A genetic study has shed light on the mystery of how fish made
the move from water to land millions of years ago.
Previous research had suggested that fish had made an abrupt
genetic jump to acquire land-friendly limbs.
But a US team has now shown this event was not an evolutionary
novelty and the transition was far more gradual.
The study, published in the journal Nature, follows the recent
discovery of a fossil described as showing the "missing link"
between fins and limbs.
In 2004, the fossilised remains of the Tiktaalik roseae
revealed an animal with fins that were equipped for a life in the
water but also for support on land.
The crocodile-like creature, which lived about 380 million years
ago, was said to "blur the distinctions" between land- and
Marcus Davis, lead researcher of the paper and a scientist at
the University of Chicago, said: "The Tiktaalik and other
recent fossil finds suggested to us that the structures that really
make land animals unique - hands and feet and fingers and toes -
just didn't appear out of nowhere."
However, he said, this was in contrast to evidence seen in
previous genetic studies, which suggested an abrupt transition from
fins to limbs.
These studies focussed on the Hox genes, which play a vital role
in limb development.
Scientists had looked at the expression of the genes in the
developing limbs of land animals (tetrapods) and the developing fins
of zebrafish, which are often used in embryological studies.