Human infants, like other newborn animals and hibernating rodents, are endowed with a built-in central heating system: Mitochondrial proton gradients are uncoupled from ATP production in brown adipose tissue, so chemical energy is converted directly into heat, which protects against the vicissitudes of an uncertain environment. Uncoupling protein 1 (UCP1), which is present only in brown adipose tissue, is critical for thermogenesis. Piglets, though, are unusual in this regard, as they lack this kind of fat and rely instead on shivering as a way to stay warm.
Berg et al. looked for and, surprisingly, found UCP1 sequences in preliminary pig genome data. But closer examination revealed that the gene is peppered with small errors and is missing exons 3 to 5, a deletion that they also found in other species of pig, wild boar, and hog, and that almost certainly renders the gene useless. The pig UCP1 sequences have randomly drifted away from those of other closely related animals, further evidence that the gene is nonfunctional and that this drift has been going on for some 20 million years, implying that the gene has been out of commission for the same period. Many pig species hail from relatively balmy environments, where such a heat-generating system would not have been needed for survival. Not so for the wild boar, which thrives in colder climes, partly because of the evolution of a nest-building behavior that compensates for the ancient loss of UCP1 and brown adipose tissue. — GR
PLoS Genet. 2, e129 (2006).