Cook Island fish evolving in real time
All mammals on Earth were originally fish swimming freely around the ocean, including humans. Our aquatic ancestors dragged themselves out of the ocean 400 million years ago and onto the beaches of the world as they were all the way back then. From there they went onto inhabit and dominate almost every corner of the planet.
We know that this was the general process of human evolution but we have never been sure of the exact details thanks to the very long timescale. The fact it happened so long ago means that the fossil record can be sporadic and we have had to fill some gaps with educated guesswork.
However, this might not be the case in the future.
Evolutionary ecologists from the University of New South Wales in Australia have been observing blenny fish on the Cook Islands and come across something quite extraordinary. Right now, today, as you are reading this, the blennies are relocating from the ocean to the land in order to escape predators.
The blennies live in shallow rock pools at low tide and haul themselves up onto the rocks when the tides come in and bring predators. They shuffle around on the rocks until the tide retreats and the waters are relatively safe again. Could this be a clue as to why our own fish ancestors once felt the need to climb out of the sea?
Once on land, the benefits are obvious for the blennies. They escape predators, obviously. Tests run at high tide with fake plasticine blenny models showed that the models were riddled with puncture wounds and scrapes by the time the tide went out again. There is a risk of being eaten by birds on land, but the rate of predation was approximately a third of that measured in the water.
The blennies also found more beneficial environments in which to lay eggs. Holes in the rocks provide a sheltered environment away from marine predators and the rough and tumble of the tides, increasing the chances of survival. In addition to this, the algae and bacteria which blennies eat are plentiful on top of rocks.
Surviving temporarily on land is a small step in the process of evolution. On Rarotonga, one of the Cook Islands, there is evidence that several species of blenny has already evolved to be able to permanently leave the water and live a life on land. They continue to breathe through their gills but have developed an ability to jump between rocks via a stronger than usual tail fin.
This is all very interesting, but the fact remains that we will likely not be around long enough to see this fish transform into anything like a mammal – if that is indeed the destiny of the blenny. What it does tell us is that our distant ancestors may have originally left the water to escape a threat and survive. This is fascinating because it is not that hard to apply similar motives to the latest set of explorers looking to secure a life for the human race beyond earth in the face of a rapidly changing climate.