The woolly mammoth vanished just after the last Ice Age but may be the best-understood prehistoric species because their massive size and demise in a geographic freezer made for near-perfect fossilization. Indeed, the fossil record has illuminated much of what we know of this animal regarding anatomical adaptations to the cold, e.g. minimizing heat loss with thick fur, thick oily skin, blubber, and small ears and tail. Interestingly, scientists have also determined that the woolly mammoth descended directly from Asian elephants that originated in tropical Africa 5–7 million years ago. What kind of evolutionary adaptations allowed a massive tropical elephant that is excellent at eliminating excess heat to move into and survive the frigid Arctic? Until recently, none of the fossilized evidence could be connected to how this animal once functioned because physiological and biochemical characteristics do not fossilize.
Based on their knowledge of blood physiology adaptations in Arctic species alive today, Kevin Campbell from the University of Manitoba in Canada and 14 colleagues from across the globe wondered if this Ice Age creature possessed similar adaptations that allowed it to move to frigid climates. In a typical mammal, haemoglobin, the O2 binding protein in the blood, releases O2 with very slight increases in temperature, thus allowing beneficial site-specific oxygen delivery to warm, working muscles. However, the haemoglobin of contemporary Arctic species is insensitive to temperature so that O2 delivery to cold extremities and appendages is maintained, despite having a warmer core, saving energy and minimizing heat loss. The team hypothesized this would be the woolly mammoth's strategy, but how do scientists analyse blood from an animal that went extinct 10,000 years ago?
Campbell's group extracted DNA from three permafrost-preserved Siberian mammoths that lived 43,000 years ago. From this DNA, they sequenced haemoglobin genes, and converted the sequences into mRNA. They then inserted the mRNA into E. coli bacteria, which manufactured the mammoth's haemoglobin. Next, the team used atomic modelling and found structural differences in mammoth haemoglobin resulting from three amino acid substitutions not found in Asian and African elephants. Finally, they performed physiological and biochemical experiments on the reassembled haemoglobin to determine how the structural differences affect function.
The mammoth haemoglobin functioned over an extremely wide temperature range compared with their tropical elephant cousins. This could be due to more chloride binding sites on the molecule, which changes how much heat is released during binding. Arctic reindeer possess similar binding sites, and elephant haemoglobin has the binding cluster but it is not used. Campbell's team believes these three substitutions in the haemoglobin sequence set mammoths apart from their elephant cousins, allowing them to oxygenate tissues even at very low temperatures, preventing costly heat loss. The team's unique multi-disciplinary approach has resulted in the first discoveries about key molecular and physiological adaptations in an extinct species. They think that the physiology behind cold-adaptation may be what facilitated the woolly mammoth's rapid expansion across a frozen environment that is no longer available for scientists to survey.