In an unprecedented experiment, researchers have taken juveniles of a fish known to occasionally walk on its fins and raised them on land. The result illustrates the kinds of changes that might have enabled fins to become limbs when some fish traded water for land, around 400 million years ago.
Lead author Emily Standen walks Nature's Kerri Smith through her experiment on teaching fish how to walk.
The evolution of walking limbs was a big step that would eventually give rise to tetrapods, which include almost all land vertebrates from amphibians to mammals. To better understand the evolutionary pressures on those early tetrapods, lead author Emily Standen, now at the University of Ottawa in Canada, and her colleagues took juvenile bichir (Polypterus senegalus) — freshwater fish that live in tropical Africa — and raised them on land. Bichirs are able to survive out of the water because they have primordial lungs in addition to their gills.
After 8 months, the terrestrially raised bichir had a more sophisticated style of walking than did aquatically raised controls. Furthermore, their bone structure and musculature changed to be more suited to a walking lifestyle.
The results provide evidence for developmental plasticity, in which organisms alter their anatomy and behaviour in response to environmental change. The team suggests that this process, as demonstrated by the bichir, could have given the earliest tetrapod ancestors the ability to venture onto land. In doing so, claims Standen, they would have become exposed to the selective pressures of a terrestrial environment, thereby speeding up the evolutionary transformation from fins for swimming into limbs for walking.
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