About 400 million years ago a group of fish began exploring land and evolved into tetrapods - the amphibians, reptiles, birds, and mammals of today. Now researchers in Canada have used a living fish to help show what might have happened when fish first attempted to walk out of the water.
The team from McGill University raised juvenile Polypterus on land for nearly a year, with an aim to revealing how these 'terrestrialised' fish looked and moved differently. Polypterus can breathe air, 'walk' on land, and looks much like those ancient fishes that evolved into tetrapods.
"Stressful environmental conditions can often reveal otherwise cryptic anatomical and behavioural variation, a form of developmental plasticity," says Emily Standen, a former McGill post-doctoral student who led the project. "We wanted to use this mechanism to see what new anatomies and behaviours we could trigger in these fish and see if they match what we know of the fossil record.”
The fish showed significant anatomical and behavioural changes. The terrestrialised fish walked more effectively by placing their fins closer to their bodies, lifted their heads higher, and kept their fins from slipping as much as fish that were raised in water. "Anatomically, their pectoral skeleton changed to become more elongate with stronger attachments across their chest, possibly to increase support during walking, and a reduced contact with the skull to potentially allow greater head/neck motion," says Trina Du, a McGill Ph.D. student and study collaborator.
"Because many of the anatomical changes mirror the fossil record, we can hypothesise that the behavioural changes we see also reflect what may have occurred when fossil fish first walked with their fins on land," says Hans Larsson, Canada Research Chair in Macroevolution at McGill and an Associate Professor at the Redpath Museum.
"This is the first example we know of that demonstrates developmental plasticity may have facilitated a large-scale evolutionary transition, by first accessing new anatomies and behaviours that could later be genetically fixed by natural selection."
You can see a short video below showing the walking behaviour of the Polypterus.
The result of the research is published in the journal Nature.
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