Box Extension 24.3 Resurrection of the Blood Hemoglobin of the Extinct Woolly Mammoth: Evidence for an Ancient Adaptation to the Challenges of Regional Hypothermia

Using genomic methods, researchers recently resurrected the hemoglobin of the extinct woolly mammoth (Mammuthus primigenius) so they could study the hemoglobin directly. The mammoth was an abundant resident of Arctic and sub-Arctic environments—in sharp contrast to its extant relatives, the African and Asian elephants. Arctic mammals often permit tissue temperatures in their appendages to fall far below the temperature of the body core (see Figure 10.32), raising the possibility that the O₂ affinity of hemoglobin might be raised to such a high level by low tissue temperatures that the appendage tissues are subjected to impaired O₂ offloading. Was this a problem for woolly mammoths? By study of the resurrected mammoth hemoglobin, the researchers concluded that, in fact, the blood hemoglobin of the woolly mammoth had adaptive specializations that made it relatively insensitive to low temperatures.

The researchers extracted DNA from a 43,000-year-old, permafrost-preserved mammoth femur collected in Siberia. They then amplified and sequenced the genes in the DNA that coded for the a- and b-globin chains of hemoglobin (they located these genes based on homology with the known genes in today’s African and Asian elephants). Using the ancient DNA nucleotide sequences to predict the amino acid sequences in the ancient a- and b-globin chains, they discovered that the mammoth’s a- and b-globin proteins differed from those of the Asian elephant at one and three amino acid positions, respectively. The researchers then, in essence, introduced the genes for Asian elephant globins into the bacterium Escherichia coli, which faithfully synthesized elephant hemoglobin. Finally, by use of site-directed mutagenesis, the researchers modified the introduced elephant genes at the positions that needed to be changed for the E. coli–synthesized globin proteins to match those once circulating in the blood of the extinct woolly mammoth. Thereafter the E. coli produced authentic woolly mammoth hemoglobin, which was purified and studied to determine its thermal and O₂-transport properties. Box Extension 24.3 discusses additional details and provides references.

Recognizing that the mammoth DNA that was sequenced was from a single individual, the researchers compared their results from that individual to sequences in two additional Siberian mammoth specimens. They found that all three specimens were the same in the mammoth-specific changes identified.

The function of the mammoth blood hemoglobin (composed of α- and β-chains) synthesized by E. coli was compared with the function of blood hemoglobin from today’s Asian elephant. For example, oxygen equilibrium curves were obtained for the two hemoglobins and compared both with and without modulators such as 2,3-DPG. The researchers found evidence that the mammoth hemoglobin had distinctive allosteric-modulator binding properties that probably rendered the mammoth hemoglobin unusually insensitive to the affinity-enhancing effect of low temperatures. This thermal insensitivity presumably ensured that the mammoth hemoglobin would not hold on too tightly to O₂ as blood circulated through the animal’s hypothermic appendages, thereby allowing O₂ offloading to match tissue metabolic requirements independent of low temperature.

Recognizing the limited sample size that characterized this study (three mammoth specimens compared in a limited way with two living species), we need to keep in mind the cautions voiced by S. J. Gould and R. Lewontin in their famous Panglossian paradigm paper (see page 27). Gould and Lewontin argued that claims for adaptation should have strong empirical support before they are accepted as being convincing. This said, the woolly mammoth researchers made an intriguing case that the hemoglobin of the ancient woolly mammoth had evolved specific, identifiable adaptive properties that aided the mammoth’s success in the cold environments it occupied.

References

Campbell, K. L., and 14 additional authors. 2010. Substitutions in woolly mammoth hemoglobin confer biochemical properties adaptive for cold tolerance. Nat. Genet. 42: 536–540.

Miller, W., and 21 additional authors. 2008. Sequencing the nuclear genome of the extinct woolly mammoth. Nature 456: 387–390.

Weber, R. E., and K. L. Campbell. 2011. Temperature dependence of haemoglobin-oxygen affinity in heterothermic vertebrates: mechanisms and biological significance. Acta Physiol. 202: 549–562.

Yuan, Y., and 9 additional authors. 2011. A biochemical-biophysical study of hemoglobins from woolly mammoth, Asian elephant, and humans. Biochemistry 50: 7350–7360.