A new biological study claims it has worked out how our ancient aquatic ancestors made that momentous leap from water to land.

Contrary to prior assumptions of the evolution of fins into limbs, a new hypothesis from Northwestern University points out that it was better vision that helped crocodile-like animals to spot easy prey on land, which then led to the creation of limbs.

Researchers studied 59 different fossils, and discovered that the eyes nearly tripled in size before the water-to-land transition.

About 12 million years before the move, the average eye socket was 13 mm (0.5 inch) in size, but by the time the animals finally crawled out of the water, that figure had almost tripled, to 36 mm (1.4 inch).

This coincided with a shift in location of the eyes from the side of the head to the top. The expanded visual range of seeing may have eventually led to larger brains in early terrestrial vertebrates, lending them the ability to plan and not merely react, as fish do.

Larger eyes meant that animals could now see 70 times farther in air than in water.

The study, “Massive Increase in Visual Range Preceded the Origin of Terrestrial Vertebrates,” was published on March 7 by the journal Proceedings of the National Academy of Sciences (PNAS).