, 

 threonine, and tyrosine. Thus laboratory studies confirm the findings in 

 fossils. - J.L.M. 



2079 



Abelson, Philip H. 1959. 



Geochemistry of organic substances. In Researches in Geochemistry. 

 Philip H. Abelson (ed.) . John Wiley & Sons, Inc., New York: 79-103. 



The protein myosin is involved in combination with ATP. In Mereenaria 

 mereenaria scars on the shell show the points of muscle attachment, and these 

 are preserved in fossils. The shape of the shell of 25 million yr old clams 

 is identical with those of today, including the muscle scar. It seems quite 

 likely that the old clams had musculature exactly like that of modern species 

 and that the conventional myosin-ATP system was employed. The amino acids in 

 M, mereenaria shells are described exactly as in Abelson's 1963 chapter, 

 abstracted in this bibliography. - J.L.M. 



577 



