Sscientists have traced the origin of fingers and toes to fish-like creatures that roamed the seas 380 million years ago, according to a new study.
The findings, published today in the British-based science journal Nature, upend the prevailing theory on the evolution of digits.
It had long been assumed that the first creatures to develop primitive fingers were tetrapods, air-breathing animals that crawled from sea to land some 10 to 20 million years later.
The need to adapt to swampy marshlands and terra firma, the theory went, is what drove the gradual shift through natural selection from fish fins suitable only for swimming to weight-bearing limbs with articulated joints.
The study, however, reveals that rudimentary fingers were already present inside the fins of the shallow-water Panderichthys, a transitional species that was nonetheless more fish than tetrapod.
“What we have shown is that the hand and the foot emerge from pre-existing bits of the fin skeleton that were just reshaped, rather than being entirely new bits that were bolted onto the existing fin skeleton,” said co-author Per Ahlberg, a researcher at Uppsala University in Sweden.
The discovery did not come from a new archeological find but from the reexamination of existing fossils, he said.
Previous research, it turns out, had simply overlooked what was there.
“The problem is that all good specimens of Panderichtys come from one location” – a brick quarry in Latvia – “where the clay is almost exactly the same color as the bones,” he said.
“With a nice big bone, that is not a problem. But if you are interested in tiny, fragile bones at the outer end of the fin skeleton, it is nearly impossible to see what is going on.”
Scientists had been thrown further off the track by the morphology of another animal from the Devonian period, which spanned from 360 to 416 million years ago.
In most ways, Tiktaalik seemed even closer to the true air-breathing tetrapods that first colonized firm land than Panderichtys, and yet its fins remained largely fish-like, lending even more credence to the theory that proto-fingers came during, not before, the transition to land.
But recent research in genetics had suggested that rudimentary digits might have emerged further back along the evolutionary tree than once suspected.
A gene that plays a key role in patterning the hands and feet in mice, for example, was found to express itself similarly in modern-day lung fish, a distant but direct cousin of the tetrapods that first crawled out of the sea.
So Mr Ahlberg and two colleagues decided it was worth taking a closer look at Panderichthys using a new technique.
They ran a specimen, still embedded in clay, through a CT scanner at a hospital.
“We could see the internal skeleton very clearly, and were able to model it without ever physically touching the specimen,” Mr Ahlberg said.
The image shows stubby bones at the end of the fin skeleton clearly arrayed like four fingers, called distal radials. There are no joints, and the bones are quite short, but there could be no doubt as to what they were.
“This was the key piece of the puzzle that confirms that rudimentary fingers were already present in the ancestors of tetrapods,” said lead author Catherine Boisvert, also of Uppsala University.