Resurrecting mammoth evolutionary dead ends
Opinion:The film Jurassic Park featured dinosaurs cloned from DNA recovered from insects preserved in prehistoric amber. The scientific and technical capacity to do this remains a long way off, but, in the meantime, powerful biotechnological techniques are being applied to study the physiology of extinct animals.
Kevin Campbell and Michael Hofreiter (Scientific American, August 2012) describe their brilliant research into how the physiology of woolly mammoths helped them survive during the ice age.
Woolly mammoths are extinct cousins of today’s Asian elephants. Woolly mammoths’ ancestors originated in sub-tropical Africa and migrated to Siberia less than 2 million years ago, at the beginning of the Pleistocene ice ages. The main problem these animals encountered in Africa was avoiding overheating, but when they moved north and the world froze they had to develop the capacity to conserve and manage heat.
Traditionally, extinct animals are studied by examining their fossilized bones and teeth. This allows the reconstruction of animal size, shape, configuration of musculature and some indication of the nature of the diet. Such studies tell us little or nothing of the physiological processes that sustained these animals, but modern biotechnology is now successfully attacking this problem.
The unit of biological organization is the cell. Every cell is controlled by instructions encoded in its genetic DNA. DNA is a very long molecule made of four different units called nucleotides strung along its length. The nucleotides are denoted by the four letters A, T, G and C, and the genetic instructions are encoded in the linear sequence of these letters.
Most of the work of the cell is carried out by proteins. There are thousands of different proteins. A protein is a long molecule made of units called amino acids strung along its length. There are 20 different types of amino acids and the types and sequence of amino acids in a protein determine the nature and the function of the protein.
The genetic DNA controls the cell by specifying what proteins are made. The linear information encoded in DNA is translated into the linear sequence of amino acids in a protein. The amount of DNA code necessary to code for a protein is called a gene.
DNA from extinct animals can be recovered, with difficulty, from fossilized remains and the nucleotide sequence of genes for critically important proteins from extinct animals can be worked out. This information is compared with the corresponding gene from the modern successor of the extinct animal. If the gene sequences are identical, then the protein products are identical. If they are different, then the extinct animal made a different protein to the modern animal and this different protein probably underpinned a different physiological regime.
If the extinct gene is different, you can modify a sample of the modern gene in the laboratory to make it match the extinct gene. The extinct gene is then incorporated into the DNA of a bacterium which is grown in culture to produce the protein product of the extinct gene. This “extinct” protein can now be studied in the laboratory to see how it behaves compared to its modern counterpart. This is how biotechnological techniques are being used to study the physiology of extinct animals.
Animals generate energy by oxidizing (“burning”) food in their cells. The necessary oxygen is taken from the air and carried through the bloodstream to the cells, bound to the protein haemoglobin. The haemoglobin releases the oxygen when it reaches the tissue cells. This release of oxygen requires energy input and its efficiency declines greatly as temperatures drop. Consequently modern animals that live in very cold environments have evolved mechanisms to help haemoglobin release its oxygen in tissues. Using the methods described earlier, Campbell and Hofreiter made woolly mammoth haemoglobin, which differs from modern elephant haemoglobin, and tested its oxygen releasing characteristics. They found that the mammoth haemoglobin releases its oxygen much more efficiently at low temperature compared with modern elephant haemoglobin. The woolly mammoth had evolved a variety of haemoglobin capable of coping with very cold conditions as part of a strategy for surviving the ice age. Future research will elucidate further details .