熊猫进化史

Panda evolution

THE GIANT PANDA is beloved of conservationists. It is the mascot of the WWF (World Wide Fund for Nature, formerly the World Wildlife Fund) and, with its striking black-and-white pelage, is one of the most recognisable large animals in the world. It is also evolutionarily weird. It is a type of bear, and therefore a member of the order of mammals known, after their usual dietary habits, as the Carnivora. But it is an obligate herbivore.

And it is rare. Optimistic estimates put the global panda population at between 2,000 and 3,000 individuals—with all those not living in zoos occupying a few fragments of bamboo forest in central China. Pessimists reckon those numbers are on the high side. It is ironic, then, that this icon of the natural world might actually be an accidental consequence of human activity. Yet this is a plausible interpretation of results just published in a paper in Current Biology, by Wei Fuwen of the Institute of Zoology, in Beijing.

Pandas are not merely herbivores, they are monovores—eating bamboo to the exclusion of almost anything else. Dr Wei wondered when this transition to monovory happened. The answer was, far more recently than anyone had expected.

Past estimates of changes in pandas’ diets have depended on studies of their skulls and genes. The jaws of 4m-year-old fossils suggest that the ancestors of modern pandas were already by then eating a lot of tough vegetable matter. Analysis of a gene called Tas1r1, responsible for the taste sensation called “umami”, which detects glutamic acid, a common component of meat, tells a similar story. It indicates that selective pressures in favour of this gene started to relax about 4.2m years ago. By around 2m years ago, conventional theory has it, pandas had completed the transition to an all-bamboo diet. Dr Wei has, however, brought a third line of evidence to bear. This is the isotopic composition of the animals’ bones and teeth.

Hydrogen apart, the most common elements in food are carbon, oxygen and nitrogen. Each of these has several isotopes (atoms of the same element whose nuclei have different numbers of neutrons within, and therefore different atomic weights). The two principal isotopes of carbon, 12C and 13C, and of nitrogen, 14N and 15N, have different ratios in different plant species—and these ratios tend to be preserved in the tissues of animals that eat those plants. The isotopes of oxygen,16O and 18O, vary in ratio according to the local climate.

Dr Wei studied carbon and nitrogen isotopes in the bones of a dozen ancestral pandas, dating from between 11,000 and 5,000 years ago, and compared them with those of modern pandas. The ancient animals had a wider range of 15N/14N and 13C/12C ratios in their bones than did the modern ones. That suggests they had broader diets. Oxygen isotopes collected from fossil teeth told a similar story. The ancient pandas had more variable 18O/16O values, suggesting that they lived in more varied environments than do their modern kin.

Whether the fossil pandas in Dr Wei’s study were still eating any meat remains unclear. What is clear, however, is that they were not yet the obligate bamboo feeders which they are today, and that they were making forest fringes, subtropical zones and open land their home, rather than dwelling solely in bamboo forests. The question is, what made them change?

Dr Wei does not speculate. But there is one obvious possible culprit: the spread of Homo sapiens. Early Chinese history is shrouded in myth, but organised states clearly existed by about 5,000 years ago. Growing human populations could easily have displaced the ancestors of modern pandas to fringe areas where there was little to eat but bamboo. And if bamboo is all there is to eat, then those that prefer to eat it will be at an evolutionary advantage. The modern, bamboo-eating panda—symbol of animals under pressure from man—may thus have been made the way it is by precisely such human pressures.

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