It is a puzzle to tax the great man himself. For, as if to mark the bicentenary of Charles Darwin's birth, scientists have come up with a remarkable discovery that poses fundamental questions about how evolution works.
The biggest study of life in the Arctic and Antarctic Oceans ever carried out, involving more than 500 polar researchers from 25 countries, has discovered at least 235 species that are common to both poles – even though they are almost 7,000 miles apart.
"This raises a whole bunch of evolutionary questions," said Professor Russ Hopcroft, of the University of Alaska, one of the leading scientists conducting the study, which is part of a 10-year worldwide census of marine life. "We think of the Arctic and Antarctic as similar habitats, but they are separated by great distances."
Professor Hopcroft and his colleagues in the census, which is supported by the UN Environment Programme and other international agencies, are now trying to work out how the same species could have evolved simultaneously at both ends of the earth, or whether they had been able to find a way of traversing the thousands of miles of warmer water between them.
In a few cases there is no problem; the species include five whales and six seabirds, which migrate vast distances around the globe. But most of the rest – including nearly 100 species of crustacean – pose far more difficult questions.
One possibility being investigated is that the species were able to travel between the poles along cold, deep-sea currents. Another is that they thrived during the height of the last Ice Age, some 20,000 years ago. The northern and southern ice masses were then much closer, and got separated when the earth warmed up. Similar evolutionary pressures in the two similar habitats might then have ensured that species did not develop differently.
The scientists are now embarking on detailed DNA studies to work out whether the species are indeed identical or whether they have begun to drift apart genetically. And they want to try to establish how long ago they were separated.