Can evolution run backwards?

In 1890, Belgian palaeontologist Louis Dollo postulated that evolution could not run backwards -- something widely accepted by the scientific community.

But now a study has claimed that the changes induced by evolution can be reversible, meaning certain animals can return to an earlier biological trait.

The remarkable discovery was made by finding birds had regrown a bone previously discarded by dinosaurs millions of years ago.

The study was carried out by a team of scientists from the University of Chile led by Dr Alexandar Vargas.

According to the Smithsonian Magazine, they studied fossils and embryos of today's birds.

They found that 230 million years ago, two-legged dinosaurs no longer required the strong wrists of their four-legged brethren, and thus they became weak.

The number of bones in wrists shrank from 11 to three, with one in particular of interest to disappear being the pisiform.

But according to research by Dr Vargas, the bone reappeared when dinosaurs evolved into birds and took flight.

The new bone, called the ulnare, appears in the same place as the pisiform once did.

The pisiform allowed bird wings to remain rigid on the upstroke. The study found it disappeared in bird-like dinosaurs, but modern birds later evolved to once again use this tiny bone.

“This idea that a bone can disappear and reappear in evolution has been resisted a lot in evolutionary biology,” Dr Vargas previously told LiveScience.

Scientists don't know how these bones disappeared, though. Prof Vargas said part of the problem is that palaeontologists look at fossils, while biologists focus on embryos.

But his team brought the research together by looking at fossils as well as studying bird anatomy.

The team also found that a half-moon-shaped bone called the semilunate in birds is the fusion of two wrist bones from dinosaurs.