By Ivan Noble BBC News Online

Valuable clues to the pace of evolution have been found in the bones of long-dead penguins recovered from the Antarctic.

DNA samples from these bones, one almost 8,000 years old, have given researchers in New Zealand a better idea of the speed of the "molecular clock" scientists use to investigate the evolutionary history of animals.

"The molecular clock is a way of dating species and how long ago they separated [from a common ancestor]," commented Tom Gilbert, of Oxford University's Biological Anthropology and Zoology department.

"There is a huge argument about how accurate it is and how applicable it is across different species," he told BBC News Online.

Ticking faster

David Lambert and colleagues at Massey University, New Zealand, dug through penguin droppings, feathers, egg remnants, soil, gravel and pebbles to unearth the preserved remains of two different lineages of Adélie penguin living in colonies in the Antarctic.

More than half a million birds nest at Cape Adare

The DNA they extracted from these remains indicated the molecular clock was ticking two to seven times faster than previously thought.

As DNA passes down the generations of every living thing, it naturally mutates , albeit very slowly.

Sometimes the mutations are the result of duplication errors as cells reproduce.

They may also occur as a result of natural exposure to radiation or to oxygen in the body.

Tricky calibration

The mutations occur at a steady rate, and as long as they do not prevent the organism reproducing, they are passed on to its descendants.

Looking at how far the DNA of one species differs from another gives an indication of how many times the molecular clock has ticked since the two species began to evolve apart.

But calibrating this clock - working out the length of a single tick - is a tricky business, and no-one is entirely sure how applicable the measure is to different types of organism.

DNA evidence has to be matched to the fossil record and to other evidence from geological samples to get a proper calibration of the technique.

Details of the penguin study appear in the journal Science.