More on Ida: overblown claims and a worrying precedent

There is no such thing as the missing link. For all its popular appeal the concept has no meaning in science because there is no fossil that can fully explain an evolutionary transition all on its own.

Even in the plural the phrase can be misleading because evolution is supremely well attested in the fossil record, as well as from other evidence such as DNA. If some of the pieces have still to be found, the overall picture is perfectly clear.

Thousands of fossils can still be thought of as missing links because their discovery adds to understanding of how evolutionary trees branch out and older species give rise to new ones. The most spectacular of these finds cause excitement because they have almost exactly the features that evolutionary theory would predict.

A great example is Tiktaalik roseae. A team of scientists led by Neil Shubin knew that the first fish to leave the ocean and walk on land lived about 375 million years ago and sought out rock formations of that age in search of a transitional fossil that made the switch. They struck gold in the Canadian Arctic in 2004 with Tiktaalik: a fish with leg-like appendages capable of moving both on land and in water.

Other transitional fossils, such as Archaeopteryx, offer a snapshot of the evolution of birds from dinosaurs. Several fossils, such as Ambulocetus natans, help to document how the ancestors of whales moved from land to water and, last month, a fossil called Puijila darwini illustrated the same process for seals and walruses.

Darwinius masillae, ancestor of the modern lemur, left, appears to be another transitional form in the same mould, on a recent branch of the human family tree. As most palaeontologists were presented with scientific details only yesterday, however, a degree of caution is still needed in this interpretation.

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