A genetic study published in 2007 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 was now able to show that this event was not an
evolutionary novelty and that the transition was far more gradual
than had previously been assumed.
The study, published in the journal Nature,
followed the recent discovery in Arctic Canada of a fossil which
was described as showing features that made it the 'missing link'
between fins and limbs (see the feature, A Key Stage in
Evolution, via the link in the sidebar).
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, was of the opinion that
the Tiktaalik and other then-recent fossil finds suggested
that the structures that really make land animals unique - hands
and feet and fingers and toes - did not simply appear out of
However, he added that 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.
In tetrapods, these studies showed that there
were two separate phases of Hox genes that turn on within the
developing appendage. Early in the development there is the first
phase, and then there is a second very characteristic phase which
plays a role in where fingers and toes form.
But if you look at a zebrafish during development,
it has the first phase, but it doesn't have this second hallmark
phase. Based on this, the hypothesis was that the second phase of
Hox expression must be a developmental and evolutionary novelty
that correlated with the origin of hands and feet.
However, Dr Davis and his colleagues decided to
repeat the studies - but this time using paddlefish, which have
a fin pattern similar to primitive fish. They found a very clear
second phase in their fins - and this shows that the second key
phase of Hox-expression is in fact a much more ancient pattern of
It seems that some fish have always had this
genetic toolkit to modify their fins. Tetrapods on the other hand
seem to have modified it in this unique and elaborate way to
produce land-dwelling animals.
The reason why some of these primitive fish went
on to become land-living animals while others remained in the water
was most probably influenced by their environment, according to
Dr Davis. A change to the ecosystem from deep water to shallow
streams may have driven some fish to make use of their genetic
The study was also interesting because it revealed
that zebrafish were the 'weirdos of the bunch'. They managed to do
something very unique - they appear to have lost the second phase
of Hox expression altogether. After this find, the expectation was
that a number of similar patterns would emerge in other fish in the