Technical Example

The table⁽¹⁾ below illustrates how the amino acid test provides evidence for paleoDNA preservation.

The table shows 26 individuals (human and animal), their radiocarbon ages, the results of the amino acid test, and the success in obtaining paleoDNA.

The primary indicator of the amino acid test is the "racemization" result for Aspartic acid (one of the most abundant amino acids in bone). Racemization is the transference of the L-enantiomer to the D-enantiomer, which occurs during diagenesis.  In simple terms, "enantiomer" is the coiling direction of the helical shape of the amino acid molecule.  The "L" enantiomer coils to the left. With diagenesis, there is a systematic change to the right-handed "D" enantiomer. Aspartic acid starts as entirely the L type (as do all amino acids).  The transference from one direction to the other is called "racemization".  Amino acid racemization has long been investigated as a dating tool (with limited success). 

Recent investigations into the application of this process (Poinar, et. al.)⁽¹⁾ have shown a correlation between the D/L ratio of Aspartic acid and the preservation of paleoDNA.  Results of this work are tabulated below.  Notice that samples with Aspartic acid D/L ratios (D/L Asp) of 0.10 or less, contained paleoDNA and those with values greater than 0.10 did not.

Other ratios are reported in the table as well.  On-going research suggests that the relative racemization of Aspartic acid to Alanine to Leucine may be an indicator of any available paleoDNA.  Amino acids which are primary to the bone should show a decreasing trend in racemization from Aspartic acid to Alanine to Leucine.  If this trend is present, it is evidence that any paleoDNA detected is primary to the individual.  A different pattern may indicate the presence of exogenous carbon (and potentially exogenous DNA) in the analyzed material.   More research is needed before making broad assumptions on this phenomenon.  The racemization of both Alanine and Leucine is usually extremely low, circumventing its application. 

CAVEAT:  A low D/L Asp value does not ensure the presence of paleoDNA.  It is an indicator.  However, no paleoDNA has been retrieved from samples with a ratio of greater than 0.20.  

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Table 1. ⁽¹⁾ The extent of racemization of Asp, Ala, and Leu and DNA amplificability for 26 archaeological and paleontological samples. DNA was extracted, amplified, and sequenced as described in the references. Briefly, for nonhuman samples 140 bp of the mitochondrial 16S ribosomal DNA (rDNA), or 120 bp of the mitochondrial 12S rDNA, were amplified, whereas for human samples primers for an 87-bp fragment of the mitochondrial control region were used. In all cases in which no DNA could be amplified, extractions were performed as in (18) and at least two attempts were made under conditions allowing amplification from single template molecules.  

(1) Poinar, Hendrik N., Hoss, Matthias, Bada, Jeffrey L., Paabo, Svante Amino Acid Racemization and the Preservation of Ancient DNA Science 1996 272: 864-866

References for table:

(5) M. Höss, A. Dilling, A. Currant, S. Pääbo, Proc. Natl. Acad. Sci. U.S.A. 93, 181 (1996)

(8) T. Lindahl, Nature 365, 700 (1993); M. Höss, M.K. Vereshchagin, S. Pääbo, ibid. 370, 333 (1994). 

(18) M. Höss and S. Pääbo, Nuceic Acids Res. 21, 3913 (1993).

(19) A. Cooper and P. Hood, in Avian Molecular Evolution and Systematics, D. Mindell, Ed. (Academic Press, New York, in press).