Eye-ing a better future on land

Richard Collins examines the possibilities for marine life should they evolve to leave the waters and come on land.

Around 440 million years ago, a fungus moved from the sea to the land. It was probably the first large living thing to do so. Life had been developing in the oceans for 3,000 million years by that time. Yet, marine animals remained relatively primitive.

Most still are today. Only the octopus, and its squid relatives, can challenge land-based animals intellectually; these invertebrate escapologists out-perform all other sea creatures at problem-solving. The other sophisticated ocean dwellers, the whales and dolphins, are ‘blow-ins’, air-breathers descended from land-based ancestors which took to seafaring 50 million years ago.

Fish, long considered to be mindless robots with limited attention spans, are more intelligent than we once thought but nowhere as brainy as some of their land-based descendants, the mammals. What held the fish back and why, once they had colonised the land, did their descendents do so well?

Researchers at Northwestern University examine these questions in a paper just published. Their new theory has been dubbed the ‘buena vista’ hypothesis.

As every scuba-diver knows, it gets dark quickly as you descend into the depths; light travels poorly under water. Visibility is 70 times better on land than in the foggy oceans. A fish’s viewing space would increase a million fold were it able to leave the water.

Being able to see only objects close to hand has led to a highly reactive lifestyle. The sudden appearance of a predator, for example, requires instant action. With no time to ponder other options, fish had no incentive to become philosophers.

Around 385 million years ago, something extraordinary happened. At that time, fish were the only backboned creatures. The ancestors of modern spiders millipedes and centipedes had already moved onto the land.

Feeding on algae, these invertebrates prospered. What temptation they must have presented to any fish which could find a way to catch and eat them. Fins evolved into primitive legs, enabling fish to make feeding sorties onto land the way mud-skippers do today. In due course, this gave rise to amphibians, like the one which left its footprints on the mud of what is now Valentia Island.

The Northwest University researchers, however, argue that changes in the position and structure of eyes, rather than the evolution of limbs, were the real driving force behind the great leap forward. Examining 59 fossil specimens dating from before, after, and during the first land invasions, the scientists found that the eyes of some fish species gradually tripled in size. Why?

Having bigger eyes would be of little benefit to creatures living in the murky depths of the sea. Most fish have eyes at the sides of the head, giving all-round vision. In bottom-dwelling species, however, they have moved to the top of the head; those of plaice and rays are modern examples.

If such eyes could protrude above the water surface, as modern crocodile ones do, a flat-fish could see the land and the abundant food it offers. Changes in the configuration and size of eyes, according to the new theory, triggered the evolution of creatures which hunted prey above the water-line and beyond.

Brain size, the researchers found, also increased during the period. With greatly enhanced vision, amphibians could see potential food sources, and spot predators, from a distance. No longer needing to react immediately to every change in circumstances, early land visitors had time to reflect and plan their actions.

However, they needed larger brains to do so effectively. Being able to see from afar, the researchers argue, led to ‘the emergence of elaborate action sequences through planning circuits in the nervous system’ and so the cerebral arms race, which would lead eventually to you and me, began.

  • Malcolm MacIver et al. Massive increase in visual range preceded the origin of terrestrial vertebrates. Proceedings of the National Academy of Sciences. 2017.


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